Integrated tunable quantum-dot laser for optical coherence

Integrated tunable quantum-dot laser for optical coherence
Integrated tunable quantum-dot laser for optical
coherence tomography in the 1.7µm wavelength region
Tilma, B.W.
DOI:
10.6100/IR712655
Published: 01/01/2011
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Citation for published version (APA):
Tilma, B. W. (2011). Integrated tunable quantum-dot laser for optical coherence tomography in the 1.7µm
wavelength region Eindhoven: Technische Universiteit Eindhoven DOI: 10.6100/IR712655
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Integrated tunable quantum-dot laser
for optical coherence tomography
in the 1.7 µm wavelength region
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 dinsdag 28 juni 2011 om 16.00 uur
door
Bonifatius Wilhelmus Tilma
geboren te Amsterdam
Dit proefschrift is goedgekeurd door de promotor:
prof.dr.ir. M.K. Smit
Copromotor:
dr. E.A.J.M. Bente
The research presented in this thesis was supported by the IOP Photonic Devices program
managed by Agentschap NL (Ministerie van Economische Zaken, Landbouw en Innovatie),
Technologiestichting STW and the NRC photonics grant.
Copyright © 2011 Bonifatius Wilhelmus Tilma
Printed in The Netherlands
A catalogue record is available from the Eindhoven University of Technology Library
Tilma, Bonifatius Wilhelmus
Integrated tunable quantum-dot laser for optical coherence tomography in the 1.7µm
wavelength region / by Bonifatius Wilhelmus Tilma. –Eindhoven : Technische Universiteit
Eindhoven, 2011.
Proefschrift. – ISBN: 978-90-386-2499-0
NUR 959
Trefw.: halfgeleiderlasers / geïntegreerde optica / 3-5 verbindingen / afstembare laser /
quantum-dots
Subject headings: semiconductor lasers / integrated optics / III-V semiconductors / tunable
laser / quantum-dots
Jen dy’t fan wysheid hâldt makket syn heit bliid.
Spr. 29:3a
Contents
Contents......................................................................................................... 5 1 Introduction ........................................................................................... 9 1.1 Optical coherence tomography ................................................................ 10 1.1.1 Requirements on tunable laser for FD-OCT....................................12 1.2 Approach to the laser design ................................................................... 14 1.2.1 Laser simulation ..............................................................................15 1.3 Thesis Outline.......................................................................................... 17 2 Integration technology ........................................................................ 19 2.1 Introduction ............................................................................................. 19 2.2 Fabrication Technology ........................................................................... 21 2.2.1 Active-passive layerstack ................................................................21 2.2.2 Active-passive processing scheme ..................................................22 2.3 Specific processing used in this thesis..................................................... 26 2.3.1 Waveguide structure at 1700nm ......................................................26 2.3.2 Top buffer layer ...............................................................................28 2.3.3 Doping passive waveguides ............................................................28 2.3.4 InP etch stop layer in the film layer.................................................29 2.3.5 Planarization and etch back Polyimide............................................29 2.3.6 Metallization ....................................................................................30 2.4 Chip mounting for measurement ............................................................. 30 3 Measurement and analysis of the gain in quantum-dot amplifiers 33 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Introduction ............................................................................................. 33 Gain measurement method, device design and fabrication ..................... 35 Gain measurement results ....................................................................... 37 Quantum-dot amplifier gain model ......................................................... 40 Simulations .............................................................................................. 46 Comparison with measurements.............................................................. 49 Conclusion ............................................................................................... 50 5
4 Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region51 4.1 Introduction ............................................................................................ 51 4.2 Tuning principle ..................................................................................... 54 4.3 Design ..................................................................................................... 56 4.4 Layerstack and waveguide design .......................................................... 57 4.4.1 High resolution filter....................................................................... 58 4.4.2 Low resolution filter ....................................................................... 59 4.4.3 Mask layout .................................................................................... 62 4.5 Fabrication .............................................................................................. 63 4.6 Measurements ......................................................................................... 64 4.6.1 Static filter characteristics............................................................... 65 4.6.2 Calibration method of the phase modulators .................................. 66 4.6.3 Calibration HR-filter ....................................................................... 68 4.6.4 Calibration LR-filter ....................................................................... 70 4.7 Tuning results ......................................................................................... 71 4.7.1 HR-filter.......................................................................................... 72 4.7.2 LR-filter .......................................................................................... 73 4.7.3 Filter Tuning speed ......................................................................... 74 4.8 Conclusion .............................................................................................. 75 5 Tunable quantum-dot ring laser simulations ...................................77 5.1 Introduction ............................................................................................ 77 5.2 Laser model ............................................................................................ 78 5.2.1 QD amplifier rate equation model .................................................. 78 5.2.2 Segmented ring laser model............................................................ 80 5.3 Simulations ............................................................................................. 82 5.3.1 Simulation 1: complete output spectrum ........................................ 83 5.3.2 Simulation 2: 1nm around laser peak (start-up).............................. 85 5.3.3 Simulation 3: 1nm around laser peak (0.2nm tuning)..................... 87 5.4 Conclusion .............................................................................................. 88 6
6 Integrated tunable quantum-dot laser in the 1.7 µm wavelength
region.................................................................................................... 91 6.1 Introduction ............................................................................................. 91 6.2 Design and fabrication............................................................................. 93 6.3 Laser characterization.............................................................................. 95 6.3.1 Laser tuning – influence of the gain spectrum ................................97 6.3.2 Laser tuning – influence of the cavity mode structure ..................100 6.3.3 Laser tuning speed .........................................................................102 6.4 Laser coherence length and effective linewidth .................................... 103 6.5 Conclusion ............................................................................................. 106 6.5.1 Tuning bandwidth..........................................................................107 6.5.2 Laser linewidth ..............................................................................108 6.5.3 Scan rate ........................................................................................108 6.5.4 Output power .................................................................................109 6.5.5 Overall conclusion .........................................................................109 References ................................................................................................. 111 List of abbreviations................................................................................. 121 Summary ................................................................................................... 123 List of publications ................................................................................... 125 International Journals ....................................................................................... 125 International Conferences ................................................................................ 125 Local Conferences ............................................................................................ 127 Acknowledgement .................................................................................... 129 Curriculum vitae ...................................................................................... 133 7
1 Introduction
The control of mankind on light sources has had large influences on human
development. Two significant technology breakthroughs in history are the invention of the
light bulb in the 19th century and the experimental proof of the laser in 1960 by Theodore
Maiman [49]. The control of emission characteristics of light sources has its impact on
many developed technologies such as the use of lasers in telecommunications, surgery and
military applications as well as other more end-user technologies such as the barcode
scanner, the optical disc reader and the laser printer.
Well-controlled light sources have also been introduced in a variety of measurement
techniques owing to the often extremely precise measurement results that can be achieved
[71]. Examples of measurement techniques in which light sources are already used range
from geographical measurement techniques such as airborne laser relief scanning [27],
structure health monitoring for example with fiber Bragg sensing [38], and different
medical measurement techniques such as optical coherence tomography (OCT) in
ophthalmology [23] spectral imaging in dermatology and hematology [86] and Sidestream
Dark Field Imaging in cardiovascular applications [30]. The required optical properties and
the related choice of light sources are different for each of the measurement techniques.
One of these measurements techniques, Optical Coherence Tomography (OCT), is an
imaging technique where cross-section images can be obtained at micrometer scale
resolution. The cross section images can be reconstructed from the phase and intensity
information in backscattered light from within a sample. This information can be retrieved
in an interferometer setup where the light scattered by different materials in the sample
interferes with light from a reference path. One of the major advantages of OCT is that it is
particularly useful for imaging of biological tissues with a resolution of several
micrometers. Images can be obtained in vivo without physical contact. In ophthalmology
this is a very useful property. It allows for obtaining detailed images of the retina, the
light-sensitive inner backside of the eye, and the anterior segment (front side of the eye).
Since useful diagnostics can be made from such OCT images, the technique is used often in
ophthalmology [72]. To image the retinal structure the wavelength ranges around 800nm
and 1050nm light are often used. Typically 1300nm is used to image the anterior segment.
The imaging depth into the tissue that can be achieved using OCT is generally dependent
on the scattering coefficient. Absorption typically tends to be less important than scattering
in biological samples. The scattering coefficient decreases in most cases with increasing
wavelength. This increase in imaging depth with increasing wavelength is however
interrupted due to the strong water absorption in the two water absorption bands between
9
Chapter 1
1400nm and 1500nm and between 1900nm and 2200nm. The spectral range between
1600nm and 1800nm is however a promising wavelength range for OCT which can be used
for deeper imaging in human tissue. The relation between wavelength and imaging depth
and the necessary spectral width needed for a 10µm resolution is given in Fig. 1-1 [22]. A
quantitative comparison of the OCT imaging depth between 1300nm and 1600nm was
investigated and presented in [40].
Fig. 1-1 The calculated imaging depth (solid curve) and bandwidth requirement (dashed curve) on the
OCT light source to maintain a resolution of 10µm as a function of wavelength. The image is
calculated for typical parameters of an OCT system, the scattering coefficient and the water
absorption spectrum. The imaging depth is defined as the depth at which the contributions of the
single and multiple scattering to the OCT signal are equal. (Figure from [22])
In this introduction chapter, an introduction to the different OCT measurement
techniques is first presented. Given a particular choice of OCT type, the requirements on
the light source are determined from the requirements on the image and the imaging setup.
In section 1.2 we discuss the general approach of the realization of a tunable laser for a
frequency domain OCT setup. The tunable laser is realized using tunable intra-cavity
filters. The requirements on the filters are determined in this section from a simple time
domain laser model. Finally an outline of the thesis is presented.
1.1 Optical coherence tomography
OCT is an interferometric three dimensional imaging technique often used in
ophthalmology. The measurement technique relies on the analysis of back-scattered light
from a tissue interfered with light from a reference arm. The basic scheme of an OCT
measurement setup is given in Fig. 1-2. The choice of light source, light detector and the
10
Introduction
interference setup is dependent on the type of OCT measurement. The main OCT types are
time domain OCT and frequency domain OCT. Both types will be briefly discussed.
Detector
Light source
Tissue
Reference mirror
Fig. 1-2 Basic measurement scheme of an OCT setup. The choice of light source, detector and the
details on the interference path depends on the type of OCT.
Time domain OCT
In a time domain (TD) OCT measurement, also known as low coherence interferometry, a
broadband light source is used to illuminate the tissue. This broadband light source has a
short coherence length that is typically in the order of a few to ten microns. The light
reflected from the tissue will only interfere constructively with the light coming from the
reference arm if the difference in path length is within the coherence length of the
broadband light source. An increase in signal intensity is measured at the detector in that
case. By changing the path length in the reference arm the depth in the tissue from which
the reflection intensity is detected can be selected. The depth image can be measured
directly by measuring the interfered light as a function of arm length difference. A major
drawback of this OCT technique is the physical arm length change necessary in the setup.
This change in path length can for example be introduced with a moving mirror. In most
cases the problem with such a mechanical moving mirror is that the scan speed seriously
limits the speed of the overall measurement.
Frequency domain OCT
In frequency domain (FD) OCT the depth image is reconstructed from measuring a
spectrally resolved interferometer signal. This can either be executed with a wavelength
scanning light source and recording the intensity of the individual reflected wavelength
components in time, which is the solution proposed in this thesis, or by illuminating the
tissue with a broadband light source and measuring the intensity of the individual
wavelength components with a spectrometer. In the latter case, the spectrometer can either
be a monochromator with a single photo-detector or a dispersive element, typically a
grating, which distributes the different wavelengths on to a detector array. Solutions based
11
Chapter 1
on detector array are much preferred since it is a more efficient detection setup by two to
three orders of magnitude. In either case, the depth image can be reconstructed by Fourier
transforming the (spectral) wavelength components to the time or space domain.
The first advantage of the measurement technique with the tunable laser is that no light
is lost in a spectrometer system so less power can be used or the signal intensity is higher.
The second advantage is that the signal level on the detector is much higher since all power
is at one wavelength. The signal from the detector is integrated only over the time that the
laser is at one particular wavelength. This means that the contribution from noise in the
detector is lowered significantly. More information about the different OCT techniques can
be found in [22].
The preferred option for FD-OCT is therefore to use a narrow linewidth laser source
for the light source and use a single photo-detector. An additional advantage of using a
tunable laser is that the measurement can benefit from the often relatively narrow linewidth
and therefore long coherence length of these lasers, the importance of which will be
discussed in the next section. The design and construction of a suitable tunable laser is
however far from trivial. Currently setups using bulk optics are used in most cases to realise
the tunable laser source.
1.1.1 Requirements on tunable laser for FD-OCT
Within this thesis the realization of a tunable laser is presented for a frequency domain
OCT system. The requirements on such a laser are imposed by the requirements on the
imaging performance of the OCT system. The following requirements on the imaging
system are given: 2.5mm imaging depth, 10µm imaging depth resolution and an
approximately 1 second measurement time for a 200 by 200 pixels image. Each of these
requirements imposes its requirements on the tunable laser system:

2.5mm imaging depth: The imaging depth that can be achieved in the OCT
measurement is dependent on the scattering coefficient and the absorption coefficient
in the tissue given a specific detection limit for the light detector. As presented above,
the wavelength range between 1600nm and 1800nm is promising for imaging
approximately 2.5mm deep into typical biological tissue. For the laser this means a
tuning bandwidth somewhere in this 1600nm to 1800nm range. For imaging at 2.5mm
depth a certain coherence length of the laser is necessary to be able to resolve the
image deeper into the tissue. For an image depth of 2.5mm the coherence length Lcoh
must be at least 5mm times the refractive index of the material (n≈1.3 for typical
human tissues, which have high water content [31]). This implies a coherence length
in vacuum of approximately 6.5-7mm. The coherence length of a laser is directly
dependent on the linewidth of the laser. For a tunable laser an effective linewidth can
be defined which is the full-with-half-maximum (FWHM) of the laser peak in the
output spectrum. The necessary effective linewidth of the laser can be calculated with:
12
Introduction



