Polarization handling in photonic integrated circuits

Polarization handling in photonic integrated circuits
Polarization handling in photonic integrated circuits
Augustin, L.M.
DOI:
10.6100/IR634815
Published: 01/01/2008
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Citation for published version (APA):
Augustin, L. M. (2008). Polarization handling in photonic integrated circuits Eindhoven: Technische Universiteit
Eindhoven DOI: 10.6100/IR634815
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Polarization Handling in
Photonic Integrated Circuits
Polarization Handling in
Photonic Integrated Circuits
PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Technische Universiteit Eindhoven,
op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn,
voor een commissie aangewezen door het College voor Promoties
in het openbaar te verdedigen op
maandag 2 juni 2008 om 16.00 uur
door
Ludovicus Marie Augustin
geboren te Maastricht
Dit proefschrift is goedgekeurd door de promotoren:
prof.dr.ir. M.K. Smit
en
prof.dr.ir. R. Baets
Copromotor:
dr. J.J.G.M. van der Tol
This work was supported by the Dutch Ministry of Economic Affairs (NRC Photonics) and the
European Community (IST- MUFINS , IST- STOLAS).
Copyright ©2008 Ludovicus Marie Augustin
Typeset using LATEX, printed in The Netherlands.
CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN
Augustin, Ludovicus Marie
Polarization handling in photonic integrated circuits / by Ludovicus Marie Augustin. Eindhoven : Technische Universiteit Eindhoven, 2008.
Proefschrift. - ISBN 978-90-386-1854-8
NUR 959
Trefw.: opto-elektronica / geintegreerde optica / optische polarisatie / 3-5 verbindingen.
Subject headings: optoelectronic devices / integrated optoelectronics / light polarisation /
III-V semiconductors.
aon pap en mam
Contents
1
Introduction
1
1.1
2
4
5
6
6
1.2
2
Integrated components and their polarization properties
2.1
2.2
2.3
2.4
2.5
2.6
3
Generic integration technology with polarization handling capability .............
1.1.1 POLARIS wavelength converter .......................................................
1.1.2 Polarization independent SOA..........................................................
1.1.3 Polarization MZI ............................................................................
Structure of this thesis ..............................................................................
Active-passive integration .........................................................................
Waveguides.............................................................................................
MMI couplers .........................................................................................
2.3.1 Design...........................................................................................
2.3.2 Reflections.....................................................................................
Spot size converters..................................................................................
2.4.1 Horizontal tapers ............................................................................
2.4.2 2D tapers .......................................................................................
SOAs .....................................................................................................
2.5.1 Design...........................................................................................
2.5.2 Measurements on the QW SOA ........................................................
Conclusions ............................................................................................
Integration technology
3.1
3.2
9
10
11
15
15
18
22
22
23
28
31
32
34
37
Introduction ............................................................................................
Active-passive integration .........................................................................
3.2.1 Active-Passive butt-joint ..................................................................
3.2.2 PIC processing ...............................................................................
vii
37
37
38
39
viii
CONTENTS
3.3
3.4
3.5
3.6
4
Polarization converters
4.1
4.2
4.3
4.4
4.5
5
High resolution lithography.......................................................................
3.3.1 Waferstepper ..................................................................................
3.3.2 Electron beam lithography ...............................................................
3.3.3 Technology ....................................................................................
Technology for packaging.........................................................................
3.4.1 Alignment on the submount .............................................................
3.4.2 Vertical taper..................................................................................
Sloped sidewalls ......................................................................................
3.5.1 Wet etch ........................................................................................
3.5.2 Masking ........................................................................................
Conclusions ............................................................................................
Introduction ............................................................................................
Principle .................................................................................................
First generation .......................................................................................
4.3.1 Design...........................................................................................
4.3.2 Fabrication.....................................................................................
4.3.3 Characterization .............................................................................
4.3.4 Integration .....................................................................................
Second generation....................................................................................
4.4.1 Design...........................................................................................
4.4.2 Fabrication.....................................................................................
4.4.3 Characterization .............................................................................
Conclusion..............................................................................................
Polarization splitters
5.1
5.2
5.3
5.4
Introduction ............................................................................................
Directional coupler polarization splitter ......................................................
5.2.1 Principle ........................................................................................
5.2.2 Simulations....................................................................................
5.2.3 Design...........................................................................................
5.2.4 Fabrication.....................................................................................
5.2.5 Characterization .............................................................................
MZI Polarization splitter...........................................................................
5.3.1 Principle ........................................................................................
5.3.2 Simulation .....................................................................................
5.3.3 Design...........................................................................................
5.3.4 Fabrication I...................................................................................
5.3.5 Characterization I ...........................................................................
5.3.6 Design and fabrication II .................................................................
5.3.7 Characterization II ..........................................................................
Conclusions ............................................................................................
42
42
43
44
45
45
47
50
50
51
52
55
55
56
60
60
63
65
66
68
69
69
73
74
75
75
77
77
81
84
84
85
87
88
89
93
94
95
96
97
98
CONTENTS
6
Wavelength converters
6.1
6.2
6.3
6.4
6.5
7
7.5
7.6
8
137
Conclusions ............................................................................................ 137
Recommendations ................................................................................... 138
Outlook .................................................................................................. 139
8.3.1 Polarization control......................................................................... 139
8.3.2 Polarization switchable laser ............................................................ 141
A Polarization description and visualization tools
A.1
A.2
A.3
117
Introduction ............................................................................................ 117
Principle of the POLARIS wavelength converter.......................................... 118
Simulation study and POLARIS concepts ................................................... 119
Fiber based POLARIS.............................................................................. 124
7.4.1 Experiments ................................................................................... 125
Integrated POLARIS ................................................................................ 128
7.5.1 Design........................................................................................... 128
7.5.2 Generic integration technology with polarization handling capability .... 130
7.5.3 Finished chip.................................................................................. 131
Conclusions ............................................................................................ 134
Conclusions and Outlook
8.1
8.2
8.3
99
Introduction ............................................................................................ 99
Principle ................................................................................................. 100
First generation ....................................................................................... 102
6.3.1 Design and fabrication..................................................................... 102
6.3.2 Static characterization ..................................................................... 102
6.3.3 Dynamic characterization ................................................................ 105
Second generation.................................................................................... 109
6.4.1 Design and fabrication..................................................................... 109
6.4.2 Packaging ...................................................................................... 111
6.4.3 Static characterization ..................................................................... 111
6.4.4 Electrical switching......................................................................... 113
6.4.5 Dynamic characterization ................................................................ 114
Conclusions ............................................................................................ 114
POLARIS
7.1
7.2
7.3
7.4
ix
143
Polarization............................................................................................. 143
Jones vector ............................................................................................ 144
Stokes parameters .................................................................................... 146
References
149
List of abbreviations
157
x
CONTENTS
Summary
159
Samenvatting
161
Dankwoord
163
Curriculum Vitae
167
List of publications
169
Journal articles ................................................................................................. 169
Conference contributions ................................................................................... 169
Chapter 1
Introduction
Optical communications has been part of human life since ancient history. The first optical
communication was based on visible transmission of messages: evolving from simple smokesignalling to the optical telegraph in 1792: semaphores with relay stations invented by Claude
Chappe. Only in the last half of the twentieth century fiber-optic communication became reality, by invention of the laser in 1960 [1], but mainly by the development of the semiconductor
laser [2] and the low-loss fiber [3] in 1970. It took until approximately 1990 for the first long
distance links to be commercially employed. This became possible by the invention of the
Erbium Doped Amplifier (EDFA) in 1987.
Optical fiber communication is the core of the internet. In the last decade the internet has
grown enormously. This growth still continues and therefore the amount of data traffic is increasing dramatically. To deal with this increasing demand, the optical networks need to be
improved even further. The bandwidth of the fibers is very large. With Wavelength Division
Multiplexing (WDM), this bandwidth can be employed to increase the capacity of the network
further. In this technique, multiple wavelengths are used in parallel in a single fiber for carrying multiple independent signals.
In the nodes of the network, optical and electrical components are present to manipulate the
light: to amplify, to regenerate, to convert to another wavelength, or to switch the light to the
desired destination. These functions can be performed in two ways: by converting the light
to an electrical signal, analyse and process it, and convert it back to the desired optical signal
again; or by doing it all-optically. All-optical signal processing will enable the highest switching speeds, because the limited speed of electronics is still a bottleneck.
Photonic Integrated Circuits (PICs) are promising devices for all-optical processing. By inte1
2
Introduction
grating optical functions in a single chip in a PIC the reliability and stability can be improved,
the size and costs can be reduced with respect to using separate bulk components, and the scalability improved.
A disadvantage of the small size of a PIC is the coupling to an optical fiber. The coupling
tolerances are tight and the losses high. A solution is to integrate spot size converters into the
PIC to ease the coupling and to decrease the losses.
Another problem is that the planar geometry of a PIC causes different behavior for the two
orthogonally polarized modes. TE polarized modes: where the electric field vector is parallel
to the PIC surface, and TM polarized modes: where the electric field vector is perpendicular to
the surface, have different boundary conditions which lead to different propagation constants
and confinement factors. Given the fact that the fibers in the network do not preserve the state
of polarization of the light, this leads to a varying performance of the PIC as the polarization
of the incoming light changes.
To overcome this problem, different approaches can be applied. One approach is to remove
polarization dependence by changing the properties of the material [4, 5] or the geometry of
the waveguides [6, 7]. This can be difficult or even impossible, and furthermore will always be
compromising with respect to optimal performance for one of the polarization states.
An alternative is polarization diversity: create subcircuits for each of the two polarization
states. In this way, components optimized for one well-defined state of polarization can be
used and the polarization can be matched to the optimal performance.
Polarization is not only a problem, it can be very useful. A consequence of the waveguide
structure and its birefringence is that the two polarizations are very stable inside the waveguides in a PIC. Conversion between them is only obtained in very short bends or specially
designed devices. Because of this, the polarization can be used for new functions [8, 9], some
of which will be discussed in this thesis.
On-chip manipulation of the polarization can help to achieve polarization diversity and to add
extra functionality based on polarization. This requires a generic technology for polarization
components that can be integrated with standard active and passive components on a PIC. The
next section will present an overview of this generic technology.
1.1
Generic integration technology with polarization
handling capability
In this thesis a generic technology for PICs with integrated polarization manipulation functions
is developed. This is achieved by extending the existing technology. The standard COBRA
active-passive technology consists of the following building blocks.
• Passive devices, such as waveguides and splitters, which are transparent for the operation
wavelengths and can be used for functions like guiding, splitting and filtering of the light
on the chip.
• Electro-optic phase modulators that are used to control the phase of the light.
1.1 Generic integration technology with polarization handling capability
3
• Semiconductor Optical Amplifiers (SOAs), which are used to amplify light and can be
used as a non-linear element, with a power-dependent transfer for both amplitude and
phase of the light.
In this thesis, the extension of the standard technology with an improved integrated spot size
converter is described. This eases the coupling of a PIC to a standard single mode fiber and
reduces the coupling losses, enabling the possibility of packaging the PIC.
In the standard components the performance is polarization dependent. These properties can
be used to manipulate the phase and amplitude difference between the two polarizations. However, for full polarization handling two additional components are required: polarization converters and polarization splitters. Polarization converters are needed to convert one polarization
state into the other. Polarization splitters are necessary to separate the different polarizations
and route them into different optical paths.
The main effort of the work described in this thesis focuses on extending the standard technology with a new type of polarization converter and its application in a novel polarization
independent wavelength converter. Furthermore a novel type of polarization splitter has been
developed that consists of a passive Mach Zehnder Interferometer and polarization converters.
Thus by only adding a polarization converter, the generic platform with polarization handling,
including a polarization splitter is obtained. Secondly by the addition of a spot size converter,
packaging of the PICs becomes feasible. Fig. 1.1 shows the full set of devices that is contained
in this generic platform.
Figure 1.1: Generic integration technology with polarization handling capability.
The next section will show some examples of improvements and new functions that will be
possible by using the new generic integration platform with on-chip polarization handling.
4
1.1.1
Introduction
POLARIS wavelength converter
The performance of a wavelength converter can be optimized by using the polarization. Wavelength converters are key components in wavelength routed optical networks. The most promising wavelength converters are based on Mach Zehnder Interferometers (MZI) with Semiconductor Optical Amplifiers (SOA) in the arms [10]. In these devices the signal from the network
(pump) changes the phase in one of the arms and the data from the pump is transferred to a
locally generated signal (probe). These devices operate best if they are used in co-directional
operation [11], but this results in the two signals being present in the output of the device. The
pump has to be filtered out. For this purpose tuneable wavelength filters are required. These
are expensive and difficult to integrate, furthermore it poses a problem if conversion to the
same wavelength (for regeneration) is required.
A solution, which allows conversion to the same wavelength and still uses a co-propagation
scheme, is the Dual Order Mode (DOMO) wavelength converter [12, 13]. In this scheme, the
pump is injected as the first-order mode, while the probe is in the fundamental mode. By using
an MMI as a modefilter, the two signals can be separated. Here mode coupling will degrade
the performance severely. Furthermore, polarization dependence problems still occur.
To overcome these problems, a new scheme for wavelength conversion is presented. This
scheme uses the polarization to facilitate the desired co-directional propagation.
The improved wavelength converter is obtained with the POLARIS concept (POlarization Labeling for Rejection and Isolation of Signals) . POLARIS is a polarization diversity scheme in
which polarization is used for signal labeling. A schematic drawing is shown in Fig. 1.2. The
signal from the network (pump signal), which carries the data, arrives in an arbitrary polarization. This signal is split into two orthogonal polarizations. In one branch, the polarization is
rotated to have the pump signal in both branches in the same polarization state (e.g. TE). These
signals are injected into the MZIs together with the locally generated CW light (probe signal)
in the orthogonal polarization (e.g. TM). After interacting in the MZI the data information is
transferred to the CW wavelength and both signals have to be separated. This is achieved by
rotating the polarization of the upper branch and then using a polarization splitter/combiner to
combine both branches. As only TE in the upper and TM in the lower input of the combiner
will couple to the output, filtering of the unwanted signal occurs.
Probe, CW
MZI
Pump signal,
arbitrary
polarization
PC
PS
PS
PC
MZI
Probe, CW
Figure 1.2: POLARIS wavelength converter
Output, Probe
1.1 Generic integration technology with polarization handling capability
5
POLARIS allows polarization independent (with polarization sensitive SOAs), co-propagating
wavelength conversion, without the need to use wavelength filters. It even allows data transfer
to the same wavelength, for regeneration and full flexibility.
The POLARIS concept can also be employed in a number of other applications where interaction between different optical signals is needed, like in all-optical switches.
Apart from the POLARIS concept, the integration of the polarization components in the generic
technology offers many more possibilities. Some examples are given next.
1.1.2
Polarization independent SOA
In SOAs the polarization dependent behavior can be problematic, leading to different propagation, amplification and non-linear phase shifts for the two orthogonal polarizations.
In a PESSOA (Polarization Effect Suppression in Semiconductor Optical Amplifiers) device,
on-chip polarization manipulation is employed to avoid polarization dependence. Two solutions are depicted in Fig. 1.3. In the first a polarization converter is placed halfway in an
SOA, causing any arbitrarily polarized signal at the input to experience TE-amplification and
TE-phase shift in one half of the device, but TM-amplification and TM-phase shift in the other
half. The net effect is in principle polarization independent, both for amplification and phase
shift.
The second solution uses polarization diversity: an incoming optical signal is split into a TEand a TM-part with a polarization splitter. The TM-part is first converted to TE with a polarization converter and then both signals are fed trough an SOA. In this way both parts experience
TE-amplification and TE-phase shift. At the output, a polarization converter is placed in the
other branch to balance the losses. In this branch the original TE-part is converted into TM, so
both parts can be combined with a polarization splitter.
SOA
Input
½ SOA
PC
½ SOA
Output
Input
PS
PS
PC
(a) Cascade PESSOA
PC
Output
SOA
(b) Polarization diversity PESSOA
Figure 1.3: Two types of PESSOA (Polarization Effect Suppression in Semiconductor Optical Amplifiers).
This example can be extended to other devices as well. Therefore, with a technology for onchip polarization handling, new solutions to polarization problems in standard components
become possible.
6
1.1.3
Introduction
Polarization MZI
The polarization can be used to provide light with two independent virtual paths in the same
physical waveguide. This can be applied to form an MZI. In a traditional MZI, two different
physical paths are used; the phase difference between the two paths results in an interference
at the output coupler.
The polarization switch (Fig. 1.4) is basically an MZI in which the two arms are separated
by using different polarizations. Nonlinear polarization rotation in an SOA can be used for
switching and wavelength conversion [14, 15]. An integrated version of this component is
possible with the proposed polarization technology. The advantage is a smaller footprint and
lower losses, because of the avoidance of bends.
At the input of the switch, the pump and the probe are converted to orthogonally circular
polarized signals in the input half PC. Both signals are injected into the SOA which functions
as a nonlinear phase shifter. At the output of the SOA, the TE and TM polarized parts of the
signals have experienced a power-dependent phase shift. The signals are combined again into
the output half PC. Depending on this relative phase shift, the probe couples to TE or TM. By
putting a polarization splitter at the output, switching can be obtained.
PC/2
SOA
PC/2
PS
Figure 1.4: Integrated polarization MZI switch.
These examples show that a generic integration technology of active and passive components
with polarization converters enables a broad variety of functions and improvements in PICs.
By also integrating a spot size converter the devices can be packaged and be versatilely used.
In this way a very strong technology platform is obtained.
1.2
Structure of this thesis
The aim of this thesis is to develop a generic technology for on-chip polarization handling.
To this end, the development of separate polarization components (converters and splitters) is
investigated together with their integration in a single PIC.
The design and polarization properties of the integrated components that are available in the
standard technology (waveguides, MMIs and SOAs), are discussed in chapter 2. In this chapter
also an improved spot size converter is introduced, that can be integrated with active components.
The required integration technology for PICs is treated in chapter 3. The standard technology
1.2 Structure of this thesis
7
is explained, as well as additional processing steps for advanced features, such as high resolution lithography, spot size converters, precision cleave etches and polarization converters.
The next chapters deal with the polarization components. In chapter 4 two generations of polarization converters are described. The design, fabrication and results are shown. A new PC is
developed that is fully integrateable in a PIC. In chapter 5 polarization splitters are discussed.
A novel splitter is introduced based on an MZI with polarization converters in its arms. Using
this device, the generic platform for polarization manipulation can be obtained by adding only
one component, the polarization converter.
A important building block of the POLARIS concept is a Mach Zehnder Interferometer. In
chapter 6 we describe two wavelength converters based on MZIs as a first step towards the
realization of a POLARIS wavelength converter. One of these SOA-MZIs is integrated with
spot size converters and has been packaged. With this device conversion at 40 Gb/s has been
demonstrated.
With these building blocks, a POLARIS wavelength converter can be constructed. The concept
is explored in more detail in chapter 7. Simulations are shown and the concept is experimentally demonstrated by using the integrated MZI from chapter 6 with the polarization manipulation done outside the chip with fiber based polarization components. Furthermore, the design
and fabrication of an integrated version of POLARIS are discussed.
Finally in chapter 8 conclusions are drawn and an outlook is presented.
Chapter 2
Integrated components
and their polarization properties
Photonic Integrated Circuits (PIC) contain different components. First of all they contain passive components: waveguides, required to guide the light on the chip, splitters, and couplers.
Furthermore, to actively control the light, active components, such as SOAs, and electro-optic
devices like phase modulators are needed.
This chapter treats the designs and polarization properties of the passive components and the
SOAs that will be used in the PICs of chapters 6 and 7. The polarization manipulating components are discussed separately in chapters 4 and 5.
All components have to be integrated in a layerstack compatible with the active-passive integration scheme. This layerstack will be described first.
In the subsequent sections the passive components are considered. First shallow and deep
waveguides, then Multi Mode Interference (MMI) couplers, used as splitters and combiners,
are discussed. Their properties and the measures taken to reduce reflections are presented and
experimentally confirmed. Furthermore fiber-chip coupling is examined. First simple lateral
tapers to improve this are described. Next a full spot size converter is introduced to couple
PICs efficiently to single mode fibers.
In the last section, the design and characterization of the SOAs are discussed, including the
measures that are developed to prevent lasing because of reflections.
9
10
2.1
Integrated components and their polarization properties
Active-passive integration
The material system of choice for PICs is InP/InGaAsP. These materials are direct bandgap
semiconductors and hence light can be generated and amplified inside them. By changing
its composition, the bandgap of InGaAsP can be adjusted, while keeping it lattice matched
to InP, which is needed to allow epitaxial growth on InP. The bandgap can be tuned over a
wide wavelength range, ranging from 1µm to beyond 1.6 µm. This makes the material system
specially suited for operation at the telecom wavelengths around 1.55 µm. The bandgap can
be tuned for passive components to be transparant for 1.55 µm, and for active functions to be
absorbing and, when current is injected, amplifying.
The layerstacks for integration of both active and passive functions are discussed here.
Passive layerstack
The passive layerstack is designed to transport light with low loss. The waveguide layer consist of a Quaternary (InGaAsP) layer with a bandgap of 1.25 µm. This value is a trade-off
between high index contrast with respect to InP for good waveguiding, high electro-optical
phase modulation, low absorbtion loss for a wavelength of 1.55 µm, and good absolute refractive index control [16]. A thickness of 500 nm is chosen to be as thick as possible for low loss,
while maintaining the mono-mode condition in the vertical direction. This Q(1.25) layer is
sandwiched between InP layers. The first 300 nm of the upper cladding are non intentionally
doped, to avoid doping induced losses. All thicknesses and doping levels are optimized according to [17]. The total passive layerstack in shown in table 2.1, the tints are used throughout this
thesis.
Active layerstack
The active layerstack contains an active region centered in a Q(1.25) waveguide layer. In the
first experiments, the active region consists of a 120 nm thick bulk Q(1.55) active region as was
used in [17, 16]1 . In later experiments the active region consists of 8 pairs of unstrained InGaAs Quantum Wells (QW)2 with strained InGaAs barriers, having a total thickness of 89 nm.
The choices made for the number and nature of the QWs is explained in section 2.5. The total
thickness of the active waveguide layer is 500 nm to match the passive waveguide layer.
The first 300 nm of the upper cladding are lowly P-doped in this case, with a doping concentration high enough to reduce the series resistance for the SOA and low enough to avoid
additional losses. Because of Zn diffusion some doping will be present in the upper part of
the Q1.25 layer. This will result in a PN junction close to the active region. The total active
layerstack in shown in table 2.2.
1 The
2 The
bulk active material was grown at JDS Uniphase, Eindhoven
QW active material was grown at the Centre for Integrated Photonics, Ipswich, UK
2.2 Waveguides
11
Table 2.1: Passive layerstack
2.2
Thickness [nm]
Material
Doping level [cm−3 ]
100
P-InGaAs
> 1 · 1019
1000
P-InP
1 · 1018
200
P-InP
5 · 1017
20
P-Q(1.25)
3 · 1017
300
i-InP
n.i.d.
500
i-Q(1.25)
n.i.d.
500
N-InP
3.5 · 1017
substrate
N-InP
2 · 1018
Legend
Waveguides
Transparent waveguides are used to guide the light on a PIC. For the purpose of this thesis, the
main properties of waveguides are the birefringence and the propagation losses. The loss in
a waveguide should be as small as possible. The birefringence can be either enhanced, to be
able to use it, or made as small as possible, to be able to neglect it.
The main design parameters are the width and the etch depth. Two types of waveguides are distinguished: shallowly and deeply etched waveguides. Shallow waveguides are etched 100 nm
into the film layer; deep waveguides are etched completely through.
The birefringence, the difference in refractive index for the two orthogonal polarizations, is
causing polarization dependent behavior. In a waveguide, the modal birefringence, the difference in effective index (∆Neff = Neff ,TM − Neff ,TE ) that the propagating polarization modes experience, is caused by a different confinement of the light for the two polarizations, by different
electromagnetic boundary conditions, and by the possible presence of material birefringence.
These different causes of birefringence are now investigated further.
Modal birefringence
Given a certain material refractive index (and birefringence, the difference in the refractive
indices ∆n), the modal birefringence can be designed. The birefringence depends on the geometry of the waveguide, especially on the width and the thickness of the topcladding.
12
Integrated components and their polarization properties
Table 2.2: Active layerstack
Thickness [nm]
Material
Doping level [cm−3 ]
100
P-InGaAs
> 1 · 1019
1000
P-InP
1 · 1018
200
P-InP
5 · 1017
20
P-Q(1.25)
3 · 1017
300
P-InP
3 · 1017
190 / 205
i-Q(1.25)
n.i.d.
120 / 89.5
i-Q(1.55) / 8 41Å InGaAs
Quantum Wells, 9 63Å InGaAs Barriers
n.i.d.
190 / 205
i-Q(1.25)
n.i.d.
500
N-InP
3.5 · 1017
substrate
N-InP
2 · 1018
Legend
The birefringence in the waveguide is calculated using the Film Mode Matching method [18].
The birefringence as a function of width for a wavelength of 1555 nm is plotted in Fig. 2.1(a)
for both deep and shallow waveguides. For deep waveguides, the birefringence is 0 for a width
of 1.5 µm (Fig. 2.1(a)). A polarization independent waveguide can be obtained for this width,
but the slope of the tangent at this point is steep, so the tolerance in width is very small. In
contrast to a deep waveguide, the birefringence in a shallow waveguide is small and cannot
be influenced much by changing the width and no polarization independent waveguide can be
obtained in this case.
In Fig. 2.1(b) the birefringence as a function of the topcladding thickness is shown for a 2 µm
wide deep waveguide. The birefringence is largest for a thin cladding, as the air-semiconductor
boundary is close to the field in that case. For a thickness above 600 nm the birefringence remains constant.
If high birefringence is needed, a thin topcladding is preferred. Narrowing the waveguides will
increase the birefringence, but will also decrease the width tolerances.
2.2 Waveguides
13
−0.002
Birefringence Neff, TM−Neff, TE
Birefringence Neff, TM−Neff, TE
0.015
0.01
0.005
0
Deep
−0.005
Shallow
−0.01
1
1.5
2
2.5
3
Waveguide width [µm]
3.5
Deep
−0.004
−0.006
−0.008
−0.01
−0.012
−0.014
−0.016
−0.018
0
4
500
1000
Thickness topcladding [nm]
1500
(a) Modal birefringence as a function of width, for a (b) Modal birefringence as a function of topcladding
1500 nm topcladding.
thickness, for a 2 µm wide waveguide.
Figure 2.1: Modal birefringence as a function of width (left) and topcladding (right).
The birefringence has a dispersive nature, which has to be taken into account when operation
over a wide wavelength range is required.
The material dispersion and refractive indices [19] as well as the dispersion of the modal
birefringence are plotted in Fig. 2.2.
−3
x 10
Birefringence Neff, TM − Neff, TE
3.45
3.4
Refractive index
Q1.25
3.35
3.3
3.25
3.2
InP
1.0
10
1.1
1.2
5
1.3
1.4
1.5
1.6
1.8
2.0
3.0
0
−5
1.4
1.45
1.5
1.55
Wavelength [µm]
1.6
1.48
1.5
1.52
1.54
1.56
Wavelength [µm]
1.58
1.6
Figure 2.2: Material dispersion (left) and calculated effective index difference between TE and TM (right) as a
function of wavelength for different waveguide widths.
The dispersion of the birefringence is largest for narrow waveguides and approaches 0 for
waveguides wider than 3 µm. It has to be noted that for waveguides with widths around 1.5 µm,
the birefringence changes sign around 1540 nm. So for shorter wavelengths Neff ,TM > Neff ,TE ,
while for longer wavelengths Neff ,TE is larger. It is important to take this into account if the
phase difference is used for the function of a device, e.g. in the polarization splitter demonstrated in chapter 5.
14
Integrated components and their polarization properties
Material birefringence
The modal birefringence can also be influenced by material birefringence. Material birefringence is caused by a different material refractive index for different orientations of the polarization. In InP/InGaAsP this phenomenon is in principle absent, but it can occur due to e.g.
photo-elastic and electro-optic effects. In a passive waveguide, only the photo-elastic effect
can cause material birefringence.
The material birefringence is influenced by strain, introduced in the growth. The birefringence
for quaternary material grown on (100) InP substrate is linearly dependent on the strain. The
difference between refractive indices parallel (nk , for TE polarized light) and perpendicular
(n⊥ , for TM polarized light) to the interface of the layers can be calculated from [20]:
αpe
ae − as
2
2
−
(2.1)
nk − n⊥ =
S11 + S12
ae
s
is the strain in the material, with as , ae , the lattice constants of the substrate and the
where aea−a
e
epitaxial layer respectively, S11 and S12 [21, 22] are the components of the elastic compliance
tensor, and αpe is the linear photo-elastic coefficient, calculated for lattice matched Q(1.25)
with the model in [23].
The resulting material birefringence as a function of the strain is plotted in Fig. 2.3. The
birefringence is positive (nTM > nTE ) for tensile and negative for compressive strain. These
−3
x 10
0.004
Unstrained
0.04% Tensile strain
eff, TE
0.003
5
0
=N
0
eff, TM
−N
10
eff
0.001
−0.001
∆N
∆n = nTM−nTE
0.002
−0.002
−0.003
−5
−0.004
−0.1
−0.05
Tensile
0
Strain [%]
0.05
0.1
Compressive
1
1.5
2
2.5
3
Width [µm]
3.5
4
Figure 2.3: Material birefringence of strained Q(1.25) Figure 2.4: Modal birefringence of 0.04% tensile
on InP as a function of strain.
strained compared to unstrained Q(1.25)
results are used to calculate the influence of strain on the modal birefringence. For a typical
value of 0.04% tensile strain, the modal birefringence is calculated as a function of width for
deep waveguides. This is plotted in Fig. 2.4. For narrow widths, the additional material birefringence has only a small influence on the modal birefringence; the confinement because of
the geometry of the waveguide plays a larger role. For wider waveguides the material birefringence is more important.
Narrow waveguides are preferred for a device tolerant to strain in the material, but wide wave-
2.3 MMI couplers
15
guides show a more width-tolerant birefringence. This is a trade-off that has to be made depending on the requirements for a device.
In a shallow waveguide the light is less confined as compared to a deep waveguide. Because
of the lower confinement, the influence of sidewall roughness is smaller. A shallow waveguide
therefore has the advantage of lower losses compared to a deep one. 3 µm wide waveguides are
used in most situations to guide light on the PIC. A disadvantage is the large bending radius,
because of the low confinement. Bends should have radii above 450 µm to avoid radiation
losses. If the chip area allows this, large shallow bends are used.
For shorter bends, deep waveguides are required. Deep waveguides can have bending radii
well below 100 µm, but unwanted polarization conversion can occur. In this thesis, the minimum bending radius taken for a 2 µm wide deep waveguide is 150 µm to minimize polarization
conversion [24].
A low-loss (< 0.1 dB) connection between shallow and deep waveguides is made by tapering
with a 100 µm long parabolic taper from the deep waveguide to a width of 3.4 µm to obtain a
mode size equal to that of the 3 µm wide shallow waveguide [25].
The 3 µm shallow as well as the 2 µm deep waveguides are multimode. Excitation of the higherorder modes can occur, but by using modefilters, the second-order modes can be stripped.
Modefilters are 1 × 1 Multi Mode Interference (MMI) couplers, which are explained in the
next section.
2.3
MMI couplers
Couplers are needed on a PIC to split and combine signals. A Multi Mode Interference (MMI)
coupler is the component of choice for this, because of its large operation bandwidth, fabrication tolerance, and polarization independence [26]. These devices are based on self imaging:
the input field profile of a multimode waveguide is reproduced in single or multiple images at
a certain (periodically repeating) distance. By injecting light from a narrow waveguide into
a wide waveguide, higher-order modes are excited in that wide waveguide. The resulting interference of these modes images the input at well defined positions along the length of the
waveguide. By placing output waveguides at these positions, the multiple images can be used
to obtain splitting.
2.3.1
Design
In our case, 1 × 2 and 2 × 2 3-dB couplers, and 1 × 1 mode filters are needed. Both shallow
and deep etched couplers are designed. In this section the design of a shallow 2 × 2 MMI is
discussed as a representative example. The other couplers are designed in a similar way. Their
properties are given at the end of this section.
16
Integrated components and their polarization properties
Paired interference couplers [26, 27] are designed. Paired interference is obtained by restricting
the number of excited modes inside the multimode waveguide. This results in shorter and more
tolerant couplers. The 2 outputs are imaged at a distance Lπ /2, where the beatlength is:
Lπ =
π
β0 − β1
(2.2)
β0 , β1 are the propagation constants for the fundamental and first-order mode in the wide
waveguide.
A schematic of the MMI coupler is shown in Fig. 2.5. A completely shallow device is designed,
having a width WMMI of 12 µm. For this width, an MMI length LMMI = Lπ /2 of 230µm is
required. The width of the input waveguides Win is 3µm and the offset is 2.2µm. This device is
simulated using the scattering matrix method developed by Leijtens et al. [28].
offset
WMMI
Win
x
z
LMMI
Figure 2.5: Schematic drawing of the MMI coupler.
The simulated field intensity profile is plotted in Fig. 2.6(left). The excitation of the higherorder modes inside the wide waveguide is clearly visible. The simulated imbalance and loss
versus the wavelength is plotted in the right figure of Fig. 2.6. The device has an expected
maximum imbalance of 0.15 dB and loss below 0.5 dB over the entire wavelength range of
100 nm.
The simulated imbalance and loss as a function of the deviation in the width are given in Fig.
2.7. It shows that the imbalance is below 0.1 dB and that the loss is below 0.5 dB for a width
deviation of ± 200 nm.
Polarization dependence
The propagation constants are polarization dependent, but as the multimode waveguide in the
MMI is wide, this difference is small. The device itself is therefore only weakly polarization
dependent. The simulated imbalance and loss of the designed MMI (shown in Fig. 2.6 and
Fig. 2.7) for both polarizations, shows this.
Other MMIs
Apart from the shallow 2 × 2 MMI, 4 other types of MMIs are needed. A special MMI is the
1 × 1 MMI, which functions as a modefilter. The fundamental mode in the input waveguide is
2.3 MMI couplers
17
0.2
0.5
Imbalance [dB]
0.15
0.4
0.3
0.05
0.2
Loss [dB]
0.1
0
0.1
−0.05
−0.1
1500
1520
1540
1560
Wavelength [nm]
1580
0
1600
Figure 2.6: Intensity profile in the designed MMI coupler (left) and the imbalance and loss of the coupler as a
function of wavelength for TE (solid) and TM (dashed).
0.1
2.5
2
−0.1
Loss [dB]
Imbalance [dB]
0
−0.2
1.5
1
−0.3
0.5
−0.4
−0.5
−0.5
0
Width deviation [µm]
0.5
0
−0.5
0
Width deviation [µm]
0.5
Figure 2.7: Imbalance (left) and loss (right) in a shallow MMI as a function of the width deviation for TE (solid) and
TM (dashed).
imaged onto the output, but the first-order mode is not, and is thus effectively filtered out.
The dimensions of the 4 other MMIs are summarized in table 2.3. Their tolerances in width
are specified in the table. The width tolerance (∆W ) is the width range in which the MMI
has an imbalance smaller than 0.2 dB and a loss smaller than 0.5 dB for both polarizations.
The maximum polarization dependence within this width range is indicated by the maximum
difference in imbalance (∆Imbalance) and loss (∆Loss) between the two polarizations.
The deep devices are to be used in the polarization splitters and are smaller and therefore
less tolerant to width deviations. The polarization dependent imbalance is negligible and the
polarization dependent loss acceptable.
18
Integrated components and their polarization properties
Description
LMMI [µm]
WMMI [µm]
Win [µm]
Offset [µm]
∆W [µm]
∆Imbalance [dB]
∆Loss [dB]
Table 2.3: Properties of the different MMIs
Shallow 1 × 2 3-dB coupler
115
10
3
2.69
-0.61 – 0.33
x
0.22
Shallow 1 × 1 Modefilter
97
6
3
0
-0.29 – 0.59
x
0.23
Deep 1 × 2 3-dB coupler
49
6.8
2
1.7
-0.10 – 0.16
x
0.24
Deep 2 × 2 3-dB coupler
142
10
3
1.7
-0.16 – 0.10
0.12
0.4
2.3.2
Reflections
The coupler can suffer from reflections that disturb the operation of the PIC. The reflections
can result in formation of an optical cavity, which can lead to unwanted lasing operation for an
integrated SOA (see section 2.5).
In literature the reflection properties of the MMI coupler are studied both theoretically [29]
and experimentally [30].
The reflections in the MMI are caused by small deviations from the design. Because of these,
the image of the input waveguide is not exactly positioned on the output waveguides, but
partly next to it. The image will reflect on the straight wall and form a new image on the input
waveguide. This results in a reflected signal into the input.
In order to reduce the reflections a design with a lossy waveguide is proposed in [31]. This
solution however complicates the device and is not generally applicable.
A more general solution to reduce the reflections from the MMI is achieved by cutting the
corners of the walls on which the reflections occur, as shown in Fig. 2.8. The light incident
next to the waveguide is now reflected out of the MMI and does no longer reach the input
waveguide.
The proposed design is applied for both shallow and deep etched devices for the 1 × 2 and
2 × 2 MMI couplers and for the shallow 1 × 1 modefilter. Here the design and results for the
shallow 2 × 2 3-dB coupler are presented.
Design
In the optical field pattern inside the MMI coupler (Fig. 2.6), the field is absent from the areas next to the access waveguides. As explained before, those areas are a potential source of
reflections. In the new design, these areas are removed, as is depicted in Fig. 2.8. The dashed
line indicates the tilted front and back walls, the original design as presented before is shown
with solid lines. The shape of the MMI coupler is obtained by cutting the corners near the
2.3 MMI couplers
19
access waveguides by an angle Θ. This angle is chosen to be larger than the divergence angle
of the light, which is inversely proportional to the effective width of the mode entering the
MMI section. This divergence angle is estimated to be around 7.5° for the waveguides used.
Because of this, the light entering the wide waveguide will not interact immediately with the
cut sidewalls of the MMI coupler. Therefore the multi-modal interference properties, and thus
the self imaging are not affected. The total length of the MMI with the cut corners remains the
same as with the original one.
Q
1
3
2
4
LMMI
Figure 2.8: Schematic of the MMI, the original (solid) and the optimized (dashed) design.
A shallowly etched (100nm into the film layer) MMI is designed. The MMI length (230µm),
the width (12µm) and the offset of the in- and outputs (2.2µm) are the same as before, the tilt
angle Θ is 20°, well above the divergence angle. On one chip both the normal and the reduced
reflection MMIs are present, as well as shallowly etched straight reference waveguides.
The fabrication of the MMIs is done using the standard technology as described in chapter 3.
The chip is cleaved in such a way that the MMI couplers are not exactly centered between the
two facets of the chip. This allows distinguishing between the different reflections involving
the facets of the chip and the MMI coupler.
Measurements
The splitting ratio of the reduced reflection MMI and the original design are measured to be
equal and around 0.46 (-0.37 dB imbalance). This indicates deviations from the design, which
may cause reflections. It also indicates that the splitting ratio of the reduced reflection MMI is
not affected by the adjustment. The method for reflection determination is based on analyzing
the transmitted spectrum by using a Fast Fourier Transformation [32]. Because of the limited
sensitivity of the high resolution optical spectrum analyzer (HR-OSA), a high input power
(5 dBm in the fiber) is required in order to monitor the possible reflections inside the chip. The
optical setup used is presented in Fig. 2.9.
An erbium-doped fiber amplifier (EDFA) is used as a source. TE polarized light is selected
using a polarizer. The light is launched in one of the inputs of the MMI coupler by means of a
microscope objective. At the straight through (bar) output port of the MMI coupler a tapered
fiber is used to collect the light. This is fed to the input of the HR-OSA, which records the
20
Integrated components and their polarization properties
Polarizer
MMI chip
EDFA
OSA
Figure 2.9: Measurement setup used for the characterization of the MMI couplers.
transmitted spectrum. The high resolution (0.8 pm) will easily monitor large cavity lengths, up
to tens of centimeters, much larger than typical chip sizes of 10 mm.
A cavity of length L will result in a period ∆λ in the spectrum according to:
∆λ =
λ2
2Neff L
(2.3)
in which λ is the central wavelength, and Neff is the effective group index.
The periods present in the spectrum can be obtained by applying a Fast Fourier Transformation
(FFT) to the recorded spectrum. Every component k of the FFT of a spectrum of 2N datapoints,
corresponds to periodicity ∆λ of N kres , in which res is the resolution at which the spectrum is
recorded. The corresponding cavity length L for every k can be obtained from eq. (2.3):
L(k) =
k
λ2
N res 2Neff
(2.4)
The results of the measurements are shown in Fig. 2.10. First, a single straight waveguide is
measured to monitor possible reflections originating from the setup itself. The analyzed data
is presented in the upper graph. The only peak present is from the cavity formed by reflection
of light on both facets of the chip. This chip-peak occurs around 5.4 mm, which matches the
chip length. No disturbing reflections are present in the setup itself. Small satellites are present
next to the chip-peak. Their origin is not identified: they possibly result from some residual
reflection in the setup. They do not pose a problem in this analysis, because the associated
cavity has a different length as those resulting from the MMI reflections.
The middle and lower graphs in Fig. 2.10 present the results for the original and the reduced
reflection MMI respectively. For these, the data is plotted for one set of in- and output ports
(port 2-4, see Fig. 2.8). The other combinations of input and output ports show a similar behavior.
For the standard MMI coupler, in addition to the chip-peak, two extra peaks can be seen around
lengths L1 = 3.16 mm and L2 = 2.55 mm. The length L1 corresponds to the cavity resulting
from the reflections between the right-hand facet of the chip and the left-hand side of the MMI.
L2 corresponds to the cavity between the right-hand side of the MMI and the left-hand facet of
the chip. As expected these are the walls on which the reflections occur.
In the case of the reduced reflection MMI coupler (lower graph) these two peaks are absent
and the graph looks similar to the one obtained for the straight waveguide. This indicates that
the reflections corresponding to the cavity length L1 and L2 are effectively suppressed.
2.3 MMI couplers
21
Figure 2.10: Fast Fourier Transformation of the recorded spectra for a straight waveguide (top), a standard MMI
(middle) and an reduced reflection MMI (bottom).
It is difficult to give an exact value of the magnitude of the reflection on the walls of the MMI
coupler, because of its multiple ports. However, the relative reduction in the reflection can be
estimated by using the measured results. The magnitude of the reflection on the MMI walls in
the case of the non-optimized MMI coupler is calculated by using the values of the strength
of the peak originating from the facets, one of the peaks around L1 or L2 , and -4.9 dB (32%)
as a value of the reflection on the waveguide facet. By assuming that every peak is originating
from a simple FabryPérot cavity, the intensity of the peak Ipeak is proportional to
p
Rchip−facet RMMI
(2.5)
where Rchip−facet , RMMI are the reflectivities of the facets of the chip and the MMI wall respectively. The value obtained from this consideration for the reflection from the MMI is
approximately -30 dB. For the reduced reflection MMI with cut corners, the reflection value
is much lower and cannot be distinguished from the trace in Fig. 2.10. This implies that the
reflection is more than 10 dB suppressed with respect to the original MMIs. The rather high
22
Integrated components and their polarization properties
reflection value obtained for the original MMI coupler is most probably due to deviation from
design parameters. Values better than -40 dB are reported in [30]. The etch depth of the fabricated MMI was 150 nm deeper than the design and the guiding layer was 50 nm thinner. These
two effects lead to increased reflections. The new design still shows the reflection suppression,
which indicates that this solution is very tolerant to fabrication errors.
2.4
Spot size converters
PICs in the InP/InGaAsP system use high-contrast narrow waveguides in a planar geometry.
This yields small and elliptical spot sizes at the output waveguides that do not match the spot
size of a fiber. Connecting a fiber to a PIC will therefore result in large overlap losses and strict
tolerances. The losses can be reduced by using tapered fibers. The reduced circular spot size
of such a fiber can be made to match the spot size of waveguide in one dimension only. The
coupling losses will decrease, but strict tolerances apply.
By enlarging the spot size of the output waveguides to match that of a standard single-mode
fiber, the coupling is simplified and the losses are substantially reduced. This enables packaging of the PIC. This becomes even more necessary if multiple in- and outputs are used, that
need to be coupled to an array of fibers. This will be the case in the wavelength converter of
chapter 6.
2.4.1
Horizontal tapers
The simplest approach for improving the fiber-chip coupling is to horizontally taper the output
waveguide. In this way a matching of the waveguide mode and the fiber mode is obtained in
one dimension. As the spot size of the waveguide is very small, tapered fibers have to be used
with this taper. In Fig. 2.11 the coupling losses based on the overlap of the field from the
tapered fiber, with a circular spot with a Mode Field Diameter (MFD)3 of 3 µm, and a shallow
waveguide are calculated as a function of the width and the horizontal offset between the fiber
and the waveguide.
From this it can be concluded that the optimal width is 5 µm. For this width the minimal losses
are 2.5 dB, a loss penalty smaller than 0.5 dB (3 dB total loss) is maintained with the largest
alignment tolerances. The standard 3 µm shallow waveguides on the PIC are therefore tapered
using 150 µm long tapers to 5 µm wide waveguides at the outputs.
Reflections
The facet reflections have to be very low, especially for active devices, where lasing might
occur. By placing the waveguide at a sufficiently large angle to the facet, the reflected light will
not be coupled back into the waveguide. For a waveguide width of 5 µm, the facet-reflection
back into the waveguide is calculated as a function of the angle of the waveguide on the chip.
The results are shown in Fig. 2.12.
3 The
Mode Field Diameter (MFD) or the spot size is 1/e2 of the intensity (1/e of the field)
2.4 Spot size converters
23
2
0.5
−3dB
Lateral offset [µm]
0
−10
−20
Reflection [dB]
1
−7dB
−6dB
−5dB
−4dB
1.5
0
−0.5
−1
−30
−40
wafer
air
−50
−60
−1.5
−70
−2
3
4
5
6
Waveguide width [µm]
7
Figure 2.11: Overlap loss of the tapered fiber with
the waveguide as a function of the waveguide width
and the offset between fiber and waveguide.
−80
0
2
4
6
o
Angle (waferside) [ ]
8
10
Figure 2.12: Reflection of the 5µm wide output waveguide as a function of the angle.
An angle of 7° is suited for a reflection wel below -40 dB. This taper has been used in a number
of circuits, but has eventually been replaced with the 2D Spot Size Converter (SSC) reported
in the next section, because of the expected improved performance of that device.
2.4.2
2D tapers
The fiber-chip coupling can be improved further. First of all, in the previous section, a tapered
fiber has to be used to reduce the mode size to be comparable to that of the waveguide. This
fiber introduces additional problems. It has to be specially made to match the waveguide, and
the coupling tolerances are rather tight because of the small mode sizes involved. It is much
easier to couple to a simple cleaved single mode fiber (SMF). For matching the spot size of
such a fiber, to decrease the coupling losses further, the integration of a spot size converter is
needed.
Principle
The SSC is based on a previous design for passive PICs [33, 34]. In this thesis an optimized
design is needed, suitable for integration with active components. The device, shown in Fig.
2.13, is made by tapering the standard waveguide both horizontally and vertically down to
a secondary waveguide layer. This waveguide layer has to accommodate a larger spot size,
matched to a single mode fiber, and is therefore referred to as the Fiber Matched Waveguide
(FMW) layer. This FMW layer is obtained by using a decreased n-doping in this layer with
respect to the substrate doping. This decrease yields a larger refractive index [35, 36] for this
layer (Fig. 2.14).
24
Integrated components and their polarization properties
3.17
Refractive index
3.165
3.16
3.155
3.15
3.145
3.14 15
10
Figure 2.13: A schematic of the implemented SSC.
16
10
17
18
10
10
Doping level [cm−3]
19
10
Figure 2.14: Refractive index of InP as a function of
the doping level.
Design
The design of the spot size converter consists of two elements. First a waveguide has to be
designed in which a mode exists that matches the spot size of a single mode fiber. Furthermore
a taper is required to couple the light from the narrow well-confined mode in the standard
waveguide to the large mode in the fiber matched waveguide.
Fiber Matched Waveguide The refractive index as a function of doping [37] is plotted in
Fig. 2.14. From the figure it is clear that the refractive index changes significantly at a doping
level above 1 · 1018 cm−3 . To obtain sufficient index contrast, the substrate doping has to be
above this level. The FMW layer has to be moderately doped, because the active devices on the
PIC use the backside as the N-contact. For the FMW layer a doping of 3.5 · 1017 cm−3 is used.
This results in a refractive index of 3.1676. A substrate with a doping level of 2 · 1018 cm−3 is
used, which has a refractive index of 3.1596 at 1550 nm.
The SSC has to be optimized for efficient coupling to a standard SMF having a circular spot
with a MFD of 10.4 µm. The spot size of the FMW has to match this in both the vertical and
the horizontal direction.
The vertical spot size is determined by the thickness of the layer and its index contrast. A
thick layer is required to match the spot size of the fiber, but above a thickness of 4.5 µm, the
FMW becomes multimode. Furthermore the growth of thick layers is difficult and expensive.
In the growth an error of ±10% can occur, so to avoid a multimode waveguide in the vertical
direction, a thickness of 4 µm is chosen. The guide can still be multimode if the index contrast
is increased due to differences from the designed doping. The doping of the substrates has
been specified to 2 ± 0.5 · 1018 cm−3 . A substrate doping of 2.5 · 1018 cm−3 will still result in a
single mode waveguide at 4.4 µm thickness. Even if the FMW is multimode, the propagation
constant of higher-order mode will not match the propagation constant in the normal waveguide and no coupling will occur.
Apart from possible multi-modal behavior, an increase in the substrate doping will decrease the
2.4 Spot size converters
25
spot size. From simulations, additional 0.15 dB loss is expected for a doping of 2.5·1018 cm−3 .
This is an acceptable penalty.
The horizontal spot size is determined by the width of the waveguide and the etch depth into
the FMW layer. The overlap with the MFD of an SMF is calculated as a function of the width
of the FMW for an etch depth of 1.7 µm (Fig. 2.15). This depth is chosen such that it can
be etched together with a standard shallow waveguide, as will be explained in chapter 3. The
optimum width is 11 µm. For this width the mode field is plotted as well. The profile is not
completely elliptical, because the mode is extending below the etched region. This decreases
the overlap by approximately 0.1 dB with respect to a completely elliptical shape.
−1.4
5
−1.5
y−position [µm]
Overlap Loss [dB]
−1.45
−1.55
−1.6
−1.65
0
−1.7
−1.75
−1.8
6
8
10
Width [µm]
12
14
−5
−6
−4
−2
0
2
x−position [µm]
4
6
Figure 2.15: Calculated overlap loss with an SMF (left) and intensity profile of the FMW (right).
Horizontal and vertical taper The mode of the standard, narrow waveguide has to be converted to the large mode inside the FMW. For this a taper is needed that tapers in both the
horizontal and the vertical direction. The design is split into two parts; the horizontal taper is
designed first. This is a parabolic adiabatic taper from 3µm to the wide 11µm waveguide. A
2D Beam Propagation Method (BPM) [38] is used to calculate the minimum length of the taper. The problem is reduced to 2D by using the Effective Index Method (EIM). The results are
given in Fig. 2.16(a). A length of 350 µm is chosen which has less than 0.01 dB loss penalty.
The vertical taper is also simulated using a 2D BPM. Here the vertical cross section in the
propagation direction is used, with the real material refractive indices. The 3rd dimension can
be neglected in this case. The resulting loss as function of length is plotted in Fig. 2.16(b).
A length of 2 mm is chosen for a reasonable loss penalty of 0.2 dB. These simulations are
performed for TE polarization, but since the tapers are adiabatic devices, the results are also
representative for TM.
26
Integrated components and their polarization properties
0
0
−0.5
−1
−0.5
Loss [dB]
Loss [dB]
−1.5
−1
−1.5
−2
−2.5
−3
−3.5
−2
−4
−2.5
100
200
300
Taper length [µm]
(a) Horizontal taper
400
500
−4.5
0
500
1000 1500 2000
Taper length [µm]
2500
3000
(b) Vertical taper
Figure 2.16: Calculated losses in the tapers as function of width (TE polarization).
Measurements
A chip on which the SSCs are present is fabricated with the process as described in the next
chapter.
The SSC is analyzed using a CCD camera to view the output field. This is plotted in Fig. 2.17.
The measured mode profile agrees very well with the simulations. From these traces the Mode
Field Diameter is obtained to be 10.4 µm in the horizontal and 4.7 µm in the vertical direction.
The overlap losses are estimated using a Gaussian approximation of the field at the Fiber
Matched Waveguide and the fiber:
Loss = 4
MFDFMW, x · MFDFMW, y · MFDSMF 2
MFDFMW, x 2 + MFDSMF 2 MFDFMW, y 2 + MFDSMF 2
(2.6)
Here, MFDFMW,x and MFDFMW,y are the MFDs of the FMW in the horizontal and vertical
direction respectively. MFDSMF is the spot size of the SMF. For the observed values, an overlap
loss of 1.3 dB is expected.
The coupling tolerance to an SMF as a function of the offset is investigated. The measured
and simulated overlap losses are plotted in Fig. 2.18. The measured tolerance agrees well to
the simulations. This results in a horizontal and a vertical alignment tolerance of ± 1.5 µm for
1 dB excess loss, as can be seen in Fig. 2.19.
Reflections
As with the horizontal taper of the previous section, for this spot size converter, the waveguides
should be at an angle to reduce reflections. Also for the FMW an angle of 7° is sufficient to
suppress the reflection below -40 dB, as can be seen in Fig. 2.20.
2.4 Spot size converters
27
1
1
Simulation
Measurement
Simulation
Measurement
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
−10
−5
0
x [µm]
5
0
−5
10
0
y [µm]
5
Figure 2.17: CCD trace of the output field of the SSC (dashed) compared to simulations (solid) for the horizontal
(left) and vertical (right) direction.
−3
−1
−3
−3
.5
−1
2
−1
0
1
xoffset [µm]
.5
−1
−0
yoffset [µm]
−2
0
−1
−1.5
−2
−3
2
−3
−3
3
−2
−1
1
−2
−2
−0.5
−1
5
−1.
−2
−1.5
.5
−0
−1
−1.5
−2
−3
−3
.5
−1 −1
−2
−2
−1
0
−1
.5
−0
1
−0.5
yoffset [µm]
.5
−1
2
3
−2
5
−1.
−2
−2
3
−3
−2
−1
0
1
xoffset [µm]
2
3
Figure 2.18: Coupling tolerance of the SSC to a SMF, simulated (left) and measured (right).
0
0
−0.5
−0.5
Excess loss [dB]
Excess loss [dB]
−1
−1.5
−2
−2.5
−1
−1.5
−2
−3
−3.5
−4
−5
−2.5
Simulation
Measurement
0
xoffset [µm]
5
−3
−3
Simulation
Measurement
−2
−1
0
1
yoffset [µm]
2
3
Figure 2.19: Coupling tolerance of the SSC to a SMF in the horizontal (left) and vertical (right) direction, simulated
(solid line) and measured (x).
28
Integrated components and their polarization properties
0
-10
Reflection [dB]
-20
-30
-40
wafer
air
-50
-60
-70
-80
-90
-100
0
2
4
6
8
10
Angle [o]
Figure 2.20: Reflection of the FMW output waveguide as a function of the angle.
2.5
SOAs
Semiconductor Optical Amplifiers are used in the PIC, either for amplification, or because of
their non-linear properties that are employed e.g. in a wavelength converter or in an all-optical
switch.
An SOA consists of a waveguide with an active layer inside the waveguide layer. Because of
the planar geometry, the propagation constants, amplification and non-linear phase shifts differ
for the two polarizations.
In the active layer of the SOA, with a material gain gm , the incoming light is amplified if
current is injected. Only a part of the light propagates through the active layer. This fraction
is expressed by the confinement factor Γ. Apart from the gain, the device has internal losses,
αint . A simple model for the net modal gain coefficient in an SOA is:
gnet = Γgm − αint
(2.7)
For an SOA of length L this leads to a linear gain G for the SOA:
G = e(Γgm −αint )L
(2.8)
Confinement factor
The confinement factor Γ usually is polarization dependent; it can be twice as large for TE
as for TM [39, 40]. This causes a polarization dependent gain, even if the material gain is
isotropic as is the case in unstrained bulk material.
For the case of an SOA with a 120 nm bulk Q(1.55) active region centered in a 500 nm thick
Q(1.25) waveguide layer, the confinement factor and propagation constants are calculated using the Film Mode Matching method [18] for an equivalent passive waveguide as specified in
Fig. 2.21. The complex part of the refractive index is not taken into account as its influence on
the propagation constants and the mode fields is negligible.
2.5 SOAs
29
w
InP (n=3.164)
Q1.25 (n=3.364)
Q1.55 (n=3.50)
Q1.25 (n=3.364)
InP (n=3.164)
Figure 2.21: Equivalent passive waveguide of the SOA used in calculations.
−0.01
Confinement factor Γ
0.24
Birefringence Neff, TM − Neff, TE
0.25
TE
0.23
TM
0.22
0.21
0.2
0.19
1
1.5
2
Width [µm]
2.5
(a) Confinement factor
3
−0.011
−0.012
−0.013
−0.014
−0.015
1
1.5
2
Width [µm]
2.5
(b) Modal birefringence
Figure 2.22: Confinement factor and modal birefringence of a shallow SOA as a function of width.
3
30
Integrated components and their polarization properties
Fig. 2.22(a) shows the confinement as a function of the waveguide width. The confinement
factor is very polarization dependent, resulting in a different net gain for TE and TM. Only
for very narrow waveguides this polarization difference disappears, at the cost of a reduced
confinement.
The modal birefringence is calculated and plotted in 2.22(b). This confirms the presence of
modal birefringence in the SOA and a phase difference between the two polarizations occurs
while they propagate through the device. Note that, similar to passive shallow waveguides,
no polarization independent propagation can be obtained by adjustment of the width. Furthermore the birefringence is larger than in a passive waveguide because of the presence of extra
interfaces between the active and passive layers.
Material gain
Apart from the geometrical properties influencing the confinement factor, and thus the net gain,
the material gain gm itself can be polarization dependent as well [14, 41]. Strain applied in the
growth is the main parameter that determines the gain for the two polarizations.
Strain introduced in the material will influence the shape of the valence band. It results in
the heavy hole subband lying above the light hole subband for compressive strain, or below it
for tensile strain as shown for a quantum well in Fig. 2.23. TM gain is caused by light-hole
transitions, while TE gain is caused by light hole (for approximately 25%) and heavy hole (for
75%) transitions [14]. By applying tensile strain, the TM gain is enhanced; compressive strain
will enhance TE gain.
E
E
E
Ec
ELH
EHH
ESO
k//
Compressive
k┴
Unstrained
Tensile
Figure 2.23: Influence of strain on the valence band structure of quantum wells [42].
By employing strain, the gain can be equalized for TE and TM to compensate for differences
in losses and confinement factors.
The gain is dependent on the optical input power Pin . As the input power increases, the gain
will saturate because of carrier depletion. The saturation of the material gain can be described
2.5 SOAs
31
as [43]:
gm,TE,TM =
g0,TE,TM
1 + Pin /Psat,TE,TM
(2.9)
Here g0,TE,TM is the unsaturated material gain for the respective polarization, Psat is the saturation power. This is the power for which the gain drops by 3 dB.
Non-linear phase shift
Apart from the linear phase shift (caused by birefringence, see Fig. 2.22(b)), a non-linear phase
change, caused by a change in refractive index as a function of carrier density (and thus gain)
is present in the SOA. In the saturation regime, the index modulation is largest. The non-linear
phase change in the SOA can be described as [43]:
∆φ =
1
∆gnet,TE,TM αTE,TM L
2
(2.10)
where αTE,TM is the α-factor or line width enhancement factor, describing the carrier induced
index change:
dgnet,TE,TM
4π dn
αTE,TM = −
,
(2.11)
λ dN
dN
where n is the effective refractive index, N is the carrier density.
The α-factor can depend on the polarization. In an SOA both the gain and the phase responses
are polarization dependent. This property can be used to actively manipulate the polarization,
for example in a polarization MZI as described in chapter 1.
2.5.1
Design
The SOA design suited for all-optical switching is based on the trade-offs and designs in [16].
A schematic cross section of the SOA is depicted in Fig. 2.24(a). A 2 µm wide shallow active
waveguide is used, the waveguide is connected using 100 µm long tapers to 3 µm wide shallow
waveguides. The active layerstack used in the first realization (chapter 6, section 6.3) consists
of a bulk Q(1.55) active layer similar to [16], with a thickness tactive of 120 nm.
In a next generation the active layer is replaced by a multiple Quantum Well (QW) layer. The
layer consists of 41Å unstrained InGaAs Quantum Wells, and 63Å strained InGaAs barriers.
The strain applied to the barriers is used to obtain the required well potential. The active layer
thickness is initially kept to 120 nm (equal to 11 QWs) to have the same optical confinement
as in the bulk case.
SOAs with different length are processed according to the standard processing in chapter 3.
The mask layout is shown in Fig. 2.24(b). The SOAs have angled output waveguides to
prevent reflections from the facets, as explained earlier.
32
Integrated components and their polarization properties
P-metal
InGaAs
InP
tactive Q1.25
Q1.25
InP
(a) Schematic cross section of the SOA.
(b) Mask layout of the SOA teststructures.
Figure 2.24: SOA cross section and layout.
2.5.2
Measurements on the QW SOA
The QW SOAs are investigated further. The devices are biased with a current source and
their output is monitored with a power meter and an Optical Spectrum Analyzer (OSA). The
recorded LI-curve and spectrum for a 900 µm long SOA is plotted in Fig. 2.25. Large ripples
are visible in the spectrum and above a threshold of 60 mA, the device starts to lase. The
reflections in the device form a cavity and cause lasing. This indicates also that the QWmaterial has a very high gain, higher than the previously used bulk material. An analysis of the
cavities is performed to identify the origin of the reflections.
2.5
−29
−30
Power [dBm]
Power [mW]
2
1.5
1
0.5
0
0
−31
−32
−33
−34
20
40
60
80
Current [mA]
(a) LI curve.
100
120
140
−35
1575 1580 1585 1590 1595 1600 1605 1610
Wavelength [nm]
(b) ASE spectrum.
Figure 2.25: LI curve and ASE spectrum of a 900 µm long SOA.
2.5 SOAs
33
Reflections
The spectrum is analyzed using an FFT, similar to the method used for the MMIs. The spectra
for a number of SOAs with increasing length are analyzed. The possible cavities vary with the
length of the SOA as can be seen in Fig. 2.24(b) and 2.26(b). This eases the identification of
the cavities. The identified cavities are shown in Fig. 2.26(b) and drawn as straight lines in the
plot in Fig. 2.26(a). The cavities obtained from the spectrum are plotted in the same graph.
The dominant cavities are indicated with circles. The dominant cavity is in most cases cavity
c6. This cavity does not contain the SOA. It nevertheless shows up in the spectrum because it
corresponds to the beating of resonances from other cavities (c3-c1, c4-c2). The devices are
clearly lasing on at least one of the butt joints and one of the facets, which indicates that the
reflections from the butt joints as well as from the facets are significant.
c4
5000
c1
c2
c3
c4
Cavity length [µm]
c2
4000
c3
c6
c5
3000
c1
2000
c5
1000
c2−c3
c6
SOA
500
1000
1500
2000
SOA length [µm]
2500
3000
(a) Cavity analysis.
(b) Cavities.
Figure 2.26: ASE spectrum and cavity analysis of the SOA.
The reflections at the butt joints are probably caused by a non-optimal connection between the
passive and the active regions. The waveguides are crossing the butt joints straight (shown in
Fig. 2.27(a)), which causes the reflections to couple back into the waveguides. By entering
the active region at an angle of 10° (shown in Fig. 2.27(b)) the reflections at the butt joint are
directed out of the waveguides [44]. This design is used in the second realization MZI (chapter
6).
Active region
(a) Old.
(b) New.
Figure 2.27: Topview of the SOA.
The reflections from the facets can be reduced further by applying an Anti Reflection (AR)
34
Integrated components and their polarization properties
coating in addition to the 7° output waveguide angle.
Gain
Apart from the reflections, another problem is present in the SOA. The ASE-spectrum (in Fig.
2.25(b)) shows that the gain peak is close to 1620 nm. This is particularly problematic for
non-linear operation; the largest α-factor, and hence the largest index modulation is expected
at the red-side of the gain peak [45, 46].
The material is designed to employ bandfilling to obtain a blueshift, so high injection currents are required. As the devices start lasing at moderate injection levels (3.3 kA/cm2 ), gainclamping is obtained. Hence the required high-current densities (in the order of 12 kA/cm2 )
cannot be achieved.
The problem can be solved by reducing the number of QWs, In that case, the active layer thickness decreases and thus the confinement factor. The simulated confinement factor as a function
of active layer thickness is plotted in Fig. 2.28. A thickness of 89.3 nm, corresponding to 8
QWs, will result in a confinement factor of 0.18, which is close to buried hetero SOAs fabricated in similar material [47]. This will yield a reduced net gain, which will prevent the device
from lasing, together with the reflection reduction measures from the previous section. It will
still provide sufficient gain for the required operation. Furthermore, because of the reduced
number of QWs, the total required carrier density to employ bandfilling is reduced, so lower
injection currents are needed. This layer stack is used in the second generation WLC (section
6.4).
Number of QWs
9
10
8
11
Confinement factor Γ
0.23
0.22
0.21
0.2
0.19
0.18
0.17
90
95
100
105
110
115
Active layer thickness [nm]
120
Figure 2.28: Confinement factor as a function of active layer thickness.
2.6
Conclusions
In this chapter passive and active devices required in a PIC are discussed. Shallow waveguides
can be used for low-loss connections on the chip with a minimum bending radius of 450 µm.
2.6 Conclusions
35
Deep waveguides are used if sharp bends are required. Furthermore if birefringent waveguides
are needed, deep waveguides are preferred. The birefringence can be enhanced by partly removing the topcladding.
The designed MMI couplers show a good splitting ratio and function almost independently of
the polarization. A design is presented to overcome the reflections that can occur from this
device. By cutting the corners from the multimode region a suppression of the reflection of at
least 10 dB can be obtained.
The fiber-chip coupling can be made more tolerant and less lossy by tapering the output waveguide laterally to 5 µm. The overall losses to a tapered fiber are still rather large: around 3 dB.
Coupling efficiency to a cleaved single mode fiber can be improved by using 2D spot size converters. A taper is designed to taper a standard waveguide to a large fiber matched waveguide
with a spot size close to that of a fiber. Experimentally determined overlap with a standard
fiber indicate coupling losses around 1.5 dB.
According to simulations, the reflections at the facets are reduced to below -40 dB in both
cases by tilting the waveguides to an angle of 7°.
SOAs are polarization dependent and can be used to manipulate the amplitude and phase difference of the two polarizations.
Devices for non-linear switching are designed based on previous designs, but problems arise in
the reflections from the butt joints in combination with very large gain. This causes lasing and
prevents bandfilling to obtain the correct operation wavelength. The reflections are reduced by
entering the SOA under an angle. The gain is reduced by reducing the number of Quantum
Wells. This will also increase the effect of the injection current on the bandfilling.
The devices presented in this chapter will be used to construct the integrated circuits of chapter
6. Together with the polarization devices from chapters 4 and 5, the circuits in chapter 7 can
be constructed.
Chapter 3
Integration technology
3.1
Introduction
Within COBRA an integration technology is used, consisting of a limited variety of components that can be integrated in a PIC. These are the standard active and passive components
from the previous chapter. The aim of this thesis is to extend this technology to a generic
technology capable of on-chip polarization handling and with the possibility for mounting and
packaging.
This chapter wil introduce the standard technology first. The butt joint integration of active
and passive material is shown and the processing of the standard components introduced in the
previous chapter is demonstrated.
After that processes for high resolution lithography are described, required for the definition
of polarization converters. Additional technology steps for mounting the chips in a submount
for packaging are discussed. Finally a process for etching sloped sidewalls in the polarization
converters (PC) is demonstrated.
3.2
Active-passive integration
The butt-joint integration of the active and passive layerstacks, introduced in chapter 2 will be
described in this section. Furthermore a processing scheme in which both active and passive
components can be integrated is considered.
37
38
3.2.1
Integration technology
Active-Passive butt-joint
The monolithic integration of active and passive components can be done in a number of ways:
by Quantum Well intermixing [48], by Selective Area Growth based on growth enhancement
near masked areas [49], by offset Quantum Wells that interact with the evanescent field [50]
and are etched in the transparant regions, by an active vertical coupler [51], by using the polarization [52], or by using selective area growth of the passive material butt-joint to the active
regions [53]. All PICs discussed in this thesis are made by butt-joint coupling of the active and
passive structures. This technique has the advantage over the other mentioned methods that
the active and the passive layerstack can be optimized separately, as long as the substrate and
the top cladding are the same for the active and passive regions. However this technique puts
strict conditions on the processing.
The integration is based on selective area Low Pressure Metal Organic Vapor Phase Epitaxy
(LP-MOVPE). The first step is to grow the active layer structure (Fig. 3.1(a)). A Silicon Oxide
(SiO2 ) mask layer is deposited on top of the grown structure. The active regions are lithographically defined on this mask layer. The mask is etched and the active layers are etched
away in the passive regions. A well defined underetch is required to obtain a sufficiently planar regrowth (Fig. 3.1(b)).
The structure is then regrown with the passive structure (Fig. 3.1(c)). The growth will only
occur on the semiconductor. On top of the SiO2 layer no deposition will take place. The buttjoint has to be well connected; voids in the connection can lead to unwanted reflections and
losses. The doping of the passive cladding is lower than the active cladding (indicated by the
lighter shade) and can be freely chosen.
After this the SiO2 mask is removed and the top cladding will be overgrown in a final growth
step (Fig. 3.1(d).
The wafer obtained in this way is used for further processing of the PICs.
(a) Active layerstack with
SiO2 mask
(b) Etching active layers in
the passive region
(c) Regrowth
passive
waveguide structure
(d) Overgrowth
topcladding and contactlayer
Figure 3.1: Active-passive butt-joint process (the materials are defined in tables 2.1 and 2.2
3.2 Active-passive integration
3.2.2
39
PIC processing
The standard technology for realizing PICs consists of the integration of shallow and deep
waveguides with SOAs. The processing has a modular approach; it can be divided in 3 major
parts:
• the definition and etching of the (waveguide) components on the PIC,
• the passivation of the PN-junction in the opto-electronic components,
• the metallization.
The standard process is depicted in Fig. 3.2. The alphabetic listing of the steps in the description below corresponds to the figure.
The first part, the definition and etching of the waveguides, consists of the following steps:
a. First the wafer is covered with 50 nm Silicon Nitride (SiNx ) by Plasma Enhanced Chemical Vapour Deposition (PECVD). Positive photoresist is spun and baked. The waveguides are defined in the resist using optical contact lithography. After development of
the photoresist the SiNx is etched using CHF3 /O2 Reactive Ion Etching (RIE).
b. The sample is covered with thick positive resist, and the regions for the deep waveguides
are opened. It is important to have the resist thick enough as the resist is attacked in the
RIE of the semiconductor. This can cause two problems: firstly the top layer becomes
hard and crusted and will be hard to remove. The second problem can occur if the resist
is attacked so much, that the underlying semiconductor is etched as well. This results in
partly etched pits in the surface.
By using a resist thickness of more than 2 µm, sufficient resist is present below the
crusted layer and this layer can be lift-off by dissolving the un-attacked resist in aceton.
c. The open areas are etched using a CH4 /H2 RIE. In this step, the difference between deep
and shallow waveguides has to be etched. The etch depth has to be deep enough so that
after the etch steps to follow, the waveguides will be etched more than 100 nm below the
waveguide layer. This depth is tolerant as long as it is deep enough. So an overetching
of approximately 100 nm is taken into account.
d. After the removal of the resist, all waveguides are etched simultaneously using the SiNx
as a mask. In this step it is important that the shallow waveguides are etched to a depth
which is shallow enough to ensure complete removal of the InGaAs contact layer in the
final etch step (f). By etching approximately 100 nm too shallow, in the next etch step (f),
the contactlayer is certainly removed. A small part of the InP top cladding (< 100 nm)
is etched as well, but this does not have a large influence on the waveguide properties.
e. The SOAs are covered with resist and the SiNx mask is removed from the passives. The
contact on the SOA remains protected by the nitride mask, while the contactlayer on the
passive waveguides is left open.
40
Integration technology
InP
Q125
Q155
InGaAs
SiNx
Resist
Polyimide
TiPtAu
(a) Waveguide definition
(b) Cover shallow waveguides
(c) Preliminary etch deep waveguides
(d) Etch all waveguides
(e) Cover the contacts on the SOA,
etch SiNx on passives
(f) Etch all waveguides to depth
and remove contact layer on passives
(g) Planarization and etch back of
polyimide
(h) Metallization of top and backside
(i) Sputter Ti-Au seed layer
(j) Lithography and Au plating
(k) Etch seed layer
(l) Etch polyimide
Figure 3.2: Standard process
3.2 Active-passive integration
41
f. The contact layer on the passives is removed by etching the top together with the lower
parts. This is a critical step, the etch depth has to be 100 nm ±50 nm into the waveguidelayer for the shallow waveguides and the SOAs. As stated before, the remaining top
cladding depends on the etch depth in step (d).
After etching of the waveguides, the next step is passivation and planarization of the structure.
g. Polyimide (PI) is used for both passivation and planarization of the SOAs. The sidewalls
of the waveguides are cleaned first by oxidation in an O2 plasma and oxide-removal in
H3 PO4 :H2 O.
The planarization has to be carefully done to ensure good adhesion of the PI. For this,
multiple layers of polyimide are spun. After every spin step the polyimide layer is baked
at slowly rising temperature starting from roomtemperature. Every layer of PI is completely cured before spinning another layer.
After a number of steps, the sample is planar to within 100 nm height difference. The
polyimide is etched back in an isotropic O2 /CF4 plasma until the level of the contactlayer.
The final steps consist of the metallization of the opto-electronic components (i.e. SOAs in
this case and possibly phasemodulators).
h. The sample is covered by negative resist and exposed using the metallization mask. A
good lift-off profile is obtained after development (Fig. 3.3(a)).
Titanium, platinum, and gold (Ti, Pt, Au) are evaporated twice on the sample, once on
the P-contact on top, and once on the backside of the sample for the N-contact. After liftoff in aceton, on the top, only the P-contacts remain, while the backside stays completely
covered to form the N-contact. A SEM photograph of an SOA with a top P-contact is
shown in Fig. 3.3(b).
i. An important parameter is the resistance of the metal contact along the SOA to achieve
uniform current-injection. A thick and wide contact is required for this. A thick contact
is obtained with Au-plating. A TiAu seed layer is sputtered over the whole sample.
j. On the seed-layer a lithography step is done to open the P-contacts. To protect the
backside, it is covered with resist. Gold is plated in an electro-chemical process and in
the openings in the resist on top of the P-contact, a thick, ≈ 2µm, gold layer is obtained.
k. The seed layer has to be removed from the sample. First all resist is removed from the
sample. After that, the seed layer is etched in two steps, by wet chemically etching first
the Au-layer and after that the Ti layer. As this is done without any masking, the contact
itself is attacked as well. The seed layer is thin compared to the thickness of the contact,
so if the etch is timed carefully, at least 1.5µm gold remains on the contacts.
l. The final step is to remove the polyimide everywhere from the sample, except where the
contacts are. This is done in an CHF3 /O2 RIE. The contacts are used as a mask. They
are not attacked if the power is sufficiently low (50 W). From Fig. 3.3(c) is can be seen
42
Integration technology
that the PI is removed without underetch and that the contacts are not attacked in this
step.
(a) Lift-off profile in negative resist
(b) SOA with evaporated P-contact (c) Polyimide removed with metal as
mask
Figure 3.3
It is possible to add other functional and technological blocks to the ones described here, without major changes to the processing steps. This becomes clear in the next sections, where
the technology for higher resolution lithography for the waveguides; the integration of alignment features for packaging, spotsize converters, and sloped sidewall etching for polarization
converters will be discussed.
3.3
High resolution lithography
The technology discussed in the previous section has a limitation in the resolution. The width
of features cannot go below approximately 1 µm, since the reproducibility of smaller widths is
a problem with contact lithography. Furthermore sub-micron alignment is very difficult using
a standard mask-aligner.
This section will discuss two options to improve both the resolution and the alignment accuracy: high resolution optical lithography with a waferstepper and Electron Beam Lithography.
These lithographical steps can replace the standard waveguide lithography, or be used in connection with it. High resolution lithography will be used for the realization of Polarization
Converters (PC), which require high accuracy and submicron size.
3.3.1
Waferstepper
The first method investigated is optical lithography using an ASML PAS5500/250 5× reduction wafer stepper. This is an i-line tool, with an exposure wavelength of 365 nm. This allows
an accurate width control, better than 20 nm on an 800 nm line. The alignment accuracy is approximately ±60 nm. This optical lithography is advantageous as compared to Electron Beam
Lithography (EBL), because it has a large writing field (22 × 22 mm2 ), better uniformity and
is suited for mass-production.
3.3 High resolution lithography
43
The machine is equipped for exposures on wafers from 4 to 8 inch. In our process quarter
pieces of a 2 inch wafer are used. To align and expose these quarter pieces, they are centered
on a 6 inch wafer and fixed using adhesive tape [54]. A schematic of the machine is presented
in Fig. 3.4(a).
Alignment marks
For the alignment, phase grating alignment marks designed by ASML are fabricated on the
quarter pieces of InP. A mask with these alignment marks (Fig. 3.4(b)) is transferred using
standard optical contact lithography. The gratings are RIE etched, 120 nm into the InP. The
same alignment marks are reused in multiple process steps. Care has to be taken not to remove
or alter the marks in processing. The alignment procedure uses red light at a wavelength of
633 nm. The photoresist is insensitive to this wavelength and the marks are not exposed during
alignment.
The thickness of the SiNx mask that is deposited on the wafer has to be such that a minimum
in the reflection is avoided. For the thicknesses used in our process this is not a problem.
(a) Optical lithography with a waferstepper
(b) Phase grating alignment marks
Figure 3.4: High resolution optical lithography
3.3.2
Electron beam lithography
A second option to achieve high resolution is electron beam lithography (EBL). This technique has the advantage that it is a direct-write technique, so no masks need to be fabricated.
A Raith-150 E-beam machine is available within COBRA, which allows a higher resolution
with respect to the high resolution optical lithography (theoretically features as small as 30 nm
can be achieved). Disadvantages are a worse reproducibility in the line width definition and a
limited write field size. The write field is the area that can be exposed without moving the stage
on which the sample is mounted. In our case this is limited to 200 × 200 µm 2 . Furthermore
every feature has to be written separately which takes a lot of time, typically 10 min. per write
44
Integration technology
field for the structures in this thesis.
Alignment marks
Because of the limited write field size and the long time needed for EBL exposures, only the
critical structures are exposed using EBL. Therefore the EBL write fields have to be aligned
to optically defined waveguides using alignment marks defined in the same mask as the waveguides. Multiple write fields can be cascaded in the same way, by defining alignment marks
for each write field. A schematic overview of the mask is shown in Fig. 3.5.
Optical waveguides
Alignment marks
EBL writefield 1
EBL writefield 2
Figure 3.5: Optical mask showing waveguides, EBL writefields and EBL alignment marks
The figure shows the optical waveguides, and two adjacent writefields. In this design, every
write field has 3 sets of 4 marks. This is needed because multiple EBL exposures are to be
done on the same sample and the marks are exposed when aligning. This is made clear by
explaining the alignment procedure. Fig. 3.6 shows the mask design with the scan regions
identified by dark gray color. The machine scans with the electron beam along these regions
and determines the middle of the cross shaped alignment mark, as shown in Fig. 3.7.
While scanning, the scan region is exposed. After development, it will be opened as can be seen
in the rightmost photograph in Fig. 3.6. This makes the mask unreliable and even unusable for
subsequent alignments.
By using a separate mark for each step, this problem is avoided.
3.3.3
Technology
For both types of high resolution lithography described in this section only positive resist is
available. This means that the exposed parts will be opened after development. This process
can be used in steps where most of the sample has to be covered and only a small area needs
to be opened.
A problem is that for the waferstepper only positive masks (i.e. a transparent pattern on a
chromium mask plate) are available, and in the EBL only a limited area can be written. This
requires a reversal process to define narrow lines. So for the definition of waveguides, a lift-off
3.4 Technology for packaging
45
2800
2600
Intensity [a.u.]
2400
2200
2000
1800
1600
1400
1200
0
1000
2000 3000 4000
Position [nm]
5000
6000
Figure 3.6: EBL alignment marks: Mask design and op- Figure 3.7: Linescan to determine middle of mark; the
dashed lines indicate the detected edges, the dash-dotted
tical photograph of an unexposed and an exposed mark
line indicates the resulting middle point.
process is used. The resist is exposed with the waveguide mask and developed. The waveguide
pattern is transferred to the resist and after this, Ti is evaporated and after dissolving the resist,
the pattern is transferred to the Ti.
3.4
Technology for packaging
Chips need to be packaged to be able to use them in systems. This requires coupling of optical
and electrical signals from and to the chip. The waveguides used in PICs are small, typically
around 3 µm. This makes the coupling to the outside world (e.g. optical fiber) difficult, because
of the submicron precision needed. Some of the chips, described in chapter 6, are packaged.
For the packaging, the PICs will be flip-chip mounted on a Si submount as shown in Fig. 3.8.
This submount will be mounted on a motherboard in which fiber-assemblies are coupled to the
in- and outputs. This is similar to hybrid integration [55] of active and passive structures.
To allow this type of packaging, additional features have to be present. The techniques to
fabricate these are explained next. To allow a relaxed fiber-chip coupling, spotsize converters
are needed. The technology to fabricate them is described afterwards.
3.4.1
Alignment on the submount
The chips have to be vertically and horizontally aligned with respect to the submount to allow
coupling to fibers and to connect the metal contacts of the PIC to the predefined metal pads
on the submount. For this recesses need to be etched and the chips have to be cleaved with
submicron precision. The technology needed is depicted in Fig. 3.9.
46
Integration technology
InP
Q125
Q155
Si
InGaAs
Polyimide
Metal
Alignment upstands
Figure 3.8: InP PIC flip-chip bonded onto a Si submount, the submount is indicated with the thick lines.
(A) The regions for grooves and recesses are RIE etched together with
the shallow waveguides.
(B) The sample is covered by resist
and the recesses are opened.
(C) The quaternary layer is selectively removed in the recesses.
(D) After planarization, the cleavearea is opened and the polyimide is
removed there.
(E) After metallization, the sample
is covered by resist and the cleavearea is opened.
(F) The deep grooves are etched into
the substrate.
Figure 3.9: Technology steps for the alignment features, the definition of the material corresponds to Fig. 3.8.
3.4 Technology for packaging
47
Technology
The sample is vertically aligned by Silicon Oxide pillars on the submount. A well-defined
height is needed on the InP PIC to align to these pillars. The substrate level, just below the
quaternary waveguide layer, is used for this.
The horizontal alignment is achieved by pushing the chip against stops on the submount. For
this, exact, i.e. sub-micron accurate, dimensions of the PIC are required. This is achieved by
wet etching deep grooves at which the chip can be cleaved. The position of these precisioncleaves has to be accurate within less than a micron with respect to the position of the in- and
output waveguides. It is thus important to define these in the same step as the waveguides.
In the standard process of the PIC, some additional steps are required. They are listed here, the
alphabetical listing corresponds to the figures in Fig. 3.9.
A. Two walls defining each precision-cleave groove are etched together with the shallow
waveguides. The difference is that the SiNx is left on top of the groove-walls in step (e)
(in Fig. 3.2(e)). In step (f) of the standard process, when the shallow waveguides are
etched into the quaternary waveguide layer, the recesses are etched.
B. In an additional step after the waveguide etching, the sample is covered with resist and
the region of the recesses and the grooves, is opened.
C. The quaternary layer is selectively removed using H2 SO4 :H2 O2 :H2 O. This etchant etches
the quaternary material fast, but does not attack the underlying InP. With this process,
the required well-defined level is achieved.
D. After planarization of the sample, a Ti lift-off process is used to cover the sample. Only
the area where the grooves are is opened. With this Ti mask, the polyimide is etched in
a O2 /CHF3 RIE to completely open the grooves.
E. After metallization, (step (h) in the standard process), the sample is covered by resist
and the cleave-area is opened. The backside is protected with resist as well.
F. Now HCl:H2 O is used to selectively etch the InP using the quaternary layer as a mask.
For the two orthogonal cleave directions the etch behaves differently. In the [01̄1] direction, the etch stops at the (112) plane, resulting in a V-groove as can be seen in Fig.
3.10(a). The depth of the groove is determined by the width of the opening, as the etch
stops completely on the crystal plane at an angle of 35.3° with respect to the surface
[56]. In the [011] direction the etch results in a ’tie’-shape. The depth of this etch is
determined by the time of etching.
With minor changes to the standard process and adding some additional steps, the alignment
features can be integrated in a PIC in this way.
3.4.2
Vertical taper
For efficient and tolerant coupling of the PIC to a fiber, spotsize converters are required. The
design and results of the spotsize converters we developed was discussed in chapter 2. Here the
48
Integration technology
(a) Etched groove in the V-groove direction.
(b) Etched groove in the dovetail direction.
Figure 3.10: SEM photographs of the precision cleave grooves.
technology for realizing them is presented. The device consists of a horizontal and a vertical
taper. The fabrication of the most difficult part, the vertical taper, is based on the process
described in [57]. This uses a tapered resist profile, which is obtained with a sliding raster
mask, similar to a sliding window method [58]. The vertically tapered profile in the resist is
transferred to the semiconductor by a non-selective etch.
The technology needed to fabricate this taper is depicted in Fig. 3.11. It is designed to be
integrated with the standard process. All these steps should precede the standard process steps
of Fig. 3.2.
A. The first step is to define the region where the taper is to be made. The taper has to extend
vertically to stop at the substrate. The slope of the taper is fixed and the quaternary layer
has to be fully removed at the end of the taper. In order to limit the length of the taper,
it will start at a level 300 nm above the quaternary waveguide layer. (Fig. 3.12) For this
the semiconductor has to be etched in the regions where the taper is to be made.
In this step SiNx is deposited on top of the wafer. Photoresist is spun and the taper region
is lithographically opened. The silicon nitride is etched using a CHF3 /O2 RIE. After the
etching, the resist is removed and the semiconductor is etched in a CH4 /H2 RIE to the
desired depth.
B. Resist is spun on the sample. A discrete transition is obtained in the previous step, but
this is not a problem as the resist used is thick enough to cover the step. The resist is first
soft-baked at 100°C to get rid of the solvents. After this, another soft-bake step at 115°C
is done to decrease the photosensitivity of the resist and obtain slow development. This
makes the development less critical.
The taper is defined by a sliding raster mask. The openings are put on the mask with a
3 µm period. They increase linearly in size. The exposure of the resist is determined by
the size of the opening. By sliding the mask over approximately 170 µm the exposure
dose varies linearly with position over the taper region and the discrete rasteropenings
3.4 Technology for packaging
49
(A) Etch of the taper region
(B) Lithography with sliding raster mask
(C) Vertical taper in resist
(D) Etch of the vertical taper using ICP
(E) Silicon Nitride mask of the waveguide
(F) Waveguide etch
Figure 3.11: Process for the vertical taper
50
Integration technology
Figure 3.12: Cross section of the vertical taper.
are averaged out. After illumination the resist is baked again to smoothen standing wave
patterns in the resist.
C. The sample is developed and a taper is obtained in the photoresist.
D. The sample is etched using an Inductively Coupled Plasma (ICP) etch. The etch conditions are tuned to have a non-selective etch for the semiconductor and the resist. The
best obtained ratio of the etchrates of resist and InP/InGaAsP is approximately 0.6. In
this way the taper transferred into the semiconductor is slightly steeper than the taper in
the resist.
E. This taper is the starting point for the standard processing. In this step SiNx is deposited
and the waveguide mask is transferred to the SiNx in the same way as in Fig. 3.2(a).
F. The next step is the waveguide etch, exactly the same as for the normal waveguides. In
the waveguide mask a horizontal taper is defined. After the etching, a vertical as well as
a horizontal taper is present in the structure.
After these steps, the standard process can continue.
3.5
Sloped sidewalls
The polarization converter (PC) consists of a waveguide with a straight and a sloped sidewall
[59]. The design is explained in detail in chapter 4. The processing of polarization converters
requires additional process steps that will be discussed next.
3.5.1
Wet etch
A sloped sidewall is obtained by wet etching. Several solutions can be employed for this.
Most etchants however are selective for InP and InGaAsP, so two etchants need to be used.
Another disadvantage is that the slopes have a different orientation for the different materials.
For stripes in the [011] direction, InP etched using HCl:H3 PO4 stops on the (112) plane which
has a slope of 35.3° [56]. InGaAsP has to be etched with H2 SO4 :H2 O2 :H2 O, the etch stops
3.5 Sloped sidewalls
(a) Underetch of
H2 SO4 :H2 O2 :H2 O.
51
quaternary
below
InP
with
(b) Smooth etch of quaternary and InP with
Br2 :CH3 OH.
Figure 3.13: Different etchants for InGaAsP and InP.
at the (111) plane which has an angle of 54.7° with respect to the surface[60]. Furthermore
there is a large underetch below the InP as is visible in Fig. 3.13(a). Such an underetch next
to the quaternary layer will result in high propagation loss. This makes these etchants less
suitable to use for producing polarization converters. Another possible etchant is bromine
(Br2 ) dissolved in methanol [56, 61] a very diluted solution (1:1500). This Br2 :CH3 OH etches
both InP and InGaAsP and stops at the (111) planes for both. This results in a very smooth
etch (Fig. 3.13(b)). Therefore this is the etchant of choice for the polarization converters.
3.5.2
Masking
An important parameter in the realization of PCs is the masking material. As stated before,
the polarization converter consists of one straight and one slanted sidewall. To obtain these
different slopes, the sides have to be opened separately, while keeping the dimensions. For
this, 2 mask materials are required that can be selectively removed with respect to one another.
Silicon Nitride and Titanium are chosen for this purpose.
The top of the PC, which has critical dimensions, is defined in Ti on top of the SiNx by using
high resolution lithography. By using resist and the Ti pattern as a mask, one side can be
opened. This is depicted in Fig. 3.14(a). With this mask, the straight side can be etched using
RIE.
Photoresist is dissolved in methanol, so this cannot be used as a mask in the Br2 :CH3 OH etch.
Therefore, when the sloped side is etched, all the rest has to be covered by SiNx . The SiNx is
deposited using a PECVD, with this technique both the top and the sides are covered. If the
initial mask is covered by another layer of SiNx , the stress in the nitride can cause a loading
effect on the material. This can lead to a ”lifting” of the mask, and thus to an underetch as is
shown in Fig. 3.14(b). The underetch is reproducible and can be corrected for in the design.
Although the nitride at the sides is slightly thinner than at the top, it can function as a mask
52
Integration technology
on the sides as well. Stress in the material can also cause an underetch in this case as is clear
from Fig. 3.14(c). In this case, it has to be taken into account that the slope starts at the
semiconductor side of the mask. The underetch is not present if the force of the stress in the
SiNx can be compensated by the adhesion to the semiconductor, or if the stress is neutralized
by having a symmetric loading. This is shown in Fig. 3.14(d).
(a) Define critical dimensions in Ti, etch SiNx using Ti and
resist as mask.
(b) Underetch below SiNx .
(c) Underetch below SiNx at the sidewalls.
(d) No underetch below SiNx if no loading is present.
Figure 3.14: Mask definition and properties for the polarization converter
The integration of this device in the standard processing and its design is investigated further
in chapters 4 and 7
3.6
Conclusions
The standard process for components on a PIC is presented in this chapter. The modular nature of the process is explained. This allows the introduction of additional steps that can be
3.6 Conclusions
53
integrated with the standard process.
For critical width definitions high resolution optical lithography or electron beam lithography
are introduced. They can replace the contact lithography where necessary. To allow accurate
alignment, some additional processing is needed to obtain alignment marks.
Precision cleaves and vertical alignment features, both required for packaging, can be introduced in the standard process without influencing the rest of the processing.
The technology for producing spotsize converters is presented. A sloped resist profile can be
obtained with a sliding raster mask and this can be used to ICP etch vertical tapers.
Polarization converters can be defined with the high resolution lithography and Ti and SiNx
are a good combination to be used as mask for this. Br2 :CH3 OH gives good quality sloped
sidewalls required for the PC. The underetch obtained in this etch has to be considered in the
design.
The modular nature of the technology platform allows introduction of additional steps without
major changes to the existing steps. This opens the possibility for creating a generic activepassive technology platform suited for on-chip polarization handling and with the capability
for packaging.
Chapter 4
Polarization converters
4.1
Introduction
Polarization conversion is an important function if on-chip polarization manipulation is required. As shown before, polarization converters (PC) that can be integrated with other functions on a chip are needed.
A well-known principle for polarization conversion is the application of electro optic effects.
This is mainly used in LiNbO3 , and has been reported for InP [62]. In InP the electro optic
effect is small, which results in very large devices. Furthermore electro-optic polarization conversion requires phasematching between the two polarizations, which is hard to achieve and
wavelength dependent.
For our application, passive polarization conversion is preferred, because of the easier fabrication and because there is no need for tuning. A number of passive PCs have been reported in
literature. Polarization conversion can be obtained by cascading sections of waveguides with
partially tilted modes. At the junctions between the sections partial conversion occurs. The
section can consist of slanted waveguides [59], periodically loaded waveguides [63, 64], or
integrated bends [24]. These device have the advantage of an easy fabrication, but are long
(close to or larger than 1 mm), and are wavelength dependent. The bandwidth is limited due to
the phase-matching that has to be obtained between the different sections.
Recently progress is made towards single section devices, full conversion occurs at the junction with a straight waveguide and only a single section is required.
Another class of short devices are converters that make use of mode-evolution [65, 66] to adiabatically couple from one polarization to the other. These devices have waveguides with a
55
56
Polarization converters
changing waveguide cross-section and do not fit in our layer stack and integration scheme.
The single sections devices that make use of tilted birefringent modes are the most promising
for integration. The most important ones are shown in Table 4.1.
The tilting of the modes is obtained by different techniques: in device A, the modes are tilted by
etching a narrow waveguide with tilted sidewalls on both sides by using Chemically Assisted
Ion Beam Etching (CAIBE), the same etching is used in device B, in which tilted slits are
etched to cause the modes to be tilted. Although both these devices show promising results,
their processing is not compatible with the active passive integration shown in chapter 3 and
these will not be considered.
In device C slits with varying depth effectively make a slanted side and tilt the modes inside the
device. The lag effect in the RIE is employed to achieve different etch depths for different slit
widths for this design, both the width and the depth have to be controlled extremely accurate,
which makes this devices very intolerant to fabricate. Device D uses a RIE etched straight
sidewall in combination with a wet etched sloped sidewall. This device is the most promising
to integrate with active and passive waveguides. A wet etch introduces more process steps as
compared to e.g. device C, but is advantageous over the dry etch, because of the tolerances. A
wet etch will stop on a crystal plane, so the slope is always well-defined. This chapter describes
the principle of the polarization converter based on a wet etched slope. Furthermore the design,
fabrication and results of two generations of such polarization converters are discussed.
4.2
Principle
The polarization converter using tilted birefringent modes is the integrated optical analogue of
the half wave plate in bulk optics. A half-wave plate has a slow and a fast axis: two orthogonal
modes can exist with differing propagation constants. When the plate is tilted with respect to
the input polarization, both modes are excited and after propagating over a certain distance
through the device, the two modes are out of phase and recombine to a different polarization
state.
The equivalent integrated device consists of a ridge waveguide with a straight and a slanted
sidewall (Fig. 4.2(a)). The waveguide is narrow, typically below 1 µm. This brings the
slanted sidewall close to the field of the mode. Because of the ‘tilted’ boundary conditions,
the hybrid nature of the modes is enlarged. Ideally this causes the modes to tilt for +45°and
-45°respectively. Furthermore, the geometry of the waveguide causes a different propagation
constant for the two modes. The angle of the sloped sidewall does not have to be 45°(in our
case it is 54°with respect to the surface). The tilt angle of the modes for a certain slope angle
is determined by the width. In Fig. 4.1 the tilt angle
θ = arctan
Ey
Ex
(4.1)
is plotted as a function of the width of the polarization converter waveguide for the modes
inside the converter.
• L = 280 µm
• c = 96%
• Loss 2.5 dB
RIE for slits and ridge in
one step. The lag-effect
is used to obtain different
depth for different slitwidth.