Lcoh=λ2/2πδλ where δλ is the effective linewidth. The necessary linewidth to obtain
6.5mm coherence length in vacuum is δλ<0.07nm.
10µm image depth resolution: The resolution of the reconstructed image in depth is
inversely proportional to the tuning bandwidth. The image resolution can be calculated
(in vacuum) to be: ΔL=0.44λ02/Δλspan [22]. Here ΔL is the image depth resolution (in
vacuum), λ0 the central wavelength of the tuning band and Δλspan the total tuning
bandwidth. Depending on the refractive index of the tissue the image resolution can be
calculated accordingly. For an image resolution of 10µm at a wavelength around
1700nm this means in typical tissues a necessary bandwidth of approximately 100nm,
as can be seen in Fig. 1-1.
Reflected signal level for reconstruction: The optical signal reflected from the tissue
should be above a certain level to discriminate the signal from the noise floor of the
detector. This reflected power is directly dependent on the output power of the light
source. It is experimentally estimated at the Academic Medical Center (AMC) of the
University of Amsterdam that the output power of the light source should be at least in
the 1-10mW range to be able to reconstruct an image from a typical biological tissue.
Variations in the output power of the light source as a function of wavelength can be
compensated for in the analysis as long as the variation is reproducible.
Image scan rate: The measurement time necessary for a single depth profile (this is
called an A-scan) is given by the time for a single wavelength scan. The time needed
for a single scan is dependent on the scan speed and the scanning bandwidth. In case
of a 100nm bandwidth, 1000 data points have to be measured to get to 10µm
resolution in 2.5mm depth (see image dept and image dept resolution above).
Therefore the sampling has to be each 0.1nm. In ophthalmology 3D images need to be
recorded. For patient comfort and the minimization of movement artifacts [39] these
need to be recorded as fast as possible. For example, with a scan rate of at least 20kHz
up to 50kHz an image of approximately 200 by 200 pixels can be built up in the time
span of 2 seconds down to 0.8 second. This means a sampling rate of at least 20MHz
(50ns samples) for the measurement of 1000 data points in a single scan of 50µs, i.e. a
scan speed of at least 2nm/µs. The exact necessary scan rate obviously depends on the
preferred number of pixels (this is called the B-scans) in the image. For real-time
imaging the calculation time also plays a role but this is not taken into account here.
To summarize the requirements on a suitable tunable laser for OCT imaging with a 2.5mm
image depth should satisfy the following:
a) Tunable in the 1600nm to 1800nm wavelength region.
b) Effective linewidth less than 0.07nm
c) Tunable over at least 100nm
d) Output power at least 1-10mW
e) Scan rate more than 20kHz which means 50ns sampling.
13
Chapter 1
1.2 Approach to the laser design
A possible approach for the realization of a tunable laser which fulfils the
requirements stated above is the integration of the laser on a single semiconductor chip.
This is possible due to the recent development of new quantum-dot (QD) gain material in
the 1700nm region with a wide gain spectrum [54]. This gain material can be implemented
in the active-passive indium-phosphide (InP) integration technology used at the
Inter-University Research School on Communication Technologies Basic Research and
Applications (COBRA) [7][60], together with electro-optically tunable wavelength
selective devices [47]. In this thesis a tunable laser is presented based on this active passive
integration technology in combination with the specially designed QD. A schematic picture
of the proposed laser design is given in Fig. 1-3. The design is based on a ring laser built up
from a QD amplifier to generate and amplify the light in the cavity and an intra-cavity
tunable wavelength filter to select the wavelength in the cavity. Part of the light is coupled
out of the ring to the output.
Tunable λ-filter
QD-amplifier
output
output
Fig. 1-3 Schematic picture of the proposed monolithically integrated tunable laser. The ring laser
basically consists of a quantum-dot amplifier to generate and amplify the light in the cavity and an
intra-cavity tunable wavelength filter to select the wavelength in the cavity. Part of the light is
coupled out of the ring to the output.
A ring laser topology is chosen above a linear laser topology. A linear laser has the
disadvantage that the light that passes through a tunable filter at the passband wavelength,
is reflected on a cleaved facet and returns through the same tunable filter again before it
returns to the amplifier again. This light can however be attenuated with more than 25dB
through losses in the filters and in reflection from the facet. Using amplifiers after the filter
and before the cleaved facet will decrease this problem. In a ring laser however the light
passes alternately through the amplifier and the filter. In principle the round trip path length
in the ring laser can be twice as short as that in the linear laser which reduces the round trip
time and therefore the switching speed of the laser because of the shorter feedback time.
Other minor advantages of a ring laser are the arbitrary choice of output coupling and the
design is independent of the location of the cleaved facets of the chip. A possible
14
Introduction
disadvantage of a ring laser is the initial bidirectional operation which reduces the output
power from one direction. To overcome this disadvantage the ring can be forced to operate
unidirectional, for example by means of a feedback loop [81] that reflects a small part of
the output light back into the ring cavity [61]. This feedback loop will further be discussed
in the chapters 5 and 6.
In the ring laser spontaneous emitted light is first generated in the QD amplifier over a
large (100-200nm) wavelength region (around 1700nm). This light travels through the ring
in both directions and is filtered by the tunable wavelength filter. After one roundtrip the
light which is attenuated by the waveguide losses and the filter is amplified again in the QD
amplifier. Due to the passband filtering in the tunable filter the light at the passband
wavelength will be stronger than the other spectral components. This effect will be
amplified each roundtrip. Due to the mode competition above laser threshold the laser peak
will be even narrowed further than the passband characteristics of the filter. Changing the
central wavelength of the passband of the tunable filter results in a suppression of the
existing laser peak and the appearance of another laser peak at the new passband
wavelength. The switching speed between two passband wavelengths is dependent on the
suppression of the initial laser peak and on the amount of small signal gain necessary to
amplify the new laser wavelength.
1.2.1 Laser simulation
In order to obtain a basic understanding of the operation of the laser and, in particular,
to determine initial requirements for the filter characteristics, a simple time-dependent
multimode laser model was developed. This model is based on simple rate equations for the
average carrier concentration in the amplifier and the cavity average photon densities of
five laser modes. The longitudinal mode spacing was assumed to be 0.05nm, based on
expectations of a 16mm long laser cavity. Modes adjacent to the target wavelength should
be sufficiently suppressed within the required 50ns after switching of the tunable filter. This
results in a 50ns switching and a linewidth of less than 0.1nm as required for the OCT
application discussed above. In this model the parameters for the amplifier structure were
that of a bulk amplifier that could supply sufficient gain. This was necessary since at that
time no data were available on the quantum-dot amplifiers. The losses for the different
modes were calculated separately, based on typical values for arrayed waveguide grating
(AWG) type of filters which can be used for the intra-cavity filtering. The continuous wave
(CW) and dynamical behavior of the laser can then be simulated for various filter
properties. From the CW state simulations one can find a set of minimum required loss
values for the unwanted laser modes. When the passband of the filter is scanned the
situation becomes more involved. As the filter is scanned the laser must change from one
mode to the next. To study the speed at which the laser mode intensities can follow the
filter tuning, one also has to consider the relative losses of the modes, the cavity length, the
laser pump level, overall cavity losses and the spontaneous emission intensity. The
15
Chapter 1
multimode dynamics in the laser and the semiconductor optical amplifier (SOA) and the
limits on the switching speed are further investigated in chapter 5. Two results from the
initial simplified simulations covering five consecutive laser modes (wl1 to wl5) are given
in Fig. 1-4 to illustrate the main issues on the wavelength scanning. On the left hand side
the evaluation of the output power is shown when the laser starts up and reaches the CW
state. All modes are building up, but the mode with the smallest loss in the filter (wl3) will
win the competition and suppress the other modes. On the right hand side the evaluation of
the output power is depicted when the laser is already on and the filter is switched from wl3
to wl2. In this case wl3 will decrease and wl2 will increase. In this simulation the tuning of
the filter is simulated by changing the wavelength dependent losses in the filter comparable
to the change in losses when a pass-band of a filter is spectrally shifted. In these figures we
immediately see that the laser can switch from one wavelength to another within 50ns. Here
we choose to have a filter with a loss difference of 0.06dB in the electric field (0.12dB in
power) at 0.05nm from the center wavelength. For a typical parabolic filter shape of an
AWG this means a filter with a 0.5nm FWHM (Power).
Fig. 1-4 Simulated output power levels for the different laser modes. (a) The laser starts up with the
filter at its center wavelength at wl3. Note that the modes at wl1 and wl5 overlap as well as the modes
at wl2 and wl4. (b)The evolution of the modes when the filter switches from wl3 to wl2 at t=0. The
legend gives the relative loss values for the different modes. The vertical line marks the 50ns
switching time.
The minimum requirement on the tunable filter is that it needs to have a filter shape
with a FWHM less than 0.5nm, assuming a parabolic filter shape, and a free spectral range
of more than 200nm to suppress all other wavelengths in the neighboring 100nm.
16
Introduction
1.3 Thesis Outline
The further research on the monolithically integrated tunable laser is described in this
thesis and is organized as follows:

Chapter 2 is concerned with the active-passive fabrication technology used to realize
the monolithic integrated devices presented in this thesis. The use of standard
active-passive integration technology of COBRA in the 1700nm wavelength region is
discussed, as well as its implications for the passive waveguide performance
requirements. The chapter further contains information on the special chip mounting
and wire bonding necessary to connect the chip to the electronics and perform
measurements.

Chapter 3 is concerned with a study into the performance of the QD amplifiers. A
series of QD amplifiers have been fabricated from which the small signal gain was
measured. This chapter contains the description of the measurement method and the
results obtained for the small signal gain as a function of current density. The results
of the fitting of a QD rate equation model to the measured gain spectra are then
presented. The change in gain spectrum with increasing injection current density is
explained and discussed.

Chapter 4 contains a description of the design, fabrication and characterization of the
tunable filters necessary for the tunable laser. A combination of two filters fulfils the
required filter characteristics as stated above. These filters can be tuned in the 1600nm
to 1800nm region with a series of electro-optic phase modulators. The characterization
of the filters is described. First the calibration of the filters and the integrated phase
modulators is discussed, followed by the results obtained for the characteristics and
tuning of the filters.

Chapter 5 contains a description of a model of the complete tunable laser as designed.
Simulations are performed to explore the laser capabilities such as the laser linewidth
and the switching speed. The model is based on the QD rate equation model discussed
in chapter 3. This model is now embedded in a segmented ring laser model.