Reactive Ion Etching
(RIE) for straight wall,
wet etch for slope.
Ridge
waveguide
with straight slits
with varying depth.
Asymmetric waveguide with a straight
and a slanted wall.
[69]
[70, 71, 72]
C
D
• L = 125 µm
• c = 95%
• Loss < 1 dB
• L = 1.6 µm
• c = 96%
• Loss 3 dB
2 times CAIBE for slits
and ridge separately.
Ridge
waveguide
with tilted slits.
[68]
B
• L = 48 µm
• c = 97%
• Loss 1 dB
[67]
Chemically Assisted Ion
Beam Etching (CAIBE)
for both sides in one
step.
Waveguide
with
tilted sidewalls.
A
Performance
Schematic
Fabrication
Principle
Reference
Table 4.1: Different single section polarization converters
4.2 Principle
57
58
Polarization converters
90
60
Tilt angle θ [°]
M1
30
0
−30
M2
−60
−90
0.5
1
1.5
Width [µm]
2
2.5
Figure 4.1: Tilt angle of the modes inside the PC as a function of waveguide width.
If light in a straight waveguide is coupled to the PC waveguide, both tilted modes are excited.
After a half beatlength,
π
Lλ /2 =
,
(4.2)
β1 − β2
where βn is the propagation constant of mode n, the modes are completely out of phase and
recombine into the orthogonal polarization in a straight output waveguide as shown in Fig. 4.2.
This is clarified in Fig. 4.2(b), where the propagation of modes M1 and M2 is shown over a
length Lλ /2 , corresponding to a phase difference φ = π rad.
M1
q
n
FILM
n
SUBSTRATE
M2
(a) Schematic cross section of the PC
(b) Propagation of the modes
Figure 4.2: Principle of the integrated polarization converter
The properties of the conversion of the PC can be explained using Jones matrices (see Appendix A). The PC with length Lλ /2 creates a phase difference φ between the two orthogonal
4.2 Principle
59
modes. In the coordinate system of the tilted modes, the transfer matrix TPC1 of the PC itself
is:
"
#
1
0
TPC1 =
(4.3)
0 e− jφ
The two modes inside the PC are in the regular coordinate system (normal along the y-axis):
sin(θ )
cos(θ )
M1 =
M2 =
(4.4)
cos(θ )
− sin(θ )
The modes inside the PC are tilted over an angle θ with respect to the modes in the straight
waveguide. The total transfer of the PC is obtained by a coordinate transformation. The tilted
modes form the new base for the calculations of the PC. The transfer with respect to the TE-TM
bases are obtained by multiplication with a rotation matrix R(θ ):
cos(θ ) sin(θ )
R(θ ) =
(4.5)
− sin(θ ) cos(θ )
Fig. 4.3(a) shows the transformation to the new bases.
M2
TM
M1
q
TE
E2
M2
M1
E1
(a) Coordinate transformation to new bases for PC
(b) Conversion of the PC plotted on the Poincaré sphere
Figure 4.3: Visualization of the conversion.
The transfer of the PC is:
TPC = R−1 (θ )TPC1 R(θ )
which is equal to
"
cos(θ )2 + sin(θ )2 e− jφ
cos(θ ) sin(θ ) − cos(θ ) sin(θ ) e− jφ
cos(θ ) sin(θ ) − cos(θ ) sin(θ ) e− jφ
sin(θ )2 + cos(θ )2 e− jφ
(4.6)
#
(4.7)
60
Polarization converters
For TE input, this yields an output E2 :
cos(θ )2 + sin(θ )2 e− jφ
E2 =
cos(θ ) sin(θ ) − cos(θ ) sin(θ ) e− jφ
(4.8)
This yields a conversion:
c =
Pconverted
= 2 {cos (θ ) sin (θ )}2 {1 − cos(φ )}
Ptotal
(4.9)
The conversion process can be visualized by plotting the State of Polarization (SOP) on a
Poincaré sphere. The two stable modes are M1 and M2 . Propagation through the PC results
in a rotation around the axis between these two points. In Fig. 4.3(b) the conversion from TE
(E1 ) to TM (E2 ) in an ideal PC (θ = π/4 and φ = π) is shown.
From eq. (4.8) it is clear that the amplitudes of the two components of E2 depend on both
the tilting angle θ and the phase difference φ . Incomplete conversion can have two different
causes.
• The PC has the correct width and therefore a required tilt θ of 45°, but the length is
incorrect. At the output the modes are not exactly out of phase. This leads to an elliptical
vertical polarization in the output as shown in Fig. 4.4.
• The PC does not have the correct width; the tilt θ is not exactly 45°. After propagation
over the beatlength, the two orthogonal modes are completely out of phase and result in
a linear polarization. However they do not recombine to a polarization orthogonal to the
input. This can be seen on the Poincaré sphere in Fig. 4.5. The two stable modes in the
structure in this case are not exactly on the ± 45° points, but are on different points on
the equator of the sphere, as indicated in Fig. 4.5(a).
The conversion of the PC is the ratio between the converted and the total power, in both examples the ratio is the same, but the output SOP is different. Thus the origin of the incomplete
conversion cannot be discovered by regarding the output powers for one device. The θ , and
thus the maximum possible conversion, can be obtained by a scan in length (i.e. a scan in φ ).
4.3
First generation
A first design for an integrateable polarization converter is made, based on the design in [72],
but now aimed at integration with other passive and active components. The design, fabrication
and characterization will be discussed in this section.
4.3.1
Design
The polarization converter is designed for active passive integration, the layerstack for this is
shown in Fig. 4.6, this is equal to stack in table 2.1. It consists of an InP substrate, a 500 nm
InGaAsP [Q(1.25µm)] waveguide layer and a 1500 nm InP topcladding. The topcladding
4.3 First generation
61
E2
TM
1
M1
E1
Ey
0.5
M2
0
−0.5
TE
−1
−1
(a) Conversion of the PC plotted on the Poincaré sphere
−0.5
0
Ex
0.5
1
0.5
1
(b) Output SOP
Figure 4.4: Incomplete conversion due to incorrect length
TM
1
M2
E2
M1
Ey
E1
0.5
0
−0.5
TE
−1
−1
(a) Conversion of the PC plotted on the Poincaré sphere
−0.5
0
Ex
(b) Output SOP
Figure 4.5: Incomplete conversion due to incorrect width
62
Polarization converters
consists of a 300 nm n.i.d. InP layer, on top of which a 20 nm thick Q(1.25) etch-stop layer
and a 1200 nm p-InP layer are grown.
For the polarization converters a thin topcladding is used because first of all an accurate control
of the width of the converter at the waveguide layer is needed, thus a critical width definition
close to the waveguide layer is required. Secondly a large birefringence is desirable, from Fig.
2.1(b) it is clear that to this end a thin topcladding is also needed.
The thin topcladding is obtained by selectively etching back the top InP cladding till the etchstop layer, and by selectively removing the etch stop layer as well. An accurately defined
topcladding of 300 nm is obtained as shown in Fig. 4.6(b). This precisely defined top is used
for the PC.
The sloped sidewall will be wet-etched and will stop on the (111) crystal plane, having an
angle of 54° with respect to the surface.
1200 nm p-InP
20 nm Q(1.25)
300 nm n.i.d.-InP
300 nm n.i.d.-InP
500 nm Q(1.25)
500 nm Q(1.25)
substrate n-InP
substrate n-InP
(a) Layerstack compatible with active and passive components
(b) Same layerstack, 1200 nm topcladding selectively removed for PC
Figure 4.6: Layerstack for the integrated PC.
For the simulations the sloped side of the PC is discretized in 60 steps. A full vectorial mode
solver (Film Mode Matching [18]) is used to calculate the tilt angle (see Fig. 4.1) and propagation constants for the optimal width and a slope of 54° for a PC in this layerstack. For a width
of 0.78 µm the modes are at ±45°. For this width a half beatlength of 102 µm is obtained.
The simulation predicts an optimal conversion of TE to TM and vice versa larger than 99%. A
conversion above 95% is expected for a width deviation of ± 50 nm as is shown in Fig. 4.7.
95% conversion is sufficient for the demonstration of the concepts shown in chapter 1 as will
become clear in chapter 7.
With this design a 3D simulation is done to optimize the coupling to straight waveguides and
to confirm the conversion.
Waveguides used on the rest of the PIC are shallowly etched waveguides, as they have the
lowest loss. The PC itself is deeply etched, so the shallow waveguides are connected to deep
waveguides using a lowloss shallow-deep transition. The deep waveguides are tapered using
50 µm long parabolic tapers to a width of approximately 1.3 µm to make the connection to the
PC as shown in Fig. 4.8. At the interface of the PC and the waveguide a small, 1 µm wide,
4.3 First generation
63
1
Conversion
0.9
0.8
taper
0.7
input
polarization
waveguide converter
wet etch border
0.6
0.5
0.7
0.75
0.8
Width [µm]
0.85
Figure 4.7: Simulated conversion as a function of
width for a fixed length
Figure 4.8: Topview of the polarization converter
wall is present to confine the wet etch for the sloped side in this area and to prevent etching of
the input waveguides. According to simulations this should introduce less than 0.1 dB loss.
4.3.2
Fabrication
The fabrication is done as much as possible with the standard technology described in chapter
3. However some changes are necessary, as the width of the device has to be defined accurately,
the critical lithography steps are carried out with a 5× reduction wafer stepper as also explained
in chapter 3.
The processing of the polarization converter is shown in Fig. 4.9. The following steps are
distinguished.
a. First the wafer is etched back using an HCl/H3 PO4 etch to selectively remove the top
1200 nm InP cladding. Next a H2 O/H2 SO4 /H2 O2 etch is used to remove the Q-etch
stop layer. Using standard contact lithography, the phase grating alignment marks are
defined and etched with RIE (not shown). They are needed for the alignment in the wafer
stepper. After this, a Silicon Nitride (SiNx ) mask layer is deposited using PECVD. The
polarization converters and waveguides are defined in Ti on top of the SiNx using the
ASML wafer stepper for the lithography in combination with a lift-off process.
b. The PC and deep waveguides are covered and the shallow waveguides are opened using
a standard contact lithography process. The SiNx is etched in a CHF3 RIE.
c. Next the shallow waveguides are etched using a CH4 /H2 RIE. Because the etch depth for
these has to be controlled accurately, this is done separately from the other structures.
d. The wafer is covered with SiNx . The nitride at the straight side of the PC and at the deep
waveguides is opened. This is a critical step as the alignment has to be done on a stripe
of approximately 800 nm wide. For this step again the wafer stepper is used.
64
Polarization converters
InP
Q125
SiNx
Resist
Ti
(a) Definition of the waveguides using High-resolution lithography and
lift-off
(b) Cover all waveguides, open shallow waveguides
(c) RIE etch shallow waveguides
(d) Cover chip with SiNx , open
straight side of PC and deep waveguides
(e) RIE etch straight side of PC and
deep waveguides
(f) Cover chip with SiNx , open
sloped side of PC
(g) Br2 :CH3 OH etch to etch sloped
sidewall
(h) Remove all SiNx and Ti using HF
Figure 4.9: Processing of the integrated polarization converter (see text).
e. The deep waveguides and the straight side of the PC are etched using RIE.
f. Everything is covered again with SiNx and the nitride at the sloped side of the PC is
opened, this step is again critical, due to the alignment tolerance which is smaller than
400 nm on the 800 nm wide stripe for the PC. For this step again the wafer stepper is
used.
g. Br2 -Methanol is used to etch the slope. This etchant etches both InP and InGaAsP with
an angle of 54.7° with respect to the surface (see section 3.5).
h. Finally all the SiNx and Ti are removed using an HF solution.
A photograph of the fabricated device is shown in 4.10.
4.3 First generation
65
The slanted side is etched deeper than the vertical wall, but that has no influence on the modes
since both etches are completely through the guiding layer. During the wet etch an underetch
of 120 nm occurred. The width of the realized converters is reduced by this amount. This has
to be taken into account in the design width.
Figure 4.10: SEM photograph of a realized polarization converter.
4.3.3
Characterization
The polarization converters are measured using the setup shown in Fig. 4.11. The device is
excited using an EDFA as a broad spectral light source with a bandpass filter set to 1555 nm.
The filter has a 2 nm bandwidth, which is large enough to average out Fabry-Pérot resonances
resulting from the uncoated facets of the chip. A polarizer at the input of the chip is used to
select the input polarization. At the output another polarizer selects the polarization that is
measured using the photodiode and the lock-in amplifier.
Lock-in
amplifier
Chopper Polarizer
EDFA
BPF
PC
Polarizer
TIA
Pinhole
PD
Figure 4.11: Setup used for characterization of the polarization converter. BPF: Bandpass filter, PD: Photodiode,
TIA: Trans impedance amplifier
The power in both polarizations at the output is measured for both TE and TM polarized light
at the input. The conversion is defined as the fraction of the converted polarization in the
output power. The conversion C for the two polarizations (i, j = TE, TM) is determined from
66
Polarization converters
the following equations:
Pi j = αi α j C Pj ; i 6= j
(4.10)
Pi j = αi α j (1 −C) Pj ; i = j
(4.11)
where Pj is the input power with polarization j; Pi j is the output power in polarization i when
the input polarization is j; αi, j are the losses for the two polarizations in the input and output
waveguides.
By solving these equations, the propagation losses in the input and the output waveguides for
the two polarizations are eliminated and the conversion C is obtained:
√
x
√
(4.12)
C =
1+ x
where
x =
Pi j Pji
Pii Pj j
; i 6= j
(4.13)
The measured conversion is plotted as a function of width in Fig. 4.12.
1
0.95
Conversion
0.9
0.85
0.8
0.75
0.7
0.65
0.6
0.75
0.8
Width [µm]
0.85
Figure 4.12: Measured conversion as a function of width of polarization converter
A conversion of 95% can be achieved for a width of 0.75 µm. For the actually designed width
of 0.78 µm a conversion of 89% is obtained. The actual widths deviate from the simulations.
This is mainly due to the under etch, which is not exactly 150 nm.
4.3.4
Integration
The next step is the integration of the PC with passive and active components. The process for
the integration of the PCs with active and passive components is subdivided into two parts, one
part for the polarization components and one in which the other components are made.
4.3 First generation
67
> 1.2mm
(a) Cross section to compare the PC and
an optical amplifier
(b) Damaged PC after RIE waveguides, including a reconstruction
Figure 4.13: Problems in integration of the PC
• The polarization components require a thin topcladding of only 300 nm, the waveguides
and the SOAs need a thick topcladding of 1.5 µm. First the area in which the polarization
converters are placed is etched down to this level. On this topography the first lithography step is done, which is common for all the waveguides, for the standard components
as well as for the polarization converters. This has to be a high resolution step for the PC
as described in the process scheme in this chapter. The standard components are covered
with an SiNx mask. The complete process as shown in Fig. 4.9 for the PC is performed.
• After this, the polarization converters are covered with SiNx and the standard process as
shown in Fig. 3.2 is performed.
The height differences, as shown in Fig. 4.13(a), between the PCs and other components on
the chip are larger than 1.2 µm. This proved to cause problems in the lithography: for critical
definition, thin resist is needed. With resist thickness below 1 µm, the large step cannot be
covered. So thicker resist is required. This limits the resolution, but tests show that this is still
useable for the definition of the PC.
Furthermore the limited depth-of-focus will pose problems in a good definition at the lowest
level, on which the width of the PC has to be defined.
Another problem occurred when the etching of the standard waveguides was performed, as
shown in Fig. 4.13(b). This SEM photograph shows a PC integrated with active and passive
components. The drawn lines show the designed structure.
There clearly was a problem in the masking of the sloped side of the PC during the RIE etching
of the waveguides. The SiNx mask is eroded on the sloped side and the PC is etched away. The
in- and output waveguides of the PC are still present, so the lithographical definition as well
as the RIE etching of the straight waveguides has been done right. (It has to be noted that the
68
Polarization converters
critical dimensions are not met.)
An improved process has to be developed to avoid these problems. First of all the critical width
definitions have to be on the same hight level, allowing thinner resist and no problems with
limited focussing depth. The etching of the PCs and the waveguides are done consecutively,
in a next realization the etching for both the PCs and the standard components can be more
integrated. The etching of the slope has to be done as a last step to avoid damage afterwards.
4.4
Second generation
The problem with the previous design is the difficulty to integrate the PC with passive and
specifically with active components. The polarization converters and active components (such
as semiconductor optical amplifiers (SOAs) and phaseshifters [73]) can be made in the same
layerstack, but the optimal thickness of the top-cladding differs by more than 1 µm. For polarization converters a large birefringence, and more importantly, a critical width definition
close to the waveguide layer is needed: hence a thin topcladding is preferred; for SOAs and
electro-optic phaseshifters, a thick topcladding (typically 1.5 µm) is needed below the contact
to avoid optical losses due to the contactlayer.
This is clarified in Fig. 4.14: an SOA, phaseshifter and a conventional polarization converter
are shown. Critical lithographical definition is not possible with these height differences as
the photoresist thickness varies over the different heights and the depth of focus is limited for
critical definitions. This complicates processing and integration of these devices.
The new design [74] is shown in the rightmost picture in Fig. 4.14. It uses the same thickness
of the topcladding as an active device (without the thin contactlayer) and can be integrated with
the other components. The birefringence with the thicker cladding is only slightly reduced with
respect to the thinner cladding. Hence the increase in length for this design is less than 40µm.
The critical width definition of the PC is kept during the etching as is explained in the next
sections.
SOA
Electro-optic
phaseshifter
First
generation
New design
Contact metal
InP
topcladding
Q1.25
waveguidelayer
InP
substrate
Figure 4.14: Cross section of active devices (SOA), phaseshifter, the first generation PC and the new design.
4.4 Second generation
4.4.1
69
Design
The layer stack is the same as used for the standard components (see Fig. 4.6(a)). Film Mode
Matching simulations [18] predict an optimal conversion of TE to TM and vice versa larger
than 99% for a width of 0.94 µm and a length of 141 µm. A conversion above 95% is expected
for a width range of 100 nm as is shown in Fig. 4.15.
1
Conversion
0.9
0.8
taper
0.7
input
polarization
waveguide converter
wet etch border
0.6
0.5
0.7
0.8
0.9
1
Width [µm]
1.1
1.2
Figure 4.15: Simulated conversion as a function of
width for a fixed length.
Figure 4.16: Topview of the polarization converter
Deviations from the calculated width and length can be expected, because the refractive indices
used in the simulations are not accurately known for these materials.
The total device is similar to the first generation PC, it contains an asymmetric waveguide as
the converter section, 1.2 µm wide deep input and output waveguides, coupled to 3 µm wide
shallow waveguides via 75 µm long tapers (Fig. 4.16).
At the interface of the input waveguides and the polarization converter section a small ridge
is present, perpendicular to the waveguide. This is needed to prevent etching of the input
waveguide in the wet etch for the slanted sidewall. However according to simulations, this
does not influence the propagation of the waveguide modes.
4.4.2
Fabrication
The polarization converters have strict tolerances in width, so high resolution lithography is
required. In this realization, the converter sections have been defined using Electron Beam
Lithography (EBL). EBL is not suited for writing large circuits, therefore all other waveguides
have been defined using standard optical lithography. The EBL written parts have to be aligned
to the optical waveguides as explained in chapter 3.
The processing of the polarization converter is shown in Fig. 4.17 and consists of the following
steps:
70
Polarization converters
InP
Q125
SiNx
Resist
Ti
(a) Optical lithographical defined
writefields and standard waveguides
(b) EBL definition of PC waveguide
(c) Straight side of the PC is openend using EBL
(d) The SiNx at the straight side of
the PC is etched
(e) All waveguides are etched using
the standard shallow deep processing
(f) The sloped side of the PC is
etched till the right depth
(g) The resist is removed and the
sample is covered with SiNx . The PC
area is opened using RIE
(h) The sloped sidewall is etched using Br2 :CH3 OH
(i) The SiNx and Ti are removed using HF
Figure 4.17: Processing of the integrated polarization converter (see text).
4.4 Second generation
71
a. First the waveguides and writefields for the EBL, including the alignment marks, are
defined optically in a Silicon Nitride (SiNx ) mask layer.
b. The polarization converters are defined in Ti on top of the SiNx using EBL and a lift-off
process.
c. Before etching any waveguide, a second EBL step is done to open the straight side of
the PC.
d. Next the nitride at the straight side of the PC is opened using the EBL-resist and the
titanium as a mask.
e. The shallow and deep waveguides, and the straight side of the PC are etched with
CH4 /H2 Reactive Ion Etching (RIE) in a double etch process [75]. This is the same
as the standard processing explained in chapter 3.
f. All shallow waveguides are covered with resist, and the PC area is opened with a noncritical optical lithography step. The SiNx at the sloped side of the PC is opened and
the InP topcladding is RIE etched until 300 nm above the waveguide layer. In this step
the critical dimensions are kept, because the Ti mask does not erode during the etch and
thus the sidewalls are vertical. While etching this side, the straight side of the PC (which
is already etched in step e.) is etched even deeper, well below the waveguide layer.
g. Silicon Nitride is deposited on the whole sample and the shallow waveguides are again
covered with resist. The PC area is opened using a non-critical lithography. The SiNx
on the PC area is etched back using CHF3 RIE. Because of the directional etching, the
etched sidewalls stay covered with SiNx , which serves as a mask for the wet etching.
h. Br2 -Methanol is used to etch the slope. This etchant etches both InP and InGaAsP with
an angle of 54.7° with respect to the surface (see section 3.5). In this step, the straight
side of the PC is etched as well. As this side is already etched well below the waveguide
layer, the etching will not influence the performance of the converter. The advantage
of this process with respect to the first generation is that here only 2 critical steps are
needed (the definition of the PC waveguide (b) and the coverage of the sloped side (c)).
A separate step to cover the deep side (step (f) in the first generation) and open the slope
side is not necessary.
i. Finally all the nitride and the Ti is removed using an HF solution.
The fabricated converters are shown in Fig. 4.18. From these figures it is clear that there is
an underetch at the shallow side (the sloped side). There is however no underetch at the deep
side. This indicates that the underetch can be caused by stress in the masking material during
the wet-etching.
72
Polarization converters
Figure 4.18: SEM photographs of the integrated Polarization converter. Left: transition of a straight waveguide to
the polarization converter. Right: cross section through the converter.
The influence of this additional underetch on the conversion performance of the device will be
small. The width definition at the InGaAsP layer is not affected by this underetch: the slope
starts at the desired point, fixed by the Silicon Nitride, and stops on the crystal plane. Fig. 4.19
shows the tilted modes for both cases (with and without underetch). The resulting difference
in the tilt angle of the modes is smaller than 1°. Using eq. (4.9), the maximum conversion (for
φ = π) can be obtained from:
c = 4 (cos (θ ) sin (θ ))2
(4.14)
So for a deviation in the tilt angle θ of ±1°, the maximum conversion is expected to be beyond
99.9%.
Mode 0
y [µm]
0.5
y [µm]
Mode 1
No underetch
Full underetch
0.5
0
−0.5
−0.5
0
−0.5
0
0.5
1
x [µm]
1.5
−0.5
2
0
0.5
1
x [µm]
Figure 4.19: E-Field of the tilted modes in the polarization converter.
1.5
2
4.4 Second generation
4.4.3
73
Characterization
1
1
0.8
0.8
Conversion
Conversion
The polarization converters are measured using the setup as shown before in Fig. 4.11 and
analyzed according to the method explained in section 4.3.3. The power in both polarizations
at the output is measured for both TE and TM polarized light at the input. The measured
conversion is plotted as a function of width and length in Fig. 4.20.
0.6
0.4
0.2
0
0.75
0.6
0.4
0.2
0.8
0.85
0.9
0.95
Width [µm]
1
1.05
0
0
100
200
300
Length [µm]
400
Figure 4.20: Simulated conversion (solid), measured values (×, ♦, ), and fit (dashed) as a function of width(left)
and length(right). The symbols indicate multiple realizations.
The maximum measured conversion from TE to TM and vice versa occurs at 131 µm length,
corresponding to the half beat length between the modes of the converter section. The conversion is back to zero at the full beat length (262 µm). The maximum conversion for this device
is 97%.
The scattered values for the measured conversion are caused by non uniformities in the width
along the device, caused in part by deviations in the etch depth, the underetch and the layer
thicknesses. A width variation of ±30 nm can explain the observed behavior. Previous research
has indicated that the width-reproducibility of our EBL is around these values [76, 72].
The right figure shows measurements for multiple realizations of the PC on different wafers,
indicated by the different symbols. This shows the reproducibility of the process of the PC. A
conversion larger than 95% is obtained in all realizations for the same device dimensions.
Fig. 4.21 shows both the wavelength and the temperature dependence of the PC. A conversion
larger than 95% is obtained over a wavelength range larger than 35 nm (limited by the available
equipment). The device is insensitive to temperature in the range from 10 to 50°C. This is an
important issue as in a PIC with active components heat is generated and the temperatures can
rise locally.
The total excess losses of the polarization converter, including input and output tapers, are
measured to be 2.4±0.3 dB for TE and 2.6±0.3 dB for TM with respect to a shallow waveguide. The losses can be separated in scattering loss, caused by sidewall roughness of the
waveguides, which has a big influence on the narrow deeply etched waveguides, and overlap
losses at the junctions (simulated to be approximately 0.25 dB). Most probably extra coupling
losses are caused by the non-optimal coupling of the waveguide and the converter because of
74
Polarization converters
1
o
10 C
o
15 C
o
21 C
o
30 C
o
40 C
o
50 C
Conversion
0.95
0.9
0.85
0.8
1530
1540
1550
1560
1570
Wavelength [nm]
Figure 4.21: Measured conversion as a function of temperature and wavelength.
the underetch of the access waveguides in the propagation direction. This can be avoided by
using a better controlled wet etch and a better masking.
4.5
Conclusion
Polarization converters are required for on-chip polarization manipulation. Two generations
of polarization converters are demonstrated. The first device shows promising results, but
has problems in integrating with active and passive components. These problems are mainly
caused by the height difference between the PC and the other components. The PC has a topcladding that is more than 1 µm thinner than that for the other components on the chip.
A second generation PC is developed that is specifically targeted to be integrated. Here a design is made that has a similar cladding thickness. Thus all the critical definitions can be done
at the same level.
Fabricated devices show the feasibility of integration with standard shallow and deep etched
waveguides. The device has a maximum conversion of 97%. A conversion larger than 95%
is needed for the demonstration of the concepts shown in chapter 1. The conversion is larger
than 95% over a large operation window: a wavelength range 35 nm and a temperature range
>40°C. It is shown that the process is reproducible and a conversion of more than 95% is obtained in all investigated process runs.
These properties demonstrate the capability of a generic integration technology with polarization conversion.
Chapter 5
Polarization splitters
5.1
Introduction
In addition to polarization converters, discussed in the previous chapter, polarization splitters
(PS) are important devices for on-chip polarization manipulation. Polarization splitters that
can be integrated with other functions on a chip are crucial in an integration technology with
polarization handling capability.
As the name implies, polarization splitters separate the TE and TM modes at the input and
route them to different outputs. As these are usually reciprocal devices, they can also be used
as polarization combiners. In principle TE and TM signals can be combined from 2 different
inputs into one output without any losses. A special case of combiners, which are needed in
certain applications (see chapter 7), is the 2 × 2 polarization splitter/coupler. This device works
as 100% 2 × 2 coupler for one polarization, but as a 0% 2 × 2 coupler for the other one.
Passive polarization splitting is preferred over active splitting to minimize power consumption
and to avoid the need for tuning. For passive splitting, birefringence is required to obtain a
difference in propagation for TE and TM polarized light.
The birefringence needed for splitting can be obtained in a number of ways. It can be by
loading a waveguide with metal [77, 78]; by using the modal birefringence of the fundamental
mode (demonstrated in InP [79, 80] and Si [81, 82]) or of higher-order modes of TE and
TM [83, 84]; or by using waveguides with a different geometry for the different polarizations
[85, 86]. The most promising devices suited for integration in InP are shown in table 5.1; for
all devices the possibility of 2 × 2 splitters is indicated.
As can be seen from the table, the splitting can be obtained by using different principles: split75
Polarization splitters
76
A
B
C
D
E
Ref.
[83]
[77]
[78]
[80]
[84]
Principle
TE+TM
TE+TM
TE+TM
TE+TM
TE+TM
Schematic
Table 5.1: Different passive polarization splitters
Mode evolution. Modal birefringence of the first-order mode (only
1 × 2).
Interference: directional coupler
employing birefringence obtained
by strip-loading one waveguide
with metal (2 × 2).
Interference: Mach Zehnder Interferometer employing birefringence
obtained by strip-loading one waveguide with metal (2 × 2).
Interference: directional coupler
employing modal birefringence of
the fundamental system modes (1 ×
2 demonstrated, 2 × 2 is possible).
Interference: directional coupler
employing modal birefringence of
first-order modes (1 × 2).
TE
TM
TE
TM
TM
TE
TE
TM
TE
TM
L = 6000 µm
SRTE =12.0 dB
SRTM =13.1 dB
Loss < 1 dB
Performance
•
•
•
•
L = 1600 µm
SRTE =12.2 dB
SRTM =30 dB
Loss 0.3 dB
Fabrication
•
•
•
•
L = 3300 µm
SRTE =16 dB
SRTM =13 dB
Loss 1.5 dB
shalRIE
Au sputtered
metal patch,
RIE etched
shallow
waveguides.
•
•
•
•
L = 3900 µm
SRTE =16 dB
SRTM =17 dB
Loss < 1.5 dB
Single
low
etch.
Au sputtered
metal patch,
RIE etched
shallow
waveguides.
•
•
•
•
L = 1100 µm
SRTE =13 dB
SRTM =20 dB
Loss < 1 dB
shalRIE
2 step selective wet etch.
•
•
•
•
Single
low
etch.
5.2 Directional coupler polarization splitter
77
ting based on mode-evolution in an adiabatic coupler (splitter A) [83], which results in tolerant,
but very long devices; or splitting based on interference. The splitters based on the latter principle (B–E) can be divided in two categories: devices that use metal loading of the waveguides
to achieve birefringence (B, C) and devices that employ modal birefringence and consist solely
of waveguides (D, E) that can be etched in a single step. The latter are advantageous over
the ones with metal on top, because of the relaxed processing. They have the advantage to be
short, have a low loss and a high splitting ratio. A drawback are the very stringent fabrication
tolerances.
The device employing the birefringence of the fundamental mode (D) has to meet even more
strict conditions, as in this case, for both TE and TM, a coupling condition has to be met. TE
has to couple through and TM has to have the exact double coupling length to couple through
and back again. In the device that uses the modal birefringence of the higher-order modes E
[84], only one condition has to be met: TE has to couple. This makes the latter device more
tolerant and shorter.
Even shorter splitters based on photonic crystal waveguides [87] are reported, but these have
the disadvantage of higher losses and more complex processing, making them difficult to integrate.
This makes splitters based on interference the most suitable for the integration with active and
passive components.
In this chapter two types of polarization splitters based on interference are described. The
principle, design, fabrication and results are discussed.
First a new tolerant 1 × 2 directional coupler based PS is reported. This device has a high
splitting ratio, but is very long and hard to integrate. Next a 1 × 2 and a 2 × 2 splitter/coupler
based on an Mach Zehnder Interferometer with polarization converters is presented. This
device is short and is fully integrateable in the standard active-passive integration scheme.
5.2
Directional coupler polarization splitter
As stated before, splitters based on modal birefringence have very stringent fabrication tolerances. Tapering both the waveguides and the coupling region increases the tolerances in directional couplers at the cost of a somewhat increased length [88]. This section will demonstrate
a polarization splitter based on modal birefringence with tapered waveguides in combination
with a tapered coupling region that has an increased tolerance in fabrication.
5.2.1
Principle
In this section the principle of the directional coupler will be explained first, and secondly the
implementation of the polarization splitter is explained.
78
Polarization splitters
Directional coupler
A directional coupler consists of two adjacent identical waveguides as depicted in Fig. 5.1. If
they are closely spaced, the evanescent field of light injected in one waveguide will excite a
mode in the other one and power exchange can take place while propagating [89].
The coupling between the two resonant waveguides can be regarded as an interference of the
system modes [90] as shown in the figure. A symmetric (with propagation constant β0 ) and an
asymmetric (with propagation constant β1 ) system mode are present in the coupler, they are
excited with equal amplitude by the mode in the input waveguide. At the input of the coupler,
the modes are in phase, so there is constructive interference in the input waveguide. After
propagation over a distance Lc , the modes will have a phase difference of π and all the power
will be transferred to the cross (upper) output waveguide (P× ). This distance is the coupling
length:
π
π
=
(5.1)
Lc =
β0 − β1
2c
in which c is the coupling constant
c =
1
(β0 − β1 )
2
(5.2)
The coupling constant depends exponentially on the gap between the waveguides. With the
coupling constant, the power in both waveguides as a function of propagation in the z direction
can be expressed as:
P= = cos2 (cz)
(5.3)
P× = sin2 (cz)
b
Px
Pin
a
b0
b1
P=
Figure 5.1: Schematic of the directional coupler with the system modes visualized
The modes propagating in the two waveguides in the z-direction can be expressed as:
E= (z) = e= (z)e(− jβ= z) ,
E× (z) = e× (z)e(− jβ× z)
(5.4)
If the waveguides are identical, β= = β× are the propagation constants of the modes in the
respective waveguides. e=,× are the amplitudes of the fields in the corresponding waveguides:
e= (z) = cos(cz),
e× (z) = j sin(cz)
(5.5)
5.2 Directional coupler polarization splitter
79
In this case synchronous coupling is possible and full power exchange between the two waveguides can take place. In the case where β= 6= β× , asynchronous coupling will occur as the
velocity of the modes in the waveguides do not match. This will result in incomplete coupling
and only a fraction of the power is transferred to the opposite waveguide [91, 92]. The distance
over which coupling takes place is shorter than in the synchronous case.
Splitter
To use the directional coupler as a polarization splitter, a polarization-dependent coupling is
required. The splitting is based on modal birefringence: the propagation constants of the
higher-order modes differ significantly for different polarizations. The splitter consists of an
asymmetric directional coupler with a narrow and a wide waveguide (Fig. 5.2). The widths are
chosen such that the propagation constant of the fundamental mode in the narrow waveguide
equals the propagation constant of a higher-order mode in the wide waveguide for one polarization only. Fig. 5.3 shows the calculated mode intensity profiles. It is seen that for TE the
fundamental and third-order waveguide modes are resonant, but for TM this is not the case.
Only the resonant polarization can couple. Over a distance equal to the coupling length, the
light in this polarization is fully transferred from the narrow to the wide waveguide. The same
principle is repeated to couple from the wide waveguide to the narrow output waveguide. In
this way the third-order mode is converted back to the fundamental mode and a double filtering
is achieved. The TM output has to bend away from the wide waveguide to prevent coupling.
TE
Input
(TE+TM)
Lcoupling
TM
Figure 5.2: Principle of the polarization splitter
The resonant modes are determined by calculating the propagation constant as a function of the
width at a wavelength of 1555 nm using the effective index method (Fig. 5.5) for a waveguide
cross section as shown in Fig. 5.4. This cross section is fully compatible with the layerstack
and processing for the first generation polarization converter as treated in section 4.3.
The propagation constants match for TE00 and TE03 for widths of 0.8 µm and 6.9 µm respectively. Furthermore, for a width of 6.9 µm no TM mode is resonant with the fundamental mode
for the narrow waveguide.
In fabrication, the width of the waveguides can deviate from the designed width, which causes
a change in the propagation constants and thereby destroys the resonance of the modes. This
80
Polarization splitters
(a)
(b)
Figure 5.3: Resonant mode for TE (a) and non-resonant mode for TM (b)
leads to a decreased coupling for one polarization and a possibly increased coupling for the
other, which will deteriorate the splitting ratio of the device.
To compensate for these errors in fabrication, the wide waveguide is tapered as is shown in Fig.
5.6 [93]. The tapering is possible because from Fig. 5.5 it can be seen that the propagation
constant for a higher-order TM mode does not match the propagation constant in the narrow
waveguide in a width range more than 750 nm. This large range yields a window for tapering,
while preserving the mismatch for TM.
In the tapered device, the coupling region (the position at which the width is such that resonance for TE occurs) shifts along the taper when a width deviation ∆w is present. This shift of
the coupling region ∆L can be calculated in the following way:
dβ
dw
dβ
dβ
∆L +
∆w =
∆w
(5.6)
dw w2 dz
dw w2
dw w1
In this equation w1 refers to the narrow waveguide, w2 refers to the wide one. The left part
of the equation is the change in the propagation constant in the wide waveguide: the leftmost
term is the change in propagation constant β as the coupling region shifts ∆L along the taper
with taper angle dw
dz . The second term is the change of the propagation constant as the width
changes with ∆w. To allow coupling, the propagation constants in both waveguides should
remain equal: the right side is the change in propagation constant in the narrow waveguide for
a width deviation ∆w.
To achieve a constant coupling for a changing width, the coupling constant has to be fixed in the
coupling region. Both the position of the coupling region and the coupling constant depend on
∆w. The coupling constant depends exponentially on the gap g between the 2 waveguides, so g
should also be tapered along the length of the device, in such a way that at the coupling region
position a fixed gap g0 , and thus a defined coupling constant, is obtained. The dependence of
g on ∆w is:
dg
∆L
(5.7)
g = g0 − ∆w +
dz
with the second term on the right-hand side indicating the direct change because of the width
deviation and the third term the effect of the tapered gap between the waveguides. These terms
5.2 Directional coupler polarization splitter
81
13.25
0
w
Q(1.25)
substrate n-InP
03
TE
13.15
03
00
500 nm
TM
TM
n.i.d.-InP
Propagation constant [1/µm ]
300 nm
TE0
13.20
13.10
04
TM
13.05
13.00
0
1
2
3
4
5
6
7
8
9
10
Width [µm ]
w1
w2
dw
Figure 5.4: Cross section
Figure 5.5: Propagation constants as a function of width for the relevant
modes. w1 (w2 ) is the width of the narrow (wide) waveguide. The horizontal
lines indicate matching propagation constants for TE00 and TE03 , but not for
TM00 and any other mode. dw is the range over which there is no matching
for TM00 .
∆w
have to cancel out, so dg
dz = ∆L . Combining (5.6) and (5.7) results in the relation between
the taper angle of the wide waveguide αT = arctan dw
dz and the taper angle of the coupling
dg
region αc = arctan dz :
arctan
dg
dz
= arctan
dw
dz
dβ
dw w
2 dβ
dβ
−
dw w
dw w
1
(5.8)
2
The derivatives of the propagation constant with respect to the width dβ
dw are obtained from
the tangent to the corresponding curves in Fig. 5.5.
The total device consists of a cascade of two of these couplers to couple the third-order TE
mode to the fundamental mode at the output waveguide. This supplies additional filtering for
the unwanted polarization (Fig. 5.6).
5.2.2
Simulations
The polarization splitter is simulated using a 2D Beam Propagation Method (BPM) [38]. The
effective index method is used to reduce the problem to 2 dimensions. These simulations are
82
Polarization splitters
P1
αT
w2 w2 +∆w
g1
g1 ’
αc
P2
Lc
Figure 5.6: Schematic of the polarization splitter, note that the vertical dimensions are
enlarged for clarity, the dashed lines visualize the broadening of the waveguides that can
be introduced in fabrication.
used to determine the length and to investigate the fabrication tolerances. The propagated field
in the splitter for both polarizations for a wavelength of 1555 nm is plotted in Fig. 5.7. It is
clearly visible that TE polarized light couples to the central (wide) waveguide and then couples
to the upper output waveguide. TM does not couple to the wide waveguide and stays in the
lower narrow waveguide.
Figure 5.7: Top view of the propagated field in the polarization splitter for TE (left) and TM (right)
In chapter 7 we show that a splitting of more than 95% is needed for a practical polarization
diversity scheme [8]. The splitting ratio (SR) of the TE polarization is defined as:
P1 (TE)
SR(TE) = 10 log
;
(5.9)
P2 (TE)
where P1 (TE) is the power in the TE-output (output 1) for TE polarized light and P2 (TE) is the
5.2 Directional coupler polarization splitter
83
power in the TM-output (output 2) for TE polarized light. For TM a similar definition is used:
P2 (TM)
SR(TM) = 10 log
.
(5.10)
P1 (TM)
The SR is calculated as a function of the width deviation1 . The results are plotted in Fig. 5.8
for both polarizations. The SR for this splitter is compared to a splitter without tapers, and to
a splitter without a tapered coupling region, but with a tapered wide waveguide. The dotted
50
50
Straight
40
30
30
20
Tapered waveguide
+ tapered gap
Tapered waveguide
10
SR [dB]
SR [dB]
TE
40
TM
Tapered waveguide
+ tapered gap
20
10
Straight
0
-100
-50
0
50
100
Width deviation [nm]
150
200
0
-100
-50
0
50
100
150
200
Width deviation [nm]
Figure 5.8: Simulated polarization splitting ratio as a function of width deviation for TE (left) and TM (right). The
dotted line indicates 13 dB splitting ratio
line in Fig. 5.8 indicates an SR of 13 dB (a splitting of 95%). From the graph it is clear that
the devices perform well for TM. Because for TM no resonance is present, even for a width
range as large as 300 nm no light will couple to the wide waveguide. The possible width range
is not as large as expected from the dispersion curves (Fig. 5.5), because also for modes that
are not completely resonant, some light can couple. The performance of the device is limited
by its behavior for TE. For TE, the straight device is only performing well for a width range of
35 nm. By tapering the wide waveguide the tolerance is improved to 95 nm. By also applying
a tapered coupling region according to eq. (5.8), a tolerance range of 145 nm can be reached.
The 2 maxima for TE in Fig. 5.8 are present because the device is not only a synchronous directional coupler [94]. In the region of the taper where the propagation constants do not match
exactly, only a small deviation from the synchronous waveguide widths is present, because
the waveguides are not far apart, the device acts as an asynchronous coupler with changing
coupling constant along the taper. The asynchronous coupling plays a role in the performance
of the device. As this effect is not taken into account in eq. (5.6), this equation does not fully
describe the coupling.
For TM the device is an asynchronous coupler. The large mismatch in propagation constants
for the narrow and the wide waveguide results in a short coupling length with a very low
coupling constant. This explains the periodic behavior and the large SR for TM.
1 The width deviation is applied to both the narrow and the wide waveguides, the position of the waveguides is
constant, so with increasing the width, the gap is decreased.
84
Polarization splitters
Table 5.2: Dimensions of the polarization splitter
5.2.3
Parameter
Symbol
Value
Taper length
Lc
1325 µm
Width input waveguide
w1
0.85 µm
Width at the start of the taper
w2s
7.27 µm
Width at the end of the taper
w2e
6.64 µm
Gap at the start of the taper
g1s
1.81 µm
Gap at the end of the taper
g1e
2.02 µm
Design
The 2D BPM simulations in combination with the effective index method can give inaccurate
results, especially for the exact coupling length. An optimization of the design is performed
by calculating the modal propagation constants of the cross section of the device with a Film
Mode Matching (FMM) mode solver [18]. The structure for the FMM simulation is chosen to
have the same index difference between the narrow and wide waveguide, and the same coupling
strength as used for the BPM calculations, both at the start and endpoint of the taper. With these
results, a full-vectorial 3D propagation simulation is done. From this the optimal dimensions
are obtained and used for the realization of the component. The optimal dimensions are shown
in table 5.2.
5.2.4
Fabrication
The devices are fabricated on an InP/InGaAsP layerstack. The layerstack consists of an InP
substrate, a 500 nm InGaAsP film layer with a bandgap at a wavelength of 1.25 µm and a
300 nm InP top cladding. This layerstack is fully compatible with the active-passive integration
scheme and allows the component to be monolithically integrated with other passive and active
devices [73].
A mask is designed on which splitters with a width variation of -100 nm to +100 nm in steps
of 20 nm are implemented. The center width of this range is 30 nm wider than in Fig. 5.8, in
order to investigate the full tolerance window. All splitters are connected with 100 µm long
tapers to 3 µm wide input and output waveguides. The lithographical definition of the waveguides is done in the 5× reduction wafer stepper as introduced in chapter 3 and used for the
first generation polarization converters. It allows a very accurate width control, better than
20 nm on a 800 nm line. This is better than the tolerance needed for the waveguides and the
taper, so the tolerances can be investigated.
The etching of the waveguides is done with CH4 /H2 Reactive Ion Etching (RIE). The devices
have to be shallowly etched, exactly until the film layer. To ensure the right etch depth, the
RIE etch is stopped slightly above the InGaAsP film layer, after which a selective wet etch dip
is applied to etch the remaining InP.
5.2 Directional coupler polarization splitter
85
An SEM photograph of the cross section is shown in Fig. 5.9. Both the narrow and the wide
waveguides are shown. The sidewall roughness which is visible is due to non-optimal process
conditions. Reducing the roughness will be important to improve the performance.
Figure 5.9: SEM photograph of the processed polarization splitter
5.2.5
Characterization
Measurements are performed using the setup shown in Fig. 5.10. The device is excited using an EDFA as a source, filtered with a 2.5 nm wide bandpass filter. This averages out the
Fabry-Pérot resonances caused by reflections from the facets of the chip, and thus increases the
stability. The polarization is selected by a free-space polarizer. The output is imaged onto an
InGaAs CCD camera. The obtained image (Fig. 5.11) is analyzed to obtain the power in both
outputs simultaneously. The intensity in both ports is obtained, by integrating the intensity
values in the lineprofile over a fixed width. From this, the splitting ratio (SR) is determined as
defined in eq. (5.9) and (5.10). With this method we can measure a maximum SR of 22 dB,
limited by the dynamic range and the noise level of the camera.
Polarizer
EDFA
PS
BPF
Figure 5.10: Measurement setup used to characterize the polarization splitters.
CCD
86
Polarization splitters
4000
TE input
TM input
3500
Intensity[a.u.]
3000
2500
2000
1500
output 2
output 1
1000
500
0
0
10
20
30
40
50
x-position [µm ]
Figure 5.11: Measurement of the power in both outputs using a CCD camera for TE (solid) and TM (dashed) at the
input.
The resulting splitting ratio as a function of the width deviation is shown in Fig. 5.12. The
measured splitting ratio is compared to the simulated results.
50
Simulation
Measurements
TE
40
40
30
30
SR [dB]
SR [dB]
50
20
10
0
-100
Simulation
Measurements
TM
20
10
-50
0
Width deviation [nm]
50
100
0
-100
-50
0
50
100
Width deviation [nm]
Figure 5.12: Splitting ratio measurements at 1560 nm, compared to BPM simulations for TE (left) and TM (right)
polarization, the dotted line indicates 13 dB splitting ratio
From the figure it is clear that the measured devices show an SR larger than 13 dB for a large
width range. For TM polarized light, the splitter is good for a range of at least 150 nm.
For TE the range is limited to approximately 100 nm. Two maxima are present in the TE
results, as is also seen from simulations.
The best splitter (40 nm wider) has an SR of 18 dB for TE and 20 dB for TM, better than any
of the devices in table 5.1.
The overall performance as shown in Fig. 5.12 is less than predicted by simulations, most
probably caused by scattering from the rough waveguide walls, which can lead to unwanted
coupling as will be confirmed by the loss measurements. Because of this, the local minimum
5.3 MZI Polarization splitter
87
for TE at 0 nm deviation is slightly lower than 13 dB.
The losses are measured using a Fabry-Pérot measurement and are compared to straight waveguides. The 1.5 mm access waveguides to the device have a loss below 0.5 dB for both TE
and TM. The excess loss of the device, including input and output tapers, is 3.4–3.6 dB for TE
polarized light and 6.5–7.0 dB for TM. In principle, the device should have very low losses,
these losses are mainly caused by sidewall roughness as is also visible in Fig. 5.9.
The high losses for TM can be explained because for TM the light will always propagate in the
very narrow waveguide, which results in a higher loss. For TE the light will be mostly situated
in the wide waveguide with a lower loss. By reducing the roughness, a very wide width range
with good polarization splitting can be obtained.
The wavelength dependence of the device having a width of 890 nm (40 nm wider) is measured.
The splitting as a function of wavelength is plotted in Fig. 5.13. The device shows polarization
25
TE
TM
SR [dB]
20
15
10
5
0
1520
1525
1530
1535
1540
1545
1550
1555
1560
1565
Wavelength [nm]
Figure 5.13: Splitting ratio as a function of wavelength (width deviation + 40 nm)
splitting better than 14 dB over the entire EDFA wavelength range from 1520 nm to 1565 nm
for both polarizations.
The splitter described here cannot be used as a 2 × 2 splitter, the design can be changed to
allow such splitting/combining.
A drawback of this design are the large height differences with respect to active and passive
components. This leads to similar lithographic difficulties as experienced with the polarization
converter in chapter 4.
5.3
MZI Polarization splitter
The polarization splitters described in the previous section have the advantage of low loss and
high splitting ratios. A drawback is their length (1 to 3 mm) which is large compared to other
components on the chip. Furthermore, the devices have to be critically defined at a lower level
88
Polarization splitters
than the most active and passive devices, which poses another problem.
A new design is investigated that solves these problems. This is a compact, 600 µm long, integrated splitter based on polarization converters. It consists solely of passive waveguides and
PCs and is thus the device of choice for a generic integration platform in which a polarization
converter can be integrated as demonstrated in chapter 4.
Both a 1 × 2 splitter and a 2 × 2 splitter/coupler are studied.
The principle of the device is explained in the next section. It is experimentally demonstrated
using the first generation PCs from the previous chapter. By using the second generation PCs,
this device is really suitable for active-passive integration. The 2 × 2 splitter is demonstrated
using the second generation PCs.
5.3.1
Principle
The device consists of a Mach Zehnder Interferometer with polarization converters in both
arms, as is depicted in Fig. 5.14. Light coupled into the input waveguide of the first Multi
MMI1
PC1
WG
PC2
MMI2
PC
output1
input
PC
output2
L
Figure 5.14: Schematic of the MZI polarization splitter/converter
Mode Interference coupler (MMI) is split into the two branches with equal power and phase.
In the upper branch a polarization converter is placed that rotates the polarization over 90°, so
after this, the orthogonal polarization propagates through this branch.
In the lower branch the light in the original polarization propagates over a distance L before
being rotated in a polarization converter. The birefringence in the waveguides causes a phase
shift between light in the arms. This phase shift however is equal in magnitude but opposite in
sign for TE and TM. When both signals are combined in the output MMI, the phase difference
causes one polarization to appear in one output while the opposite polarization goes to the
other output. To achieve the desired splitting, the phase difference between the branches needs
to be π2 radians. This is obtained when:
L=
π
,
2 (βTE − βTM )
(5.11)
where βTE,TM are the propagation constants for both polarizations.
Apart from 1 × 2 devices, a 2 × 2 polarization splitter/combiner is possible, based on the same
concept. The device is depicted in Fig. 5.15. Light coupled into the input waveguide of the
first 2 × 2 MMI is split into the two branches with equal power. In this case, the signals in both
5.3 MZI Polarization splitter
89
DL
MMI2
PC1
WG
PC2
MMI2
PC
input1
output1
input2
output2
PC
L
Figure 5.15: Schematic of the MZI polarization splitter/converter
arms have a phase difference of π2 . This phase difference results in destructive interference in
one output and constructive in the other. This is depicted in Fig. 5.16. The two polarizations
1×2 MZI
2×2 MZI
1
1
0.6
φTE
Normalized output power
Normalized output power
output2
output2
0.8
φTM
0.4
0.2
output1
0.8
0.6
φTE
φTM
0.4
0.2
output1
0
−1
−0.5
0
0.5
Phase [π radians]
1
0
−1
−0.5
0
0.5
Phase [π radians]
1
Figure 5.16: Principle of the 1 × 2 and 2 × 2 MZI splitter
always have a phaseshift that is equal in amplitude, but opposite in sign. The arrows in the
figure indicate this phaseshift. For the 2 × 2 MZI, no polarization dependent splitting can be
obtained. To achieve polarization splitting, an additional polarization independent phaseshift
of π2 is required, to obtain the same situation as in the 1 × 2 MZI. In the polarization splitter,
in one arm a path length difference ∆L is present to achieve this polarization independent
phaseshift.
5.3.2
Simulation
The polarization splitter circuit is simulated by concatenating the transfer matrices of each of
the relevant sections in the device (Fig. 5.14 and 5.15): the input coupler (MMI1/MMI2); a
phase compensation section for the 2 × 2 device (∆L); a Polarization Converter in the upper
arm (PC1); straight waveguides of length L in both branches (WG); a Polarization Converter
in the lower arm (PC2) and the output coupler (MMI2). Four different signals can propagate
90
Polarization splitters
through the device, which are represented by a 4 element vector:


Sin = 

TE upper branch
TE lower branch
TM upper branch
TM lower branch




(5.12)
For the 1 × 2 device, only 2 input signals (TE and TM) are possible. Therefore in that case the
”lower branch” input signals are zero.
The transfer matrices of the separate sections indicated in Figs. 5.14 and 5.15 are described
next.
The two polarization splitters differ only in the input section. From eq. (5.16) on, the splitters
are the same. The input section is described first for the 1 × 2 splitter. Here an input 1 × 2 MMI
(MMI1)is present, the transfer of this section is:
TMMI1
 p
pk1TE
 k1
TE
= 

0
0
0
0
0 p0
0 pk1TM
0
k1TM

0
0 

0 
0
(5.13)
Here k1TE , k1TM are the coupling constants of the MMI coupler for the respective polarization.
The coupling constant is defined as the fraction of the power in the crossed output of the MMI.
For a 1 × 2 MMI both outputs are equal as it is a fully symmetric device.
In the case of the 2 × 2 PS, the input section consists of an input 2 × 2 coupler (MMI2) and the
phase compensation (∆L). The transfer matrix of the input coupler is:
p
p k2TE
 j 1 − k2
TE
= 

0
0

TMMI2
p
j p
1 − k2TE
k2TE
0
0
0
0
p
p k2TM
j 1 − k2TM

0


p 0
j p
1 − k2TM 
k2TM
(5.14)
where k2TE , k2TM are the coupling constants of the MMI coupler for the respective polarizations.
The transfer matrix for the phase compensation is:

TWG∆L
1
0
 0 e jβTE ∆L
= 
 0
0
0
0

0
0

0
0


1
0
jβ
∆L
0 e TM
(5.15)
Here ∆L is the additional path length in the lower arm. Ideally this should result in a phase
difference of π/2 for both polarizations.
5.3 MZI Polarization splitter
91
From this point on, both the 1 × 2 and 2 × 2 PS are equal. Both polarization splitters have a PC
in the upper arm for section PC1 :
 √