Chapter 6 consists of a presentation of the full design and characterization of the
monolithically integrated tunable laser. This chapter is builds upon the results
presented in chapter 3 and 4. The full tuning capabilities of the laser are presented, as
well as a detailed conclusion on the performance and the improvements which can be
made. The chapter ends with a demonstration of the laser in a free space Michelson
interferometer setup, the first step towards an OCT measurement.
17
Chapter 1
The research presented in this thesis was supported by the IOP Photonic Devices
program managed by Agentschap NL (Ministerie van Economische Zaken, Landbouw en
Innovatie), Technologiestichting STW and the NRC photonics grant.
18
2 Integration technology
Abstract – In this chapter the active-passive integration technology used to fabricate
the different devices presented in this thesis is discussed. This integration technology based
on a generic integrated technology used at COBRA is slightly adapted for the work
presented in this thesis. The special design and fabrication considerations made are
discussed. The chapter ends with a description of the mounting and bonding of the chip
necessary to perform measurements on the chip.
2.1 Introduction
The integration of optical devices on semiconductor material has some major
advantages above the use of bulk optics, especially if multiple components can be
integrated together on a single chip. The integration can often benefit from advantages such
as: compactness, less power consumption, faster, robust, potentially cheaper, and in some
cases compatible with electronic integration technologies.
There are however many different integration approaches, using different material
systems and even more different fabrication technologies. Some of the main material
systems are Silicon (Si) [37], Gallium- Arsenide (GaAs) and Indium- Phosphide (InP) [78].
Each of these material systems has its own capabilities.
For a light source in the 1600nm to 1800nm wavelength region InP would be the first
choice. InP has already been used the last couple of decades for the integration of light
sources in the 1550nm telecom wavelength region. An InGaAsP layer can be grown lattice
matched on InP resulting in a direct bandgap material with an emission wavelength in the
1550nm wavelength region. This emission wavelength in lattice matched “bulk” InGaAsP
is however limited to approximately 1670nm [26]. Recent breakthroughs in new types of
gain materials, the so called Quantum Well (QW) gain material and the Quantum-dot (QD)
gain material [54] make it possible to extend the wavelength region in InP/InGaAsP
towards the 1600nm to 1800nm wavelength region. For quantum wells this can be done by
the growth of thin layers of non lattice matched (strained) material in the InGaAsP film
layer. A proper choice of material and growth conditions allows for fabrication of InAs
quantum wells that can emit light with wavelengths up to 2300nm [58]. Quantum-dots on
the other hand can also be grown in the InGaAsP film layer. These quantum-dots are grown
by metal-organic vapour-phase epitaxy (MOVPE) using self-organized strain induced
island formation. The size of these dots, and so also the corresponding emission
19
Chapter 2
wavelength, can be tuned potentially up to emit light at 2000nm wavelength due to the
recent development of a method to control the sizes of the quantum-dots through the
insertion of ultrathin GaAs interlayers beneath the quantum-dots [54]. These quantum-dots
have already been used to fabricate lasers in the 1500nm to 1600nm wavelength region [2].
Both quantum wells and quantum-dots have their own advantages and disadvantages.
The advantage of QWs is that a higher gain per unit length can be obtained per well than
can be obtained with QDs per dot layer. Also the growth process is relatively
straightforward. The gain bandwidth of QWs is however limited to approximately 20 to
40nm which is ideal for narrowband lasers but not for widely tunable lasers of over 100nm
bandwidth. Theoretically it may be possible to stack different QWs with different emission
wavelengths to broaden the bandwidth, however this has not been explored for such long
wavelength quantum wells to our knowledge. QDs on the other hand have a wide gain
spectrum due to the inhomogeneous broadening. In the gain material there is a variety of
dots with different dot sizes and since each dot size has its characteristic gain wavelength,
the QD amplifier can amplify light over a broad spectrum up to 200nm (Chapter 3) [69].
This makes the QDs very suitable for widely tunable lasers. The drawback of QDs is
however the low gain per unit length per layer with respect to QWs.
The integration of light sources and other components in the InP/InGaAsP material
system has already been explored extensively for the use in telecommunication systems.
Large scale integrated circuits have already been successfully brought to the market [51].
The recent activities on the development of a generic integration technology platform, both
in Europe and the United states, are expected to boost the integration technology of
InP/InGaAsP. The standardization of a limited number of integrated component building
blocks and the availability of multi-project wafer runs should also open up the market for
smaller companies. The prospects are that the generic integration technology platform will
reduce the development costs by at least one order of magnitude.
The tunable laser presented in this thesis uses this standard integration technology
developed at COBRA. It relies on the experience COBRA has with the so called
“active-passive” integration technology used for photonic integrated circuits in the 1550nm
telecom wavelength region. With the active-passive integration technology is meant the
integration of active components together with passive components on a single chip. In this
technology one is able to combine active components that generate, amplify or absorb light,
with passive components which are transparent for light at 1550nm and are used to
manipulate the light. For the use of this integration technology for components operating in
the 1600nm to 1800nm wavelength region two major aspects have to be explored: the
integration of long wavelength QDs in the active regions and the applicability of standard
passive components in the 1600nm to 1800nm wavelength region. The QDs themselves
have already successfully been integrated in the COBRA active-passive integration
technology [75] and their applicability will be discussed in this chapter.
This chapter continues in section 2.2 with a description of the fabrication technology
starting with the fabrication of the active-passive wafer followed by a step-by-step
20
Integration technology
description of the active-passive processing scheme used. Section 2.3 describes the
adaptations made to the standard integration technology necessary to fabricate the tunable
laser in the 1600nm to 1800nm wavelength region. The chapter ends with a description of
the mounting of the chip before measurement.
2.2 Fabrication Technology
The fabrication of a semiconductor integrated optical device can basically divided into
two main parts: the fabrication of the semiconductor wafer with a specific layerstack and
the fabrication of the optical devices on the wafer. In this section the active-passive
layerstack is discussed first, as well as the fabrication of the active-passive wafer. Then the
processing scheme that was used to fabricate the tunable laser is explained in detail.
2.2.1 Active-passive layerstack
The complete monolithically integrated tunable laser consists of active layerstack
components (semiconductor optical amplifiers (SOAs)) as well as passive layerstack
components (waveguides, arrayed waveguide gratings (AWGs), multi-mode
interferometers (MMIs) and phase modulators (PHMs)). To be able to integrate these active
and passive components on a single chip, a single wafer with active areas and passive areas
is required. These active-passive wafers where fabricated at COBRA, in collaboration with
the MiPlaza division of Philips Research, using the butt-joint integration approach [11].
(see Table 2-1) In this integration approach, based on MOVPE, the active layerstack is
grown first on an n-doped InP substrate, ending with the 20nm not-intentionally-doped
(n.i.d.) InP layer on top of the InGaAsP film layer (Q1.25) (performed at COBRA). Within
the Q1.25 waveguiding film layer five InAs QD layers are stacked with an ultrathin GaAs
interlayer underneath each QD layer to control the size of the dots [2]. The QD are
optimized to emit light in the 1600 to 1800nm wavelength region, as presented in [28][54].
The QD layers are 40nm separated from each other with InGaAsP (Q1.25). Furthermore,
the film layer also contains a 10nm n.i.d. InP etch stop layer 20nm underneath the QD
layers to be used in the etch-back process. The active areas are then defined in a SiNx layer
by means of a photolithography process and the layerstack is etched back down to the InP
etch-stop layer 20nm underneath the QD layers (performed at COBRA). In the first
re-growth step (performed at Philips) the passive InGaAsP (Q1.25) film layer is grown
ending with a 20nm n.i.d. InP layer. After the removal of the SiNx mask a common
cladding is grown in the second re-growth (performed at Philips). The final layer structure
is shown in Table 2-1.
21
Chapter 2
Passive Layerstack
Layer
Material
Doping
[cm-3]
E3-6
p-InGaAs
1.51019
7.210
18
p-Q1.2
4.710
18
p-InP
E3-5
E
p E3-4
i
E3-3
p-Q1.4
3 E3-2
Active Layerstack
Thickness
[nm]
280
Color Color Thickness
code code
[nm]
280
10
10
Doping
[cm-3]
Material
Layer
1.51019
p-InGaAs
E3-6
7.210
18
p-Q1.4
E3-5
4.710
18
p-Q1.2
E3-4
E
p
i
10
10
11018
1000
1000
11018
p-InP
E3-3
p-InP
51017
300
300
51017
p-InP
E3-2
E3-1
i-InP
n.i.d.
180
180
n.i.d.
i-InP
E3-1
E2-2
i-InP
n.i.d.
20
20
n.i.d.
i-InP
E1-8
i-Q1.25
n.i.d.
330
100
n.i.d.
i-Q1.25
E1-7
5x40
n.i.d.
QD/i-Q1.25
E1-6
20
n.i.d.
i-Q1.25
E1-5
10
n.i.d.
i-InP
E1-4
E
p
i
1
E E2-1
p
i
2
E1-3
E
p E1-2
i
E1-1
1 E1-0
i-Q1.25
n.i.d.
170
170
n.i.d.
i-Q1.25
E1-3
i-InP
n.i.d.
70
70
n.i.d.
i-InP
E1-2
n-InP
51017
430
430
51017
n-InP
E1-1
≈350µm
≈350µm
1-41018
n-InP
E1-0
n-InP
1-410
18
3
Table 2-1 Active-passive layerstack. The thick lines show the different growth steps: First the active
layerstack is grown on the substrate E1-layers. Then the passive areas are selectively etched back till
the InP stop layer (E1-4) in the film layer. In the second growth step the passive waveguide layers are
grown (E2-layers). In the third growth step the common cladding layers and contact layers are grown
(E3-layers).
2.2.2 Active-passive processing scheme
The active-passive integration technology used at COBRA limits the number of
different components/waveguide structures that can be used on a single chip. These
components are passive waveguides, PHMs and SOAs. The passive waveguides and the
PHMs can be fabricated as shallowly etched or deeply etched, whereas the SOA can only
be shallowly etched. A shallowly etched component has a low contrast ridge waveguide
where the area around the ridge is etched away down to 100nm into the film layer. In a
deeply etched waveguide this area around the waveguide is etched away all the way
through the film layer. Thus a high contrast ridge waveguide is formed that is suitable for
small bend radii. For the tunable laser it was decided to use only the shallowly etched
waveguides, PHMs and SOAs to minimize the complexity of the processing. In the
22
Integration technology
following step-by step description of the processing scheme diagrams are added in Fig. 2-1
to illustrate the process. In these pictures the different components are presented from left
to right; SOA, PHM, passive waveguide with electrical lead crossing it, bond pad and an
electrical isolation waveguide at the far right-hand side. Note that the dimensions of the
picture are not realistically scaled.
a)
Optical waveguide lithography - A 50nm thick SiNx layer is deposited on the wafer
using a PECVD process. The optical waveguide pattern is defined in a 750nm thick
layer of positive photoresist (HPR504) using optical contact lithography. This pattern
is transferred to the SiNx layer using a CHF3 reactive-ion etching (RIE) dry etch
process. After removal of the photoresist the optical waveguide pattern remains in the
50nm SiNx which is used as a hard-mask in the subsequent InP RIE etching steps.
b)
First InP RIE etch step - In the first InP RIE dry etch step the height difference
between the top of the isolation sections in the waveguides and the etch depth of the
shallow waveguides in the film layer is realisedusing an optimized CH4/H2 RIE
process alternated by an O2-descum process. In this etch step also the estimated lag
effect has to be taken into account. This is an effect which results in small etch depth
differences between areas directly beside a ridge waveguide and areas further from the
waveguide where the etch depth is measured. The aiming etch depth is 380nm (200nm
top cladding, 100nm film layer and 80nm lag effect).
c)
Isolation section lithography - The SiNx mask is removed from the areas where
isolation sections have to be made in the optical waveguides. First the isolation
sections are defined in a 3µm thick layer of positive photoresist (AZ4533) using
optical lithography (dark field mask). Afterwards the SiNx is etched away wet
chemically using a buffered hydrofluoric acid (BHF) solution.
d)
Second InP RIE etch step - In the second InP RIE etch step the height difference
between the top of the passive waveguides and the top of the isolation sections in the
waveguides is made using the same etch process as the first InP RIE etch step. Here it
is important to realize that at the end of the thirds RIE etch the top of the passive
waveguides must be underneath the highly doped InGaAs contact layer to minimize
the waveguide losses. This means in this second RIE etch step the etch depth should
be a little less than the difference between the top of the passive waveguide and the
isolation section. In this etch step the aim is to etch 1200nm. The top of the passive
waveguides is then 100nm underneath the contact layer.
e)
Contact layer lithography - The SiNx mask is removed from the passive waveguides
which do not need the InGaAs contact layer for metal contacting. It is removed by
defining the contact areas in a 3µm thick layer of positive photoresist (AZ4533) using
optical lithography and etching away the SiNx wet chemically from the passive
waveguides using a BHF solution.
23
Chapter 2
f)
Third InP RIE etch step - In the third and last InP RIE etch step the height difference
between the top of the contact layer and the top of the passive waveguides is made
using the same etch process as the first and second InP RIE etch steps. More important
is the final etch depth of the shallow waveguides in the film layer which has to be
180nm (including lag effect) which means it is 100nm near the ridge. The expected
etch step is 400nm however the etch depth of the shallow waveguides determines the
final etch step. After removing the remaining SiNx mask wet chemically using a BHF
solution the waveguide topology is finished.
g)
Planarization - The wafer is planarized by spinning six layers of polyimide (PI2723)
which are cured after each layer at a temperature up to 325C. The polyimide is first
etched back using a CHF3/O2 RIE dry etch process till approximately 100nm above the
top of the contact layer. The areas where contact openings have to be made in the
polyimide are defined in a 2.5µm thick layer of negative photoresist (MaN-440) using
optical lithography. Finally the polyimide is selectively etched further using the
CHF3/O2 RIE dry etch process till approximately 100nm underneath the top of the
contact layer.
h)
Metallization - The metallization pattern is defined in a 2.5µm thick layer of negative
photoresist (MaN-440) with a lift-off profile using optical lithography. To clean the
surface of the contact layer to get a low contact resistance, the top 100nm of the
contact layer is etched away wet chemically using H2SO4:H2O2:H2O in proportions
1:1:100. Then the Ti/Pt/Au 60/75/300 nm metal contacts are deposited by means of
e-beam evaporation and lift-off in acetone. The backside of the wafer is metalized with
evaporated Ti/Pt/Au 60/75/200 nm to create one common n-contact. No waver
thinning process was employed before this backside metallization. The wafer is
annealed at 325C for 30s. More than one annealing cycle at 325C for 30s did not
improve the contact resistance, only increased the risk of defects in the p-metal.
i)
Plating - To assure a uniform current injection in the long SOAs, the thickness of the
p-side metal contacts on the SOAs is increased using an electroplating process. For
this process first a 100nm Au seed layer is sputtered on the wafer. Then the plating
pattern is defined in a 3µm thick layer of positive photoresist (AZ4533) using optical
lithography (dark field plating mask). A 1.7µm thick layer of gold is deposited on the
SOA contacts using an electroplating process. After the photoresist has been removed
the metallization pattern is again defined in a new 3µm thick layer of positive
photoresist (AZ4533) using optical lithography (same mask as for the p-side
metallization). The seed layer in between the p-metal contacts is now removed by
etching the gold layer in a potassium cyanide (KCN) solution. The reason to
selectively etch back the seed layer is to keep the seed layer on the unplated PHMs and
their leads to the bonding pads to improve the reliability of the electrical connection
when crossing a height difference in the polyimide.
24
Integration technology
j)
Cleaving - Finally the approximately 350-360µm thick chip was cleaved out of the
wafer and mounted on a copper chuck using an electrical and thermal conductive
epoxy resin. Further details of this mounting are given in the section on chip mounting
at the end of this chapter.
a
SOA
PHM
WG
PAD
Iso
b
SOA
PHM
WG
PAD
d
SOA
PHM
WG
PAD
Iso
PHM
WG
PAD
Iso
j
SOA
PHM
WG
PAD
Iso
SOA
PHM
WG
PAD
Iso
e
SOA
PHM
WG
PAD
g
SOA
Iso
c
Iso
f
SOA
PHM
WG
PAD
Iso
h
SOA
PHM
WG
PAD
Iso
i
SOA
PHM
WG
PAD
Iso
Fig. 2-1 Different processing steps. The different components are
presented from left to right; SOA, PHM, passive waveguide with
electrical lead crossing it, bond pad and an electrical isolation
waveguide at the far right-hand side. The used color codes are as
presented in Table 2-1.
(a) Optical waveguide lithography (b) First InP RIE etch step
Isolation section lithography (d) Second InP RIE etch step
Contact layer lithography (f) Third InP RIE etch step
Planarization (h) Metallization (i) Plating (j) Cleaving. Note that
dimensions of the picture are not realistically scaled.
25
(c)
(e)
(g)
the
Chapter 2
2.3 Specific processing used in this thesis
The active-passive layerstack and the processing scheme described above are used for
the realization of the tunable laser. They are based on the experience of previous chip
fabrication at COBRA. In this section the differences compare to the standard integration
technology will be discussed.
2.3.1 Waveguide structure at 1700nm
The standard integration technology described above is optimized for the use in the
1550nm wavelength region. The layerstack consists of a 500nm waveguide core layer to
confine the optical mode in between the InP top and bottom cladding. In the standard
integration technology the waveguides are fabricated to be 2µm wide and the area around
the waveguides is etched 100nm into the film layer to confine the light in the lateral
direction. Using these waveguides in the 1600nm to 1800nm wavelength region has some
influence on the mode size and the waveguide losses. In this subsection we describe what
we can expect and what influence it has on the waveguide losses.
The waveguide wide of 2.0µm at 1550nm is chosen just underneath the 3rd order
lateral mode cut-off width. The existence of the antisymmetric 2nd order lateral mode in the
waveguide is not considered problematic. It has considerably higher losses than the lowest
order mode due the higher intensity of the fields at the edges of the waveguide. And the
antisymmetric field distribution couples less easily to the fundamental mode than the
3rd order mode. For the use of these waveguides in the 1700nm region the width is
increased to 2.2µm. The width is chosen as large as possible without enabling higher order
modes to minimize the overlap of the fundamental mode with the rough side walls.
The intrinsic material losses changes proportionally to the change in refractive index
of the material. For the InGaAsP (Q1.25) film layer this change in refractive index is from
3.364 at 1550nm to 3.354 at 1700nm (-0.8%) [24]. This change is due the increasing energy
distance from the absorbing band edge in the material and thus the optical absorption of the
material is decreasing. But this is almost negligible when the loss at 1700nm is compared to
that at 1550nm.
The other difference is the change in mode size. The change from 1550nm to 1700nm
resulted in an increase in mode size due to the longer wavelength. This results in a larger
overlap of the optical mode with the doped cladding layers. Especially the p-doped top
cladding has a high loss as can be seen in Table 2-2 [14][77]. To estimate waveguide losses
due to doping in the cladding layers we calculated the confinement in each of the layers.
These confinement factors are given in Table 2-3 for a 2.0µm waveguide at 1550nm and
1700nm and for a 2.2µm waveguide at 1700nm. The total waveguide losses can be
calculated with:
26
Integration technology
 p,n   i i
(2-1)
i
Here αp,n is the total waveguide losses, Γi the confinement in the ith layer and αi the
waveguide losses in layer i due to doping. The calculated waveguide losses due to doping
in the different layers are also given in Table 2-3 for the different configurations.
Doping type
Doping level
[cm-3]
p
p
p
n
n
n
1e18
5e17
1e16 (n.i.d.)
1e16 (n.i.d.)
5e17
2.5e18
Loss αi
[dB/cm]
1550nm
99.0
49.5
1.0
0.1
5.8
42.5
Loss αi
[dB/cm]
1700nm
128.2
64.1
1.3
0.1
7.0
51.1
Table 2-2 Material losses for different p and n doping levels calculated [14] for 1550nm wavelength
and 1700nm wavelength. For not-intentionally-doped (n.i.d.) layers a 1e16 cm-3 doping level is
estimated.
1000
Confinement
Γi
2.0µm
1550nm
0.0036
Loss Γiαi
[cm-1]
2.0µm
1550nm
0.0821
Confinement
Γi
2.0um
1700nm
0.0060
300
0.0276
0.3151
0.0347
0.5120
0.0362
0.5346
200
0.0863
0.0197
0.0908
0.0268
0.0936
0.0276
500
0.7318
0.0110
0.6912
0.0124
0.6917
0.0125
Layer
Doping
-3
[cm ]
Thickness
[nm]
p-InP
1e18 (p)
p-InP
5e17 (p)
i-InP
1e16 (p)
i-InGaAsP
1e16 (n)
Loss Γiαi Confinement
[cm-1]
Γi
2.0µm
2.2um
1700nm
1700nm
0.1760
0.0064
Loss Γiαi
[cm-1]
2.2µm
1700nm
0.1876
i-InP
1e16 (n)
70
0.0573
0.0009
0.0599
0.0011
0.0587
0.0011
n-InP
5e17 (n)
430
0.0859
0.1042
0.1042
0.1668
0.1010
0.1617
n-substrate
2.5e18 (n)
Total losses
[dB/cm]
0.0072
0.0703
0.0129
0.1523
0.0121
0.1428
2.7
4.6
4.6
Table 2-3 Calculated confinement and loss components in the different layers for a 2.0µm waveguide
at 1550nm and 1700nm and for a 2.2µm waveguide at 1700nm
Furthermore the increase in mode size also results in an increase in overlap of the
optical mode with the etched side walls. This causes increasing waveguide losses as well,
due to light scattering on sidewall roughness. The estimated increase in losses due to
sidewall roughness has not been calculated.
If we assume that the waveguide losses are mainly due to doping and scale
comparably with the calculated losses due to doping, an increase of approximately 75% in
the waveguide losses can be expected in a 2.2µm waveguide at 1700nm in comparison to
the 2.0µm waveguide at 1550nm.
27
Chapter 2
2.3.2 Top buffer layer
In the active-passive layerstack growth process the first epitaxial growth ends with an
InP layer (E1-8 in Table 2-1) on top of the InGaAsP film layer (E1-7 in Table 2-1). In
active-passive layerstacks that were used previously in our research institute, this first
epitaxial growth ended with a 200nm p-doped (31017 cm-3) InP buffer layer above the
InGaAsP film layer [4][20][34]. The p-doping in this buffer layer was included to improve
the current flow to the active regions. After the etch back and the selective area re-growth
of the passive film layer, a 200nm n.i.d. InP buffer layer was grown on top of the passive
film layer. This regrown layer is n.i.d. to minimize the passive waveguide losses.
In the active-passive wafer fabricated for the tunable laser we chose to end the first
growth with a 20nm n.i.d. InP layer above the film layer (layer E1-8 in Table 2-1). This
layer is grown this thin to minimize the height of the selectively re-grown passive film
layer. This in turn reduces the height variations that appear at the active-passive butt-joint.
In the second re-growth (layers E2 in Table 2-1) this 20nm InP is completed with an extra
180nm n.i.d. InP layer (layer E3-1 in Table 2-1) to end up with a 200nm n.i.d. InP buffer
layer above the film layer for both the active and the passive areas. This n.i.d. buffer layer
is also chosen to minimize the waveguide losses in the waveguides. The fact that the buffer
layer above the amplifiers is now also n.i.d. can result in slightly higher voltage over the
amplifiers during operation when compared to previously used wafers with a lightly
p-doped cladding.
2.3.3 Doping passive waveguides
For the passive layerstack one has to make a tradeoff between minimum waveguide
losses and optimal phase shift efficiency in the phase shifters. An n-doping in the film layer
results in a higher phase shift efficiency [10][82][83] due to the increasing carrier effects as
will be discussed in chapter 4. However, this n-doping in the film layer also results in a
higher optical loss due to the increasing free carrier absorption as can be seen above. The
calculated film layer loss increases from 0.08dB/cm for a 11016cm-3 doping level to
0.59dB/cm for a 61016cm-3 doping level (1700nm)[14]. Measured phase shift efficiencies
for doping concentrations of 61016cm-3 vary between 0.2rad/Vmm [35] and 0.4rad/Vmm
[82]. The decrease in phase shift efficiency expected when reducing the doping in the film
layer from 61016cm-3 to 11016cm-3 is however very small [83].
Because of the long path lenghts of passive waveguides in the tunable laser circuit we
chose to have a 11016cm-3 n-doping in the film layer to reduce waveguide losses. The
expected small decrease in phase shift efficiency can be compensated with slightly longer
phase modulators (chapter 4).
28
Integration technology
2.3.4 InP etch stop layer in the film layer
During the active-passive wafer fabrication the film layer is selectively etched-back
after the first growth in the areas where passive waveguides have to be realized
(section 2.2.1). To have a uniform and precisely defined etch back over the complete
etch-back area, a 10nm InP etch-stop layer is included in the film layer, 20nm underneath
the QD layers where the etch-back has to stop. The selective wet-chemical etch-back will
stop on this InP etch-stop layer. The InP etch stop layer is removed in the passive area
during the bromine-methanol cleaning step prior to the regrowth. The influence of this layer
on the optical field is assumed to be negligible. The electrical influence is not known
exactly; however, most probably the carriers will tunnel through this 10nm InP layer.
2.3.5 Planarization and etch back Polyimide
After the etching of the waveguide structures, the wafer needs to be planarized before
metal evaporation. For the planarization a method is used that consists of spinning six
layers of polyimide alternated with a curing at 325C and a surface treatment as described
above in section 2.2.2(g). The advantage of applying six layers over the method that is
often used with one or two layers of polyimide is the uniformity of polyimide. After six
layers the height variation is reduced to approximately 150nm whereas these variations
after two layers can be more than 500nm. These height variations need to be less than
200nm to be able to open all contacts approximately at the same time when etching back
the polyimide.
Another important advantage of these six layers is the surface roughness of the
polyimide after etching back. The roughness of the surface is proportional to the etch-back
time. In case of six layers of polyimide this etch time is approximately 35minutes. With
two layers of polyimide applied 3minutes can be enough which leaves a much smoother
surface. To be able to make a good adhesion between the metallization and the polyimide
the surface of the polyimide should be rough. Previous chips fabricated with only one or
two layers of polyimide or BCB did not have this strong adhesion between the metal and
the polyimide which resulted in release of contact pads from the polyimide during the
bonding process or during measurements with probe needles. Various chips with these six
layers of polyimide did not show this release of contact pads.
The tunable laser was fabricated using the six layers of polyimide. For the separate QD
amplifiers fabricated on a different wafer for the QD gain measurements described in
chapter 3 only two layers of polyimide where used. The six polyimide layers where not
necessary in this case. This was due to the absence of separate probe pads and the uniform
distribution of the amplifiers on the wafer resulting in smaller height differences.
29
Chapter 2
2.3.6 Metallization
For the metallization a distinction has been made between the metal contacts on the
amplifiers and the metal contacts on the PHM and the associated bonding pads. The metal
contacts of the amplifiers need to be plated to minimize the electrical losses in the contacts
[34]. However, the PHM and their associated bonding pads do not need this plating due to
the low current through the PHM in reverse bias. Furthermore, bonding is preferably done
on non-plated pads due to the fact that plated gold contains more residuals from the plating
bath. These residuals make the adhesion of a bonding on plated gold more difficult. For this
reason, only the amplifiers where fabricated with a plating on top of the evaporated
Ti/Pt/Au.
During the fabrication of the tunable laser the gold seed layer has not been completely
removed after plating but has been etched away selectively with the p-contact metallization
mask. This introduces an extra lithography step (not an extra mask) but has the advantage
that the seed layer remains on the PHM contacts and their contact pads. This prevents that
part of the evaporated gold is etched away when etching away the seed layer. Furthermore
it improves the electrical leads from the PHM to the contact pads especially where a step
has to be made between different heights in the polyimide. The seed layer improves this
due to the tilted evaporation of the seed layer whereas the evaporated Ti/Pt/Au layers are
evaporated right from the top.
2.4 Chip mounting for measurement
The tunable laser described in this thesis has to be controlled with 36 voltage sources
connected to the PHMs as will be described later in chapter 6. The most common
connection method used in a research environment for such number of connections is by
means of a multi-probe connection. The multi-probe is typically placed directly on the gold
connection pads on the fragile chip. To avoid the need for direct probing, which easily
damages the fragile chip, a wire bonding connection method was used for the
measurements described in this thesis. The optical chip is glued on a copper mount together
with a printed circuit board (PCB). The PHMs on the chip are connected individually via
the bond pads on the chip to bond pads on the PCB by means of wedge-bonding. On the
PCB high bandwidth (1GHz) multi-pin connectors, each connecting eight voltage signals,
are positioned to connect the laser to the electronics [70]. A picture of the measurement
setup is given in Fig. 2-2.
The PCB is designed to transport the voltage signals from the connectors to the bond
pads on the PCB. The electrical leads from connectors to bond pads on the PCB have been
shielded to avoid electrical crosstalk at 100MHz (maximum) operation. An 8-layer PCB is
designed to transport all electrical signals to the small (10mm wide) connection area close
to the optical chip. Two layers contain the signal lines from the connectors to the individual
30
Integration technology
bond pads. The individual ground signals from each of the signal lines are carried by two
other layers in the PCB. These ground signal lines all come together in a common ground
bond pad in the bottom layer of the PCB directly underneath the bond pads. This common
ground pad is glued to the copper mount and thus connected to the p-side of the optical
chip. The four other layers on the PCB are used as vertical shielding layers in between the
signal and ground layers to avoid electrical crosstalk. The vertical crosstalk between signal
and ground leads is minimized by alternating these signal and ground leads with shield
leads connected to the common shielding. The PCB frequency response capabilities have
not been tested separately.
6
5
4
2
3
5
1
4
5
6
Fig. 2-2 Image of measurement setup. The setup contains the following components: 1)
Tunable laser chip (within dotted lines). 2) Printed circuit board. 3) Bond pads on PCB to
connect the contacts of the PHMs on the laser chip to the PCB. 4) Multi-pin connectors to
connect the control electronics of the PHMs to the PCB. 5) Probe needles to inject current
in the three QD amplifiers. 6) Fiber stages to collect the light from the laser chip with
lensed fibers.
The copper mount is fixed on a measurement setup. Within this measurement setup
cooled temperature stabilized water can be guided just underneath the copper mount to
remove heat generated in the chip. The optical chip can be approached with lensed fibers
from both sides of the chip to collect the light from the cleaved facets.
The described connection method of the optical chip to the electronics was used
successfully for the tunable laser over more than 4 months. This demonstrates that this
connection method is very stable and robust for the measurement of optical chips with a
large number of electrical connections.
31
3 Measurement and analysis of the gain in
quantum-dot amplifiers
Abstract - Small signal modal gain measurements have been performed on
two-section ridge waveguide InAs/InP (100) quantum-dot amplifiers that we have
fabricated with a peak gain wavelength around 1700nm. The amplifier structure is suitable
for monolithic active-passive integration and the wavelength region and wide gain
bandwidth are of interest for integrated devices in biophotonic applications. A 65nm blue
shift of the peak wavelength in the gain spectrum has been observed with an increase in
injection current density from 1000A/cm2 to 3000A/cm2. The quantum-dot amplifier gain
spectra have been analyzed using a quantum-dot rate-equation model that considers only
the carrier dynamics. The comparison between measured and simulated spectra shows that
two effects in the quantum-dot material introduce this large blue shift in the gain spectrum.
The first effect is the carrier concentration dependent state filling with carriers of the bound
excited and ground states in the dots. The second effect is the decrease in carrier escape
time from the dots to the wetting layer with decreasing dot size.
3.1 Introduction
The behavior of semiconductor quantum-dots (QDs) has been studied extensively over
the last fifteen years. The use of QDs in monolithic optical devices, such as semiconductor
optical amplifiers (SOAs) and semiconductor lasers, can have a number of benefits over the
use of bulk or quantum well gain material. These advantages are due to the three
dimensional carrier confinement in QDs. Some of the benefits that have been
experimentally demonstrated are a comparatively low lasing threshold current density [46]
and ultra fast gain recovery in optical amplifiers [12][13]. Another major advantage of QDs
over bulk or quantum well material is the wavelength tunability. The wavelength of a
generated photon depends first of all on the discrete energy levels in the quantum-dot and
can be changed only slightly due to the temperature dependent homogeneous broadening
[62]. The discrete energy levels in the QD are however directly dependent on the size (and
shape) of the dots [36] which means that the distribution of the size of the dots in an
amplifier determines to a large extent the gain spectrum. In this chapter we present results
on InAs QD on an InP (100) substrate. The average QD size in this system can be
controlled in the growth process which makes it possible to tune the average wavelength
33
Chapter 3
[2][28][54]. Most results that have been published up to now on this material are in the
1550nm telecom region where wide gain spectra (e.g. 80nm wide) have been observed.
However, the average dot size can also be tuned to provide gain material which has a centre
emission wavelength in the 1700nm region or even longer wavelength regions.
Such wavelengths in combination with a wide gain bandwidth are desirable for other
applications such as tunable lasers for gas detection [79] or optical coherence tomography
[23]. This extension of the wavelength range of an amplifier structure that can be used in
active-passive optical integration schemes [7][75] opens up the possibility to develop
integrated light sources for these applications.
In order to be able to use this gain material optimally in devices for applications in
these longer wavelength ranges, we have measured the small signal gain spectra of
InAs/InP(100) ridge waveguide amplifiers as a function of injection current and modeled
this QD material to understand its behavior. This can be utilized for instance for increasing
the gain bandwidth of the optical amplifier by using multiple amplifier sections which are
operated at different injection current densities [80].
In this chapter we start with presenting the measurement method, the InAs/InP QD
devices used and the characterization results in the 1600nm to 1800nm region. Then the
QD amplifier model that was used to explain the dependency of the observed gain spectra
on injection current is presented. The modeling of QD materials is more complicated than
the modeling of bulk or quantum well gain material due to the inhomogeneous character of
the QD gain material. Extensive QD models have been presented especially for
InxGa1-xAs/GaAs QD materials [25][63] but also for InAs/InP QD materials [29]. Here we
have simplified a commonly used QD amplifier model to calculate the small signal gain
spectrum of the amplifiers and to fit a number of the model parameters to the experimental
data. Three important parameters are the electron-hole transition energies of the wetting
layer (WL), the excited state (ES) and the ground state (GS). From InAs QD on GaAs
substrate these transition energies can be determined with photoluminescence (PL)
measurements or comparable measurement techniques. In these PL measurement results
two individual peaks can be observed, one for the GS and one for the ES. PL measurements
from InAs QD on InP substrate show however only one wide individual peak and no
feature is observed that can be attributed to the wetting layer. The peaks from the GS and
ES do overlap, which makes it impossible to extract the transition energies of the dots and
the WL with this PL measurement technique [1][18]. Using the model we have been able to
determine the transition energies of the GS and ES and an effective transition energy for the
wetting layer.
The fitted results are discussed in the last section together with the explanation that
was found for the large shift in peak wavelength in the gain spectrum with the change in
injection current density.
34
Measurement and analysis of the gain in quantum-dot amplifiers
3.2 Gain measurement method, device design and
fabrication
Small signal gain spectra of optical amplifiers are often determined with the well
known Hakki-Paoli technique [32][4]. In this method the optical gain is derived from the
contrast ratio or shape of the modulations in the spectrum of the Amplified Spontaneous
Emission (ASE) caused by the resonances of a laser cavity operating below threshold. Our
quantum-dot amplifiers have a relatively low modal gain and therefore need to be typically
at least a few millimeters long to generate detectable signal. The spectral measurements
therefore have to be done using a high resolution spectrometer in order to resolve the
spectral modes and not to distort the observed contrast ratio and/or linewidth of the modes.
Since such a high resolution spectrometer in the 1600-1800nm wavelength region is not
available to us, we have used a measurement technique based on the analysis of ASE
spectra from different lengths of amplifiers as described by [55][67]. By this method the
gain can be calculated from the ASE spectra over a wide range of injection current
densities.
Under the condition that there is no optical feedback and there is no gain saturation in
the amplifier, the net modal gain G of a SOA of length L is related to its ASE output power
P according to [67]:
P
Psp
G
 eGL  1,


(3-1)
where Psp is the spontaneous emission (SE) power (per unit length) and L is the SOA
length. Note that the net modal gain G relates to the material gain g according to: G=Γg-αi
where Γ is the optical confinement factor and αi is the internal loss. Note that this loss value
is generally quite high for SOAs, in the order of 10 cm-1 to 20 cm-1. We have extracted the
gain by using the ASE spectra from a series of amplifiers of different lengths at constant
current density and fitting equation (3-1) on the data at each wavelength to extract the
parameters G and Psp. This fitting has several advantages over using only two lengths
(L and 2L) as presented by Thompson et al. [67] to extract the gain. First of all, the
different ASE spectra always contain noise or small deviations in comparison to the ideal
situation described by equation (3-1). These deviations will be averaged out by the fitting
over multiple devices. Secondly, measurement points that deviate over 10% from the
expected value on the curve are detected easily. Such points were found to deviate due to
misalignment or fabrication imperfections. When this is observed at one current setting for
one device, it turned out there was an alignment issue and a new measurement was done
which typically resulted in a measurement point close to the curve formed by the other
points. When such a large deviation is observed at all current settings on one particular
device, we have concluded that the device is not working properly, most probably due to
35
Chapter 3
fabrication imperfections. Measurements from such devices were excluded from further
analysis.
We have realized a chip with a series of parallel amplifiers with different lengths that
have two contact sections. One of the two sections is forward biased to produce the ASE
and the second section is reverse biased in order to absorb the ASE from the first section to
prevent any significant feedback which is particularly important when using this method.
The ratio of the length of the amplifier and absorber sections is varied so the different
lengths of amplifiers are realized on a single chip.
This series of two-section ridge waveguide amplifiers was realized on a single chip
starting from the QD gain material grown on an n-type InP (100) substrate by metal-organic
vapor-phase epitaxy (MOVPE), as presented in [2]. In the active region, five InAs QD
layers are stacked with an ultrathin GaAs interlayer underneath each QD layer to control
the size of the QDs (3 ML InAs, 1 ML GaAs interlayer, and 40nm InGaAsP separation
layers). These QD layers are placed in the center of a 500nm InGaAsP (Q1.25) optical
waveguiding core layer. The QD layers are designed to produce a gain spectrum in the
1600nm to 1800nm wavelength region, have a PL wavelength at 1770nm and an area
density of 3.1·1010 cm-2. The bottom cladding of these devices is a 500nm thick n-InP
buffer and the top cladding is a 1.5µm p-InP layer with a compositionally graded 300nm pInGaAs(P) top contact layer. This layerstack is compatible with a butt-joint active-passive
integration process for possible further integration [75]. The single mode 2µm wide ridge
waveguides are etched 100nm into the InGaAsP waveguiding layer using an optimized
CH4/H2 two step reactive-ion dry etch process. To achieve electrical isolation between the
two sections, the most highly doped part of the p-cladding layer is etched away between the
two sections. The structures are planarized using polyimide. Two evaporated (Ti/Pt/Au)
and plated (Au) metal pads contact the two sections to create two contacts. The backside of
the n-InP substrate is metalized to create a common ground contact for the two sections.
The structures are cleaved perpendicularly to the waveguide ridge, and no coating was
applied. Finally the devices are mounted on a copper chuck, p-side up and contacted with a
probe.
The results presented in this chapter have been obtained from 22 out of 26 two-section
ridge waveguide devices with a total length of 7mm. Four devices could not be used due to
damage during fabrication or measurements. All devices are parallel on a single chip and
are separated from each other by 250µm. The absorber section in different devices
comprises between 5% and 30% of the total ridge length and is separated from the amplifier
section by an isolation section. Therefore, the length of the amplifiers would be 6.48mm to
4.94mm. A photograph of a part of the chip is given in Fig. 3-1
36
Measurement and analysis of the gain in quantum-dot amplifiers
Amplifier
Absorber
7mm
Fig. 3-1 Photograph of a part of the realized devices with different amplifier and absorber length
combinations. The amplifier and absorber sections are indicated.
3.3 Gain measurement results
The QD amplifier gain spectrum has been determined by first measuring the
continuous wave (CW) ASE with different injection current densities while keeping the
temperature in the copper just underneath the chip constant (288K). The long section is
used to inject a current which is adjusted to get a certain injection current density. On the
smaller section of the ridge waveguide a -3V reverse bias voltage is applied to work as an
absorber and so prevent optical feedback. The single-pass amplified spontaneous emission
generated in the QD ridge amplifiers was collected with a lensed fiber and the spectrum
recorded with a 0.5nm resolution spectrometer over 300nm in 3000 data points. As an
example, in Fig. 3-2a the recorded ASE spectra for a 4.97mm amplifier section are given
for injection current densities between 500A/cm2 and 5000A/cm2. From this figure we
observe a 130nm shift in the peak wavelength as a function of the injection current density
from 1778nm at 500A/cm2 to 1649nm at 5000A/cm2. In Fig. 3-2b the peak wavelengths
with respect to the injection current density are given. Note that in these figures no clear
spectral features can be observed that could be identified with ASE from specifically the
GS, the ES or any other higher energy state even at the highest current density (5kA/cm2).
In measured ASE spectra from InAs QD on GaAs the separation between GS and ES can
clearly be seen. In these spectra two different peaks can be distinguished at high injection
currents, one for the GS and one for the ES [25].
37
Chapter 3
160
1790
Wavelength [nm]
140
Power [nW/nm]
b
a
120
100
80
60
500A/cm2 5000A/cm2
40
1760
1730
1700
1670
20
0
1640
1550
1600
1650
1700
1750
1800
1850
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Current density [kA/cm2]
Wavelength [nm]
Fig. 3-2 (a) ASE spectrum from the 4.97mm amplifier section under different injection current
densities between 500A/cm2 and 5000A/cm2 in steps of 500A/cm2. (b) Peak wavelength of the ASE
spectrum from the 4.97mm amplifier with respect to the injection current density
To determine the gain spectra, the ASE is measured for 22 amplifier lengths and 5
injection current densities. Injection current densities above 3kA/cm2 could not be used
with the available devices due to the fact that the absorber started to be bleached at these
injection current densities and optical feedback started to occur. At all lower injection
current densities this feedback could be neglected. To ensure that the feedback through the
absorber is negligible we measured the optical spectrum from the absorber side output. By
comparing the amplifier output spectrum and the absorber output spectrum we were able to
determine the total roundtrip gain and the fraction of amplifier output signal originating
from the feedback through the absorber. For the most unfavorable combination of amplifier
current density and absorber length used (3kA/cm2, 730 µm) the wavelength dependent
roundtrip gain was at most -18dB at the most unfavorable wavelength. This means that less
than 2% of the recorded power at that wavelength is caused by the back reflection under the
most unfavorable circumstances used. The total amount of light which comes back in the
amplifier through the absorber was determined to be -49dBm. At the other end of the scale,
injection current densities below 1kA/cm2 could not be used due to the low signal level. A
nonlinear least square algorithm fitting algorithm was used to fit the parameters in equation
(3-1) to the data. In Fig. 3-3 the measured ASE power versus the length of the amplifiers is
given for situations with a high gain (a), intermediate gain (b) and weak absorption (c). In
the figures also the 10% error bars of the ASE measurements are given as well as the fitted
gain curves. For each wavelength channel a weighted fit is made on the recorded power
data for one wavelength over up to 22 different amplifier lengths and the zero length data
point. For this zero length data point the dark spectrum of the spectrum analyzer is used
which is approximately -75dBm/nm over the full bandwidth. The weighting factor for the
data points including the zero point is Wi=1/((εPi)2+σd2), where ε=0.1 is the relative error
38
Measurement and analysis of the gain in quantum-dot amplifiers
a
20
100
50
Power [nW/nm]
150
Power [nW/nm]
Power [nW/nm]
estimate and σd is the standard deviation of the dark spectrum of the spectrum analyzer. The
influence of including the zero point is small. The fitting without the zero data point
resulted in a slightly higher (less than 10%) gain value but the same spectral shape. Besides
the gain parameter also the spontaneous emission power (per unit length) Psp is extracted at
each wavelength from the fitting. In Fig. 3-4a the modal gain spectra which are determined
from the ASE spectra with the fittings are given for the different injection current densities.
Fig. 3-4b shows the 68% confidence bounds for three of the derived gain curves. Note that
this measurement technique is not suitable for absorption measurements due to the low
signal level from attenuated ASE far from the output of the amplifier. Obviously a similar
blue shift with increasing injection current density as in the ASE spectra is observed.
b
15
10
5
0
0
0
1
2
3
4
5
Length [mm]
6
7
3
c
2
1
0
0
1
2
3
4
5
6
Length [mm]
7
0
1
2
3
4
5
Length [mm]
6
7
Fig. 3-3 Recorded ASE power versus amplifier length, the 10% error bars and the fitted curve. (a)
λ=1700nm, I=3000A/cm2, Gfit=5.54cm-1 and 19 data points (The 3 longest amplifiers could not be
used due to the presence of feedback in the ASE spectrum). (b) λ=1750nm, I=2000A/cm2,
Gfit=2.24cm-1 and 22 data points. (c) λ=1640nm, I=1500A/cm2, Gfit=-1.07cm-1 and 22 data points.
8
8
a
4
6
Gain [1/cm]
Gain [1/cm]
b
3000
2500
6
2000
2
1500
0
3000
4
2000
2
0
1000
-2
1000
-2
-4
-4
1550
1600
1650
1700
1750
1800
1550
Wavelength [nm]
1600
1650
1700
1750
1800
Wavelength [nm]
Fig. 3-4(a) Modal gain spectra determined from measured ASE spectra for different injection current
densities between 1000A/cm2 and 3000A/cm2 (b) Selected model gain spectra including the 68%
confidence bounds.
39
Chapter 3
The shift in peak wavelength with current density is the most notable aspect of what
we have observed in the gain spectrum. In practice it means that when this material is used
in a laser without wavelength selective devices, e.g. a simple Fabry-Pérot laser, the lasing
wavelength is strongly dependent on the length of the device. The longer the laser, the
lower the gain per unit length or the lower the injection current density needs to be, and
thus the laser will operate at a significantly longer wavelength then one that is say twice as
short. A similar effect also occurs with quantum well or bulk gain material but we found it
is more significant in the InAs/InP quantum-dot materials. The shift in the peak of the gain
spectrum due to temperature changes is relatively small and has been observed to be
approximately 1nm per degree K. No change in shape of the gain spectrum was observed
over a range of 20 degrees at constant current density.
3.4 Quantum-dot amplifier gain model
To understand the origin of the shape and in particular the blue-shift of the gain
spectrum with increasing injection current density in our amplifiers, we have set up a
simulation of the QD amplifier gain with a QD rate equation model as presented in [25][63]
which has been simplified for our purpose to reduce the calculation time. In this model we
assume that the dots are neutral in electrons and holes since there is no doping in the dots.
We treat an electron and a hole as an exciton and assume that the dynamics are controlled
by the electron and that the hole immediately follows. We therefore use the same time
constants for electrons and holes for the relaxation processes as well as the escape and
recombination processes. The energy states of the hole in the dots and in the WL are
assumed to be equal and are used as zero energy reference level. In this chapter we use
electron energy levels in the calculations which are equal to the transition energy values
because of the zero reference level of the holes. The carrier losses in the dots due to
stimulated emission were neglected because we only look at the low power ASE spectra far
below the lasing threshold. Therefore we did not include a series of equations for the
photons, which reduces the number of equations to be solved by nearly one third. In the
model the homogeneous and inhomogeneous broadening are taken into account as well as
the occupation dependent capture times and temperature and energy level dependent escape
times. Furthermore we did not include the Auger effect [29], due to their neglect ability at
low injection current densities, or any other many body effects, nor was direct relaxation
from WL to GS [29] or any other higher energy states in the dots [57] incorporated.
Inclusion of these aspects would significantly increase the number and complexity of the
equations and increase calculation times too much. Including direct relaxation from the WL
to the GS in the model using values reported in literature [50] does not significantly
influence the results from the model.
A schematic picture of the dynamics in the QD amplifier gain model is given in Fig.
3-5. The model describes a separate confinement heterostructure (SCH) where the carriers
40
Measurement and analysis of the gain in quantum-dot amplifiers
are injected and a WL that acts as a common carrier reservoir for the QDs. To include the
inhomogeneous dot size distribution, the ES and the GS of the QDs are allocated into N sub
groups, each represent a different average dot size with a different average ES and GS
energy level. The dot size distribution is assumed to be Gaussian [63] with a full-widthhalf-maximum (FWHM) of Γ0 and with Gn being the fraction of the n-th dot group. Carriers
are injected in the SCH layer with a constant rate I/e. From this SCH layer they can relax to
the WL with rate 1/τs. Carriers which are in the WL are able to escape back to the SCH
layer with a rate 1/τqe or can be captured into a QD from the n-th subgroup with a rate 1/τc
which is dependent on the average capture time from the WL to the ES 1/τc0 assuming an
empty ES, and dependent on the filling probability PESn of the ES. In the ES the carriers can
escape back to the WL with a rate 1/τeESn or can relax to the GS of the dot with a rate 1/ τd
which is dependent on the average capture time from the ES to the GS 1/τd0 assuming an
empty GS, and dependent on the filling probability PGSn of the GS. From the GS the carriers
can escape back to the ES with a rate 1/τeGSn. Carriers can also recombine through radiative
and non-radiative processes from the SCH, the WL and the two energy states in the dots
with a rate 1/τsr, 1/τqr and 1/τr respectively.
I/e
SCH
τqe
τs
τsr
WL
τc
τeES
τqr
τd
τeGS
τr
ES
GS
τr
Fig. 3-5 Schematic of the energy band diagram of the QD amplifier active region. The carrier capture
and escape rates from the various states are indicated
The resulting rate equation system is as follows:
Nq
dN s
N
I N
  s  s 
,
dt
e  s  sr  qe
dN q
dt