√
1 − cPC 0
j cPC
0

0
1 √ 0
0 

TPC1 = 
(5.16)
 j√cPC
0
1 − cPC 0 
0
0
0
1
where cPC is the conversion of the individual PCs, the transfer of the lower arm is 1.
The next section consists of the birefringent waveguides (WG) of a length L. The birefringence
causes a phase difference of (βTM − βTE ) L between the polarizations. The transfer matrix of
this section is:

 jβ L
e TE
0
0
0

 0
e jβTE L
0
0

(5.17)
TWG = 
jβ
L

 0
TM
0
e
0
0
0
In section PC2, a polarization converter is present
section is described by:

1 √ 0
 0
1 − cPC
TPC2 = 
 0
0
√
0
j cPC
0
e jβTM L
in the lower branch. The transfer of this

0
0
√
0
j cPC 


1 √ 0
0
1 − cPC
(5.18)
In the final section (MMI2), a 2 × 2 MMI is used to combine the signals from both branches.
The transfer matrix of this output MMI is the same as in eq. (5.14).
Using these matrices, the outputvector can be calculated. For the 1 × 2 splitter the complete
transfer is:
S1×2out = TMMI2 · TPC2 · TWG · TPC1 · TMMI1 · Sin
(5.19)
For the 2 × 2 splitter, it is:
S2×2out = TMMI2 · TPC2 · TWG · TPC1 · TWG∆L · TMMI2 · Sin
(5.20)
The 1 × 2 splitter is described in detail, the 2 × 2 splitter is investigated in the same way. For
the 1 × 2 splitter with TE polarized light at the input, the output vector is:
jLβ
p
p

 p
TE
k
(1
−
c
)
k
+
j
1
−
k
1
PC
2
2
TE
TE
TE
p
e jLβ
p
p

k2TE + j 1 −pk2TE e TE 
PC )

p k1TE (1 − cp
(5.21)
S1×2out (TEin ) = 
 k1 cPC − 1 − k2 e jLβTE + j k2 e jLβTM 
TM
TM
p
p TE
p
k1TE cPC − 1 − k2TM e jLβTM + j k2TM e jLβTE
The converted part of the output power (TM polarized in this case) at the upper output (output
1) can be calculated as:
p
p
π
PTMout1 = k1TE cPC
k2TM e j(LβTM + 2 ) − 1 − k2TM e jLβTE
·
p
(5.22)
p
π
k2TM e− j(LβTM + 2 ) − 1 − k2TM e− jLβTE
92
Polarization splitters
This yields:
PTMout1 = k1TE cPC
1−
q
π
k2TM (1 − k2TM )2 cos L(βTM − βTE ) +
2
and similarly for the lower output (output 2):
q
π
PTMout2 = k1TE cPC 1 − k2TM (1 − k2TM )2 cos −L(βTM − βTE ) +
2
(5.23)
(5.24)
The unconverted part (TE) in both outputs is the same and is in this case:
PTEout1,2 = k1TE (1 − cPC )
(5.25)
The power of TE polarized light from the outputs is only zero if the conversion in the arms
cPC = 1. For a lower conversion, the non-converted part is split equally over the two branches.
With this model the performance of the MZI-PS can be investigated as a function of the parameters of the various components.
The simulation results as a function of the conversion (cPC = PTEP+TMPTM ) of the polarization
converters in the arms, with TE polarized light at the input, are shown in Fig. 5.17(a). In the
simulations all other sections are assumed to have ideal performance.
Fig. 5.17(b) shows the splitting ratio and the net circuit conversion as a function of the conversion of the PCs in the arms.
The Splitting Ratio of the splitter defined as the total power in the desired port (output1 for TE
input) divided by the total power in the undesired port:
PTMout1 + PTEout1
SR (TEin ) = 10 log
(5.26)
PTMout2 + PTEout2
For a splitting ratio larger than 13 dB, a conversion cPC above 90% is needed.
The net circuit conversion cPS is the conversion of the whole polarization splitter circuit, this
is the conversion obtained at the desired output port (port 1 for TE input):
cPS (TEin ) =
PTMout1
PTEout1 + PTMout1
(5.27)
This cPS is larger than 95%, for cPC above 90%, because the unconverted part is split equally
over the outputs.
The conversion of the polarization converters depends critically on the width as is shown in
chapter 4 an accuracy of ±60 nm is needed for cPC > 90%, so these devices are considered to
be the limiting factor in the fabrication.
The dependence on the other parameters is investigated as well. In these simulations, the
conversion cPC is 95%. The influence of the coupling coefficient of the couplers is considered
to be less important as MMIs can be made tolerant to width deviations [27].
In the 1 × 2 case, the input coupler will always be balanced as this is a symmetric device, this
cannot cause a problem. The 2 × 2 splitter however, can be unbalanced, and apart from this,
5.3 MZI Polarization splitter
93
1
(TE )
out1
in
0.6
0.4
0.2
0
0.75
TEout1(TEin), TEout2(TEin)
0.8
0.85
0.9
0.95
Conversion polarization converters (cPC)
1
(a) Output power from output ports 1 and 2
(TMout2 (TEin ) is always zero in this (L = 2(β π−β ) )
TE
TM
case)
24
22
20
0.95
18
16
14
0.9
12
Splitting Ratio (SR) [dB]
TM
0.8
Net circuit conversion (cPS)
Normalized Output Power
1
10
0.85
0.75
0.8
0.85
0.9
0.95
Conversion polarization converters (cPC)
1
(b) Conversion and splitting ratio.
Figure 5.17: Simulated performance of the integrated polarization splitter/converter as a function of the polarization
conversion of the converters in the branches for TE input.
have a polarization dependent coupling constant. A polarization dependent output coupler will
result in a difference in splitting for TE and TM.
In the 2 × 2 PS, the problem is larger. Here a 2 × 2 MMI is used both in the input and in the
output. The MMI at the input couples the orthogonal polarization with respect to the output
MMI. In Fig. 5.18(a) the influence on the coupling constant of both couplers on the output SR
is plotted for the 2 × 2 case. The SR is above 13 dB if the coupling constant is better than 0.42.
The length of the waveguides between the PCs determines the phaseshift between the polarizations and hence the splitting. The SR as a function of length is plotted in Fig. 5.18(b). The
length has to be accurate within ±0.1π rad (corresponding to approximately ±10 µm up to
±15 µm, depending on the choice of the waveguides).
For the 2 × 2 case, the splitting ratio as a function of the length of the phase compensation section (∆L) is shown in Fig. 5.18(c). The path length difference has to be correct within ±25 nm
to achieve a splitting ratio larger than 13 dB. The obtained phase difference is somewhat different for TE and TM, but as the pathlength difference is only 120 nm, the error in the phase is
smaller than 5 · 10−3 rad.
The used components in the splitter operate over a broad wavelength range, thus the wavelength dependence is expected to be small. This is confirmed by the simulated wavelength
dependence, plotted in Fig. 5.18(d). The device has an SR above 15 dB for the whole wavelength range from 1500–1600 nm.
5.3.3
Design
A first design is made, compatible with the first generation PC, using waveguides in the splitter
that are 2 µm wide, and deeply etched into a layerstack having a 300 nm InP topcladding, and
a 500 nm Q(1.25) waveguide layer on an InP substrate. This yields a ∆β = βTM − βTE of
94
Polarization splitters
16
20
15
14
10
SR [dB]
SR [dB]
12
10
5
0
−5
8
−10
6
4
0.25
−15
0.3
0.35
0.4
0.45
Coupling constant MMI2
(a) Splitting ratio as a function of the coupling constant
of the output MMI.
20
20
15
15
10
5
0
0
−0.5
0
0.5
Phase difference L(βTM−βTE) [π radians]
1
(b) Splitting ratio as a function of length of the waveguides.
SR [dB]
SR [dB]
−20
−1
0.5
10
5
50
100
150
Length ∆L [nm]
200
250
(c) Splitting ratio as a function of the phase compensation path length ∆L.
0
1.5
1.52
1.54
1.56
Wavelength [µm]
1.58
1.6
(d) Splitting ratio as a function of wavelength.
Figure 5.18: Splitting ratio for the splitter in case of deviations of the parameters. All simulations are done for
cPC = 0.95.
-0.030 µm −1 , so for this device an offset L between the converters of 52 µm is needed. The
total length of the device, including in- and output MMIs, is about 600 µm.
This splitter can be made shorter by using smaller MMIs. By using the new PCs, the device
can be made compatible with active-passive processing. This is demonstrated in the second
design.
5.3.4
Fabrication I
The processing of the first design is equal to that of the first generation PC (section 4.3). The
MZI-PS and the separate polarization converters are processed on the same chip. A photograph
of the finished device is shown in Fig. 5.19. Some roughness is visible on the sidewalls of
the waveguides, this is mainly caused by an non-optimal lift-off process for the waveguide
5.3 MZI Polarization splitter
95
definition.
Figure 5.19: SEM photograph of the input MMI and 2 Polarization converters in the arms of the MZI
5.3.5
Characterization I
The splitter is examined with the setup shown in Fig. 5.20. The devices are excited using an
Lock-in
amplifier
Chopper Polarizer
EDFA
BPF
PS
Polarizer
TIA
Pinhole
PD
Figure 5.20: Setup used for characterization of the polarization splitter and converter. BPF: Bandpass filter, PD:
Photodiode, TIA: Trans impedance amplifier
EDFA as a source and a 2.5 nm wide bandpass filter, set to a central wavelength of 1555 nm.
This signal is chopped and the polarization is fixed using a polarizer. The light is coupled into
the chip and the output is coupled through a polarizer to determine the output polarization. It
is detected with a photodiode connected to a transimpedance amplifier and a lock-in amplifier.
From chapter 4, section 4.3 it can be seen that a conversion of 95% can be achieved for a width
of 0.75 µm. The polarization converters used in the splitter are 0.8 µm wide. According to the
96
Polarization splitters
measurements in the previous chapter, this converter will have a conversion of 88%.
The full splitter/converter is measured and the results are listed in table 5.3.
Table 5.3: Measured output powers and resulting Splitting Ratio and Conversion
TEout1
TEout2
TM out1
TM out2
conversion cPS
SR
Pout (TEin ) [a.u.]
4.5
2.6
43.7
3.4
91%
9.1 dB
Pout (TMin ) [a.u.]
1.3
47.3
2.5
4.9
91%
11.4 dB
The conversion cPC of the converters in the branches for TE (or TM) can be calculated by
dividing the output power in TM (TE) in both outputs by the total power from both outputs,
under the assumption that both PCs are equal:
cPC (TEin ) =
PTMout1 + PTMout2
× 100%
PTEout1 + PTEout2 + PTMout1 + PTMout2
(5.28)
This conversion equals 87%. The net conversion of the device cPS is 91% for both polarizations. This is less than the expected net conversion from Fig. 5.17(b). This is probably caused
by an imperfect output coupler.
The resulting splitting ratio, defined in eq. (5.26), is 9.1 dB for TEin and 11.4 dB for TMin . The
low splitting ratio is caused by a deviation from the actual L to the optimal L = 2(β π−β ) .
TE
TM
Probably caused by deviations from the calculated propagation constants of the modes.
The difference in splitting ratio for TE and TM is caused by the polarization dependence of the
output coupler. The coupler is not a perfect 50-50 coupler for TM.
The excess losses are 5.0±0.1 dB compared to a straight waveguide. This is mainly caused by
the waveguide roughness which is visible in Fig. 5.19.
These problems can be corrected for in a next realization.
5.3.6
Design and fabrication II
A next generation of the PS is designed in which the problems from the previous realization are
corrected. The PCs are replaced by the second generation devices, discussed in section 4.4. All
waveguides in this realization have a topcladding to be fully compatible to the active-passive
scheme.
In addition to a 1 × 2 device, a 2 × 2 polarization splitter is demonstrated, based on the same
concept.
The waveguides used in the splitter are 1.21 µm wide to couple to the PCs without tapers, and
have single-mode waveguides. They are deeply etched into a layerstack having a 1500 nm
InP topcladding, and a 500 nm Q(1.25) waveguide layer on an InP substrate. Because of the
thicker topcladding, the birefringence is reduced with respect to the first realization. In this
5.3 MZI Polarization splitter
97
case ∆β = βTM − βTE = -0.020 µm −1 , so for this device an offset L between the converters
of 78 µm is needed. The total length of the device, including in- and output MMIs is about
800 µm for the 2 × 2 case, a similar 1 × 2 device is 630 µm long.
The device is coupled with tapers to 2.5 mm long 3 µm wide, shallow waveguides (etched
100 nm into the waveguide layer) at both the in- and outputs.
The pathlength difference of 120 nm (, π/2), in the arms of the MZI is achieved by tilting the
in- and output MMI coupler at an angle of 1°, see Fig. 5.21. In this way sharp deeply etched
bends, in which unwanted polarization conversion can occur [24], are avoided.
Figure 5.21: SEM photograph of the 2 × 2 MZI-PS. The vertical dimensions are 3× enlarged for clarity.
The fabrication is based on the processing for the second generation polarization converters
using Electron Beam Lithography (EBL) for the lithographical definition. In this case, the
complete PS, including the MMI couplers, is defined using EBL. The access waveguides are
defined by optical lithography to which the EBL pattern is aligned as shown in chapter 3.
The device spans multiple EBL writefields and critical alignment is needed per field.
5.3.7
Characterization II
The integrated splitter is examined on a setup similar to Fig. 5.10, only an extra polarizer is
used to determine the output polarization. As before, the output is detected with an InGaAs
CCD camera to observe the 2 outputs at the same time. The obtained image is analyzed to
obtain the power in both outputs simultaneously by integrating the intensity in both ports over
a fixed width. From this, the splitting ratio (SR) is determined.
Devices with increasing L are fabricated, but show random behavior. A 2 × 2 splitter with a
designed L of 78 µm shows polarization splitting with a splitting ratio of 7 dB for TM and 4 dB
for TE. The polarization converters in the arms have a conversion of 88%.
According to Fig. 5.17(b) this conversion should yield a splitting ratio of approximately 13 dB.
The low splitting ratio can be explained by a deviation from π in the phase difference between
TE and TM. A phase error in the branches can have multiple causes. First of all, due to the
proximity of the waveguides inside the splitter, in the EBL exposure their width can be influenced.
Furthermore, the device spans multiple EBL writefields and critical alignment is needed per
field. Misalignment can cause small gaps or offsets in the connection between the fields, these
98
Polarization splitters
can yield to phase-errors in the arms.
The difference between the splitting for TE and TM can only result from polarization dependent MMIs, although the MMIs are designed for polarization independent splitting.
5.4
Conclusions
Polarization splitters are crucial components for on-chip polarization manipulation. Polarization splitting based on interference is the method of choice for short, low loss, and high splitting
ratio devices. Because of the lack of material birefringence, modal birefringence needs to be
employed to achieve splitting.
Two different classes of devices are presented. One is a 3 mm long directional coupler. Tapered waveguides together with a tapered coupling region are shown to enhance the tolerance.
From simulations it follows that a width deviation of 145 nm yields a splitting ratio of at least
13 dB. This is an increase in the width tolerance of more than a factor of 4 as compared to a
straight device.
The best realized splitter has a splitting ratio of 18 dB for TE and 20 dB for TM. The devices show a splitting ratio of 13 dB over a width range of approximately 100 nm and for a
wavelength window larger than 45 nm. The measurements agree well with the simulations.
Reduction of the sidewall roughness will lower the losses, improve the polarization splitting of
the device, and increase the width tolerance.
This splitter has the disadvantage that it is hard to integrate with other components.
Another type of splitter is presented, based on an MZI with polarization converters. This
device is shorter (below 1 mm) and has the ability to be integrated with active and passive
components.
The device is simulated using the transfer matrix method. Splitting ratios larger than 13 dB are
expected for conversion ratios of the converters of more than 90%. The first generation MZIPS shows a splitting of 9.1 dB for TE, 11.4 dB for TM and a conversion of 91%. This device
uses a thin topcladding of the waveguides and is therefore not suited for easy integration. A
next generation device is fabricated, suited for integration. The 2 × 2 splitter made in this case
shows a maximum splitting of 7 dB and a conversion close to 90%.
This device shows that the generic platform for polarization manipulation can be obtained by
only adding the polarization converter to the standard technology.
Chapter 6
Wavelength converters
6.1
Introduction
The POLARIS wavelength converter, introduced in chapter 1, is used as a vehicle for the
demonstration of the generic technology with polarization handling capability. In this chapter
the (polarization dependent) wavelength converter is described, that forms the building block
for the POLARIS polarization diversity wavelength converter.
A large variety of wavelength converters is described in literature [10]. In this thesis, integrated
SOA - Mach Zehnder Interferometer (SOA-MZI) based wavelength converters (WLC) will be
regarded as they are compact, stable, give low chirp and high extinction ratio [95, 96].
Apart from wavelength conversion, SOA-MZIs offer interesting properties for all-optical switching. Their multi-functional nature can be applied to achieve more all-optical functions such as
2R regeneration, and logical functions such as a full-adder. Here the focus will be on the design
and characterization of the MZIs used as wavelength converters. These WLCs can be used as
building blocks for the POLARIS concept as demonstrated in the next chapter.
The present chapter will discuss two generations of designs of the SOA-MZI. The first design
is used to test the feasibility of the active-passive technology for wavelength conversion and
will be used to demonstrate the POLARIS principle in the chapter 7. The second generation is
an array of SOA-MZI switches suited for packaging. In this array the integration of spot size
converters in the generic technology is demonstrated.
In this chapter, first the principle of an SOA-MZI wavelength converter is discussed. Next the
design and fabrication and the static and dynamic characterization of the first generation device
is treated. Problems are identified and solved in a next generation. The second generation
99
100
Wavelength converters
device demonstrated is the SOA-MZI array suited for packaging. On the packaged device both
static and dynamic measurements are performed.
6.2
Principle
The principle of operation for a wavelength converter using cross-phase modulation (XPM) is
based on refractive index changes caused by the optical input signal. The change in refractive
index results in a change in phase, this is exploited in a Mach Zehnder interferometer with
SOAs in each branch, as shown in Fig. 6.1.
Light from a continuous wave (CW) laser is split in the multimode interference coupler (MMI)
and distributed over the two branches. It is combined again in the output MMI after passing through the SOAs. The phase difference from both arms determines whether the signals
combine destructively or constructively. The currents in the SOAs are chosen to balance the
interferometer. This results in destructive interference in the bar-port when no input signal is
present.
If a signal is injected in one of the interferometer arms, the injected power changes the refractive index of the SOA in this arm, and thus the phase. A phase change of π results in
constructive interference, and the light from the CW laser can now reach the output port. If the
input signal is modulated, the modulation will be transferred to the CW-light.
PPump
PProbe
Pump
SOA
SOA
P1
Pout Probe
P2
Figure 6.1: Wavelength converter based on XPM
Co-directional conversion (the pump and probe signals propagate in the same direction, as depicted in Fig. 6.1) has a better performance (higher extinction ratio (ER)), larger input dynamic
range) at high speed than counter-directional operation, because the interaction time between
the two signals in the SOA is longer with co-directional propagation [11].
However, in the co-propagation case both the pump and the probe are present in the output
and tuneable filters are needed to separate them. These filters are expensive and difficult to
integrate. Conversion to the same wavelength is impossible in this way, but is desirable to keep
full flexibility to provide the implicit regeneration of the wavelength converter and to strip the
frequency and phase noise from the data signal.
A simple model of the MZI is used to give insight into the functioning of the MZI and to draw
conclusions on the effect of the SOA parameters on the conversion. In the model, all MMIs
are exact 50–50 splitters, and both SOAs are assumed to be equal: they have the same length
6.2 Principle
101
L, unsaturated gain g0 , and saturation power Psat .
The modal gain gnet in the SOA for an input power P can be expressed as:
gnet =
g0
,
1 + P/Psat
(6.1)
At a power Psat , the gain of the SOA has decreased 3 dB, caused by depletion of carriers in the
device at high power levels. This depletion results in a change of the refractive index so that
the phase of the optical signals changes. This is expressed in the alpha-factor, which couples
the refractive index change to a change in gain:
dgnet
4π dn
,
(6.2)
α = −
λ dN
dN
where n is the refractive index, N is the carrier density in the SOA and gnet the modal gain.
The outputpower Pout can be calculated:
Pout
=
1
2
√
P1 + P2 + 2 P1 P2 cos(∆φ )
(6.3)
where P1 is the power at the output of the upper SOA:
g0 Psat L
P1 = 1/4 Pprobe e Psat +1/2 Ppump +1/4 Pprobe
(6.4)
P2 is the total power at the output of the lower SOA:
g0 Psat L
P2 = 1/4 Pprobe e Psat +1/4 Pprobe
and ∆φ is the phase difference between the arms of the MZI:
1
1
∆φ = 1/2 α L g0 Psat
−
Psat + 1/2 Ppump + 1/4 Pprobe Psat + 1/4 Pprobe
(6.5)
(6.6)
Some conclusions that can be drawn from the model are: a large g0 will yield a large Pout . For
a large g0 , a high current is required, it is a trade-off as the maximum current will be limited
by heating effects.
Long SOAs will lead to large phase shifts. The length of the SOAs will be limited by the
amount of Amplified Spontaneous Emission (ASE). In a too long SOA, the ASE itself will
saturate the device and furthermore contribute to the noise.
A low Psat will require low pump powers to saturate the SOA.
A high α will result in a large phase change, the α-factor is highest for wavelengths longer
than the gain-peak [46, 97].
High extinction (Pout =0 when Ppump =0), can only be obtained when both arms are identical: P1
is exactly equal to P2 in case Ppump =0, and no additional phasedifferences are present between
the arms.
It has to be noted that g0 , Psat and α usually are polarization dependent and that the conversion
will therefore be sensitive to the polarization.
102
6.3
Wavelength converters
First generation
The first generation SOA-MZI is shown in this section. This device is developed for the
STOLAS (Switching Technologies for Optically Labeled Signals) project. In that project, a
packet-switched data network is used. The data and the label for the packets are orthogonally
modulated: On-Off Keying (OOK) is used for the data, the label is an Frequency Shift Keying
(FSK) modulated signal on top of this. In order to preserve the label while traveling through
the network, a limited Extinction Ratio (ER) is needed to have sufficient power in the zerolevel of the data. Therefore an ER of 6 dB is expected at the input of the WLC [98]. At the
output of the WLC, the ER cannot be large either. The output ER can always be reduced by
slightly unbalancing the MZI arms by changing the SOA currents.
An important feature of the WLC is the ability to strip the label from the converted signal, for
this purpose also conversion to the same wavelength is required.
6.3.1
Design and fabrication
The integration of MZIs is a first step towards the realization of a POLARIS wavelength converter. The design of the MZI is based on the device in [16]. The 1 × 2 MMIs are replaced
by 2 × 2 MMIs. In the case of a 2 × 2 MZI, destructive interference is obtained in the bar-port
(required for non-inverting operation) when the interferometer is balanced. This will enhance
the bandwidth with respect to a 1 × 1 MZI [16, 99]. In the 1 × 1 case, a balanced MZI will
yield constructive interference in the output port, and a π phaseshift is needed in one of the
arms to obtain destructive interference. To obtain this phase difference, the SOAs have to be
set to different currents, this condition is expected to be wavelength dependent.
The design of the device is presented in Fig. 6.2(a). The in- and outputs are situated on one
facet, all ports are looped back using shallowly etched bends. The waveguides are placed at
a pitch of 250 µm, this allows the usage of a fiber-array in which the fibers are placed at this
pitch.
The device is fabricated on bulk Q(1.55) active-passive material. The standard processing described in chapter 3, is used except for the passivation and metallization. For the passivation
300 nm SiNx is used, no planarization is done. Sidewall coverage is obtained by sputtering of
the contacts in stead of evaporating. Fig. 6.2(b) shows a photograph of the finished chip.
6.3.2
Static characterization
A suitable operation point of the SOA-MZI based wavelength converter is first determined with
static measurements. All measurements reported here are performed on an MZI with 1000 µm
long SOAs in the branches. The 1000 µm is a trade-off between long SOAs for a large phaseshift and short devices to limit the ASE.
6.3 First generation
103
pump1
probe1
probe2
pump2
output2
SOA
MMI
MMI
output1
MMI
SOA
MMI
(a) Schematic drawing of the design
(b) Photograph
Figure 6.2: Mask design and fabricated MZI wavelength converter.
Experimental setup
The experimental setup used to perform the static measurements is shown in Fig. 6.3. The CW
probe with wavelength λ CW is generated by a Tunable Laser Source (TLS). The light coming
from this TLS has a well defined polarization. Since the operation of the SOA-MZI WLC is
sensitive to the state of polarization of the probe, a polarization maintaining fiber-array is used
to feed the light to the input of the device. The used array consists of cleaved fibers, having a
large spotsize, which leads to an overlap loss of approximately 10 dB.
Another TLS is used for the pump signal with wavelength λ s . The pump is amplified by means
of an Erbium Doped Fiber Amplifier (EDFA) followed by a Band Pass Filter (BPF) for suppressing the ASE noise before launching it in the MZI-WLC. At the output, a second BPF
(centered on λ CW ) is used to filter the ASE noise from the SOAs in the WLC, and the reflected
pump signal. The output is detected using a photodiode.
The measurements are performed in contra-propagation (pump and probe propagate in opposite direction), this configuration has the advantage over co-propagation that it allows conversion to the same wavelength.
Electrical switching
An optimal working point has to be determined. This point is obtained by fixing the current
of one SOA (SOA1 in this case), while sweeping the current of the second SOA. By sweeping this current, the carrier density and thus the phase and the amplitude of the light traveling
through this SOA are affected. No pump signal is present, hence by combining the light with
that coming from the SOA1 in the output MMI, an interference curve can be obtained as shown
in Fig. 6.4.
104
Wavelength converters
Figure 6.3: Measurement setup for static measurements
In this graph, the interference curves for different currents of SOA1 are present: 130 mA,
150 mA and 180 mA. The probe has a power PCW is 2 dBm, measured at the input of the
fiber-array, and a wavelength of 1560.62 nm. The chip placed on a heatsink that is kept at a
constant temperature of 10°C.
−20
Output power [dBm]
−25
−30
−35
−40
I
SOA1
−45
0
50
100
150
Current SOA [mA]
200
250
2
Figure 6.4: Switching curve for the wavelength converter, λ CW = 1560.61 nm, PCW = 2 dBm and T=10°C. ISOA1 =
130 mA, 150 mA and 180 mA.
A current of 130 mA for SOA1 yields the largest interference. By setting the current in SOA2
to 107 mA, non-inverting operation is achieved.
The difference between the currents in the SOAs in this working point indicates that the SOAs
are not equal and the interferometer is not balanced. Full destructive interference is not obtained in this case, and a limited ER is expected.
6.3 First generation
105
Optical switching
For the optical switching curves, the currents of both SOAs and the probe power are set, the
pump power is changed. The current of SOA1 is ISOA1 = 130 mA, for SOA2 ISOA2 = 107 mA as
determined before. The probe power PCW is increased to 5 dBm into the fiber array to achieve
a higher output ER. The optical switching curves are obtained for the conversion between 4
combinations of the wavelengths of 1555.75 nm and 1560.61 nm (Fig. 6.5).
For an input pump power range changing between 9 and 15 dBm, an output ER of at least 6 dB
is obtained for all wavelengths.
The need for high pump power is mainly caused by te usage of cleaved fibers, which results in
high overlap losses (about 10 dB per fiber-chip coupling).
−28
−30
Output Power [dBm]
Output Power [dBm]
−30
−28
1560 to 1555
−32
−34
−36
1555 to 1555
−38
−40
−42
0
1555 to 1560
−32
−34
−36
1560 to 1560
−38
−40
5
10
Input Power [dBm]
15
−42
0
5
10
Input Power [dBm]
15
Figure 6.5: Optical switching curves for the Mach-Zehnder Interferometer, the label indicate the conversion of λpump
to λprobe
The largest modulation depth is obtained for a probe wavelength of 1560 nm, because the current setting for the MZI is optimized for this wavelength and complete interference is only
obtained at this wavelength. This confirms that the device is unbalanced, therefore the setting
cannot be optimal for 1555 nm. Furthermore a higher α is expected for higher wavelengths.
6.3.3
Dynamic characterization
Dynamic all-optical wavelength conversion is performed in non-inverting operation for counterpropagation. A schematic of the setup is shown in Fig. 6.6. The setup is similar to the setup
used for the static experiments. The difference is that the pump signal is modulated with a
2.5 Gb/s PRBS signal. The output power is controlled with an attenuator and the signal is
amplified and detected using a photodiode and an oscilloscope.
A new working point is determined for which non-inverting operation is obtained at higher
output powers. The currents in the SOAs are 187 mA for SOA1 and 241 mA for SOA2 . For
all measurements the input ER is approximately 7 dB (6.5 dB for 1560.62 nm to 7.3 dB for
1555.74 nm). The input eye is shown in Fig. 6.7.
106
Wavelength converters
Figure 6.6: Measurement setup for testing the WLC in counter-propagation
(a) Input, λpump = 1555.74 nm, ER=7.3 dB
(b) Converted signal, λprobe = 1560.62 nm
Figure 6.7: Eye diagrams (200 ps/div)
The device performance depends on the polarization of the input signals. The polarization
is optimized to obtain maximum eye opening. The eye diagram of the converted signal at
1560 nm is plotted in Fig. 6.7(b). From the eye diagrams the extinction ratio is measured to
be 10.5 dB. From the output eye it can be seen that the rise and fall times are large. They
are approximately 400 ps, which is equal to the bittime for 2.5 Gb/s. Furthermore the pattern
effect is visible as a double line. This is also a result of the slow behavior of the device.
Rise and fall times
For operation at higher bitrates, shorter rise times are required. To investigate the behavior,
the converted bitpattern (Fig. 6.8(a)), and the amplified pump signal (Fig. 6.8(b)) is recorded.
Furthermore the Cross Gain Modulated (XGM) signal is obtained by biasing only one SOA in
the MZI. As expected, also this signal (Fig. 6.8(c)) recovers at the same speed.
To understand the origin of this behavior, the SOA is simulated using a rate-equation model
[100]. First a XGM simulation with one SOA is done for two values of the injection current.
6.3 First generation
107
(a) Co-propagation XPM
(b) Co-propagation pump
(c) Co-propagation XGM
Figure 6.8: Output bitpatterns
The measured input signal is used for the simulation. The results are plotted in Fig. 6.9(a).
The simulation shows that for low currents, no fast response can be obtained.
With the settings for the low current, the XPM performance is investigated. The simulated
output pulse is compared to the measurement and plotted in Fig. 6.9(b). A similar behavior is
visible.
1
Output pulseshape XGM, Psignal=0.5 mW
0.9
0.9
Normalised output power
Normalised output power
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
I=100 mA, PCW=0.1 mW
I=200 mA, PCW=0.1 mW
0.1
0
0
Output pulses XPM
1
0.5
1
Time [ns]
1.5
Measured
Simulated
0.2
2
(a) Simulated output pulse shape as a function of
current in the SOA for XGM
0.1
0
1
2
3
Time [ns]
4
5
(b) Simulated output pulse shape compared to
measured pulse
Figure 6.9: Simulated output pulse shapes
The speed of the device is clearly influenced by the current density, and the measured slow
response is most probably caused by limited current injection into the SOA. The reason for the
bad injection is a problem in the design of the metal contact of the SOA. This contact consists
of a 20 µm wide metal stripe with thickness of 250 nm. The gold is the significant conductor,
so the resistance of this 500 µm long line (a 1000 µm long SOA contacted in the middle) is:
LSOA
= 5Ω
(6.7)
wt
This is a high value compared to the resistance of the SOA, which is approximately 7 Ω at
R = ρAu
108
Wavelength converters
150 mA. The current distribution in the SOA is approximated by a resistor network, this yields
an approximate current distribution as shown in Fig. 6.10. This distribution is the cause that
outer regions of the SOA are only slightly pumped. Thus the current density is too low in a
part of the device to restore the carrier concentrations fast enough.
1
Normalized current
0.8
0.6
0.4
0.2
0
0
100
200
300
x [µm]
400
500
Figure 6.10: Current distribution inside SOA
To overcome this problem in the next realization, the contacts have to be enlarged to a width
of 100 µm and a thickness of more than 1 µm. This will lower the resistance to below 0.5 Ω.
For this case the current distribution will be equally spread along the device.
Conclusions
An MZI-WLC is realized in an active-passive integration technology. The device is characterized both statically and dynamically. Conversion up to 2.5 Gb/s is found. In counterpropagation the device is working well as ER and output power are concerned. The speed limit
of the counter-propagation with respect to co-propagation is not an issue at the low bitrates
used in these measurements. The speed in this case is limited by other, more severe, parameters and is not influenced by the propagation direction.
A large rise and fall time is encountered, most probably due to low current injection as can be
seen from simulations.
The device performance depends on the polarization of the input signal. The switching speed
of the MZI can be improved by decreasing the resistance of the contacts yielding a better current distribution in the device.
Furthermore by operating in a push-pull configuration [101], the switching speed can be increased. This is not possible with the current device, so a different design is needed in which a
pump signal can be input in both arms.
6.4 Second generation
6.4
109
Second generation
The second generation SOA-MZIs are developed for the MUFINS (MUlti - Functional INtegrated arrays of interferometric Switches) project. In this project the multi-functional nature
of the SOA-MZI is employed to obtain several optical functions [102, 103, 104]. For these
functions multiple MZI switches are needed and hence an array of 4 switches is integrated
with spot size converters. In order to use the device in system experiments, the device has to
be packaged. For this a number of additional features are integrated as discussed in chapter 3.
The device is operated as a wavelength converter in this section.
6.4.1
Design and fabrication
A single switch consists of an MZI with SOAs in the arms as shown in Fig. 6.11. 2 × 2
MMIs with angled sides are used to reduce reflections. In both arms an additional coupler is
placed to inject a pump signal into the arms to use the device in a push-pull operation [101]
as indicated in the figure. For push-pull operation, the delay between the two pump signals
determines the output pulse. First a pulse is sent into input Pump1, the carriers are depleted
almost instantaneously, and a phase shift occurs. The carriers start to recover slowly, but after a
time ∆t, shorter than the bit-time, a pulse is send to port Pump2, and the phases are equalized.
This results a fast pulse at the output.
Pump1
Probe
SOA
Probe
SOA
Pump2
Pump1
Pump2
Probe out
Figure 6.11: Schematic overview of the SOA-MZI switch
The chips are designed for flip-chip mounting on a submount that will be placed on a motherboard in which fiber-assemblies are coupled to the in- and outputs of the switches. This is
similar to hybrid integration [55]. The mask layout of the chip is shown in Fig. 6.12. In the
figure the additional features are indicated.
At all in- and outputs Spot Size converters (SSC) are present. The SSC consists of a vertical
and a horizontal taper and is designed to have a spotsize that matches that of a cleaved single
mode fiber as is shown in chapter 2.
110
Wavelength converters
Vertical alignment
recess
Spotsize converters
Precision cleave
openings
Additional metal
Figure 6.12: Mask layout of the SOA-MZI switch
For the vertical alignment, oxide pillars are placed on the submount. On the InP chip a welldefined level is needed. The bottom of the waveguide layer is used for this. Recesses are etched
into the chip using a selective wet etch.
To horizontally align the chip, the chips is pushed to stops on the submount. To ensure exact
dimensions of the chip, as well as exact distance from the edge to the in- and outputs, precision
cleave openings are defined lithographically at the same mask as the waveguides. After the chip
is finished, the cleaving can be initiated at these precision openings.
The SOAs in the arms consist of a 2 µm wide shallow waveguide with an active region of
9 strained InGaAs barriers and 8 unstrained InGaAs Quantum Wells (QW) inside a Q(1.25)
waveguide layer, with a confinement factor of 0.19 for TE. Bandfilling of the QWs is required
to shift the operation wavelength to the desired 1550 nm.
As shown in chapter 2, the waveguide enters the SOA at an angle of 10° to avoid reflections
from the active-passive butt-joint [44].
The chip is kept in place by soldering the metal contacts onto thick gold solderbumps on the
submount. Because of this, the contacts on the chip are just thin evaporated metal pads, as
a good thermal and electrical contact is ensured by the solder. For a good adhesion of the
metal contacts to the solder, the metal has to be planarized. Polyimide is used to passivate and
planarize the contacts of the SOAs. Apart from these contacts, additional metal is present to
enlarge the metal surface and thus enhance the adhesion of the chip.
The full fabrication process is described in section 3.4. A photograph of the finished chip is
shown in the left of Fig. 6.13.
6.4 Second generation
6.4.2
111
Packaging
Figure 6.13: Photograph of the integrated array of 4 MZI switches (left) and the packaged array (right)
The finished chip is mounted P-side up on a different submount. In this way the chip can be
quickly packaged to assess the functioning, but an optimal performance is not possible in this
case. In a flip-chip mount it is easy to get rid of the heat at the P-contact. By placing the
chip P-side-up, the heat transfer is severely compromised. Furthermore the electrical contact
is not good enough to obtain a good uniform injection into the SOAs. Nevertheless for the
experiments shown here, the chip packaged in this way (Fig. 6.13 right) can be used.
6.4.3
Static characterization
−35
−35
−40
−40
−45
−45
400 mA
−50
320 mA
−55
240 mA
Power [dBm]
Power [dBm]
The array consists of 1250 µm long SOAs in the MZI. The InGaAs Quantum Wells used are
designed to employ bandfilling to shift the peak wavelength to the right operation wavelength
around 1550 nm [46]. The shift is investigated by recording ASE spectra different currents
(Fig. 6.14(a)).
−50
−55
−60
−65
−70
1500
250
206 mA
mA
94 mA
72 mA
50 mA
1550
1600
Wavelength [nm]
(a) DC
800 mA
640 mA
−60
160 mA
−65
80 mA
1650
−70
1500
1550
1600
Wavelength [nm]
(b) Pulsed (corrected for 1% duty cycle)
Figure 6.14: ASE spectra of the packaged MZI switch for different bias conditions.
1650
112
Wavelength converters
The peak wavelength shifts over approximately 40 nm because of bandfilling, but due to heating the shift is not large enough to obtain the peak around 1550 nm. The desired current density
to completely employ the bandfilling effect cannot be obtained as around 200 mA thermal rolloff starts to occur. It is anticipated that the device will have low gain and will not show proper
cross-phase modulation around 1550 nm as the α-factor is low at the blue side of the gain
peak. This poor thermal behavior is most probably caused by the non-optimal mounting of the
chip.
Pulsed current injection is used to investigate the thermal characteristics further. 100 ns long
pulses with a repetition rate of 100 kHz (i.e. 1% duty cycle) are used to reduce heating of the
SOA. The recorded ASE spectra are shown in Fig. 6.14(b). Now a significantly larger shift
of more than 100 nm is obtained. The gain-peak can be shifted to approximately 1520 nm.
Furthermore the maximum power is increased for more than 20 dB. This demonstrates the
feasibility of this device to operate with high gain for wavelengths around 1550 nm.
Thermal Crosstalk
The MZIs are integrated in an array and the thermal crosstalk between them is an issue, when
adjacent MZIs are operated. For the investigation of the thermal crosstalk, a TLS is set to a
wavelength of 1580 nm and a power level of 0 dBm. The laser is modulated to be able to
detect the outputsignal with a lock-in amplifier. Two SOAs in MZI3 (Fig. 6.15) are biased
to have complete destructive interference in one of the outputs. In this case ISOA5 = 200 mA,
ISOA6 = 206 mA. The current in the adjacent SOA (SOA4 ) is swept and the resulting interference is observed, by recording the output power in both outputs.
−15
P
tot, MZI
−20
P
Pout [dBm]
out, Single SOA
Inverting output
−25
−30 Non−Inverting output
−35
−40
−45
0
Figure 6.15: Photograph of the integrated array
50
100
150
Current in adjacent SOA [mA]
200
Figure 6.16: Thermal crosstalk in MZI switches
The heating of the branches of the MZI leads to a destroyed interference condition. Biasing
the adjacent SOA causes a significant phaseshift between π/2 and π.
The gain in the SOA is not much influenced. The black solid line in Fig. 6.16 shows the
transmission through SOA5 when SOA6 is not biassed. The gain is reduced by approximately
0.2 dB.
6.4 Second generation
113
Thermal crosstalk causes an additional phaseshift in the arms of the MZI, but the effect on the
gain is small (≈ 0.2 dB decrease). In an array in which all SOAs are switched on, a steady state
can be reached and the additional phase error can be corrected for. In that case the outermost
arms of the outer MZIs will have a different temperature than the rest of the array and these
MZIs will be unbalanced. In a flip-chip package the heat will be removed through the P-contact
and the heating of the adjacent MZI will be reduced.
6.4.4
Electrical switching
For using the MZI in a system, the device has to be operated under DC bias. For determining
a working point, one SOA is biased as high as possible without entering the thermal roll-off:
at 200 mA, the current into the other SOA is swept. At the input, a Tunable Laser Source
(TLS) is used, modulated at 1 kHz to detect the light using a lock-in amplifier. A polarization
controller is used to set the polarization for maximum output power. The input power Pprobe is
set to 0 dBm.
The interference curve is recorded for input wavelengths of 1550 nm and 1580 nm (which is
the maximum wavelength possible with the used equipment, and is closest to the gain-peak).
The resulting switching is plotted in Fig. 6.17.
−10
−15
Pout [dBm]
−20
1580 nm
−25
1550 nm
−30
−35
−40
−45
0
50
100
150
Current [mA]
200
250
Figure 6.17: Electrical switching
From this plot it is seen that a large extinction ratio (≈ 25dB) is found for 1580 nm. As
expected for 1550 nm both the output power and the ER are lower (the ER is limited to 14 dB).
The shape of the curve for 1580 nm around 150 mA is possibly caused by a thermally induced
phase shift.
For 1580 nm a sharp destructive interference is present at 206 mA. The currents in both SOAs
are almost equal, this indicates that the MZI is very well balanced. This is a large improvement
in comparison to the first generation. Furthermore output power is larger and the ER higher.
This is a result of the reduced coupling losses by using the integrated spot size converters.
114
6.4.5
Wavelength converters
Dynamic characterization
The static results are promising and the dynamic performance of the MZI is investigated. The
switch is used as an all-optical wavelength converter. For this experiment a setup is used as
shown in Fig. 6.18. The pump signal is obtained by modulating the short pulses (≈ 7 ps)
from a Mode Locked Laser (MLL) with a 10 Gb/s PRBS signal. A 40 Gb/s signal is obtained
by multiplexing the 10 Gb/s signal. This signal has a wavelength of 1560 nm. The switch is
operated in a push-pull configuration as stated before.
A TLS set to a wavelength of 1570 nm is used for the probe signal. This is the highest wavelength possible in this setup. It is still far from the gain peak.
Figure 6.18: Setup for testing dynamic switching of the MZI
The resulting output signals for 10 Gb/s and 40 Gb/s switching are shown in Fig. 6.19. In the
10 Gb/s case a sequence of ones is shown. A pattern-effect is visible, but it is not very strong.
An average output ER of more than 10 dB is obtained in this case.
In the 40 Gb/s case more noise is present, but an ER of approximately 9 dB can be obtained.
Figure 6.19: output bitpattern at 10 Gb/s (left) and 40 Gb/s (right)
6.5
Conclusions
In this chapter two generations of SOA-MZI wavelength converters are discussed. The first
generation shows conversion up to 2.5 Gb/s for counter-propagation operation offering the
6.5 Conclusions
115
possibility for conversion to the same wavelength. Large recovery times are encountered, most
probably due to low and non-uniform current injection as is inferred from simulations.
In the second generation improvements are made to the design, reflection free MMIs and SOAs
are used and the device is designed for flip-chip packaging. The device is mounted P-sideup and the resulting thermal behavior causes heating which prevents a blue-shift of the peak
wavelength. Pulsed measurements confirm that a blue-shift and high gain is possible at higher
injection currents. This indicates the feasibility of this device to operate with high gain for
wavelengths around 1550 nm. Flip-chip packaging will most probably reduce the heating of
the device and it is very likely that the MZI will operate at 1550 nm under DC bias in the latter
case.
Integration of active structures with SSCs, vertical and horizontal alignment features is successfully demonstrated.
The fiber-to-fiber gain of the second generation device is higher than for the first one, this is a
result of the SSC. The device is balanced, which will yield a large operation bandwidth.
Dynamic measurements show switching up to 40 Gb/s.
Chapter 7
POLARIS
7.1
Introduction
The generic platform for polarization handling developed in this thesis consists of active and
passive components and polarization converters. In this chapter the capability of polarization
handling is demonstrated by the application of polarization diversity in a wavelength converter.
The polarization is used to add functionality. The polarization is very stable inside a PIC and is
not easily transferred from one into the other. The polarization can thus be used to label signals,
and by making use of this, select and filter them. This is the principle of POLARIS (POlarization Labeling for Rejection and Isolation of Signals). It is a polarization diversity scheme
in which the polarization is used to identify signals. It allows filter-free, co-propagation wavelength conversion and conversion to the same wavelength.
As an additional advantage POLARIS allows polarization independent operation. The polarization dependence of most Photonic Integrated Circuits (PICs) is problematic. By the creation
of subcircuits for each of the two polarization states, components optimized for one certain
state of polarization can be used and the polarization can be matched to the optimal performance.
This chapter will first explain the concept of POLARIS as applied to a wavelength converter
(WLC) and an all optical switch. With a simulation study of the POLARIS WLC the concept is investigated. Next the concept is experimentally demonstrated by using the WLC from
chapter 6 and external fiber-based polarization components. Finally the design and fabrication
of the integrated version of POLARIS is discussed. With this the full processing of the generic
integration platform with polarization handling capability is introduced.
117
118
7.2
POLARIS
Principle of the POLARIS wavelength converter
The POLARIS concept can be applied for devices in which two optical signals interact and
have to be separated at the output. The basic idea is to give both signals, the pump and the
probe signal, well defined, but orthogonal polarizations. The polarization thus is a label for
the signals and can be used for filtering. A number of different configurations are possible as
will be explained in this chapter. POLARIS has the additional advantage that it can be made
polarization insensitive (even when using polarization sensitive SOAs) for one of the signals.
The basic principle, applied to a wavelength converter, will be explained first.
Fig. 7.1 shows the schematic configuration of the POLARIS wavelength converter.
In the WLC case, the assumption is made that the probe signal, which is locally generated, has
a fixed and stable polarization, whereas the pump signal, coming from the fiber-network, has
an arbitrary (and varying) polarization.
Probe, CW
MZI
Pump signal,
arbitrary
polarization
PC
PS
PS
PC
Output, Probe
MZI
Probe, CW
Figure 7.1: POLARIS wavelength converter
In the figure the arrows give an indication of the state of polarization of the signals in the
device. TE polarized light is indicated as horizontal, TM as vertical. The pump signal is
indicated by the dashed circles, the probe by the solid circles.
The signal from the network (pump signal) arrives in an unknown polarization state. This
signal is split into two orthogonal polarizations in the input polarization splitter (PS). In the
lower branch, the polarization is rotated in the polarization converter (PC) to have the pump
signal in both branches in the same polarization (TE). These signals are injected into the MZIs
together with the locally generated CW light (probe signal) in the orthogonal polarization
(TM). After interacting in the MZI the signal information is transferred to the probe wavelength
and both signals have to be separated. This is done by rotating the polarization of the upper
branch and then using a polarization splitter/combiner to combine the (probe) signals from
both branches. As only TE in the upper and TM in the lower input of the combiner will couple
to the output, filtering of the unwanted remains of the pump signal occurs. Note that this is
not an interferometric coupling; hence the phase difference between the signals will not be
important. It will influence the state of polarization of the output light, but not the amplitude.
7.3 Simulation study and POLARIS concepts
119
If the polarization converters are not perfect, the small unconverted part of the probe in the
upper branch can interfere in the output polarization splitter if this splitter is also imperfect.
The same will happen for the pump signal, here the influence is even smaller as this signal is
filtered twice, first at the input, and at the output. In both cases it is a very small fraction and
will not cause any problems.
7.3
Simulation study and POLARIS concepts