Ns
s

Nq
N
  eESn   qr
n
ESn
(3-2)

Nq
 qe

Nq
 c0
 1  PESn Gn ,
n
41
(3-3)
Chapter 3
dN ESn N qGn 1  PESn  NGSn 1  PESn  N ESn N ESn N ESn
1  PGSn ,





 c0
 eGSn
r
 eESn
d0
dt
(3-4)
n=0,1,…,N-1


dN GSn N ESn
1  PGSn   N GSn  N GSn 1  PESn ,

 d0
r
 eGSn
dt
(3-5)
n=0,1,…,N-1
The rate equation systems consists of a series of rate-equations (3-2)-(3-5) which
represent the total number of carriers in the SCH reservoir Ns, the total number of carriers
in the WL reservoir Nq, and two series of N rate-equations which represent the carriers in
the ES of the n-th dot group NESn and the carriers in the GS of the n-th dot group NGSn. The
carrier escape time τeGSn and τeESn from the GS to the ES and the ES to the WL respectively
are according to the principle of detailed balance related to the carrier capture time τd0 and
τc0 in the following way:
 eGSn   d 0
 GS
e
 ES
EESn  EGSn
k BT
(3-6)
,
n=0,1,…,N-1
 eESn   c 0
 ES N D AD
e
WLeff AWL
EWL  EESn
k BT
(3-7)
,
n=0,1,…,N-1
where µGS=2 and µES=4 are the degeneracy of the GS and the ES levels; EWL, EESn and EGSn
are the electron energy levels of the WL and the n-th ES and GS; ND the dot density; ρWLeff
the effective density of states in the WL; AD the total area of the QD amplifier active region
and AWL the total area of the WL. NDAD is in (3-7) the total number of dots in the amplifier
and ρWLeffAWL the total number of effective states in the WL. AD and AWL have the same
value.
The coupled rate equations have been solved in the time domain with a solver based
on the second order modified Rosenbrock formula [56]. The set of equations are integrated
in time until a steady state has been reached. From this situation the gain from different
dots to the optical spectrum, which has been divided in M spectral groups, has been
calculated with the equations:
42
Measurement and analysis of the gain in quantum-dot amplifiers
g mn
GS
  GS C g

N w N D PGS
H act
2
2PGSn  1Gn Bcv E m  EGSn ,
EGSn
m=0,1,…,M-1
n=0,1,…,N-1
 2
P
N w N D ES
g mn
  ES C g
2 PESn  1 Gn Bcv Em  E ESn ,
H act E ESn
ES



m=0,1,…,M-1
(3-8)

(3-9)
n=0,1,…,N-1
where gmnGS and gmnES are the contribution to the gain from the n-th dot group to the m-th
spectral mode from the GS and the ES respectively. Cg= πe2ћ/cnrε0m02 is a constant; NW is
the number of QD layers; Hact is the total active layer thickness (Fig. 3-6); |PσGS,ES|2 are the
transition matrix elements of the GS and ES recombination [63]; Bcv the Lorentzian
homogenous broadening function [63] with a 2ħΓcv FWHM and Em the recombination
energy of the m-th spectral group. The total modal gain for each spectral group Gm could be
calculated with:
Gm  
 g mn
n
GS

 g mn ES   i ,
m  0,1,..., M  1,
(3-10)
m=0,1,…,M-1
where Γ is the optical confinement factor and αi the internal modal loss. All the values of
the parameters used in the simulations are given in Table 3-1.
43
Chapter 3
Table 3-1 Used parameter values
Simulation parameter
FWHM of inhomogeneous broadening
Effective density of states in the WL
FWHM of Homogeneous broadening
Degeneracy of the GS
Degeneracy of the ES
Relaxation time from SCH to WL
Capture time from WL to ES
Capture time from ES to GS
Carrier escape time from WL to SCH
Carrier recombination time in SCH
Carrier recombination time in WL
Carrier recombination time in ES and GS
Quantum-dot density
Number of QD layers
Total active layer thickness
Total quantum-dot area
Total wetting layer area
Refractive index
Optical confinement factor
Transition matrix elements
Internal modal loss
Temperature
Number of QD sub groups
Number of spectral groups
Value:
Γ0 = 41 meV
ρWLeff = 2.4·1011 cm-2 (T = 288 K)
2ħΓcv = 40 meV
µGS = 2
µES = 4
7 ns < τs < 20 ns
τc0 = 1 ps
τd0 = 1 ps
τqe = 3 ns
τsr = 4.5 ns
2 ps < τqr < 100 ps
τr = 1 ns
ND = 3.1·1010 cm-2
NW = 5
Hact = 200 nm
AD = 7·10-4 cm2
AWL = 7·10-4 cm2
nr = 3.261
Γ = 0.34
|PσGS,ES|2 = 2.70·m0·EGS,ES kgeV
αi = 10 cm-1
T = 288 K
N = 31
M = 31
Reference:
[12]
Fit
Fit
[12]
[12]
[25]
[25]
Fit
[12]
[2]
Fit, [63]
The number used for the FWHM of the inhomogeneous broadening Γ0 is based on
time resolved differential reflection spectroscopy measurements and PL measurements at
4.8K applied on similar QD [12]. The FWHM of the homogeneous broadening 2ħΓcv is not
known from previous research. It is used to fit the spectral width of the simulations to the
measured spectrum. The resulting 40meV can to a certain extent be explained by the
previous measurements [12]. In these measurements the PL-spectrum at room temperature
show a FWHM of 93meV while at 4.8K a FWHM of 41meV is measured. The difference
between these values comes from the homogeneous broadening. The effective density of
states in the WL per unit area is given by: ρWLeff=(meWLkBT/πħ2). The total carrier capture
time is measured to be approximately 2.2ps [12]. Based on these measurements the carrier
capture time from WL to ES, τc0, and from ES to GS, τd0, are chosen to be 1ps to have a
total carrier capture time from WL to GS of 2ps. The carrier recombination time, τr, is set at
1ns based on previous measurements [12]. Note that the spontaneous emission lifetime τsp
which is a part of the total recombination time τr can be calculated from the transition
matrix element [64]:
44
Measurement and analysis of the gain in quantum-dot amplifiers
 0 m 0 2 C 3  2
 sp 
2
P
e n r E GSn , ESn GS , ES
2
,
(3-11)
The relaxation time from SCH to WL, τs, and the carrier recombination time in the
WL, τqr, together give a limitation to the number of carriers that can relax to the QDs. These
two time constants are unknown and have been fitted to match the simulation results to the
measurements. In the table a range is given over which the time constants were changed.
The carrier escape time from WL to SCH τqe and the carrier recombination time in the SCH
τsr do not have a large influence on the gain. They are chosen to be 3ns and 4.5ns
respectively [25]. The optical confinement factor, Γ, is calculated from the overlap integral
of the optical mode with the active area of thickness Hact containing the QD layers (Fig.
3-6). The number of QD sub groups and the number of spectral groups is chosen to be 31 to
limit the amount of calculation time. Increasing the number of subgroups and spectral
groups does not influence the gain spectrum and so does not give extra information.
p-InGaAs
p-InP
100nm
5 x QD
Hact=200nm
InGaAsP
500nm
n-InP
Fig. 3-6 Schematic picture of the ridge waveguide including the 5 QD layers. The elliptical optical
mode in the InGaAsP film layer is depicted with the dashed line.
45
Chapter 3
3.5 Simulations
There are two major effects in the QD material which can explain the blue shift in the
gain spectrum. We will first discuss both effects, show theoretical simulation results from
limited parameter fits on the QD model, illustrating the effect on the calculated small signal
gain spectra and compare them. Note that the simulations of the two effects are purely
hypothetical. The values are chosen to isolate a particular effect and make clear what
influence it has on the shape of the gain spectrum.
The first and most commonly used explanation for the spectral shift with current
density is based on the contribution of higher energy states in the quantum-dots to the
optical gain. Carriers which are captured in a dot relax to the lowest energy state which has
a free position with a rate (1- PESn,GSn)/τc0,d0. If the two positions in the ground state are
occupied (PESn=1), the carriers are not able anymore to relax to the GS and will fill the
fourfold ES and if filled also other higher energy states. In the gain spectrum this means
that at low injection current densities only a peak appears around the central GS
wavelength. When the injection current density is increased, the GS start to be fully
occupied and a second peak starts to appear at the central ES wavelength. Due to the
separate transition energies of the GS and ES two peaks will appear one at the GS and one
at the ES wavelength. At a certain injection current density the ES peak will take over the
peak wavelength. If the GS and ES are too close to each other, the peaks will overlap and
there will be no clear distinction between the GS and ES peak. We have tried to reproduce
the shape of the experimental gain spectra assuming this is the dominant effect. That has
been done by fixing the WL wavelength at 1130nm (EWL=1.096eV) (this WL wavelength
is a theoretical value to better explain the effect, in reality the WL has a lower transition
energy). The central GS wavelength was fixed at 1730nm (EGS=0.716eV) and the central
ES wavelength at 1630nm (EES=0.760eV). These values gave the best similarity of
simulations with the measured gain. Due to the large energy gap between EES and EWL
electron energy levels there is a low escape rate from the ES directly back to the WL. If a
carrier is captured in a dot there is nearly no chance that it will be able to go to another dot
via the WL due to re-excitation. All the dots will be filled equally and the gain contribution
from the GS and the ES are proportional to the inhomogeneous dot size distribution. In Fig.
3-7a the contributions from the GS (solid) and ES (dashed) to the total gain, Fig. 3-7b, are
given for different injection current densities. First the GS starts to contribute to the gain
and with increasing injection current density also the ES starts to contribute to the gain and
takes over the peak wavelength.
The second effect in the QD material which can explain the blue shift in the gain
spectrum is based on a dot-sized dependent escape rate from carriers in the dots to the
wetting layer. The smaller dots have relatively high electron energy level which is closer to
the electron energy level of the WL than the electron energy levels of large dots. The rate
46
Measurement and analysis of the gain in quantum-dot amplifiers
1/τeESn in (3-7) at which the carriers escape from the dots to the WL is larger for the smaller
dots compared to the large dots due to the smaller energy differences. The relaxation time τc
from the WL to the dots is however not energy level dependent which means that all dots
will be filled at the same rate assuming that there are enough free states. The large dots will
therefore be populated more and these dots also start to contribute to the gain at first time.
When the injection current density is increased also the smaller dots will be more populated
with carriers and start to contribute to the gain. So the large dots first start to contribute to
the gain in the longer wavelength region and while increasing the injection current density
this shifts to the smaller dots which contributes to the gain in the shorter wavelength region.
Again we have tried to reproduce the shape of the experimental gain spectra but now under
conditions where the dot size dependent depopulation is the dominant effect. To simulate
this we fixed the central GS wavelength at 1660nm (EGS=0.746eV), the central ES
wavelength at 1640nm (EES=0.755eV) and the WL wavelength at 1540nm (EWL=0.805eV).
Again these values are chosen to illustrate the effect, in reality we expect that the transition
energy of the WL is higher and that the GS transition energy and the ES transition energy
are more widely separated. In this case the different electron energy levels are close to each
other and carriers can easily escape to a higher energy level due to the high escape rates
1/τeESn,GSn and so switch to another dot via the wetting layer. The escape rate 1/τeESn is still
dependent on the ES electron energy level EESn in the dots (3-7). Small dots have a
relatively high energy level which increases the escape rate with respect to the large dots. In
Fig. 3-7c the contribution from the GS (solid) and ES (dashed) to the total gain Fig. 3-7d
are given for different injection current densities. In this figure one can see directly that at a
certain injection current density part of the dots (long wavelengths) contribute to gain and
another part still absorbs light.
47
Chapter 3
10
10
- - ES
–– GS
a
Gain [1/cm]
Gain [1/cm]
b
6
6
2
-2
-6
2
-2
-6
-10
-10
-14
-14
1550
1650
1750
1550
1850
1650
Wavelength [nm]
10
10
- - ES
–– GS
c
1850
d
6
Gain [1/cm]
Gain [1/cm]
6
2
-2
-6
-10
2
-2
-6
-10
-14
-14
1550
1650
1750
1850
1550
1650
Wavelength [nm]
10
10
- - ES
–– GS
e
1750
1850
Wavelength [nm]
-––
f
measured
model
6
Gain [1/cm]
6
Gain [1/cm]
1750
Wavelength [nm]
2
-2
-6
-10
2
-2
-6
-10
-14
-14
1550
1650
1750
1850
1550
Wavelength [nm]
1650
1750
1850
Wavelength [nm]
Fig. 3-7 Simulated gain spectra to show the different effects. Spectra are
simulated for different injection current densities between 500A/cm2 and
3000A/cm2 in steps of 500A/cm2. In all figures the lowest line is the 500A/cm2
spectrum and the upper line the 3000A/cm2 spectrum. On the left side (a,c,e) the
spectra are split up in contributions from the GS (solid) and the ES (dashed) and
on the right side (b,d,f) the total gain spectra are given. (a,b) Simulated modal
gain spectra where the shift in peak wavelength is due to the shift from GS to ES.
(c,d) Simulated modal gain spectra where the shift in peak wavelength is due to
the dot size dependent filling due to dot size dependent escape rates. (e,f)
Simulated modal gain spectra where the shift in peak wavelength is due to both
effects. In (f) also the measured modal gain is given (dashed)
48
––– 3000A/cm
2
––– 2500A/cm
2
––– 2000A/cm
2
––– 1500A/cm
2
––– 1000A/cm
2
––– 500A/cm
2
Measurement and analysis of the gain in quantum-dot amplifiers
3.6 Comparison with measurements
If we compare the two simulations with the measurement results, we see that neither of
the optimized simulations follows the same trend as the measurements. In the first
simulation we see that the peak wavelength starts at the GS wavelength and at a given
moment switches to the ES wavelength, a kind of step function. This kind of step-like
behavior from GS to ES gain is very familiar in InAs QD on GaAs substrate. In this
material system the GS and ES do not overlap or only for a small part due to the
inhomogeneous broadening. An extensive theoretical study has been done on this kind of
behavior by Asryan et al. [3]. They also predict that the transition from GS to ES becomes
more smooth if the GS peak and ES peak overlap more due to a smaller energy difference
or a larger inhomogeneous broadening, for example the case in InAs QD on InP substrate.
However our simulations show that this cannot be the only effect which causes the smooth
transition from GS to ES gain.
The second simulation does have the continuously changing wavelength with respect
to the injection current density like in the measurements, but the amount of peak
wavelength shift is only 45nm while the measurements show a shift of at least 65nm.
Besides the magnitude of the shift in the peak wavelength, the spectral width is also not
comparable with the measured gain spectra. The width of the gain spectrum from the GS is
determined by the inhomogeneous broadening of the QD size, and is dependent on the
homogeneous broadening. Gain from only one state in the QDs or from two states which
are very close to each other like in the second simulation, does not give the same spectral
width as measured given the 41meV FWHM of the inhomogeneous broadening and the
40meV FWHM of the homogeneous broadening.
In order to have a satisfactorily agreement between the simulation and the experiment
a third simulation is presented where we did choose the location of the GS, the ES and the
WL in such way that both effects are involved. In this simulation the GS wavelength is
located at 1730nm (EGS=0.716eV), the ES at 1630nm (EES=0.760eV), and the WL at
1470nm (EWL=0.843eV). If we now take a look in Fig. 3-7e at the contributions from the
GS (solid) and the ES (dashed) to the total gain at different injection current densities, we
see that at low injection current densities the coupling to the WL has a dominant effect on
the peak wavelength of the gain. This coupling shifts the peak wavelength to longer
wavelengths than the central GS wavelength at low injection current densities. This also
explains the peak wavelength of 1780nm in the measured ASE spectrum at an injection
current density of 500A/cm2 if we assume that the peak in the ASE spectrum corresponds to
the peak in the gain spectrum. At higher injection current densities, the contribution to the
gain from the ES forces the peak wavelength to the shorter wavelength region. In the
contribution of the ES the influence of the coupling to the WL can still be seen which shifts
the peak wavelength of the ES contribution to longer wavelengths, if the injection current
density is decreased. The total gain is shown in Fig. 3-7f together with the measured gain.
From this figure we clearly see the simulated gain spectra following the same trend as the
49
Chapter 3
measured gain spectra. We expect that the two mechanisms both shift the peak wavelength
of the gain spectrum to even shorter wavelengths if there are other higher energy states
available in the dots between the ES and the WL. The amount of shift originating from
these higher energy states will decrease with respect to the increasing injection current
densities due to the increasing number of states per energy range [36].
From the simulations we also noticed that the relaxation time from SCH to WL τs and
the carrier recombination time in the WL τqr do not have influence on the change in shape
of the gain spectrum, but only on the transparency current density of the GS and ES gain.
The value for the transition matrix element determined in the fitting procedure is used to
calculate the spontaneous emission lifetime (3-11) and found to be 2.5ns which is
compatible with a total carrier recombination time of 1ns for the GS and ES levels.
3.7 Conclusion
In this chapter we have presented the measured modal gain spectrum from the
InAs/InP (100) QD gain material designed to emit light in the 1600nm to 1800nm
wavelength region. The data have been measured using the ASE signals from a series of
amplifiers with varying length, a method that can provide accurate data over a wide range
of current densities of samples that are several millimeters or more long. The origin of the
observed blue shift in this gain spectrum with increasing current and the spectral width has
been analyzed by modeling the QD gain material with a QD rate-equation model. The
simulations suggests that the large blue shift is due to a combination of two effects which
are present in the material. First the shift from GS to ES with respect to the injection current
density and secondly the dot size dependent escape rates from the ES to the WL which
decreases the gain for the short wavelengths and increases the gain for the long
wavelengths. The latter effect is dominant at lower current densities. This knowledge of the
behavior of the amplifier under different injection current densities can be used to optimize
an integrated optical QD amplifier by e.g. the use of two section amplifiers biased at
different injection current densities.
50
4 Tunable wavelength filters in the 1.6 to
1.8 µm wavelength region
Abstract – In this chapter we present the design, fabrication and characterization of
two monolithically InP based integrated electro-optically tunable filters. These filters are
designed to be used in an integrated tunable laser source in the 1600nm to 1800nm
wavelength region using active-passive integration technology. This shows that this
integration technology, originally designed to be used around 1550nm wavelength, can also
be used successfully in the 1600nm to 1800nm wavelength region without a large penalty
in performance. The two filters, a high resolution arrayed waveguide grating (AWG) type
filters and a low resolution multi-mode interferometer (MMI)-tree type filter are made
tunable using 5mm long electro-optically phase modulators in the arms of the waveguide
arrays. Measurements show that these filters can be tuned over a wavelength range of more
than 100nm with an accuracy of 0.1nm (1% of the free spectral range) for the high
resolution filter and an accuracy of 9nm (4% of the free spectral range) for the low
resolution filter.
4.1 Introduction
Monolithically integrated tunable optical filters have been an active research area for
over twenty years. Two important applications where a fast (down to nanoseconds) tunable
integrated filter is desirable are the monolithically integrated continuous tunable lasers [19]
and more recently also all-optical wavelength routing in waveguide-division multiplexing
(WDM) systems [66]. The demand of lasers that can be tuned continuously and rapidly is
currently growing quickly due to the increase in the use of such coherent light sources in
measurement systems. Examples are the use of scanning laser sources in medical imaging
such as frequency domain optical coherence tomography (FD-OCT) for ophthalmology
[23] and spectral imaging in dermatology and hematology [86]. Also in other measurement
systems such as gas sensing [79] and fiber Bragg strain sensing [38] fast scanning tunable
lasers are required. In order to realize a monolithically integrated continuously tunable
laser, a semiconductor based wavelength filter has to be integrated. Currently there are
three types of monolithically integrated wavelength selective components that have been
demonstrated and applied; the tunable distributed Bragg reflector (DBR) [19], the tunable
ring resonator [53] and the more complex tunable arrayed waveguide grating (AWG) [47].
51
Chapter 4
For hybrid integrated tunable laser sources it has to be noted that the microelectromechanical-systems (MEMS) in hybrid integrated tunable laser sources can be a
good alternative as e.g. discussed in [45].
Tunable DBRs and the more complex related structures such as the segmented DBR
(SG-DBR) are most often used as intra-cavity tuning elements in a laser cavity. Such DBRs
are be tuned by current injection where the injected carriers cause a change in the refractive
index of the grating elements. Typically two different DBRs have to be combined to realize
a widely tunable laser using the Vernier principle [19]. Another option is the digital
supermode (DS)-DBR laser that uses a tunable DBR in combined with a relatively
broadband digitally chirped grating to realize a widely tunable laser [76]. For OCT a fast
continuous tuning over many tens of nanometers is required. This is however more difficult
to achieve due to the Vernier-like effect and the current control used in Bragg grating based
devices.
A second drawback of the tuning technique though current injection in the DBR
mirrors is that heat is generated in the waveguide thus changing its temperature. Also the
injected carriers have an effect on the total roundtrip gain in the laser [15]. The amount of
heat depends on the amount of current and therefore on the detuning of the DBR. Since the
relaxation of the temperature to equilibrium typically takes milliseconds in these devices,
tunable lasers with DBR type of filters can have a stabilization time in the order of
milliseconds. Furthermore due to ageing the current-detuning relation of the DBRs changes
over time and this has to be corrected for over the lifetime of the device, this aspect is
currently well managed in production DBR designs.
AWG filters, as discussed in [59], can also be used in tunable lasers. When an
active/passive integration scheme is used in the fabrication such a passive optical filter can
be combined on a single chip with an optical amplifier [9][21]. AWGs can be tuned in
different ways. The simplest way is to change the temperature of the AWG [35] however
the tuning speed is limited by the change of temperature in time, which is in the millisecond
range. The tuning range is limited by the operating temperature range and the approximate
0.12nm/C detuning of the AWG wavelength [6]. Another more interesting approach is the
introduction of a series of phase modulators (PHMs) in each arm of the AWG [47]. As will
be discussed, by introducing a linear increasing phase shift over the series of PHMs, the
AWG can be continuously tuned over its total free spectral range to a desired wavelength.
The PHMs in these AWGs can be current or voltage controlled. In case of current control
the phase shifting is based on carrier injection which introduces carrier induced effects [66]
[73][48], however this brings the same issues as with tunable DBRs. In case of a voltage
controlled PHM, the phase shifting is based on field-induced effects [84][48]. The
advantage of reverse biased controlled PHMs is the relatively low current (in the nA to µA
range for reverse bias and in the 1-10mA range for forward bias) which flows through a
PHM. This makes that there is negligible heat dissipation which prevents unwanted
temperature tuning of the filter, it reduces the settling time of the filter and improves its
long term stability. Furthermore the switching speed of the PHMs in reverse bias can be up
52
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
to several tens of GHz and theoretically up to 200GHz [10][85] whereas in forward bias the
switching speed is limited to 1GHz [66].
An AWG has a periodic transmission spectrum. Consequently for wide tuning ranges
such as those required for FD-OCT a combination of two filters will have to be used to
select one of the transmission peaks of the AWG. However with the combination of two
filters very large continuous tuning ranges of over 200nm can be realized. Obviously there
are also disadvantages of such tunable AWG filters. They tend to be relatively large, due to
the length of voltage controlled phase modulators, which can be up to several millimeters,
and the large number of waveguides required. Also a fairly large number of electrode
voltages needs to be accurately controlled.
Another important aspect is the wavelength scan range required in applications such as
OCT. OCT imaging is typically applied in material that has a high water content and that is
highly scattering. This makes that the wavelength range around 1700nm is of interest for
OCT [40]. This wavelength range is in between two water absorption peaks and the
Rayleigh scattering of the light is lower compared to light at wavelengths in the 1300 and
800nm ranges that are currently used routinely. Devices such as a tunable AWG can be
modified relatively easy to operate in this wavelength range.
In this chapter we demonstrate two different InP-based tunable filters designed to be
used in a monolithically integrated tunable laser source for FD-OCT purposes. Both filters
are based on the principle of a tunable AWG filter with an array of voltage controlled
PHMs in the arms of the filter array. These types of filters are chosen for three main
reasons. Firstly the combination of these two filters can be tuned over more than 200nm in
the 1600-1800nm wavelength region. Secondly the filters can in principle be tuned within a
nanosecond due to the fast electro-optical response in the reversely biased PHMs [83]. The
third reason is that these filters can be fabricated within the active/passive integration
technology of COBRA [7][60] and integrated laser systems in the required wavelength
range can be realised.
The tunable filters have been realized on a single InP-chip and are positioned in a
monolithically integrated ring laser structure. In this way the filter inputs are connected to
quantum-dot (QD) optical amplifiers. Consequently an on-chip 1600-1800nm amplified
spontaneous emission (ASE) light source is available that can be used to characterize the
AWGs. This chapter focuses entirely on the results from the tunable AWG filters. Such
results are of a wider interest than just for intra-cavity laser applications: for example, they
may be used for switching of optical signals in the wavelength domain or space domain
[66]. Two types of such filters are presented. The first configuration is the more traditional
AWG design with free propagation regions to couple light into and out of the array of
waveguides. In the second configuration MMIs are used for distributing and collecting the
light to and from the waveguide array. To our knowledge this is the first time this
integrated MMI-based filter is presented.
First the principle of the tunable AWG is presented. Then the design issues of the
filters are discussed, the issues relating to the new wavelength range and the fabrication
53
Chapter 4
limitations. After that, the calibration procedure and control of the tunable AWGs is
presented and the tuning capabilities are demonstrated over the 1630 to 1790nm
wavelength range. The measurement results demonstrate that the AWG operates
satisfactorily thus demonstrating for the first time that the ridge waveguide InP based
optical integration technology can be applied in the 1600-1800nm wavelength range. The
results on the performance of ring laser devices using the filters will be presented
elsewhere. Details on the QD-amplifier material can be found in Chapter 3 [69].
4.2 Tuning principle
In an AWG the light entering at the input is distributed over an array of n waveguides.
At the other end of the array the output of the waveguide array is combined again. The
physical path length difference ΔL between adjacent array waveguides is such that the
optical path length difference is an integer number m times the wavelength λc devided by
the effective index of the array waveguides Neff. The central wavelength of the AWG can be
expressed as [59]
c 
L  N eff
(4-1)
m
From this equation it can be seen that the order m of the filter can be increased to
(m+1) by increasing the optical path length difference with one wavelength without
affecting the central wavelength
c 
L  N eff  c
(4-2)
m 1
Furthermore we know that this increase in optical path length difference causes a shift
in the central wavelength when the filter is treated as an mth order filter. This shift in central
wavelength is called the free spectral range (FSR) and equals [59]
 FSR 
c N eff
m