The POLARIS concept is studied by simulations using a commercial circuit simulator [105].
The performance of the device is investigated to obtain the window of operation for different
parameters of the components.
The polarization components used in the simulations are assumed to have efficiencies (conversion and splitting) of 95% or 99%, and excess losses of 2 dB. The SOA is modeled using a
length-averaged rate-equations model. The effective alpha-factor of the SOA is a parameter in
the simulation. In the simulations values of 5 and 8 are used. ASE is added as white noise to
the output of the SOA. The SOA is wavelength and polarization independent.
The SOA-model can be extended by placing a polarization dependent attenuator in front of the
SOA, this will mimic polarization-dependent gain. In the POLARIS concept the polarization
definition at the input of the SOA is fixed and therefore no significant influence is expected
from polarization dependence of the SOA.
The probe has a power of -8 dBm in each branch, and is TM polarized. For the pump signal,
the power and polarization are varied. The pump signal is modulated with a 10 Gb/s PRBS
data signal with an Extinction Ratio of 7 dB.
The basic POLARIS design (as shown in Fig. 7.1) is investigated when it consists of wellperforming components: SOAs with an α-factor of 8, and polarization components with 99%
efficiency.
The Bit Error Rate (BER) as function of the input power for different polarizations is plotted
in Fig. 7.2.
There is a large input power range of 15 dB for which the BER is well below 10−12 for all
polarizations. The small difference between the polarizations is explained by the different
losses for the different paths. For TE polarized pump light (0°), 99% of the light is coupled
to the upper POLARIS branch. No additional losses are present in this path, so almost all the
power reaches the SOA. Thus for TE the lowest input power is required.
For TM polarized pump light (90°), 99% of the light is coupled to the lower branch, here a
PC is present with 2 dB loss, hence the BER curve is shifted to 2 dB higher input powers. For
45°, the worst case, the pump light is split equally into the 2 branches, so 3 dB more power
is needed to compensate for this. Furthermore, the TM part has an additional 2 dB loss. This
explains the shift with respect to TE.
Apart from the BER, the Extinction Ratio (ER) and the filtering of the pump is investigated.
The filtering of the pump is expressed as the isolation, defined as the ratio between the probe
and pump signal at the output of the device.
In Fig. 7.3 the results are given for all three quantities. A comparison is made between the use
120
POLARIS
0
10
−5
10
−10
0o
BER
10
90o o
45
−15
10
−20
10
−25
10
−30
−25
−20
−15
Input Power [dBm]
−10
−5
Figure 7.2: BER simulation for POLARIS, 99% efficient polarization components, polarization independent SOA
with α=8, changing polarization.
of a polarization-independent SOA and a polarization-dependent SOA. In the latter, the TMgain is 3 dB lower, and the probe signal is increased by 3 dB to compensate for this. These
simulations were performed for the worst case: a polarization of 45° for the pump signal.
0
10
12
11
20
10
−10
ER [dB]
10
−15
10
9
15
8
10
7
6
−20
10
Isolation [dB]
−5
10
BER
25
5
5
−25
10
−30
−25
−20
−15
Input Power [dBm]
(a) Bit error Rate
−10
−5
4
−30
−25
−20
−15
Input Power [dBm]
−10
0
−5
(b) Extinction ratio and isolation
Figure 7.3: Simulation results for POLARIS, 99% efficient polarization components, solid lines for a polarization
independent SOA, dashed lines for a polarization dependent (TM gain 3 dB lower) SOA.
The results indicate that the use of a polarization-dependent SOA does not have a large influence on the performance. The largest difference is in the isolation. This is 1.5 dB higher for the
polarization dependent SOA, because the unwanted TM fraction of the pump is less amplified.
In the power range from -20.5 dBm – -7.5 dBm the BER is lower than 10−12 , and the ER is
above 7 dB, so there is regeneration. If an isolation of more than 20 dB is desired, this can
only be achieved for powers of -7.5 dBm (or higher).
7.3 Simulation study and POLARIS concepts
121
Improvements
From the previous section, it is clear that only a limited operation range is possible. Some
modifications to the concept are necessary to improve the performance of the wavelength converter.
The PCs in the branches can introduce additional losses. To equalize the loss in both arms,
an additional attenuator can be used in the opposite branch of the PCs in both the in- and the
outputs. This attenuator is a PC with twice the length. In chapter 4, in Fig. 4.20 it is shown that
conversion completely back to the original signal occurs at the full beat length of the converter.
As the junction losses dominate the loss of the converter, this attenuator will have similar losses
as the normal PC.
The input pump signal into the MZI can be extra filtered to achieve a better polarization definition. The same filtering can be done before reaching the output PS. A polarization filter can
be achieved by using the respective output of a polarization splitter. This improved scheme is
depicted in Fig. 7.4.
Probe, CW
Att.
Pump signal,
arbitrary
polarization
MZI
PC
PS
PS
PS
PS
PC
Output, Probe
PS
MZI
Att.
PS
Probe, CW
Figure 7.4: POLARIS wavelength converter with additional attenuators and filters.
This concept is tested with the worst-case components, in which the α-factor is 5 and the
polarization efficiency is 95%.
The BER, ER and isolation are plotted in Fig. 7.5 for a varying input power. In Fig. 7.5(a)
the BER is plotted for different input polarizations. The addition of the attenuators in the arms
removes the polarization dependence for full TE and TM, for 45° polarized light, the required
power is 3 dB higher because the signal is split into two branches. The ER and isolation are
shown for an input polarization of 45°. The device performs much better than the previous
case, even with worse components. The additional filtering and equalized losses yield a larger
operation range.
The ER and the isolation will increase further for a higher α-factor. For this purpose an optimization of the SOA is required. This is entirely possible; α values up to 32 are reported in
literature [46].
Another improvement can be achieved by replacing the coupler for the pump signal in the arms
of the MZI by polarization splitters. The improvement achieved now is twofold: additional
polarization filtering of the signals is obtained, and 3 dB coupling loss is avoided for both the
pump and the probe in the coupler. Fig. 7.6 shows an MZI in which the couplers are replaced
by polarization splitters. They can be used in the POLARIS concept to replace the normal
122
POLARIS
SOA-MZI.
This has the same effect as using the filters as is done in the previous case. The improvements
with respect to the previous case are the lower required input power and a reduced number of
components.
0
22
10
14
18
12
o
0
16
ER [dB]
BER
90o 45o
10
14
12
8
−20
10
10
6
−25
8
6
−30
10
Isolation [dB]
−10
10
20
−20
−15
−10
−5
Input Power [dBm]
0
4
−20
5
(a) Bit error Rate for a changing polarization
−15
−10
−5
Input Power [dBm]
0
(b) Extinction Ratio and isolation
Figure 7.5: Simulation results for POLARIS, 95% efficient polarization components, α=5. Including loss compensation and additional polarization filters.
Pump
PS
Probe
SOA
MMI
Output, Probe
MMI
PS
SOA
Figure 7.6: MZI with polarization splitters in the arms.
Polarization MZI
A consequence of the polarization diversity approach is the condition that 2 MZIs have to be
used. Each MZI contains 2 SOAs, so a total of 4 SOA is required. This results in a large
footprint of the POLARIS WLC. By replacing the MZIs with polarization MZIs (PolMZI)
[14] introduced in chapter 1, the number of SOAs can be reduced. In Fig. 7.7 the POLARIS
WLC scheme in this case is depicted.
The polMZI used in the branches is an MZI in which the two arms are separated by using
different polarizations instead of different physical paths. At the input of the polMZI, the
7.3 Simulation study and POLARIS concepts
123
Probe, CW
PS
Pump signal,
arbitrary
polarization
PC/2
SOA
PC/2
PC
PS
PS
PC
PS
PC/2
SOA
Output, Probe
PC/2
Probe, CW
Figure 7.7: POLARIS with polarization MZIs.
pump and the probe are converted to orthogonally circularly polarized signals in the input half
PC. Both signals are input into the SOA, which functions as a nonlinear phase shifter. At the
output of the SOA the TE and TM polarized parts of the signals have experienced a power
dependent phase shift. The signals are recombined into the output half PC. Depending on the
relative phase difference between the two polarizations, the probe couples to TE or TM. TE
polarized light in the upper output is converted to TM and in the output PS, only TM from the
upper arms and TE from the lower arm will couple to the output.
The polMZI should be designed such that the probe is not rotated in case there is no pump
signal present. In that case the probe will not couple to the output. An other important issue
for the POLARIS filtering to work is that the self-phase modulation (SPM) of the pump signal
in the SOA causes a phase shift of exactly π radians. In that case the pump at the output of
the SOA is completely orthogonal to the pump at the input and will be filtered by the output
PS. Ideally the SPM is equal to the cross-phase modulation of the probe and the probe will be
coupled to the output of the PS. A deviation from this will result in a small loss on the probe
signal, but the suppression of the pump is not influenced.
The PS used for coupling the probe into the arms is used as an additional filter as explained in
the previous design. A good PS will yield low-loss coupling of the pump and probe into the
branches. By using an MMIs at these points, 3 dB inherent loss is introduced.
POLARIS all-optical switch
As stated before, in a wavelength converter, the signal coming from the network can have
an arbitrary polarization. In that case this signal is used as pump. In an all-optical switch,
a different optimization might be preferred as here the signal from the network is the probe
which is switched by a local signal. In this case, the POLARIS scheme looks as depicted in
Fig. 7.8.
As the POLARIS solution is a polarization diversity solution, the switch consists of two MZIs.
In the scheme presented here, both MZIs share the same in- and output MMI. One MZI, used
for a TE-polarized probe signal (referred to as ”TE-MZI”), uses the inner arms of the device
(indicated in the figure by thick lines), the other, the ”TM-MZI” consists of the outer arms,
indicated by the dashed lines.
The probe signal, in an arbitrary polarization, is split in the input MMI into the two arms. In
the polarization splitters, the signal in both arms is split into two orthogonal polarizations. The
124
POLARIS
Pump,
50% TE, 50% TM
SOA
PC
PC
SOA
PS
Probe,
arbitrary
polarization
PS
Output1, Probe
MMI
MMI
PC
SOA
PS
Output2, Probe
PS
SOA
PC
Figure 7.8: POLARIS all optical switch.
TE part into the inner outputs, the TM part into the outer branches.
The pump signal (the control signal), has to have a polarization with equal power in both TE
and TM (the phase between the polarizations is not important), so that it is split in the upper
input polarization splitter into equal power parts. This signal is now orthogonal to the probe in
both upper arms.
The polarization of the signals in the inner arms is rotated by 90° by the polarization converters.
Now in all branches the signals entering the SOAs have equal polarization (pump in TE, probe
in TM). The signals interact in the upper SOAs and the phase is modulated. The signals in the
outer arms are rotated in the PCs to have the corresponding signals orthogonal at the input of
the output PS. In those output PSs, TE polarized light is coupled to the cross-port, while TM
polarized light is coupled to the bar-port. In this way, only the probe signal is coupled to the
inner outputs of the PSs. The signal is combined in the output MMI and depending on the
presence of the pump signal is output at output 1 or 2.
Note that for this concept 2 × 2 polarization splitters are required. The coupling in the input
polarization splitters is different than in the output PSs. At the input TE polarized light is
coupled to the bar-ports, while at the output it is coupled to the cross-port. This can be easily
achieved with the 2 × 2 splitter demonstrated in chapter 5 by exchanging the position of the
PCs in this PS. If another PS is to be used with a fixed coupling mechanism, crossings are
needed to couple the respective outputs to the output MMI, which complicates the design.
For the all-optical switch similar modifications as shown for the wavelength converter can be
made to improve the performance.
In this section the POLARIS concept is explained and explored. Simulations show that even
with moderately performing components, good performance of the whole circuit can be expected if loss balancing and sufficient filtering are used. The concept can be applied to WLC
and all-optical switches, and can be simplified with polMZIs.
7.4
Fiber based POLARIS
The POLARIS concept is experimentally tested by realizing the proposed circuit using the first
generation integrated wavelength converter of chapter 6. The polarization handling is realized
7.4 Fiber based POLARIS
125
with fiber-based polarization splitters and rotators.
7.4.1
Experiments
The measurements are performed on a standard transmission setup, similar to the setup shown
in Fig. 7.10. Light is coupled into the chip using an array of cleaved Polarization Maintaining
Fibers (PMF).
A single branch of POLARIS is tested. This is done by injecting the CW probe and pump
signal with orthogonal polarizations. For the probe signal we make use of a TLS with well
defined output polarization and a PMF output.
The polarization of the pump signal is defined by using a polarization controller and polarization splitter. The power in the unwanted polarization is monitored and corrected manually. The
polarization splitter has a polarization ER of 15 dB. This results in 3% of power in the wrong
polarization being also injected in the device. At the output a polarization splitter, also with an
ER of 15 dB, is used as a filter.
Static characterization
The probe and pump signal wavelengths are spaced 0.5 nm to be able to measure the isolation
using an Optical Spectrum Analyzer. The recorded spectra are shown in Fig. 7.9. An isolation
−20
No pump
POLARIS
No filter
−25
Power [dBm]
−30
−35
−40
−45
−50
−55
−60
−65
1555
1560
Wavelength [nm]
1565
Figure 7.9: Output spectrum at the output of POLARIS.
of 10 dB is achieved in this way. For signals with a wider wavelength separation, isolation
up to 13 dB is obtained. This demonstrates one of the strengths of POLARIS: signals spaced
this closely are very difficult to separate using conventional wavelength filtering as it requires
extremely narrow filters.
The level of isolation achieved here reflects the quality of the fiber based polarization components. With integrated devices, with their inherently better definition of the polarization state,
higher isolation can be expected.
126
POLARIS
2.5 Gbit/s
PRBS
PS
TLS
PC
Modulator
EDFA
PC
ATT
BPF
Psignal
ISOA1
SOA
PCW
BER
counter
PS
ATT
SOA
TLS
EDFA
AWG
ATT
PD
ISOA2
Figure 7.10: Measurement setup for testing the POLARIS principle. TLS: Tuneable Laser Source, PC: Polarization
Controller, MOD: Intensity Modulator, ATT: Attenuator, BPF: Band Pass Filter, PS: Polarization Splitter, AWG: 8channel WDM demultiplexer, PD: Photo Diode
Dynamic characterization
The POLARIS WLC is measured dynamically at 2.5 Gb/s. The setup for testing a single
branch of POLARIS is shown in Fig. 7.10.
Standard co-propagation is compared to POLARIS. For co-propagation operation we replace
the polarization splitter at the output of the device with a tuneable filter.
−3
10
−14
−6
10
−16
Output Power [dBm]
Bit Error Rate
back to back
co−dir. TE−TM
POLARIS TM−TE
POLARIS TE−TM
co−propagation
POLARIS
−18
−20
−22
−24
−26
−9
10
−32
−30
−28
−26
−24
Received Power [dBm]
−22
(a) BER vs. received power
−20
−28
4
6
8
Input Power [dBm]
10
(b) Output vs. input power
Figure 7.11: Co-propagation operation compared to POLARIS λpump =1555 nm, λprobe =1560 nm.
The results are presented in Fig. 7.11. From Fig. 7.11(a) it is seen that for POLARIS a
power penalty of 5 dB is encountered, in contrast with 4 dB for wavelength filtering. This is
probably caused by ASE, which is filtered almost completely by the wavelength filter in the
co-propagating case, but only half in the POLARIS case.
7.4 Fiber based POLARIS
127
In Fig. 7.11(b) the output power as a function of the input power is plotted. The output power is
2 dB higher in case of POLARIS. This is caused by the higher insertion loss of the wavelength
filter in the co-propagation case as compared to the polarization filter.
Conversion to the same wavelength is possible in co-propagation with POLARIS. This is another advantage of POLARIS: co-propagation is preferred over contra-propagation for high
speeds as explained previously. When converting to the same wavelength a lot of noise is
present in the output signal, as can be seen in Fig. 7.12.
(a) λprobe =1560.40 nm
(b) λprobe =1560.70 nm
(c) λprobe =1560.90 nm
Figure 7.12: Output bitpattern when converting to almost the same wavelength λpump =1560.66 nm
Because of the imperfect isolation, resulting from the limited performance of the fiber based
polarization components, the received signal will constain not only the probe signal, but also
remains of the pump. The wavelengths of both signals are almost the same. The difference
frequency will be within the electrical bandwidth of the detector, and is present as noise in
the output. A small de-tuning of the wavelength, larger than 25 GHz, is enough to place the
difference frequency outside the bandwidth of the detector, then this beating noise disappears.
The beating noise results from heterodyne mixing between the signals, and has a power of:
p
2Pprobe
Pmix = 2 Ppump Pprobe = √
A
(7.1)
where A is the isolation ratio between probe and pump.
This mixed signal is considered as the main contribution to the noise. The Signal-to-NoiseRatio (SNR) is calculated as the ratio of Pprobe and Pmix :
√
Pprobe
A
SNR =
=
(7.2)
Pmix
2
With an isolation of 20 dB, this will yield an SNR of 7 dB, which should be sufficient to allow
error-free operation.
The measurements demonstrate the feasibility of the POLARIS concept. The limited isolation
obtained translates to noise in the signal, particularly if conversion to the same wavelength is
regarded. The main cause of the unwanted signal being present in the output is an imperfect
128
POLARIS
definition of the polarization. This is the result of the fiber-connectors having a limited polarization ER of 15 dB, an accumulation of polarization errors in the connectors, and the 15 dB
ER of polarization splitter.
According to the simulations, higher values of the isolation can be obtained by using higher
ER polarization filters or by cascading them, and by improving the performance of the MZI.
By using SOAs with higher α-factors, a lower pump power can be used, which yields lower
remains of the pump power at the output. These improvements will lead to an isolation of more
than 20 dB with good BER performance, as shown in the simulations.
Filtering of signals through an active-passive integrated MZI based on polarization is demonstrated. This avoids the need for (tuneable) filters. Because of the possibility of conversion
to the same wavelength the POLARIS WLC simplifies wavelength management in a WDM
network.
7.5
Integrated POLARIS
The feasibility of the POLARIS concept is demonstrated in the previous section. One of the
problems in that experiment is the imperfect definition of the polarization and the accumulation
of polarization errors in connecting polarization maintaining fibers and polarization splitters.
By integrating the whole concept, these polarization errors can be avoided.
In chapter 4 a polarization converter with a conversion of more than 95% is demonstrated,
which should be sufficient to demonstrate POLARIS according to the simulations. By using
the MZI polarization splitter from chapter 5, a fully integrated POLARIS circuit can be realized. This is a demonstration of the generic integrated polarization handling technology by
integrating a polarization converter in the standard active-passive technology.
7.5.1
Design
An integrated version of POLARIS is designed using the building blocks discussed previously
in this thesis. The active material is the same as is used for the second generation MZI switch,
containing 8 Quantum Wells (see section 6.4).
Three types of the POLARIS concept are present on the mask. First of all the POLARIS
wavelength converter according to the schematic of Fig. 7.1. The corresponding mask design
is depicted in Fig. 7.13.
As can be seen in the figure, all in- and outputs are positioned at an angle of 7° to avoid reflections, they are all horizontally tapered to a width of 5 µm, to maximize coupling tolerances.
Higher-order modes can be excited in these waveguides, therefore modefilters are placed after
the bends in every in- and output. These modefilters, as well as all other MMIs, have their corners cut to reduce reflections as explained in section 2.3. Reflections from the active-passive
butt-joint are prevented by entering the active region at an angle of 10°.
Multiple inputs have to be used. This requires the usage of an angled fiber array with a pitch
of 250 µm. Because the waveguides are put at an angle of 7°, the refraction angle is 23° in air.
The waveguides are thus placed at a pitch of 250/ cos(23o )=271.6 µm.
7.5 Integrated POLARIS
horizontal modefilters
o
tapers at 7
129
polarization splitters
and converters
pump
output
probe
SOAs
EBL alignment marks
Figure 7.13: Mask design of the POLARIS wavelength converter.
The MZIs use 2 × 2 input and output MMI couplers to improve the balance of the MZIs (see
chapter 6). The non-POLARIS in- and outputs are also led to the facets. This gives the possibility to test the MZI separately by circumventing the polarization components.
All waveguides are 3 µm shallow waveguides that are designed to have low loss. Only the
waveguides inside the polarization splitters and converters are deeply etched. EBL write fields
with alignment marks are included in the waveguide design. In these write fields the polarization splitters and converters can be defined. No additional polarization filters are used in this
case due to a lack of space.
Apart from the wavelength converter design, also the all-optical switch (schematic in Fig. 7.8)
is designed. The mask layout for this is shown in Fig. 7.15. Here 2 × 2 polarization splitters
(chapter 5) are used.
Moreover a POLARIS wavelength converter with polarization MZIs is designed according
to the schematic of Fig. 7.7. The mask design is shown in Fig. 7.14. The much simpler and
smaller mask layout is evident here. For all circuits the same designs are used for the individual
components.
pump
probe
output
Figure 7.14: Mask design of the POLARIS wavelength converter with polarization MZIs.
pump
output1
output2
probe
Figure 7.15: Mask design of the POLARIS all-optical switch.
130
7.5.2
POLARIS
Generic integration technology with polarization handling
capability
In this section the active-passive integration technology from chapter 3 is extended by adding
the processing of the polarization converter.
The proposed processing scheme, in which active and passive components and polarization
converters are combined, is shown in Fig. 7.16. It is based on the second generation designs of
the polarization converter (section 4.4), polarization splitter (section 5.3) and the MZI (section
6.4).
The process consists of the following steps:
a. First the contactlayer is removed from all passives components. To this end a lithography
step is done in which the active regions are covered. The InGaAs contact layer is wet
chemically removed by H2 SO4 :H2 O2 :H2 O. This is a selective etch and will stop on the
InP topcladding. The contact layer thickness is only 100 nm in our case, therefore the
coverage of the etched step with resist or masking material later on in the process is not
a problem.
b. Silicon Nitride (SiNx ) is deposited on the sample. On this, the waveguides, and writefields for the EBL, including the alignment marks, are optically defined. The SiNx mask
layer is etched using CHF3 /O2 RIE.
c. The sample is covered with EBL resist in which the polarization converters are defined.
d. Ti is evaporated and the polarization converters are defined in Ti on top of the SiNx using
a lift-off process.
e. A second EBL step is done to open the straight side of the PC. The nitride at the straight
side of PC is opened using the resist and the titanium as a mask.
f. All shallow waveguides are covered with resist. The deep waveguides and the straight
side of the PC are opened by optical lithography.
g. The deep waveguides and the straight side of the PC are etched with CH4 /H2 RIE. Here
the same considerations are taken into account as for the double etch process in the
standard technology.
h. The resist is removed from the chip and all waveguides are etched using CH4 /H2 RIE.
The shallow waveguide depth is critical in this step.
i. All waveguides are covered with resist, the PC area is opened with a non-critical optical
lithography step. The SiNx at the sloped side of the PC is opened.
j. The InP topcladding is RIE etched until 300 nm above the waveguide layer. In this step
the critical dimensions are kept, because the Ti mask does not erode during the etch and
thus the sidewalls are vertical. While etching this side, the straight side of the PC (which
is already etched in step h.) is etched even deeper, well below the waveguide layer.
7.5 Integrated POLARIS
131
k. Silicon Nitride is deposited on the whole sample. The PECVD deposition also covers
the sidewalls.
l. All waveguides are again covered with resist. The PC area is opened using a non-critical
lithography. The SiNx on the PC area is etched back using CHF3 /O2 RIE. Because of the
directional etching, the etched sidewalls stay covered with SiNx , which serves as a mask
for the wet etching in the next step.
m. Br2 -Methanol is used to etch the slope. This etchant etches a slope in both InP and
InGaAsP, with an angle of 54.7° with respect to the surface. In this step, the straight
side of the PC is etched as well. As this side is already etched well below the waveguide
layer, the etching will not influence the performance of the converter.
n. The nitride and the Ti are removed using an HF solution. The SOAs are passivated and
planarized by spinning polyimide on the sample. The polyimide is etched back till the
contacts are opened.
o. The sample is covered by negative resist and the contacts are opened. Titanium, platinum, and gold (Ti, Pt, Au) are evaporated twice on the sample, once on the P-contact
on top, and once on the backside of the sample for the N-contact. After lift-off in aceton,
the P-contacts remain on top. The backside is completely covered for the N-contact.
For a low resistance contact, the metal has to be made thicker. This is achieved by electro
plating as explained in chapter 3. After this thick gold P-contacts remain.
p. The final step is to remove the polyimide everywhere from the sample, except below
the P-contacts. This is done in an CHF3 /O2 RIE. The metal contacts serve as a mask.
They are not attacked if the power is sufficiently low (50 W). The polyimide needs to
be removed in order to cleave the sample. Furthermore the PC is designed to have air
surrounding the device. The correct index contrast is not obtained if the polyimide is left
on the sample.
In the standard COBRA technology, electro-optic phase modulators can be integrated as well.
This possibility is taken into account in this scheme. The phase modulators themselves are
processed in exactly the same way as the SOAs. Phase modulators are reversely biased, so
isolation sections are required in between them. These sections are waveguides from which
the highly doped top cladding is removed to increase their resistance. Their top cladding can
be etched in step (j) to reach the same level as the start of the sloped side of the PC.
7.5.3
Finished chip
The POLARIS chip is fabricated. A photograph of part of the chip is shown in Fig. 7.17. Here
2 POLARIS all-optical switches and 2 POLARIS WLCs with polMZIs are visible.
The fabricated chip shows the integration of polarization converters and splitters with other
passive and active components. Inspection with an optical microscope and a SEM shows
that the realization of a large number of the SOAs, the passive waveguide devices and the
132
POLARIS
InP
Q125
Q155
InGaAs
SiNx
Resist
Polyimide
TiPtAu
(a) Covering actives and wet-selective removal
of the contact layer on passives.
(b) SiNx deposition, waveguide lithography
and SiNx etching.
(c) EBL to define PC waveguides.
(d) Ti evaporation and lift-off for PC definition.
(e) EBL to cover the sloped side of the PC.
Etching SiNx at the straight side.
(f) Covering all shallow waveguides with resist.
(g) Preliminary etching of deep waveguides.
(h) Resist removal and etching of all waveguides.
7.5 Integrated POLARIS
133
(i) Covering all waveguides, opening of the PC
area. Etching SiNx to reveal the sloped side of
the PC.
(j) Etching the sloped side of PC to the right
depth.
(k) Covering the whole chip with SiNx .
(l) Covering the whole chip with resist and
opening the PC area. Etch SiNx in the PC area.
(m) Etching the slope with Br2 :CH3 OH.
(n) Removing all SiNx . Polyimide planarization and etching back.
(o) P- and N-contact metallization.
(p) Etching all the polyimide with the metal as
a mask.
Figure 7.16: integration technology with polarization handling capability
134
POLARIS
Figure 7.17: Photograph of the POLARIS all-optical switch and POLARIS WLC with polMZIs.
polarization converters has succeeded.
The fabrication of the polarization splitters and converters within the full integration scheme
was successful. The 2 × 2 polarization splitters from chapter 5 are fabricated in this realization.
As stated before, they show a conversion of 88% and a splitting ratio of 7 dB. This limited
performance is most probably caused by a non-optimal dose in the EBL. A better proximity
effect correction is required for structures close to each other as in the polarization splitter.
Due to bad adhesion of the EBL resist on part of the SOAs in step (e), these devices are etched
away. Furthermore, an error during the metallization damaged the contacts on a number of the
SOAs. The undamaged SOAs show proper operation.
A number of all the relevant components (SOAs, PCs, PSs) on the chip showed reasonable
functioning. The overall yield however was too low to demonstrate the integrated POLARIS
circuits. An optimization of the process is required, but due to time constraints this has not
been done.
The integration, and the operation of the various components, demonstrate the feasibility of
the generic integration scheme with polarization handling capability.
7.6
Conclusions
A new configuration is studied for a wavelength converter based on an SOA-MZI, which uses
polarization components to isolate the pump and probe signals. The concept is useable for
different circuits in which optical-optical interactions are used for all-optical switching. The
concept is investigated for a wavelength converter. The configuration is promising since it
works in co-propagation and allows conversion to the same wavelength. An input power range
of 15 dB is found in simulations in which the BER is lower than 10−12 . High ER and high
isolation can be obtained at the same time. This is achieved by using components with a performance that is already realized.
The concept is experimentally demonstrated by using an integrated SOA-MZI and external
fiber-based polarization handling. An isolation of 10 dB is achieved in this way. This demon-
7.6 Conclusions
135
stration shows one of the strengths of POLARIS: signal wavelengths spaced as close as 0.5 nm
are very difficult to separate using conventional wavelength filtering as it requires extremely
narrow filters.
By using SOAs with higher α-factors a lower pump power can be used, which yields lower
powers at the output in both the unwanted and wanted polarization. These improvements can
lead to an isolation of more than 20 dB.
An integrated version of POLARIS has been designed. A generic integrated polarization handling technology is demonstrated by realizing this circuit. This technology is an extension of the
standard technology where the only addition is a polarization converter. The realization clearly
showed that the integration scheme is feasible. Unfortunately due to not sufficiently optimized
processing steps no POLARIS operation could be shown yet with the integrated device.
Chapter 8
Conclusions and Outlook
8.1
Conclusions
In this thesis a technology for polarization manipulation in Photonic Integrated Circuits (PIC)
is studied. Applying polarization offers a broad variety of functions. On-chip polarization
handling, such as polarization diversity, can solve polarization problems without changing
waveguide or material properties and thus without compromising the performance.
A generic technology platform with polarization handling capability is proposed. For full control of the polarization on a PIC, polarization converters and polarization splitters are required
that fit in the standard technology for realizing of active and passive components.
A new polarization converter is designed that can be fully integrated in the standard process.
The polarization converter shows a maximum conversion of 97%. A conversion larger than
95% is obtained over a wavelength range larger than 35 nm and a temperature range larger
than 40°C. As these measured ranges were limited by the equipment used, actually much
larger ranges are inferred.
Two novel types of polarization splitters are designed. One tolerant device with a large splitting ratio (larger than 13 dB) and a large window of operation (width range of approximately
100 nm, wavelength range of more than 45 nm). It has however the disadvantages that it is
long (2.7 mm), and that it is hard to integrate with other components.
Another type of polarization splitter is presented as well, based on a passive MZI with polarization converters in the arms. This device is shorter (600 – 800 µm) and, because it consists only
137
138
Conclusions and Outlook
of passive components and PCs, it has the ability to be integrated in the standard active-passive
scheme. Simulations show the possibility of high conversion, 9 dB splitting is experimentally
achieved. A 1 × 2 and a 2 × 2 polarization splitter are realized.
The demonstration of the PC and PS show that the generic platform for polarization manipulation can thus be obtained by extending the standard technology with the addition of a polarization converter only.
Apart from the polarization handling capability, technology for packaging PICs is required.
For this purpose an spot size converter is developed that can be integrated with active and passive components. Experimentally determined overlap with a standard fiber indicates coupling
losses around 1.5 dB and a coupling tolerance of ± 1.5 µm for 1 dB excess loss.
The feasibility of the packaging technology is demonstrated by the fabrication of integrated
MZI switches with spot size converters. An array of these switches is packaged and wavelength conversion at 40 Gb/s is achieved.
The feasibility of polarization handling is demonstrated by a new type of integrated wavelength converter: POLARIS (Polarization LAbelling for Rejection and Isolation of Signals).
This wavelength converter uses the polarization of the light to label the original and the converted signals. By using a polarization splitter, the two signals can be separated and filtered.
This approach can also be used in all-optical switches.
The POLARIS concept is demonstrated by simulations and experimentally verified with external fiber based polarization manipulating components.
An integrated version of POLARIS is designed. The generic integrated polarization handling
technology is demonstrated by realizing this circuit. The realization clearly showed that the
integration scheme is feasible, because working examples of all relevant components were
present on the chip. Unfortunately due to time constraints not all processing steps were sufficiently optimized leading to a too low yield of working components, therefore no POLARIS
operation could be shown with the integrated device.
8.2
Recommendations
The POLARIS principle and a number of the polarization handling circuits introduced in chapter 1 can be realized with the standard technology extended with polarization converters. Some
improvements are required in the components and in the process. They are discussed here.
The losses of the integrateable polarization converter are high (around 2.5 dB) compared to
simulations. Excess loss of the device should be below 1 dB, similar to earlier reported devices
[72]. Most probably extra losses are caused by the non-optimal coupling of the waveguide and
the converter, because of an underetch of the access waveguides. This can be avoided by using
a different masking material, with less strain.
The splitting ratio of the integrateable polarization splitter has to be increased. For an integrated POLARIS, values above 13 dB are required. An optimized proximity effect correction
8.3 Outlook
139
on the mask for the EBL exposure is required, because the width definition of the waveguides,
the MMIs and the PCs is critical. Furthermore the EBL exposure spans multiple write fields.
An imperfect connection between them can cause gaps or overlaps that lead to overexposure.
Small rotations of the write fields can cause phase errors between the arms of the MZI. The
alignment of the EBL write fields has to be improved to be able to use the EBL for these devices. By using high-resolution optical lithography, with a reduction stepper, these problems
are avoided.
The roughness of the waveguides that is present in the polarization splitters has to be decreased.
To this end the lithographical process for lift-off has to be optimized. The resist profile has to
be suited for this purpose to reduce the edge roughness. For the EBL defined structures the
resist profile is straight and lift-off of Ti did not lead to mayor roughness.
In the standard COBRA technology, electro-optic phase modulators can be integrated as well.
This is taken into account in the development of the generic platform developed in this thesis.
By using the polarization dependence of the electro-optic Pockels effect, the phase between
the two polarizations can be controlled. This opens a new variety of possibilities. In the next
section some of these are introduced.
8.3
Outlook
With the integrated polarization handling technology presented in this thesis and by also using
electro-optic phase modulators, a number of other polarization based functionalities can be obtained. This section will briefly show some new concepts that can be monolithically integrated
with the new standard.
8.3.1
Polarization control
Several solutions for polarization dependence of PICs are proposed in this thesis. One of the
solutions is a POLARIS scheme for the all-optical switch as depicted in Fig. 7.8. In this
schematic the pump polarization should have equal power in TE and TM. A circuit shown in
Fig. 8.1 can change an arbitrary state of the polarization (SOP) of the incoming light to achieve
this. On the Poincaré sphere in Fig. 8.2 the relevant manipulations of the SOP are illustrated.
A signal with an arbitrary polarization (E1 in Fig. 8.2) is fed into the circuit. The phase modulator equalizes the phase to obtain a linear polarization E2 (SOP at the equator of the Poincaré
sphere). The half polarization converter equalizes the amplitudes of both polarizations (E3 at
the meridian in the S2 S3 -plane). The resulting polarization has equal power in both polarizations but an arbitrary phase between them. The MMI at the output of the PC is a 90/10 MMI,
by which 90% of the light is led to the output. The other part of the light is led to the PS. At
the outputs of the PS photodiodes (PDs) measure the intensity of the light in each polarization.
This information can be fed back to control the phase modulator at the input.
This circuit allows conversion from an arbitrary SOP to a SOP with equal power in both polarizations.
140
Conclusions and Outlook
E3
PD
PS
PHM
PC/2
E1
MMI
PD
Figure 8.1: Schematic of the 50–50 polarization controller.
E2
Figure 8.2: Poincaré sphere
indicating the SOP in the polarization controller.
This concept can be extended to convert an arbitrary polarization at the input to the desired
polarization at the PIC. As usually TE polarization is preferred, because of its higher gain, a
circuit is presented in which any SOP is converted to TE. The schematic is shown in Fig. 8.3.
The SOPs are visualized on the sphere in Fig. 8.4
E3
E1
PD
PHM
PC/2
PHM
PC/2
PS
E2
E5
E4
Figure 8.3: Schematic of the polarization controller.
Figure 8.4: Poincaré sphere
indicating the SOP in the polarization controller.
The first part (the phase modulator and half PC) of this polarization controller is the same
as the previous one. The signal with SOP E3 enters the second phase modulator. The phase
is changed to achieve a left-handed circular polarization (E4 ). With the last half polarization
converter this circular polarization is transformed to the TE output polarization (E5 ).
The detection of the SOP at the output of this controller is much simpler. A PS is used to split
the signal in the two polarizations, the TM output is connected to a photodiode. As the desired
8.3 Outlook
141
output polarization is fully TE, the phase modulators need to be adjusted to have minimum
power in TM.
8.3.2
Polarization switchable laser
The devices in the previous paragraphs are used as controllers at the input of a PIC. Similar
circuits can also be used to achieve polarization control at the output of a PIC. The integration
of a laser with a polarization controller is depicted in Fig. 8.5. The integration of the laser
depends on the application; it can be a simple Fabry-Pérot laser, a short integrated DBR laser
[106] or a multi-wavelength laser [17].
The functioning of the device is the same as in the previous controllers as can be seen in 8.6.
The laser has a fixed and stable output polarization, normally TE (E1 in 8.6). The first half PC
converts the polarization to a circular polarization (E2 ). The phase modulator determines the
eventual output polarization. The output PC converts the signal back to a linear polarization. If
both PCs are the same, a phase shift of π will result in TE output polarization (E4 ), zero phase
shift will result in TM. Any other phase shift results in a linear SOP with a controllable angle
of polarization.
E2
laser
PC/2
PHM
PC/2
Figure 8.5: Schematic of the laser with switchable polarization.
E1=E4
E3
Figure 8.6: Poincaré sphere
indicating the SOP in the polarization switchable laser.
The examples treated here, and in the introduction in chapter 1, illustrate the many possibilities that can be achieved if polarization manipulating components are integrated in standard
active-passive PICs. On-chip polarization control is very useful in the coherent detection of
Differential Phase Shift Keying (DPSK) modulated signals [107, 108], and is required for polarization multiplexing [109, 110].
The generic integration platform for on-chip polarization handling developed in this thesis
makes this feasible.
Appendix A
Polarization description
and visualization tools
In this appendix the polarization of light in a PIC is described. Jones vectors and matrices
are introduced to model the polarization transformation inside optical components. Stokes
parameters and the Poincaré sphere are used to visualize the state of polarization.
A.1
Polarization
A plane wave incident on an interface of two media, as shown in Fig. A.1(a) is reflected at that
interface. Two polarizations are observed: TE (Transverse Electric), when the electrical field
is parallel to the interface and TM (Transverse Magnetic), when the magnetic field is parallel
to the interface. These polarizations differ in the electromagnetic boundary conditions, as the
tangential field components of both the Electrical Field E and the magnetic field H have to be
continuous at the boundary. So for TE, the tangential components Ex and Hz are continuous
over the interface while for TM this holds for Hx and Ez .
A planar waveguide can be obtained by creating a second reflecting interface, parallel to
the first one. The planar waveguide consists of three layers with refractive indices nFILM >
nSUBSTRATE , nCLADDING .
The light will be confined in the layer having the highest index (nFILM ), because of total internal
reflections.
Two polarization modes for a wave propagating in the z direction can be found. The definitions
143
144
Polarization description and visualization tools
given before, hold for the polarizations: TE polarized light has its electrical field vector E
parallel to the interfaces, i.e in the x direction, so Ey , Ez = 0. . TM polarized light has its
magnetic field vector in the x direction, so Hy , Hz = 0.
Fig. A.1(c) shows a ridge waveguide used in a PIC. In these waveguides, an additional set of
TM
TE
y
n
y
FILM
n
FILM
E
H
n
E
n
k
SUBSTRATE
TM
y
H
k
n
CLADDING
n
FILM
n
SUBSTRATE
TE
SUBSTRATE
x
x
z
(a) TE reflection
x
z
(b) TM reflection
z
(c) ridge waveguide
Figure A.1: Waveguides showing the definition of the polarization modes
boundary conditions applies. Because of the presence of the ridge, also two vertical interfaces
are present. This ensures that the field is also confined in the x-direction. In this case pure TE
and TM do not exist. A TE polarized mode also has an electric field component perpendicular
to an interface: the vertical boundary of the ridge. But, as Ey (and Ez ) is close to zero for
quasi-TE and Ex ≈ 0 (and Ez ) for quasi-TM, the modes can be approximated by pure TE and
TM modes.
The H and E are related and a wave can be described by regarding the electrical field vector
only. Thus, in this thesis, the polarization will be described using the electric field components.
Furthermore, the stable polarized modes in waveguide devices will be approximated with TEmodes (only Ex ) and TM-modes (only Ey ).
A.2
Jones vector
The polarization of a mode is defined by the direction of the electrical field vector at a position z. The polarization can be described as a 2D vector, the Jones vector, containing the two
complex amplitudes (Ex , Ey ) of the electrical field. The transfer function of an optical component can be described by a 2 × 2 Jones matrix. By multiplying the vector with the matrix,
the transformation of the polarization state can be described and the output Jones vector can
be obtained. In this way a transfer matrix method can be used to model the effect of a set of
components on the polarization state.
The electrical field of a mode, propagating in the z-direction and expressed as a Jones vector,
is [111, 112]:
Ex
|Ex | e jφx
E =
=
(A.1)
Ey
|Ey | e jφy
A.2 Jones vector
145
Here |Ex,y | is the amplitude of the electrical field in the respective direction, φx,y is the phase.
For TE polarized light, the electrical field vector is directed in x direction, hence |Ey | = 0. For
TM |Ex | = 0, the Jones vectors for the two polarizations are thus:
0
|Ex | e jφx
TE :
TM :
(A.2)
0
|Ey | e jφy
Any state of polarization can be decomposed into the two orthogonal polarizations present
inside a waveguide. Some common cases are plotted in Fig. A.2. The magnitude of the
electrical field vectors and the phase between them, ∆φ = φy − φx , is indicated.
|E |=1, |E |=0
x
1
0
0
−1
−1
0
1
y
|Ex|=|Ey|=1/√2, ∆φ=π/2
E
|Ex|=0, |Ey|=1
y
1
−1
−1
1
1
0
0
−1
−1
0
1
0
1
|Ex|=|Ey|=1/√2, ∆φ=−π/2
−1
−1
0
1
|E |=|E |=1/√2, ∆φ=0
|E |=|E |=1/√2, ∆φ=pi
1
1
0
0
x
−1
−1
y
0
1
x
y
−1
−1
Ex
0
1
Figure A.2: Different states of polarization constructed from TE and TM polarized modes.
The Jones matrix of an optical component has the form:
J11 J12
T =
J21 J22
(A.3)
Here J11 (J22 ) is the transfer from the TE (TM) at the input to TE (TM) at the output. J12 and
J21 are the cross-polarization terms.
146
A.3
Polarization description and visualization tools
Stokes parameters
A convenient way to visualize the change in the state of polarization (SOP) will help to understand the polarization behavior of devices. For this, the SOP can be expressed in terms of
Stokes parameters [113]. Using the definitions from (A.1), these parameters are written as:
s0
s1
s2
s3
2 2
= Ex + Ey 2 2
= Ex − Ey = 2 Ex Ey cos(φy − φx )
= 2 Ex Ey sin(φy − φx )
(A.4)
(A.5)
(A.6)
(A.7)
q
Here s0 is the total intensity of the light, equal to s21 + s22 + s23 . The SOP is expressed by s1 ,
s2 , s3 . s1 and s2 describe the portion of the linearly polarized light, s3 describes the circularly
polarized fraction.
A visual interpretation of these Stokes parameters can be obtained by plotting them as coordinates on a sphere having a radius s0 . From this representation the SOP can be easily observed.
Points on the poles of this Poincaré sphere (Fig. A.3) correspond to circularly polarized light:
left-handed circular at the south pole, right-handed at the north pole. At the equator the light is
linearly polarized, the intersection with the s1 axis is fully TE or fully TM. At the nearer point,
s1 = s0 , TE polarized (0°). The further point, s1 = −s0 indicates TM (90°) polarized light. The
left and right intersections with the s2 -axis are the points in which the light has equal amount
of TE and TM polarized light (± 45°). The areas between the poles and the equator describe
various elliptical polarization states.
The sphere is very well suited for showing the change in polarization, if the light travels through
a photonic device.
This is explained by the example of a polarization-dependent phase shift in a waveguide into
which 45° linearly polarized light is fed. The light incident into the waveguide (E1 ) can be
decomposed into the two orthogonal stable polarization modes present inside the waveguide:
P1 and P2 , in this case TE and TM, respectively, as indicated in Fig. A.4(b).
The change in polarization is calculated using the Jones matrix of the phase shifter and is plotted on a Poincaré sphere.
A Jones matrix can be employed to calculate the phase change. The normalized input polarization is:
1
1
(A.8)
E1 = √
2 1
The transfer matrix of the waveguide employing ∆φ phase shift is given by:
T =
e j∆φ
0
0
1
(A.9)
A.3 Stokes parameters
147
s3
s2
s1
Figure A.3: Poincaré sphere for visualizing the state of polarization.
The output polarization E2 is obtained from the matrix multiplication:
j∆φ 1
e
E2 = T E1 = √
1
2
(A.10)
Fig. A.4(a) shows the propagation of the modes through the device. The corresponding
Poincaré sphere is plotted in Fig. A.4(b). Those two modes correspond to two points on
the sphere, opposite to each other. While propagating through the structure, phase differences
between these two stable modes (P1 and P2 ) occur, which is equivalent to rotating around the
axis through these modes. So a phase shift of π translates to a rotation of π radians around the
s1 -axis.
The calculation and visualization methods discussed here will be used to describe the polarization behavior of optical components in PICs. More details can be found in [111, 112, 113].
148
Polarization description and visualization tools
P2
E2
P1
(a) Propagation
(b) Poincaré sphere
Figure A.4: The change of the polarization when π phaseshift is applied.
E1
References
[1] T. Maiman, “Stimulated optical radiation in ruby,” Nature, vol. 187, pp. 493–494, Aug.
1960.
[2] I. Hayashi, M. Panish, P. Foy, and S. Sumski, “Junction lasers which operate continuously at room temperature,” Appl. Phys. Lett., vol. 17, no. 3, pp. 109–111, 1970.
[3] F. Kapron, D. Keck, and R. Maurer, “Radiation losses in glass optical waveguides,”
Appl. Phys. Lett., vol. 17, no. 10, pp. 423–425, 1970.
[4] M. Itoh, Y. Shibata, T. Kakitsuka, Y. Kadota, and Y. Tohmori, “Polarization-insensitive
SOA with a strained bulk active layer for network device application,” IEEE Photon.
Technol. Lett., vol. 14, pp. 765–767, June 2002.
[5] J.-Y. Emery, T. Ducelier, M. Bachmann, P. Doussière, F. Pommereau, R. Ngo, F.Gaborit,
L. Goldstein, G. Laube, and J. Barrau, “High performance 1.55 µm polarization-insensitive
semiconductor optical amplifier based on low-tensile-strained bulk GaInAsP,” Electron.
Lett., vol. 33, pp. 1083–1084, June 1997.
[6] S. Kitamura, K. Komatsu, and M. Kitamura, “Very low power consumption semiconductor optical amplifier array,” IEEE Photon. Technol. Lett., vol. 7, pp. 147–148, Feb.
1995.
[7] P. Doussiere, P. Garabedian, C. Graver, D. Bonnevie, T. Fillion, E. Derouin, M. Monnot,
J. Provost, D. Leclerc, and M. Klenk, “1.55 µm polarization independent semiconductor
optical amplifier with 25 dB fiber to fiber gain,” IEEE Photon. Technol. Lett., vol. 6,
pp. 170–172, Feb. 1994.
[8] R. Hanfoug, J. van der Tol, L. Augustin, and M. Smit, “Wavelength conversion with
polarisation labelling for rejection and isolation of signals (POLARIS),” in Proc. 11th
Eur. Conf. on Int. Opt. (ECIO ’03), pp. 105–108, Prague, Czech Republic, April 2–4
2003.
[9] J. van der Tol, L. Augustin, U. Khalique, and M. Smit, “Polarization control and its
application to waveguide devices,” in Proc. 13th Micro optic Conf. (MOC ’07), p. C1,
Takamatsu, Japan, Oct. 28–31 2007. Invited paper.
[10] K. Stubkjær, A. Kloch, P. Hansen, H. Poulsen, D. Wolfson, K. Jepsen, A. Clausen,
E. Limal, and A. Buxens, “Wavelength converter technology,” IEICE Trans. on Comm.,
vol. E82-B, pp. 390–400, Feb. 1999.
149
150
References
[11] C. Joergensen, S. Danielsen, K. Stubkjaer, M. Schilling, K. Daub, P. Doussiere, F. Pommerau, P. Hansen, H. Poulsen, A. Kloch, M. Vaa, B. Mikkelsen, E. Lach, G. Laube,
W. Idler, and K. Wünstel, “All-optical wavelength conversion at bit rates above 10 Gb/s
using semiconductor optical amplifiers,” J. of Sel. Topics in Quantum Electron., vol. 3,
pp. 1168–1180, Oct. 1997.
[12] J. Leuthold, J. Eckner, P. Besse, G. Guekos, and H. Melchior, “Dual-order mode (DOMO)
all-optical space switch for bidirectional operation,” in Techn. Digest Opt. Fiber Comm.
(OFC ’96), pp. 271–272, San Jose, California, USA, Feb. 20–25 1996.
[13] D. Wolfson, T. Fjelde, A. Kloch, C. Janz, F. Poingt, I. Guillemot, F. Gaborit, and M. Renaud, “Detailed experimental investigation of an all-active dual-order mode Mach-Zehnder
wavelength converter,” in Techn. Digest Opt. Fiber Comm. (OFC ’00), pp. 72–74, Baltimore, Maryland, USA, March 7-10 2000.
[14] H. Dorren, D. Lenstra, Y. Liu, M. Hill, and G. Khoe, “Nonlinear polarization rotation in
semiconductor optical amplifiers: Theory and application to all-optical flip-flop memories,” IEEE J. Quantum Electron., vol. 39, pp. 141–147, Jan. 2003.
[15] Y. Liu, M. Hill, E. Tangdiongga, H. de Waardt, N. Calabretta, G. Khoe, and H. Dorren,
“Wavelength conversion using nonlinear polarization rotation in a single semiconductor
optical amplifier,” IEEE Photon. Technol. Lett., vol. 15, pp. 90–92, Jan. 2003.
[16] R. Broeke, A Wavelength Converter Integrated with a Discretely Tunable Laser for
Wavelength Division Multiplexing Networks. PhD thesis, Delft University of Technology, Delft, The Netherlands, 2003.
[17] J. den Besten, Integration of Multiwavelength Lasers with Fast Electro-Optical Modulators. PhD thesis, Technische Universiteit Eindhoven, Eindhoven, The Netherlands,
2004. ISBN 90-386-1643-0.
[18] Photon Design. FimmWave/FimmProp. http://www.photond.com.
[19] S. Adachi, Physical properties of III-V semiconductor compounds. New York: John
Wiley and Sons, 1992.
[20] J. Soole, C. Caneau, H. LeBlanc, N. Andreadakis, A. Rajhel, C. Youtsey, and I. Adesida,
“Suppression of modal birefringence in InP-InGaAsP waveguides through use of compensated tensile strain,” IEEE Photon. Technol. Lett., vol. 9, pp. 61–63, Jan. 1997.
[21] EMIS Datareviews Series No. 6, Properties of Indium Phosphide. London and New
York: INSPEC, 1991. ISBN 0-85296-491-9.
[22] S. Adachi, Properties of group-IV, III-V and II-VI semiconductors. Chichester, England:
John Wiley and Sons, 2005. ISBN 0-470-09032-4.
[23] S. Adachi and K. Oe, “Internal strain and photoelastic effects in Ga1−x Alx As/GaAs and
In1−x Gax Asy P1−y /InP crystals,” J. Appl. Phys., vol. 54, pp. 6620–6627, Nov. 1983.
[24] C. van Dam, L. Spiekman, F. van Ham, F. Groen, J. van der Tol, I. Moerman, W. Pascher,
M. Hamacher, H. Heidrich, C. Weinert, and M. Smit, “Novel compact polarization convertors based on ultra short bends,” IEEE Photon. Technol. Lett., vol. 8, pp. 1346–1348,
Oct. 1996.
[25] D. Maat, InP-based integrated MZI switches for optical communications. PhD thesis,
Delft University of Technology, Delft, The Netherlands, 2001. ISBN 90-9014700-4.
[26] L. Soldano and E. Pennings, “Optical multi-mode interference devices based on selfimaging: Principles and applications,” J. Lightwave Technol., vol. 13, pp. 615–627,
Apr. 1995.
References
151
[27] M. Hill, X. Leijtens, G. Khoe, and M. Smit, “Optimizing imbalance and loss in 2 × 2
3-dB multimode interference couplers via access waveguide width,” J. Lightwave Technol., vol. 21, pp. 2305–2313, Oct. 2003.
[28] X. Leijtens, P. Le Lourec, and M. Smit, “S-matrix oriented CAD-tool for simulating
complex integrated optical circuits,” J. of Sel. Topics in Quantum Electron., vol. 2,
pp. 257–262, June 1996.
[29] E. Pennings, R. van Roijen, M. van Stralen, P. de Waard, R. Koumans, and B. Verbeek,
“Reflection properties of multimode interference devices,” IEEE Photon. Technol. Lett.,
vol. 6, pp. 715–718, June 1994.
[30] D. Erasme, L. Spiekman, C. Herben, M. Smit, and F. Groen, “Experimental assessment
of the reflection of passive multimode interference couplers,” IEEE Photon. Technol.
Lett., vol. 9, pp. 1604–1606, Dec. 1997.
[31] Y. Gottesman, E. Rao, and B. Dagens, “A novel design proposal to minimize reflections
in deep-ridge multimode interference couplers,” Electron. Lett., vol. 12, pp. 1662–1664,
Dec. 2000.
[32] Y. Barbarin, E. Bente, C. Marquet, E. Leclère, J. Binsma, and M. Smit, “Measurement
of reflectivity of butt-joint active-passive interfaces in integrated extended cavity lasers,”
IEEE Photon. Technol. Lett., vol. 17, pp. 2265–2267, Nov. 2005.
[33] F. Soares, F. Karouta, B. Smalbrugge, S. Oei, R. Baets, and M. Smit, “InP-based photonic integrated circuit with WDM switched optical delay lines for true-time-delay
beamstearing of a 40 GHz phased-array antenna,” in Proc. 12th Eur. Conf. on Int. Opt.
(ECIO ’05), pp. 129–132, Grenoble, France, April 6–8 2005.
[34] F. Soares, F. Karouta, E. Geluk, J. van Zantvoort, and M. Smit, “A compact and fast
photonic True-Time-Delay beamformer with integrated Spot-Size Converters,” in Technical Digest Integr. Photon. Res. and Apps. (IPRA ’06), p. IMF5, Uncasville, USA, Apr.
24–Apr. 28 2006.
[35] L. Chusseau, P. Martin, C. Brasseur, C. Alibert, P. Hervé, P. Arguel, F. Lozes-Dupuy, and
E. Rao, “Carrier-induced change due to doping in refractive index of InP: Measurements
at 1.3 and 1.5 µm,” Appl. Phys. Lett., vol. 69, pp. 3054–3056, Nov. 1996.
[36] P. Martin, E. M. Skouri, L. Chusseau, C. Alibert, and H. Bissessur, “Accurate refractive
index measurements of doped and undoped InP by grating coupling technique,” Appl.
Phys. Lett., vol. 67, pp. 881–883, Aug. 1995.
[37] F. Fiedler and A. Schlachetzki, “Optical parameters of InP-based waveguides,” Solid
State Electron., vol. 30, no. 1, pp. 73–83, 1987.
[38] C2V. OlympIOs. http://www.c2v.nl.
[39] T. Visser, B. Demeulenaere, J. Haes, D. Lenstra, R. Baets, and H. Blok, “Confinement
and modal gain in dielectric waveguides,” J. Lightwave Technol., vol. 14, pp. 885–887,
May 1996.
[40] T. Visser, H. Blok, B. Demeulenaere, and D. Lenstra, “Confinement factors and gain in
optical amplifiers,” IEEE J. Quantum Electron., vol. 33, pp. 1763–1766, Oct. 1997.
[41] Y. Liu, All-optical buffering based on nonlinear optical processing with semiconductor
optical amplifiers. PhD thesis, Technische Universiteit Eindhoven, Eindhoven, The
Netherlands, 2004. ISBN 90-386-0922-1.
152
References
[42] P. Thijs, Strained layer InGaAs(P)/InP Quantum Well semiconductor laser grown by
organometallic vapour phase epitaxy. PhD thesis, Technische Universiteit Delft, Delft,
The Netherlands, 1994.
[43] G. Agrawal and N. Olsson, “Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers,” IEEE J. Quantum Electron., vol. 25,
pp. 2297–2306, Feb. 1989.
[44] Y. Barbarin, 1.55 µm integrated modelocked semiconductor lasers. PhD thesis, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2007. ISBN 978-90-3862013-8.
[45] N. Storkfelt, M. Yamaguchi, B. Mikkelsen, and K. Stubkjaer, “Recombination constants
and α-factor in 1.5 µm MQW optical amplifiers taking carrier overflow into account,”
Electron. Lett., vol. 28, pp. 1774–1776, Sept. 1992.
[46] R. Manning, A. Kelly, A. Poustie, and K. Blow, “Wavelength dependence of switching
contrast ratio of semiconductor optical amplifier-based nonlinear loop mirror,” Electron.
Lett., vol. 34, pp. 916–918, Apr. 1998.
[47] D. Rogers. CIP. Private communication.
[48] E. Skogen, J. Barton, S. Denbaars, and L. Coldren, “A quantum-well-intermixing process for wavelength-agile photonic integrated circuits,” IEEE J. Sel. Topics in Quantum
Electron., vol. 8, pp. 863–869, Jul./Aug. 2002.
[49] N. Futakuchi, X. Song, D. Miyashita, M. Kato, and Y. Nakano, “Fabrication of InGaAsP/InP
Mach-Zehnder interferometer optical amplifier switches by metalorganic vapor phase
selective area epitaxy,” in Proc. IPRM01 conference, pp. 583–586, Nara, Japan, May
14–18 2001.
[50] L. Xu, M. Gokhale, P. Studenkov, J. Dries, C.-P. Chao, D. Garbuzov, and S. Forrest, “Monolithic integration of an InGaAsP-InP MQW Laser/Waveguide using a TwinGuide structure with a mode selection layer,” IEEE Photon. Technol. Lett., vol. 9, pp. 569–
571, May 1997.
[51] R. Varrazza, I. Djordjevic, and S. Yu, “Active vertical-coupler-based optical crosspoint
switch matrix for optical packet-switching applications,” J. Lightwave Technol., vol. 22,
pp. 2034–2042, Sept. 2004.
[52] U. Khalique, J. van der Tol, F. Groen, F. Karouta, E. Geluk, and M. Smit, “Polarization
based integration scheme (POLIS) for active and passive components,” in Proc. 11th
Eur. Conf. on Int. Opt. (ECIO ’03), pp. 137–140, Prague, Czech Republic, April 2–4
2003.
[53] J. Binsma, M. van Geemert, F. Heinrichsdorff, T. van Dongen, R. Broeke, and M. Smit,
“MOVPE waveguide regrowth in InGaAsP/InP with extremely low butt joint loss,”
in Proc. IEEE/LEOS Symposium (Benelux Chapter), pp. 245–248, Brussels, Belgium,
Dec. 2001.
[54] 3M. Scotch Magic Tape 810. http://www.3M.com.
[55] G. Maxwell, A. Poustie, C. Ford, M. Harlow, P. Townley, M. Nield, T. Lealman, S. Oliver,
L. Rivers, and R. Waller, “Hybrid integration of monolithic semiconductor optical amplifier arrays using passive assembly,” in Electronic Components and Technology Conference (ECTC ’05), vol. 2, pp. 1349–1352, 31 May-3 June 2005.
References
153
[56] S. Adachi and H. Kawaguchi, “Chemical etching characteristics of (001) InP,” J. Electrochem. Soc., vol. 128, pp. 1342–1349, June 1981.
[57] F. Soares, Photonic integrated True-Time-Delay Beamformers in InP technology. PhD
thesis, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2006. ISBN
90-386-1833-6.
[58] D. Trommer, R. Steingrüber, R. Löffler, and A. Umbach, “A novel flexible, reliable and
easy to use technique for the fabrication of optical spot size converters for InP based
PICs,” in Proc. IPRM99 conference, pp. 12–13, Davos, Switzerland, May 1999.
[59] J. van der Tol, J. Pedersen, E. Metaal, F. Hakimzadeh, Y. Oei, F. Groen, and I. Moerman,
“Realization of a short integrated optic passive polarization converter,” IEEE Photon.
Technol. Lett., vol. 7, pp. 893–895, Aug. 1995.
[60] A. Stano, “Chemical etching of InGaAs/InP and InAlAs/InP heterostructures,” J. Electrochem. Soc., vol. 134, pp. 448–452, Feb. 1987.
[61] P. Bowman, E. Ko, and P. Sides, “Potential hazard in preparing bromine-methanol solutions,” J. Electrochem. Soc., vol. 137, pp. 1309–1311, Apr. 1990.
[62] M. Schlak, C. Weinert, P. Albrecht, and H.-P. Nolting, “Tunable TE/TM-mode converter
on (001)-InP-substrate,” IEEE Photon. Technol. Lett., vol. 3, pp. 15–16, Jan. 1991.
[63] Y. Shani, R. Alferness, T. Koch, U. Koren, M. Oron, B. Miller, and M. Young, “Polarization rotation in asymmetric periodic loaded rib waveguides,” Appl. Phys. Lett., vol. 59,
pp. 1278–1280, Sept. 1991.
[64] H. Heidrich, P. Albrecht, M. Hamacher, H. Nolting, H. Schroeter-Janssen, and C. Weinert, “Passive mode converter with a periodically tilted InP/GaInAsP rib waveguide,”
IEEE Photon. Technol. Lett., vol. 4, pp. 34–36, Jan. 1992.
[65] M. Watts and H. Haus, “Integrated mode-evolution-based polarization rotators,” Opt.
Lett., vol. 30, pp. 138–140, Jan. 2005.
[66] T. Barwicz, M. Watts, M. Popović, P. Rakich, L. Socci, F. Kärtner, E. Ippen, and
H. Smith, “Polarization-transparent microphotonic devices in the strong confinement
limit,” Nature Photonics, vol. 1, pp. 57–60, Jan. 2007.
[67] D. Beggs, M. Midrio, and T. F. Krauss, “Compact polarization rotators for integrated
polarization diversity in InP-based waveguides,” Opt. Lett., vol. 32, pp. 2176–2178,
Aug. 2007.
[68] M. Kotlyar, L. Bolla, M. Midrio, L. O’Faolain, and T. F. Krauss, “Compact polarization
converter in InP-based material,” Optics Express, vol. 13, pp. 5040–5045, June 2005.
[69] B. Holmes and D. Hutchings, “Realization of novel low-loss monolithically integrated
passive waveguide mode converters,” IEEE Photon. Technol. Lett., vol. 18, pp. 43–45,
Jan. 2006.
[70] F. Groen, Y. Zhu, and J. van der Tol, “Compact polarisation converter on InP/InGaAsP
using an asymmetrical waveguide,” in Proc. 11th Eur. Conf. on Int. Opt. (ECIO ’03),
pp. 141–144, Prague, Czech Republic, April 2–4 2003.
[71] H. El-Refaei, D. Yevick, and T. Jones, “Slanted-rib waveguide InGaAsP-InP polarization converters,” J. Lightwave Technol., vol. 22, pp. 1352–1357, May 2004.
[72] U. Khalique, Y. Zhu, J. van der Tol, L. Augustin, R. Hanfoug, F. Groen, P. van Veldhoven, M. Smit, M. van de Moosdijk, W. de Laat, and K. Simon, “Ultrashort polarization
converter on InP/InGaAsP fabricated by optical lithography,” in Technical Digest Integr.
Photon. Res. and Apps. (IPRA ’05), p. IWA3, San Diego, USA, Apr. 11–Apr. 13 2005.
154
References
[73] J. Binsma, R. Broeke, and J. den Besten, “InP-based photonic integration technology,”
in Technical Digest Integr. Photon. Res. (IPR ’04), p. IFB1, San Francisco, USA, Jun.
30–Jul. 2 2004. Invited paper.
[74] L. Augustin, J. van der Tol, and M. Smit, “A compact passive polarization converter
for active-passive integration on InP/InGaAsP,” in Proc. 13th Eur. Conf. on Int. Opt.
(ECIO ’07), p. WA3, Copenhagen, Denmark, April 25–27 2007.
[75] J. den Besten, M. Dessens, C. Herben, X. Leijtens, F. Groen, M. Leys, and M. Smit,
“Low-loss, compact, and polarization independent PHASAR demultiplexer fabricated
by using a double-etch process,” IEEE Photon. Technol. Lett., vol. 14, pp. 62–64, Jan.
2002.
[76] Y. Zhu, U. Khalique, J. van der Tol, E. Geluk, F. Groen, F. Karouta, and M. Smit,
“Ultrashort, highly efficient integrated optical polarization converter,” in Proc. 12th Eur.
Conf. on Int. Opt. (ECIO ’05), pp. 96–99, Grenoble, France, April 6–8 2005.
[77] P. Albrecht, M. Hamacher, H. Heidrich, D. Hoffmann, H. Nolting, and C. Weinert,
“TE/TM mode splitters on InGaAsP/InP,” IEEE Photon. Technol. Lett., vol. 2, pp. 114–
115, Feb. 1990.
[78] L. Soldano, A. de Vreede, M. Smit, B. Verbeek, E. Metaal, and F. Groen, “MachZehnder interferometer polarization splitter in InGaAs/InP,” IEEE Photon. Technol.
Lett., vol. 6, pp. 402–405, Mar. 1994.
[79] A. Vellekoop and M. Smit, “A small-size polarization splitter based on a planar optical
phased array,” J. Lightwave Technol., vol. 8, pp. 118–124, Jan. 1990.
[80] F. Ghirardi, J. Brandon, M. Carré, A. Bruno, L. Menignaux, and A. Carenco, “Polarization splitter based on modal birefringence in InP/InGaAsP optical waveguides,” IEEE
Photon. Technol. Lett., vol. 5, pp. 1047–1049, Sept. 1993.
[81] I. Kiyat, A. Aydinli, and N. Dagli, “A compact silicon-on-insulator polarization splitter,”
IEEE Photon. Technol. Lett., vol. 17, pp. 100–102, Jan. 2005.
[82] H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S. i. Itabashi,
“Ultrasmall polarization splitter based on silicon wire waveguides,” Optics Express,
vol. 14, pp. 12401–12408, Dec. 2006.
[83] J. van der Tol, J. Pedersen, E. Metaal, Y. Oei, H. van Brug, and I. Moerman, “Mode evolution type polarization splitter on InGaAsP/InP,” IEEE Photon. Technol. Lett., vol. 5,
pp. 1412–1414, Dec. 1993.
[84] J. van der Tol, J. Pedersen, E. Metaal, J.-W. van Gaalen, Y. Oei, and F. Groen, “A short
polarization splitter without metal overlays on InGaAsP-InP,” IEEE Photon. Technol.
Lett., vol. 9, pp. 209–211, Feb. 1997.
[85] R. M. de Ridder, A. Sander, A. Driessen, and J. Fluitman, “An integrated optic adiabatic
TE/TM mode splitter on silicon,” J. Lightwave Technol., vol. 11, pp. 1806–1811, Nov.
1993.
[86] M. Watts, H. Haus, and E. Ippen, “Integrated mode-evolution-based polarization splitter,” Opt. Lett., vol. 30, pp. 967–969, May 2005.
[87] V. Zabelin, L. Dunbar, N. L. Thomas, R. Houdré, M. Kotlyar, L. O’Faolain, and T. Krauss,
“Self-collimating photonic crystal polarization beam splitter,” Opt. Lett., vol. 32, pp. 530–
532, Mar. 2007.
[88] M. G. Wilson and G. Teh, “Tapered optical directional coupler,” IEEE Transactions on
Microwave Theory and Techniques, vol. 23, pp. 85–92, Jan. 1975.
References
155
[89] H. Zappe, Introduction to Semiconductor Integrated Optics. Boston: Artech House
Publishers, 1995. ISBN 0-089006-789-9.
[90] K. Chiang, “Performance of the effective-index method for analysis of dielectric waveguides,” Opt. Lett., vol. 16, pp. 714–716, Oct. 1991.
[91] H. Unger, Planar optical waveguides and fibres. Oxford: Clarendom press, 1977.
[92] E. Patent, Optical Self-Switching Effects in Mach-Zehnder Interferometers. PhD thesis,
Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2005. ISBN 90-7444571-3.
[93] L. Augustin, J. van der Tol, R. Hanfoug, and M. Smit, “Design of a single etchstep
fabrication-tolerant polarisation splitter,” in Proc. 12th Eur. Conf. on Int. Opt. (ECIO ’05),
pp. 125–128, Grenoble, France, April 6–8 2005.
[94] W.-P. Huang, “Coupled-mode theory for optical waveguides: an overview,” J. Opt. Soc.
Am. A, vol. 11, Mar. 1994.
[95] C. Janz, B. Lavigne, F. Poingt, I. Guillemot, F. Gaborit, B. Dagens, D. Chiaroni, and
M. Renaud, “Low-penalty 10 Gbit/s operation of polarization-insensitive Mach-Zehnder
wavelength converters based on bulk-tensile active material,” in Techn. Digest Opt.
Fiber Comm. (OFC ’98), pp. 101–102, San Jose, California, USA, February 22-27 1998.
[96] C. Joergensen, S. Danielsen, T. Durhuus, B. Mikkelsen, K. Stubkjaer, N. Vodjdani,
F. Ratovelomanana, A. Enard, G. Glastre, and D. R. R. Blondeau, “Wavelength conversion by optimized monolithic integrated Mach-Zehnder interferometer,” IEEE Photon.
Technol. Lett., vol. 8, pp. 521–523, Apr. 1996.
[97] T. Keating, J. Minch, C. Chang, P. Enders, W. Fang, S. Chuang, T. Tanbun-Ek, Y. Chen,
and M. Sergent, “Optical gain and refractive index of a laser amplifier in the presence
of pump light for cross-gain and cross-phase modulation,” IEEE Photon. Technol. Lett.,
vol. 9, pp. 1358–1360, Oct. 1997.
[98] J. Vegas Olmos, Label-controlled optical switching nodes. PhD thesis, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2006. ISBN 978-90-386-1843-2.
[99] R. Hanfoug, L. Augustin, J. van der Tol, R. Broeke, and M. Smit, “Optical bandwidth of
Mach-Zehnder interferometer wavelength converters,” in Technical Digest Integr. Photon. Res. (IPR ’04), p. JWB19, San Fransisco, USA, Jun. 30–Jul. 4 2004.
[100] M. Heck, E. Bente, Y. Barbarin, D. Lenstra, and M. Smit, “Simulation of mode-locked
ring lasers including integrated passive components for dispertion compensation,” in
Proc. IEEE/LEOS Symposium (Benelux Chapter), pp. 159–162, Ghent, Belgium, Dec.
2004.
[101] B. Mikkelsen, M. Vaa, H. Poulsen, S. Danielsen, C. Joergensen, A. Kloch, P. Hansen,
K. Stubkjaer, K. Wünstel, K. Daub, E. Lach, G. Laube, W. Idler, M. Schilling, and
S. Bouchoule, “40 Gbit/s all-optical wavelength converter and RZ-to-NRZ format adapter realised by monolithic integrated active Michelson interferometer,” Electron. Lett.,
vol. 33, no. 2, pp. 133–134, 1997.
[102] D. Tsiokos, P. Bakopoulos, O. Zouraraki, D. Apostolopoulos, D. Petrantonakis, G. Maxwell, A. Poustie, and H. Avramopoulos, “Integrated MZI-based all-optical clock and
data recovery for asynchronous variable packet length traffic,” in Proc. 31st Eur. Conf.
on Opt. Comm. (ECOC ’05), p. We4.P.022, Glasgow, Scotland, Sep. 25–29 2005.
156
References
[103] P. Bakopoulos, D. Tsiokos, O. Zouraraki, H. Avramopoulos, G. Maxwell, and A. Poustie,
“Compact all-optical packet clock and data recovery circuit using generic integrated
MZI switches,” Optics Express, vol. 13, pp. 6401 – 6406, Aug. 2005.
[104] H.-D. Jung, I. Monroy, A. Koonen, and E. Tangdiongga, “All-optical data vortex node
using an MZI-SOA switch array,” IEEE Photon. Technol. Lett., vol. 19, pp. 1777–1779,
Nov. 2007.
[105] Virtual Photonics Incorporated. VPItransmissionMaker. http://www.vpiphotonics.com.
[106] B. Docter, T. Segawa, T. Kakitsuka, S. Matsuo, T. Ishii, Y. Kawaguchi, Y. Kondo,
H. Suzuki, F. Karouta, and M. Smit, “Short-cavity DBR laser using vertical groove gratings for large-scale photonic integrated circuits,” IEEE Photon. Technol. Lett., vol. 19,
pp. 1469–1471, Oct. 2007.
[107] C. Doerr, Zhang, S. Chandrasekhar, and L. Buhl, “Monolithic DQPSK receiver in InP
with low polarization sensitivity,” IEEE Photon. Technol. Lett., vol. 19, pp. 1765–1767,
Nov. 2007.
[108] C. R. Doerr, L. Zhang, L. L. Buhl, J. H. Sinsky, A. H. Gnauck, P. J. Winzer, A. L.
Adamiecki, and N. J. Sauer, “High-speed InP DQPSK receiver,” in Techn. Digest Opt.
Fiber Comm. (OFC ’08), p. PDP23, San Diego, USA, Feb. 2008.
[109] D. van den Borne, S. Jansen, E. Gottwald, P. Krummrich, G. Khoe, and H. de Waardt,
“1.6-b/s/Hz spectrally efficient transmission over 1700 km of SSMF using 40 × 85.6Gb/s POLMUX-RZ-DQPSK,” J. Lightwave Technol., vol. 25, pp. 222–232, Jan. 2007.
[110] C. R. Doerr and L. Zhang, “Monolithic 80-Gb/s dual-polarization On-Off-Keying modulator in InP,” in Techn. Digest Opt. Fiber Comm. (OFC ’08), p. PDP19, San Diego,
USA, Feb. 2008.
[111] B. Saleh and M. Teich, Fundamentals of Photonics. John Wiley & Sons Inc., 2007.
ISBN 0-471-35832-9.
[112] C.-L. Chen, Elements of optoelectronics and fiber optics. USA: Irwin, 1996. ISBN
0-256-14182-7.
[113] M. Born and E. Wolf, Principles of Optics. Pergamon Press, sixth ed., 1993.
List of abbreviations
ASE
BER
BPF
BPM
CW
DC
DPSK
EDFA
EIM
ER
FSK
FFT
FMW
MFD
MMI
MOVPE
MUFINS
MZI
OOK
OSA
PC
PECVD
PESSOA
PI
PIC
PMF
PolMZI
Amplified Spontaneous Emission
Bit Error Rate
Bandpass filter
Beam Propagation Method
Continuous Wave
Direct Current
Differential Phase Shift Keying
Erbium Doped Fiber Amplifier
Effective Index Method
Extinction Ratio
Frequency Shift Keying
Fast Fourier Transformation
Fiber Matched Waveguide
Mode Field Diameter
Multimode Interference
Metal-Organic Vapour-Phase Epitaxy
MUlti - Functional INtegrated arrays of interferometric Switches
Mach-Zehnder Interferometer
On-Off Keying
Optical Spectrum Analyzer
Polarization Converter
Plasma Enhanced Chemical Vapour Deposition
Polarization Effect Suppression in Semiconductor Optical Amplifiers
Polyimide
Photonic Integrated Circuit
Polarization Maintaining Fiber
Polarization Mach Zehnder Interferometer
157
158
List of abbreviations
POLARIS
PS
QW
RIE
SMF
SOA
SOP
SPM
SSC
STOLAS
TE
TM
TLS
WDM
VPI
WLC
XPM
POlarization LAbeling for Rejection and Isolation of Signals
Polarization Splitter
Quantum Well
Reactive Ion Etching
Single Mode Fiber
Semiconductor Optical Amplifier
State Of Polarization
Self Phase Modulation
Spot Size Converter
Switching Technologies for Optically Labeled Signals
Transverse Electric
Transverse Magnetic
Tuneable laser source
Wavelength Division Multiplexing
Virtual Photonics Incorporated
Wavelength Converter
Cross Phase Modulation
Summary
Polarization Handling in Photonic Integrated Circuits
Photonic Integrated Circuits (PICs) are usually polarization dependent. A changing polarization of the light coupled into these circuits can severely degrade their performance. On-chip
manipulation of the polarization can help to improve this and to add extra functionality based
on polarization.
The aim of this thesis is to develop a generic integration technology with polarization handling
capability. The main effort of the work focusses on extending the standard technology for PICs
with a new type of polarization converter. Furthermore a novel type of polarization splitter has
been developed that consists of a passive Mach Zehnder Interferometer and polarization converters. Thus by only adding a polarization converter, the generic platform with polarization
handling, including a polarization splitter is obtained. Moreover by the addition of a spot size
converter packaging of the PICs becomes feasible.
The polarization can be applied to add functionality. For example the performance of a wavelength converter can be optimized using the polarization. Wavelength converters are key components in optical telecommunications networks, but the available devices have several problems. Firstly they need expensive tunable wavelength filters at the output, and secondly they
are highly polarization dependent.
The application of polarization handling is demonstrated by a new type of integrated wavelength converter: POLARIS (POlarization LAbelling for Rejection and Isolation of Signals).
This wavelength converter uses the polarization of the light to label the original and the converted signals. By using a polarization splitter, the two signals can be separated and filtered.
This approach can also be used in all-optical switches. In this way tunable filters and polarization dependence are avoided.
On-chip polarization manipulation can be used in a number of other circuits as well to enable
a broad variety of functions and improvements (for example: polarization independent optical
amplifiers, on-chip polarization controller, a laser with a switchable output polarization).
159
160
Summary
To demonstrate the generic integration platform, the development and realization of polarization converters and polarization splitters, together with standard passive (waveguides, couplers)
and active (semiconductor optical amplifiers) components is needed.
The standard components are designed and a standard fabrication process is developed in
which all these components can be integrated.
Two generations of polarization converters are realized. The first device has an efficient and
short design, but it proved to be difficult to integrate it with active components. A second
generation converter is designed, fabricated and characterized. This device is well suited for
integration and has a high conversion.
Furthermore, two types of polarization splitters are demonstrated. Also these devices need to
fit in the standard fabrication. One design is a relatively long device, tolerant to fabrication
variations, but leading to complications with integration. A second design is shorter and consists only of a passive Mach Zehnder interferometer with polarization converters in the arms.
This splitter fits exactly in the integration scheme, so this is the device of choice for the generic
integration technology.
Moreover an array of Mach Zehnder Interferometers with SOAs in the arms is designed and
fabricated. This circuit can be used in wavelength converters and all-optical switches. The
device is integrated with spotsize converters to enable packaging. With the packaged device
wavelength conversion up to 40 Gb/s is demonstrated.
The POLARIS concept is demonstrated by simulations and experimentally verified. An integrated version of POLARIS is designed. The generic integrated polarization handling technology is demonstrated by realizing this circuit. The realization clearly showed that the integration
scheme is useable, because working examples of all relevant components were present on the
chip. Unfortunately due to time constraints not all processing steps were sufficiently optimized, leading to a too low yield of working components; therefore no POLARIS operation
could be shown with the integrated device.
This thesis describes the theory, design, fabrication and characterization of polarization handling components, as well as passive and active components, integrated in InP/InGaAsP. A
generic integration technology for Photonic Integrated Circuits is developed. Circuits constructed with the components of this technology can be made polarization insensitive and can
have additional functionality based on polarization.
Samenvatting
Polarisatie beheer in geı̈ntegreerde fotonische schakelingen
Geı̈ntegreerde fotonische schakelingen (PICs = Photonic Integrated Circuits) zijn meestal polarisatie afhankelijk. Een veranderende polarisatie van het licht aan de ingang van deze circuits
kan hun functionaliteit aanzienlijk laten afnemen. Flinke verbeteringen kunnen verkregen worden door de polarisatie te manipuleren op de chip. Bovendien kan de polarisatie gebruikt worden om nieuwe functies te realiseren op de chip.
Het doel van dit proefschrift is het ontwikkelen van een generieke integratie technologie met de
mogelijkheid om de polarisatie te manipuleren. Het hoofddeel van het werk is gericht op het
uitbreiden van de standaard technologie voor PICs met een nieuw type polarisatie omzetter.
Verder is een nieuw soort polarisatie splitser ontwikkeld die bestaat uit een passieve Mach
Zehnder interferometer en polarisatie omzetters. Dus door het toevoegen van de technologie voor polarisatie omzetters aan de bestaande standaard ontstaat een generiek technologie
platform voor geı̈ntegreerd polarisatie beheer, inclusief polarisatie splitsers. Door de toevoeging van een bundelgrootte-aanpasser aan deze technologie, wordt het mogelijk om glasvezels
makkelijker en nauwkeuriger uit te lijnen ten opzichte van de chips en zo de mogelijkheid te
hebben om de chips af te monteren.
De polarisatie kan gebruikt worden om extra functies toe te voegen. Zo kunnen de prestaties
van bijvoorbeeld een golflengte omzetter geoptimaliseerd worden door gebruik te maken van
polarisatie. Golflengte omzetters zijn cruciale componenten in optische telecommunicatie
netwerken, maar de huidige beschikbare componenten hebben een aantal problemen. Allereerst
is het noodzakelijk om dure verstembare filters toe te passen aan de uitgang om het originele
en het omgezette signaal te scheiden. Bovendien zijn ze erg polarisatie afhankelijk.
Het beheer van de polarisatie is aangetoond door de toepassing hiervan in een nieuw soort
geı̈ntegreerde golflengte omzetter: POLARIS (POlarization LAbelling for Rejection and Isolation of Signals). Deze golflengte omzetter benut de polarisatie van het licht om een label
161
162
Samenvatting
toe te kennen aan het originele en omgezette signaal. Met een polarisatie splitser kunnen deze
twee signalen van elkaar gescheiden worden. Op deze manier hoeven geen verstembare filters
gebruikt te worden en wordt polarisatie afhankelijkheid vermeden.
Voor de toepassing van de generieke integratie technologie, is het noodzakelijk om polarisatie
omzetters en polarisatie splitsers, en standaard passieve (golfgeleiders, koppelaars) en actieve
(optische versterkers) componenten te realiseren. De standaard componenten zijn ontworpen
en een standaard fabricage technologie is ontwikkeld waarmee deze componenten geı̈ntegreerd
kunnen worden.
Twee generaties polarisatie omzetters zijn ontwikkeld. De eerste generatie omzetter heeft een
efficiënt en kort ontwerp, maar is moeilijk te integreren met actieve componenten. Een tweede
generatie is ontworpen, gerealiseerd en gekarakteriseerd. Deze component is zeer geschikt om
te integreren en heeft een hoge efficiënte omzetting.
Verder zijn twee types polarisatie splitsers gedemonstreerd. Deze moeten ook passen in de
standaard fabricage technologie. Het eerste ontwerp is een relatief lange component, tolerant
voor fabricage variaties, maar lastig om te integreren. Een tweede ontwerp is korter, dit bestaat
alleen uit een passieve Mach Zehnder interferometer met polarisatie omzetters in de armen.
Deze component past precies in het voorgestelde integratie schema, dus dit is de component
die zich het beste leent voor een generiek integratie platform.
Bovendien is een geı̈ntegreerde reeks van Mach Zehnder interferometers met optische versterkers in de armen ontworpen en gerealiseerd. Deze schakeling kan toegepast worden als
golflengte omzetter en als volledig optische schakelaar. Het circuit is geı̈ntegreerd met bundelgrootte-aanpassers zodat het mogelijk is om de chip af te monteren. De afgemonteerde schakeling is gebruikt als golflengte omzetter en werking op 40 Gb/s is gedemonstreerd.
Het POLARIS concept is aangetoond met behulp van simulaties en experimenteel geverifiëerd.
Een geı̈ntegreerde versie van POLARIS is ontworpen. De generieke technologie voor polarisatie beheer is aangetoond met de realisatie van deze schakeling. Deze realisatie bewijst
dat het voorgestelde integratie schema bruikbaar is, want er zijn werkende voorbeelden van
alle op de chip aanwezige componenten. Helaas zijn door tijdgebrek niet alle proces stappen voldoende geoptimaliseerd. Dit heeft geleid tot een onvoldoende hoge opbrengst van
werkende componenten; hierdoor kan het POLARIS principe niet aangetoond worden met de
geı̈ntegreerde schakeling.
Dit proefschrift beschrijft de theorie, het ontwerp, de fabricage en de karakterisatie van zowel
polarisatie componenten, als passieve en actieve componenten, geı̈ntegreerd in InP/InGaAsP.
Een generieke integratie technologie voor geı̈ntegreerde fotonische schakelingen is ontwikkeld.
Schakelingen, die gebruik maken van componenten gemaakt met deze technologie, kunnen
ongevoelig voor polarisatie gemaakt worden en bovendien extra functionaliteit bezitten, gebaseerd op polarisatie.
Dankwoord
Dan is het zover, aan het eind van het schrijfwerk ben je eindelijk op het punt beland dat je aan
het dankwoord mag beginnen. Al snel loopt het over van de cliché’s, die allemaal even waar
zijn: ”promoveren kun je niet alleen” en zeker niet in een zo multidisciplinaire vakgroep als
OED. Daarom wil ik dan ook iedereen bedanken die zijn of haar steentje heeft bijgedragen aan
het tot stand komen van dit proefschrift.
Uiteraard wil ik als allereerste Meint bedanken. Het is een fijne tijd geweest bij OED, en dat is
het nog steeds! Bedankt voor je kritische blik op mijn artikelen en zeker op mijn proefschrift,
dat was voor mij erg nuttig. Ik ben blij dat ik nog een tijdje mag blijven in je groep en zo kan
meewerken aan de volgende stap.
Natuurlijk wil ik Jos enorm bedanken, niet enkel als copromotor, maar zeker als begeleider en
mentor. Met veel plezier heb ik met jou samengewerkt, en veel van je geleerd. Ik ben blij dat ik
de passie voor polarisatie met je mag delen! Bedankt voor het motiveren als een chip mislukt
was en voor het bijschaven van zo ongeveer alles wat ik de afgelopen jaren geproduceerd heb:
van ontwerpen, tot masker layouts, tot en met dit proefschrift. Bovendien was het altijd weer
gezellig om samen te reizen naar conferenties en vergaderingen van Japan tot de VS en van
Griekenland tot Noorwegen.
Prof. Roel Baets, mijn tweede promotor, van harte bedankt voor het luisteren en vragen stellen
tijdens de twee-maandelijkse sessies in Eindhoven. Deze waren steeds een nuttige aanvulling.
Verder wil ik de leden van de promotie commissie bedanken: Prof. Alfred Driessen, bedankt
voor uw opmerkingen op mijn manuscript. Prof. Anton Tijhuis, hartstikke bedankt voor uw
grondige en zeer zinvolle commentaar.
Graeme Maxwell, thanks a lot for your participation in my committee, for your comments and
questions and for the fun time we had on the MUFINS meetings. Rabah, thanks for the nice
time we had together when you were working here. Thanks for participating in the committee.
Verder wil ik op deze plaats onze cleanroom-technici bedanken. In alfabetische volgorde, dat is
163
164
Dankwoord
het eerlijkste want jullie hebben me allemaal geweldig geholpen. Barry, de OED-troubadour, je
hebt me al fantastisch geholpen met mijn gepruts met kleine stukjes GaAs voor mijn VCSELs
tijdens mijn afstuderen. En je hulp is sindsdien alleen maar toegenomen, van broom-methanol
etsen tot goud-platen en van stain-etsen tot klieven. Ik hoop dat je 2 juni je gitaar bij je hebt.
Ben, wat hebben we veel aan jou te danken! Zowel technisch, wat betreft de mooie nieuwe
machines maar zeker ook sociaal: de OED-borrels, koffiepauzes en groepsuitjes.
Erik Jan, dit proefschrift zou dun zijn zonder jouw hulp. Hartstikke bedankt voor al die uren
SEMmen en EBLen. Je hebt me hier veel over geleerd en ze zijn een belangrijk onderdeel
van mijn werk geworden. Het is prijzenswaardig hoe je je rust bewaart, zelfs als het platina
bijgevuld moet worden in een opgedroogde FC2000.
Kitty, wat is het jammer dat je weg bent! Jij was een aanwinst voor de groep, je verhelderende
kijk op onze processing heeft me echt grote stappen vooruit geholpen. Aan jou heb ik bijna
een heel hoofdstuk te danken: de schuivende maskers, lastige ICP etsen en de diepe precisieklief-etsen hebben erg mooie resultaten opgeleverd. Hartstikke bedankt!
Tjibbe, van harte bedankt voor de vele hulp. Er is geen chip te vinden waar jij niet aan hebt
meegewerkt. Van polyimide spinnen tot RIE etsen, dat heb ik allemaal van jou geleerd. Buiten
het technische deel was het altijd weer gezellig tijdens en na de borrels en kerstvieringen.
Vanzelfsprekend wil ik alle andere collega’s bedanken. Fouad en Siang, bedankt voor jullie
hulp op technologisch gebied, voor het antwoord op de vele vragen en voor de ontwikkeling
van standaard en niet-zo-standaard technologie. Xaveer, bedankt voor alle hulp op software
en modelleer gebied. Erwin bedankt voor alle hulp wat betreft metingen en karakteriseren van
mijn chips.
Hans, hartstikke bedankt voor je hulp bij zo ongeveer alles: van computers, bureaus, bestellingen, tot en met de koffie. Ik ga geen poging wagen om een opsomming te maken, die zou toch
niet toereikend zijn.
Martijn en Els, als generatiegenootjes hebben we veel samen opgetrokken. Martijn, jij hebt het
al gezegd, we hebben veel positieve maar ook frustrerende momenten meegemaakt. Bedankt
voor het luisteren en voor jullie adviezen, en natuurlijk voor de discussies over en op het werk,
maar zeker ook daarnaast.
Boudewijn, mijn kamergenoot. Jij hebt flink wat te verduren gehad het afgelopen jaar, met
die twee gestreste collega’s. Bedankt voor je begrip en succes met jouw onderzoek. Milan,
bedankt voor je gezelligheid en veel succes met je lasertjes.
Uzma and Youcai, thanks for your help and cooperation in making the polarization converters.
Fokke bedankt voor je hulp en ideeën voor de polarisatie metingen.
Bauke, Pietro, Ling, Omar, Jose, Stefano, Hugo, Frederic, and Martin, thanks for being nice
colleagues.
Susan, José en zeker Els, bedankt voor de ondersteuning op velerlei gebied.
Genia, heel erg bedankt voor de fijne tijd die we samen hebben gehad bij OED. Ik heb veel
van je geleerd en ben goed bevriend geraakt met jou. Vooral onze etentjes moeten we blijven
voortzetten.
Jan Hendrik, Mirvais, Mark, Roger, Francisco, Ronald, Yohan, Thieu, Aert, de oud-gedienden,
ik heb jullie altijd als fijne collega’s ervaren.
Dankwoord
165
Buiten onze eigen groep is er nog een aantal mensen dat ik wil bedanken. Iedereen bij voormalig JDS-Uniphase in Eindhoven, en met name Hans Binsma, bedankt voor de wafers.
I want to thank everybody at CIP for the wafers that were the basis of many of my successful experiments. Special thanks to Dave Rogers and Alistair Poustie, for your questions and
answers, and of course for the good time on the Greek islands.
Ik wil iedereen bij ASML, en met name Wim de Laat, Michel van de Moosdijk en Paul van
Dijk bedanken voor de vele hulp in het maken van de polarisatie componenten op de ASML
machines. Het was flink klussen, maar het heeft hele mooie resultaten opgeleverd!
Johan van Zantvoort, bedankt voor de hulp bij het meten met de fiber-arrays, met de technische
hulp van allerlei aard, en natuurlijk voor de gezelligheid op de borrels, barbecues en andere
gelegenheden.
Hyun-Do thanks for helping me with the dynamic measurements.
Bas, bedankt voor de fijne samenwerking, de chips die we samen hebben gemaakt hebben een
mooi lijstje publicaties opgeleverd! Het is erg plezierig om werk en vriendschap te kunnen
combineren.
Naast fijne collega’s is het cruciaal om mensen om je heen te hebben waar je te allen tijde op
kunt rekenen voor steun en zeker voor ontspanning. Nu heb ik de mogelijkheid om jullie te
bedanken, de vrienden van Praeclara, mijn familie, en uiteraard mijn vrienden in Maastricht.
Ivo en Gerard wil ik hierbij speciaal noemen. Bedankt dat jullie als mijn paranimfen willen
optreden.
In het bijzonder wil ik Gwennie bedanken, “hiel erreg bedaank veur alles, veur dien interesse,
gedöld en steun”. Ik wil afsluiten met het bedanken van mijn ouders, “pap en mam, hartstikke
bedaank veur de motivatie, begrip en höllep, zonder uuch waor dit noets gelök!”
Luc
Curriculum Vitae
Ludovicus (Luc) M. Augustin was born in Maastricht, The Netherlands, in 1978. He received
the M.Sc. degree in electrical engineering from Eindhoven University of Technology, Eindhoven, The Netherlands, in 2002.
His masters thesis work was carried out at the COBRA Research institute, Eindhoven University of Technology, on the manufacturing and characterization of polarization controlled
VCSELs.
In 2002 he started his Ph.D. research in the Opto Electronic Devices group. He was involved
in research on the integration of polarization components with active and passive structures on
InP-based material.
He is currently working as a researcher in the same group working on the optimization of the
active-passive integration technology.
167
List of publications
Journal articles
• L.M. Augustin, J.J.G.M. van der Tol, E.J. Geluk and M.K. Smit, “Short polarization converter
optimized for active-passive integration in InGaAsP/InP,” IEEE Photon. Technol. Lett., vol. 19,
no. 20, pp. 1673-1675, Oct. 2007.
• L.M. Augustin, R. Hanfoug, J.J.G.M. van der Tol, W.J.M. de Laat, and M.K. Smit, “A Compact
Integrated Polarization Splitter/Converter in InGaAsP/InP,” IEEE Photon. Technol. Lett., vol. 19,
no. 17, pp. 1286-1288, Sep. 2007.
• B. Huiszoon, L.M. Augustin, E.A.J.M. Bente, H. de Waardt, G.D. Khoe, M.K. Smit, and A.M.J.
Koonen, “Integrated Mach-Zehnder based spectral amplitude OCDMA on a passive optical network,” IEEE J. Sel. Topics in Quantum Electron., vol. 13, no. 5, pp. 1487-1496, Sep./Oct. 2007.
• B. Huiszoon, L.M. Augustin, R. Hanfoug, L. Bakker, M. Sander-Jochem, E. Fledderus, G.D. Khoe,
J.J.G.M. van der Tol, H. de Waardt, M.K. Smit, and A.M.J. Koonen, “Integrated parallel spectral
OCDMA en/decoder,” IEEE Photon. Technol. Lett., vol. 19, no. 7, pp. 528–530, Apr. 2007.
• L.M. Augustin, J.J.G.M. van der Tol, R. Hanfoug, W.J.M. de Laat,M.J.E. van de Moosdijk,
P.W.L. van Dijk, Y.S. Oei, and M.K. Smit, “A single etch-step fabrication-tolerant polarization
splitter,” J. Lightwave Technol., vol. 25, no. 3, pp. 740–746, Mar. 2007.
• R. Hanfoug, L.M. Augustin, Y. Barbarin, J.J.G.M. van der Tol, E. Bente, F. Karouta, D. Rogers,
S. Cole, Y. Oei, X. Leijtens, and M.K. Smit, “Reduced reflections from multimode interference
couplers,” Electron. Lett., vol. 42, pp. 465–466, Apr. 2006.
Conference contributions
International conferences
• L.M. Augustin, J.J.G.M. van der Tol, E.J. Geluk, Y.S Oei, and M.K. Smit, “Method for polarization
effect suppression in semiconductor optical amplifiers,” accepted for Proc. 14th Eur. Conf. on Int.
Opt. (ECIO ’08), Eindhoven, The Netherlands, Jun. 2008.
169
170
List of publications
• J.J.G.M. van der Tol, L.M. Augustin, A.A.M. Kok, U. Khalique, and M.K. Smit, “Use of polarization in InP-based integrated optics,” in 2008 Conference on Lasers and Electro-Optics (CLEO),
San Jose, USA, May 2008. Invited paper.
• B. Huiszoon, A. Leinse, D.H. Geuzebroek, L.M. Augustin, E.J. Klein, H. de Waardt, G.-D. Khoe,
and A.M.J. Koonen, “Multi-stage cascade and tree en/decoders integrated in Si3 N4 -SiO2 for spectral amplitude OCDMA on PON,” in Techn. Digest Opt. Fiber Comm. (OFC ’08), p. OMR4, San
Diego, USA, Feb. 2008.
• J.J.G.M. van der Tol, L.M. Augustin, U. Khalique, and M.K. Smit, “Polarization control and its
application to waveguide devices,” in Proc. 13th Micro Optic Conf. (MOC ’07), p. C1, Takamatsu,
Japan, Oct. 28-31 2007. Invited paper.
• L.M. Augustin, J.J.G.M. van der Tol, and M.K. Smit, “A compact passive polarization converter
for active-passive integration on InP/InGaAsP,” in Proc. 13th Eur. Conf. on Int. Opt. (ECIO ’07),
p. WA3, Copenhagen, Denmark, Apr. 25–27 2007.
• L.M. Augustin, J.J.G.M. van der Tol, R. Hanfoug, W.J.M. de Laat, and M.K. Smit, “A short integrated polarization splitter/converter on InP/InGaAsP,” in Proc. Int. Symposium on Contemporary
Photonics Technology, pp. 101–102, Tokyo, Japan, Jan. 2007.
• J.J.G.M. van der Tol, U. Khalique, L.M. Augustin, and M.K. Smit, “Using polarization for optoelectronic integrated devices,” in Proc. Int. Symposium on Contemporary Photonics Technology,
pp. 57–60, Tokyo, Japan, Jan. 2007. Invited paper.
• B. Huiszoon, M.K. Smit, A.M.J. Koonen, L.M. Augustin, R. Hanfoug, L. Bakker, M. Sander Jochem, E. Fledderus, G.D. Khoe, J.J.G.M. van der Tol, and H. de Waardt, “Novel building block
for multiple encoding and decoding in spectral amplitude encoded OCDMA,” in Proc. IEEE/LEOS
Annual Meeting (LEOS ’06), pp. 905–906, Montreal, Canada, Oct. 29 – Nov. 2 2006.
• L.M. Augustin, J.J.G.M. van der Tol, R. Hanfoug, W.J.M. de Laat, and M.K. Smit, “Design and
fabrication of a single etch-step polarization splitter on InP/InGaAsP with increased width tolerance,” in Technical Digest Integr. Photon. Res. and Apps. (IPRA ’06), p. ITuG4, Uncasville, USA,
Apr. 24–Apr. 28 2006.
• U. Khalique, Y. Zhu, J.J.G.M. van der Tol, L.M. Augustin, R. Hanfoug, F. Groen, P. van Veldhoven, M.K. Smit, M. van de Moosdijk, W.J.M. de Laat, and K. Simon, “Ultrashort polarization
converter on InP/InGaAsP fabricated by optical lithography,” in Technical Digest Integr. Photon.
Res. and Apps. (IPRA ’05), p. IWA3, San Diego, USA, Apr. 11–Apr. 13 2005.
• L.M. Augustin, J.J.G.M. van der Tol, R. Hanfoug, and M.K. Smit, “Design of a single etchstep
fabrication-tolerant polarisation splitter,” in Proc. 12th Eur. Conf. on Int. Opt. (ECIO ’05), pp. 125–
128, Grenoble, France, April 6–8 2005.
• L.M. Augustin, R. Hanfoug, J.J.G.M. van der Tol, J. Binsma, Y.S. Oei, and M.K. Smit, “Polarisation based isolation in wavelength converter,” in Proc. 30th Eur. Conf. on Opt. Comm. (ECOC ’04),
p. We4.P.072, Stockholm, Sweden, Sep. 5–9 2004.
• R. Hanfoug, L.M. Augustin, J.J.G.M. van der Tol, R.G. Broeke, and M.K. Smit, “Optical bandwidth of Mach-Zehnder interferometer wavelength converters,” in Technical Digest Integr. Photon.
Res. (IPR ’04), p. JWB19, San Fransisco, USA, Jun. 30–Jul. 4 2004.
• R. Hanfoug, J.J.G.M. van der Tol, L.M. Augustin, and M.K. Smit, “Wavelength conversion with
polarisation labelling for rejection and isolation of signals (POLARIS),” in Proc. 11th Eur. Conf.
on Int. Opt. (ECIO ’03), pp. 105–108, Prague, Czech Republic, Apr. 2–4 2003.
List of publications
171
Local conferences
• L.M. Augustin, J.J.G.M. van der Tol, M.J.H. Sander-Jochem, R. Hanfoug, F. Karouta, H.D. Jung,
D. Rogers, and M.K. Smit, “Monolithically integrated SOA-MZI array in InP/InGaAsP suited for
flip-chip packaging,” in Proc. IEEE/LEOS Symposium (Benelux Chapter), pp. 75–78, Brussels,
Belgium, Dec. 2007.
• L.M. Augustin, J.J.G.M. van der Tol, E.J. Geluk, and M.K. Smit, “Short 2 × 2 polarization splitter
in InP/InGaAsP using polarization converters,” in Proc. IEEE/LEOS Symposium (Benelux Chapter), pp. 71–74, Brussels, Belgium, Dec. 2007.
• B. Huiszoon, A. Leinse, D.H. Geuzebroek, L.M. Augustin, E.J. Klein, H. de Waardt, G.D. Khoe,
and A.M.J. Koonen, “Multi-stage en/decoders integrated in low loss Si3 N4 -SiO2 for incoherent spectral amplitude OCDMA on PON,”in Proc. IEEE/LEOS Symposium (Benelux Chapter),
pp. 159–162, Brussels, Belgium, Dec. 2007.
• L.M. Augustin, J.J.G.M. van der Tol, and M.K. Smit, “Polarization based filtering in a wavelength
converter,” in Proc. IEEE/LEOS Benelux Workshop on Progress in Optical Devices and Materials,
pp. 9–10, Eindhoven, The Netherlands, May 2007.
• L.M. Augustin, J.J.G.M. van der Tol, R. Hanfoug, W.J.M. de Laat, and M.K. Smit, “An integrated
polarization splitter and converter,” in Proc. IEEE/LEOS Symposium (Benelux Chapter), pp. 89–
92, Eindhoven, The Netherlands, Dec. 2006.
• R. Hanfoug, M.K. Smit, L.M. Augustin, Y. Barbarin, J.J.G.M. van der Tol, E. Bente, F. Karouta,
D. Rogers, Y.S. Oei, and X. Leijtens, “A multimode interference coupler with low reflections,” in
Proc. IEEE/LEOS Symposium (Benelux Chapter), pp. 97–100, Mons, Belgium, Dec. 2005.
• U. Khalique, Y.C. Zhu, J.J.G.M. van der Tol, L.M. Augustin, R. Hanfoug, F. Groen, M. van de
Moosdijk, W.J.M. de Laat, K. Simon, P. van Veldhoven, and M.K. Smit, “Polarization converter on
InP/InGaAsP fabricated with optical reduction wafer stepper,” in Proc. IEEE/LEOS Symposium
(Benelux Chapter), pp. 131–134, Ghent, Belgium, Dec. 2004.
• L.M. Augustin, J.J.G.M. van der Tol, R. Hanfoug, and M.K. Smit, “Improved tolerance in polarisation splitters,” in Proc. IEEE/LEOS Symposium (Benelux Chapter), pp. 123–126, Gent, Belgium,
Dec. 2004.
• R. Hanfoug, L.M. Augustin, J.J.G.M. van der Tol, R. Broeke, and M.K. Smit, “Static extinction
ratio bandwidth of Mach-Zehnder interferometer wavelength converters,” in Proc. IEEE/LEOS
Symposium (Benelux Chapter), pp. 73–76, Enschede, The Netherlands, Nov. 2003.
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