(4‐3) Ng
where Ng is the group index in the waveguide mode. In other words, the central wavelength
of the filter λc can be shifted with ΔλFSR when the optical pathlength difference is increased
with one wavelength λc.
The increase in optical pathlength difference can be introduced with an array of PHMs
in the arms of the AWG. Including an array of PHM in the arms of the filter with an equal
length LPHM does not influence the filter characteristics. The optical path length of PHM i
can be expressed as
54
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
opt
S PHMi
 LPHM  N PHMi , eff
(4-4)
Applying a reverse biased voltage on PHM i increases the effective index in the
waveguide [73] resulting in an increase in optical path length
opt
Vi   LPHM  N PHMi ,eff Vi 
S PHMi
(4-5)
where ΔNPHMi,eff(Vi) is the change in effective index when a reverse bias voltage Vi is
applied on PHM i. A linear increasing optical path length difference ΔSPHM in the array of
PHMs can be introduced if
opt
Vi   i  S PHM
S PHMi
for i=1,2,….n (4-6)
where PHMi=1 is the innermost PHM with the shortest initial arm length. If this linear
increasing optical path length difference ΔSPHM introduced by the array of PHMs equals one
wavelength λc, we know from the equations above that either the order m of the AWG is
increased to m+1 or the central wavelength of the filter is shifted over one FSR Δλc=ΔλFSR.
When ΔSPHM is a fraction of λc it also shifts the central wavelength over a fraction of the
FSR. The shift in central wavelength due to the introduction of ΔSPHM in the PHMs can be
expressed as
c 
S PHM
c
 FSR
(4-7)
This linearly increasing extra optical path length difference ΔSPHM with respect to the
central wavelength λc can also be expressed as an introduced extra optical phase difference
Δ.
c 

FSR
2
(4-8)
The introduced extra optical path length in each of the PHMs can also be expressed as
an extra introduced optical phase
PHMi  i  
(4-9)
Before tuning the filter to a desired wavelength relative to the central wavelength λc,
the extra optical phase difference Δ has to be determined with (4-8). Then the necessary
extra optical phase for each PHM can be calculated with (4-9).
When scanning the tunable AWG over one FSR, there is one arm in the waveguide
array of which the optical path length is not changed. This arm can be chosen to be any arm
with number ic. The other arms numbered i are then varied over an optical path length
which is (i – ic)Δ. Since only the phase is relevant for the interference at the output, the
55
Chapter 4
path length change can be limited to at most 2π phase delay. This limiting of the path length
change does not change the transmission for the central wavelength, but does introduce a
small change in the filter transmission for other wavelengths which we neglect. This small
change occurs due to the fact that the AWG does not have an accurately defined order
number when the extra introduced optical phases are truncated to modulo 2π. This
truncation also makes it possible to choose the arm with number ic arbitrarily. Thus the
PHMs never have to be driven to a delay over 2π. However this has an important technical
consequence for the control. When a continuous scan is made over the free spectral range
of the AWG, all arms except for the arm ic and its direct neighboring arms, will have to
have large changes in their control voltages at the points where the 2π phase jumps occur.
Together with the accuracy requirement on the phase setting, this will mean a strict
requirement on the control electronics in terms of bandwidth and slew-rate to be derived
from the required scan speed.
4.3 Design
The requirements on the filter are imposed by the requirements of the laser in which
the filter is going to be used (Chapter 1). These requirements are that the filter should be
tunable between 1600nm and 1800nm with a filter bandwidth less than 0.5nm and a
parabolic filter shape [59]. This means for the filter a free spectral range (FSR) larger than
200nm and a full-width-half-maximum (FWHM) less than 0.5nm. A single AWG filter
with a 0.5nm FWHM and an FSR of 200nm can be designed on InP however it requires a
large AWG with more than 200 arms in the AWG. To make such a filter tunable implies
that more than 200 individually controlled phase shifters should be included in the arms of
the AWG. In practice this means also more than 200 parallel voltage sources, all
individually bounded to the chip. From the practical point of view this becomes very
inconvenient. Also the size of the filter would become prohibitively large.
We chose to design two filters, the first filter a high resolution (HR) filter with a
FWHM of 0.5nm and a FSR of 10nm, and the second filter a low resolution (LR) filter with
a FWHM of 29nm and a FSR of 210nm. The HR-filter is used as a narrow wavelength filter
to allow a maximum of 3 ring laser cavity modes with a 0.02nm mode spacing and
suppressing other neighboring ring cavity modes. This HR-filter however has a periodical
passband response in the wavelength domain with a periodicity of the FSR, in this case
approximately 10nm. The LR-filter is used to select one passband of the HR-filter and
suppress the other passbands. In Fig. 4-1 an example of usable transmission characteristics
of the two filters are given.
56
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
Power transmission
1.0
High resolution filter
Low resolution filter
0.8
0.6
0.4
0.2
0.0
1600
1650
1700
1750
1800
Wavelength [nm]
Fig. 4-1 Example of filter characteristics of the high resolution filter (solid) and low resolution filter
(dashed)
The tunable filters presented in this chapter are designed to be used for transverse
electric (TE) polarized light in the 1600-1800nm wavelength region. This optimization for
TE polarization is chosen since the intended use of the filter is in a TE polarized laser.
4.4 Layerstack and waveguide design
The filters will be fabricated using the active/passive integration technology of
COBRA [7][60]. This integration technology is optimized for the 1550nm wavelength
region. However as we will demonstrate it can also be used in the 1600nm to 1800nm
wavelength region without large modifications to the layerstack and the fabrication process.
The standard integration technology uses a 500nm (Q1.25) InGaAsP film layer which is not
intentionally doped (n.i.d.) which in practice means slightly n-doped (61016cm-3) in our
technology. Two types of ridge waveguides are available in our integration technology. The
first is a 2.0µm wide low contrast waveguide where the ridge is etched 100 nm into the
index guiding InGaAsP layer (shallow etch) to achieve the lowest propagation loss. The
second type is a 1.5µm wide deeply edged high contrast waveguides for sharp bends where
the etch is at least 100nm through the index guiding layer.
Deeply etched high contrast waveguides, which have a higher waveguide loss
compared to shallow etched waveguides, are not used in the designs presented here. This
was to minimize the fabrication process complexity and thus fabrication reliability. The size
reductions achievable through the use of deep etched waveguide were minimal due to the
necessary long QD amplifiers and PHMs. As second advantage the absence of these deeply
etched areas reduces height differences over the wafer which increases the wafer uniformity
after polyimide planarization (will be discussed in section 4.5).
Using the same standard shallow waveguides in the 1600 to 1800nm region has an
influence on the waveguide performance. The absorption coefficient in InGaAsP (Q1.25)
57
Chapter 4
reduces due to the larger distance to the band gap [24]. This absorption is however
estimated to be negligible. The size of the optical mode however increases due to the longer
wavelength. The overlap of the optical mode width the n-doped InP bottom cladding and
the p-doped InP top cladding increases due to this optical mode increase. Especially the
increasing overlap with the p-doped InP top cladding introduces more losses. These losses
are estimated to increase from approximately 2.7dB/cm to 4.6dB/cm (Chapter 2.3)[14][77].
Furthermore, the increase in mode size increases the overlap with the sidewall and so the
losses due to sidewall roughness. For this reason the waveguide width is increased from
2.0µm to 2.2µm, just underneath the cut-off of the 3rd order mode. The total waveguide
losses in the 1600 to 1800nm wavelength range are therefore expected to be approximately
75% higher than at 1550nm due to the increase in mode size.
The film layer as well as the 200nm InP layer on top of the film layer is n.i.d. to
reduce waveguide losses. Experiments on test structures show that the phase shift
efficiency in p-doped waveguides increases due to the increase in carrier induced electrooptical effects, however it also increases the losses due to the increase in free carrier
absorption.
In Fig. 4-2 the layerstack as well as the schematic cross-section of the different
components is presented.
1500nm Au
Polyimide
2.2µm
300nm Ti/Pt/Au
200nm p-InGaAs
1300nm p-InP
200nm i-InP
Isolation Waveguide
5 x QD-Layer
500nm i-InGaAsP
70nm i-InP
SOA
n-InP (substrate)
PHM
Fig. 4-2 Layerstack and schematic cross-section of the different used components. Note that the
dimensions are not scaled
4.4.1 High resolution filter
For the high resolution filter we choose to use a 154th order orthogonal shaped type
AWG [65], containing 28 arms with a physical arm length difference of ΔL=81.16 µm. The
layout of this orthogonal shaped AWG has been adapted in two aspects. The first is that an
array of PHMs has been added in the arms of the AWG; with these PHMs the filter can be
tuned. The second is that the waveguide layout was modified so the PHM waveguides had a
58
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
fixed 30 µm distance between them. The fixed distance ensures that the distribution of
waveguides over the chip area of the PHMs is uniform. This is necessary for a step in the
processing of the chip. It is required to obtain a sufficiently uniform polyimide
planarization before p-metal contact evaporation. This increases the reliability of the
fabrication of the electrical contacts on top of the PHMs. The adaption in the layout has
been performed by tilting the vertical parts of the arms in the AWG as shown in Fig. 4-3a.
Keeping the innermost arm of the AWG as a reference arm, all the other arms can be placed
on equal distances from each other by choosing the proper tilting angle for each arm. The
AWG filter has been simulated with an S-matrix oriented CAD-tool [42] resulting in a filter
with a FWHM of 0.50nm and a spectral suppression of 0.064dB at 0.04nm (two ring cavity
modes with 0.02nm mode spacing) from the central wavelength, both determined from the
power spectrum.
4.4.2 Low resolution filter
For the LR-filter a standard AWG filter shape could not be used. The arm length
differences for this 7th order low resolution filter have to be only 3.69µm. In a standard
AWG design [59] it is not possible to get such a small arm length difference with the
preferred 30µm spacing between the phase shifters. For this LR-filter two different filter
shapes are considered; an AWG laid out in an S-shape [66] and a filter based on an
MMI-tree. The S-shaped AWG is a zero-order filter adapted on one side to introduce the
small arm length differences as schematically depicted in Fig. 4-3b. In the MMI-tree based
filter the light is distributed over the arms of the waveguide array in the filter by means of a
series of 1x2 MMI couplers called an MMI-tree (Fig. 4-3c). The arm length differences in
this layout are introduced in between the MMI-tree and the array of PHMs on both sides.
There is a difference in filter response between these two filter arrangements when an
equal number of arms and equal arm lengths are used. This is mainly due to the difference
in light distribution over the arms. In the S-shaped AWG the light is distributed among the
arms in the free propagation region (FPR). This leads to a Gaussian distribution of the light
over the arms in the array which leads to a Gaussian spectral filter response. In the
MMI-tree based filter the light is distributed equally over all the arms with the balanced
1x2 MMI couplers. This rectangular shaped distribution over the arms results in a sinc2
response in the power spectrum.
59
Chapter 4
PHM
(b)
(a)
(c)
Fig. 4-3 (a) Schematic design of the HR-filter. The dotted lines represent the original array arms
which are moved to get equal distances between all arms. (b) S-shaped LR-filter. The dotted lines
represent the original array arms which are moved to introduce the small arm length differences. (c)
MMI-tree shaped LR-filter. The MMI-trees on both sides are tilted to introduce the small arm length
differences.
The filter response from both LR-filter types has been simulated with the CAD-tool.
The S-shaped AWG filter has been simulated with 11 arms and the MMI-tree shaped filter
with 8 arms (3-level MMI-tree). The calculated spectral filter characteristics are given in
Fig. 4-4. From this figure one clearly sees that the MMI-tree filter has a sharper filter
response than the S-shaped AWG filter despite the lower number of arms. The penalty for
the MMI-tree filter is in the spectral response outside the passband. For the MMI-tree
shaped filter the spectral suppression is more than 13dB whereas the spectral suppression
for the S-shaped AWG filter is more than 30dB. This difference is a direct consequence
from the Gaussian and sinc2 spectral response from the S-shaped AWG filter and the
MMI-tree filter respectively. This lower spectral suppression in the MMI-tree filter outside
the passband is however no problem for the use in the ring laser. An extra suppression from
13dB to 30dB in one roundtrip does not make any difference.
60
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
0
Power transmission [dB]
S-AWG
MMI-tree
-10
-20
-30
-40
1600
1650
1700
1750
1800
Wavelength [nm]
Fig. 4-4 The simulated filter responses of an S-shaped AWG filter including 11 arms (solid) and a
MMI-tree filter including 8 arms (dashed). Both filters have the same arm length difference between
adjacent arms.
More important is the suppression at approximately 10nm from the central wavelength
of the filter. This LR-filter has to select one passband from the HR-filter and suppress all
others. From the simulations could be determined that the S-shaped AWG filter has a 29nm
FWHM resulting in 1.44dB suppression at 10nm from the central wavelength. The
MMI-tree filter has a 25nm FWHM resulting in 1.97dB suppression at 10nm from the
central wavelength.
Based on these simulations it was decided to use the MMI-tree filter as a LR-filter in
the tunable ring laser. It has a sharper filter response using less arms (also less PHM).
Furthermore the layout of the MMI-tree filter is more compact, resulting in less waveguide
losses and a smaller chip area necessary. PHMs are included in the arms of the filter to
make the filter tunable. These PHMs are separated with a fixed 30µm pitch for the same
reason as in the HR-filter.
61
Chapter 4
(a)
(b)
(c)
Fig. 4-5 (a) The designed waveguide and PHM mask of the complete integrated tunable laser system.
Also plotted is the metallization of the phase modulators to indicate their position. (b) Zoom in on
FPR of the HR AWG filter showing the design features to prevent back reflections and the positions
of the in- and output waveguides. (c) Zoom in on MMI-tree from the LR-filter showing how the path
length differences have been realized.
4.4.3 Mask layout
In Fig. 4-5a the mask design is presented for the complete integrated tunable laser
system. All waveguides are designed to be 2.2µm wide ridge shallow waveguides etched
100nm into the film layer. In the center of the mask design the orthogonal shaped tunable
AWG HR-filter is located including the 28, 5mm long PHMs. These PHM are parallel to
the [0-11] crystal direction to maximize the Pockels effect providing a positive refractive
index change in this direction [83]. The linear phase shift efficiency in these type of PHMs
turned out to be in the order of 0.4rad/(Vmm) at 1550nm [82] but also depends on the final
doping in the waveguide [83]. The control electronics available has a maximum output
voltage of 10V. To keep the control voltages as low as possible and to make sure that a 2π
phase shift can be obtained, 5mm length was chosen for the PHMs.
On both sides of the AWG multiple input and output waveguides are connected to the
FPR as can be seen in Fig. 4-5Fig. 4-5b. Five waveguides are equally spaced in the center
of the FPR with a 1.2nm channel spacing. The central waveguide is used as in- or output in
the ring laser cavity, the other four waveguides are leading to a cleaved facet and can be
used as test in- or output. Besides these five waveguides, two more waveguides are
62
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
positioned close to the corner of the FPR. These are positioned at the higher diffraction
order of the waveguide array when the zero order image is focused on the central output
waveguide. These test waveguides also lead to a cleaved facet. In between the five central
waveguides and the waveguides in the corner a triangular shaped FPR is added to prevent
reflection of the ends of the FPR directly back into the AWG (Fig. 4-5b).
Below the HR-filter, the LR MMI-tree filter is located. In Fig. 4-5c one side of the
filter is depicted including the MMI-tree. It clearly shows how the MMI-tree is slightly
tilted on both sides with respect to the PHMs to introduce the small arm length differences.
Both filters are positioned next to each other with the PHM from both filters parallel to
each other keeping the 30µm pitch spacing in between the PHMs. In this way a large area
was created with a uniform PHM distribution to improve the polyimide planarization. For
the same reason 20 extra test/dummy PHMs and waveguides are included underneath and
above the PHMs in the filters.
All input and output waveguides exit the chip at an angle of 7 with respect to the
normal of the cleaved facet in order to minimize facet reflections [33].
4.5 Fabrication
The devices were fabricated on wafers that contained active as well as passive areas to
realize both active layerstack components (Semiconductor Optical Amplifiers (SOA)) as
well as passive layerstack components (waveguides, AWGs, MMIs and PHMs). The activepassive layerstack has been fabricated in collaboration with the MiPlaza division of Philips
Research using the butt-joint integration approach [11]. The active layerstack is first grown
on an n-type InP (100) substrate by metal-organic vapor-phase epitaxy (MOVPE), as
presented in [2]. In the active region, above the 500nm n-InP buffer layer, five InAs
quantum-dot (QD) layers are stacked with an ultrathin GaAs interlayer underneath each QD
layer to control the size of the QDs. These QD layers are placed in the center of a 500nm
InGaAsP not-intentionally-doped (n.i.d.) (Q.125) optical waveguiding core layer. The QD
layers are designed to produce a gain spectrum in the 1600nm to 1800nm wavelength
region (Chapter 3)[69]. The passive areas are selectively etched back till 20nm underneath
the QD layers. In the first regrowth step the passive InGaAsP (n.i.d. Q1.25) film layer is
grown. In the second regrowth step the common 1.5µm p-InP top cladding is grown
followed by a compositionally graded 300nm p-InGaAs(P) top contact layer.
The devices are fabricated using a three-step CH4-H2 reactive-ion dry etch process to
create shallow etched waveguides with or without contact layer and isolation section to
prevent electrical crosstalk between PHMs and SOAs. The structures are planarized using
six layers of polyimide. These six layers of polyimide are necessary to increase the surface
flatness of the polyimide which in case is necessary to open all PHM and SOA at the same
time prior to metal evaporation. For this reason the PHMs are also equally spaced with a
fixed 30µm pitch to reduce non-uniform polyimide planarization. Height variations in the
63
Chapter 4
polyimide cause height variations in the opening of the PHM and SOAs. This leads either
to polyimide in between part of the PHMs and the metal when the polyimide is not enough
etched away or leads to areas where too much polyimide is etched away. This leads to
higher waveguide losses due to the reduced spacing between the metal and the optical
mode. Furthermore, using the six layers, a thicker total layer of polyimide has to be etched
away which results in a rough surface. This roughness increases the adhesion of the metal
to the polyimide.
Evaporated Ti/Pt/Au metal pads contact the PHMs and SOAs to apply a voltage or a
current. The SOA contact pads are thickened with plated Au to reduce the electrical
resistance. The backside of the n-InP substrate is metalized to create a common ground
contact.
The structures are cleaved off from the rest of the wafer and no coating is applied to
the facets. A picture of the 10 by 6 mm chip is depicted in Fig. 4-6.
Fig. 4-6 (a) Photograph of the resulting 10x6mm chip (b) A scanning electron microscope (SEM)
picture of the a phase modulator waveguide cross-section including the p-contact metallization and
polyimide planarization. The core wave guiding layer is indicating between the two white dashed
lines.
4.6 Measurements
The laser chip including the two filters is mounted p-side up on a copper chuck. This
copper chuck is cooled with a constant water flow of 13, 15mm underneath the chip, to
keep the chip stable and at a constant temperature. The temperature of the chip is not
actively controlled with a Peltier element. This is not necessary due to the constant current
injection in the amplifiers and the low reverse bias currents through the electro-optic PHMs
of the tunable filters. The bond pads of the PHMs are bonded individually to a printed
circuit board (PCB) which is also mounted on the copper chuck. On this PCB each
64
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
connection leads to a connector to which the electronics can be connected. This method
avoids the fragile use of multiprobes directly on the chip.
Each PHM is controlled by a 13 bit arbitrary waveform generator with a voltage range
between -10V and +10V. (We will only use the -10V to 0V range)[70]. A waveform pattern
can be uploaded into a 4096 word memory for each waveform generator. A common clock
and a common trigger signal can be used to step through each waveform pattern for the
PHMs at the same time. The minimum step size is 10ns and settling time of 4ns between
+0.5V and -0.5V (40ns between +5V and -5V). These coupled arbitrary waveform
generators can be used to control all PHMs parallel with a 100MHz rate.
All results presented here are obtained with TE light emitted by the QD SOAs. The
output light from cleaved output facets is collected with a lensed fiber and measured with a
spectrum analyzer with a minimum resolution of 0.05nm (YOKOGAWA AQ6375).
First the static filter characteristics are presented and discussed followed by a
description of the PHM calibration method and the calibration results for both filters. In the
second part the tuning results of both filters are presented.
4.6.1 Static filter characteristics
The waveguide loss was measured at 1535nm which is the standard wavelength at
which optical losses of all our devices are measured using a Fabry-Pérot based loss
measurement technique [74]. This makes that the results can be compared with other
waveguides fabricated. The losses were determined to be (1.9 ±0.4) dB/cm (TE) which
indicates a good optical material quality and waveguide etching quality. Direct
measurement of the waveguide losses around 1700nm was not possible due to the fact that
we did not have suitable single mode tunable light sources available in that wavelength
region. The waveguides losses around 1700nm wavelength are expected to be slightly
higher 3.3dB/cm (75% higher) as discussed above. An estimate of the waveguide losses
and the losses in the filters can be made by comparing the optical spectrum from a QD
amplifier with the optical spectrum from a similar QD amplifier through a filter. The
difference in peak power at the central passband wavelengths gives in indication of the total
losses. This has been done for the AWG filter. The total losses were approximately 10.5dB.
These losses include the 9.2mm passive waveguide losses outside the filter. Assuming the
same waveguide loss as measured at 1535nm, the optical loss in the AWG is 8.8dB. To
compare this value with common AWG loss figures one must also subtract the expected
loss in the 5mm long PHM waveguides in the filter which leaves 7.8dB. This loss value is
relatively large compared to a typical excess loss value of 4dB in an InP AWG [8]. It
indicates that the waveguide losses are slightly higher.
The first aspect to consider for the electrical characterization of the PHMs is the dark
current. The dark current gives an indication on the quality and reliability of the PHM. A
dark current in the nA range at -10V results in an operating current in the µA range due to
residual photocurrent (dependent on the light intensity in the PHM). Higher dark current
65
Chapter 4
can indicate a leakage current in the PHM or the electrical circuit. This leakage current can
increase during the first couple of measurements (less than 10 measurements) and
sometimes exceeding 50mA at -10V. This leads to unwanted heat generation possibly in the
chip which detunes the central wavelength of the filters. In worse case the PHM will be
damaged resulting in an uncontrollable PHM.
The dark current has been measured in four 5mm long test PHMs under reverse bias.
These test PHMs are used instead of the PHMs in the filters to prevent damage to the bond
pads during probe contacting. Two of these PHM show a similar V-I curve with a dark
current of less than 60nA at -10V. The two other PHMs had a dark current of 178nA and
1.35µA at -10V indicating a leakage current. The path of the leakage current is not yet
known and can only be determined by the destructive removal of the chip from the setup.
So far we only could see that the high leakage current does not directly influence the
performance of the PHM. PHMs with low leakage current show comparable relation
resulting phase shift efficiencies. This is an indication that the leakage path is outside the
PHM.
The arbitrary waveform generators however cannot provide this high leakage current.
This results in a nonlinear voltage drop from the voltage setting to the actual voltage on the
PHM. These PHMs with a high leakage current still work as a proper phase modulator but
will not be taken into account when the phase shift efficiency to the PHMs is discussed.
4.6.2 Calibration method of the phase modulators
To be able to tune the filters the central wavelength(s) of the passband(s), the FSR and
the electro-optical behavior of the PHMs has to be known. The electro-optical behavior of
the PHMs defining the relation between the applied voltage and the optical phase shift can
be expressed in a quadratic polynomial function:
i Vi   aVi 2  bVi  c
(4-10)
where a and b are the quadratic and linear phase change originating from field induced
electro-optical effects and free carrier depletion based electro-optical effects which change
the refractive index in the PHM [73]. The c term is the phase offset originating from phase
errors in the arms of the filters due to variations in layer thicknesses over the wafer and
fabrication imperfections. This relation between applied voltage and phase can be
determined for each PHM by tuning that particular PHM in the AWG filter over a voltage
range and measuring the optical response of the filter in a small wavelength region (factor 2
smaller then the width of the passband of the filter). The transmitted optical power will vary
when the voltage is scanned due to the constructive or destructive interference of the light
from that particular arm with the light in the other arms. The output power can be described
according to:
66
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region

P  A  C  cos aVi 2  bVi  c

(4-11)
Here P is the measured optical power, A is the mean output power and C the
modulation of the output power due to the varying phase shift in the single PHM. The
coefficients a, b and c describe the phase shift characteristics. A voltage dependent
reduction in the modulation depth caused by an increase in absorption due to the band edge
shift [83] is not observed. Also no linear power fluctuation due to the applied voltage was
observed. Both effects are therefore omitted.
The a, b and c coefficients describing the phase shift characteristics have been
determined using an automated measurement routine. In this measurement routine the ASE
from one of the QD-amplifiers is used as a light source and the transmitted optical power
through one of the passbands of the filter is recorded with a 0.2nm resolution spectrum
analyzer. The measurement routine scans the voltage on a single PHM over at least 2π
phase shifting (0V to -7 in 0.1V steps) and records the optical output power through the
filter at the central wavelength of the passband. The a, b and c terms are extracted by a least
square fitting routine of (4-11) to the recorded data. This routine is executed on each PHM
for several wavelengths as discussed below. After each routine the PHM is set to the
voltage for zero phase shift (found by setting equation (4-10) equals 0). In Fig. 4-7 two
examples of recorded scans in the 1680.8nm passband are presented. The transmitted
optical power is recorded versus the reverse bias voltage on the PHM in the central arm of
the AWG filter (circles) and on the PHM in the outermost arm of the AWG filter (squares).
The fitted curve is also given in this figure, showing a very good agreement with the
measurements.
62
Power [nW/nm]
59
56
53
50
meas. central arm
fit central arm
meas. outer arm
fit outer arm
47
44
0
1
2
3
4
5
Reverse bias voltage [V]
6
7
Fig. 4-7 Two examples of the measured transmitted optical power versus the applied voltage and the
fitted curve for the PHM in the central arm of the tunable AWG filter (circle) and for a PHM in the
outmost arm of the tunable AWG filter (squares) (1680.8nm passband used).
67
Chapter 4
4.6.3 Calibration HR-filter
The central wavelengths of the passbands in the HR-filter have been determined by
measuring the optical spectrum through the HR-filter at one of the monitor outputs using
the ASE from one of the QD-SOAs as light source. The FSR and shape of the transmission
channels could be determined from this spectrum. The measured spectrum and the
wavelength dependent FSR is given in Fig. 4-8a. The envelope in the spectrum originates
from the ASE spectrum of the SOA. The filter is designed to have a passband at the central
wavelength of 1700nm. This means that the monitor output has a designed passband 1.2nm
from this central wavelength at 1698.8nm. The measured passband is located at 1701.2nm
which means a 2.4nm spectral shift with respect to the designed value. This spectral shift
originates from a difference between the effective index of the waveguide mode in the filter
compare to the used value in the design. The measured FSR around 1700nm corresponds to
the targeted 10nm FSR at 1700nm within 0.04 nm. The FWHM of the passband at
1701.2nm is 0.46nm where 0.5nm at 1700nm is the design value. In Fig. 4-8b the measured
central passband of the HR filter is given as well as the designed filter characteristics
(corrected for the shift in central wavelength and the absolute power level). These
measurements indicate that the phase errors in the AWG arms are relatively small as will be
discussed below.
14
14
1.10
a
1.06
8
6
1.04
4
FSR (○) [THz]
1.08
10
measured
designed
b
12
Power [nW/nm]
Power [nW/nm]
12
10
8
6
4
1.02
2
2
1.00
0
1620
1650
1680
1710
1740
Wavelength [nm]
1770
0
1800
1700
1700.5
1701
1701.5
Wavelength [nm]
1702
Fig. 4-8 (a) The static optical response of the HR-filter using the SOA as an ASE light source (solid).
Measured FSR between two adjacent passbands (circles) (b) Zoom in on the measured central
passband at 1701.2nm (solid) and the designed filter characteristics (dashed) (compensated for a
1.2nm shift and adjusted in power level).
Within the HR-filter 18 out of 28 PHMs worked properly, 8 PHMs had a high leakage
current between 10mA and 50mA which means they have significantly different calibration
parameters, and the voltage of two PHMs could not be controlled due to an open circuit in
the electronic circuit on the PCB. The a, b, and c coefficients have been determined at all
central passband wavelengths of the HR-filter between 1630nm and 1800nm for the 18
68
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
proper working PHMs and the 8 PHMs with a high leakage current. The offset value, c, is
ideally 0rad for all PHMs at the central wavelength in a passband of the filter and in worse
case it varies between 0rad and 2πrad (more than 2π can not be discriminated from a value
between 0rad and 2πrad). The maximum difference between measured c values for all
PHMs at a certain wavelength gives an indication about the phase error in the arms of the
filter. The measured c values for the 18 proper working PHMs in the HR-filter are depicted
in Fig. 4-9a. For these PHMs the maximum difference between measured c values is less
than 2.2rad (at all wavelengths). Asuming that the phase error never exceeds 2π, based on
the fact that the maximum difference in phase offset only varies less than 2.2rad, indicates
that the error in arm lengths due to wafer non-flatness and fabrication imperfections is less
than 0.04% taking an average arm length of 10mm in the HR-filter.
The linear phase shift term, b, is expected to be the same for all PHMs at the same
wavelength. In Fig. 4-9b the average b coefficients over the 18 proper working PHMs is
given including the statistical error (standard deviation devided by square root of n-1) in the
1630nm to 1800nm wavelength region. The wavelength dependency of the b coefficient is
attributed to the change in optical mode size and the wavelength dependency in the electrooptical effects.
The average linear phase shift efficiency in these PHM can be calculated from Fig.
4-9b and varies between 0.33 rad/Vmm at 1633nm to 0.26rad/Vmm at 1796nm. The
quadratic phase shift term, a, was in all cases between -0.04rad/V2 and +0.04rad/V2 (5mm
long PHM) indicating a minor influence on the phase shifting but it could not be neglected.
2.0
a
1.5
Linear phase shift b [rad/V]
Phase offset c [rad]
2.0
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
b
HR-filter
1.8
LR-filter
1.6
1.4
1.2
1.0
1620
1650
1680
1710
1740
1770
1800
1620
Wavelength [nm]
1650
1680
1710
1740
1770
1800
Wavelength [nm]
Fig. 4-9 (a) Measured phase offset values, c, for all 18 proper working PHMs in the HR-filter. Each
PHM has a different marker. The two dotted lines indicate the maximum difference in offset of
2.2rad. (b) Wavelength dependent average linear phase shift term b and there statistical error for the
HR-filter (diamond) and the LR-filter (circle)
69
Chapter 4
4.6.4 Calibration LR-filter
The determination of the central wavelength of the passband of the LR-filter and the
FSR of the filter is less straightforward than that of the HR-filter. The LR-filter is
positioned in between two SOAs on the chip and could only be measured with one SOA as
ASE light source and the other SOA used as output amplifier. The central wavelength could
however not directly be determined from a measured spectrum due to the convolution of
the ASE spectrum of the first SOA with the passband spectrum of the filter and the
wavelength dependent gain and ASE from the second SOA. Also the FSR could not be
measured directly, owng to the fact that the next passband of the filter is designed to be
210nm away from the designed 1700nm central passband of the filter. These passbands are
far outside the ASE spectrum of the SOAs.
SOA 1
(Ring)
LR
Filter
SOA 2
(Output)
Fig. 4-10 Schematic diagram of the component on the chip used to calibrate the LR-filter. The ring
amplifiers (SOA 1) is used as a ASE light source, the output amplifier (SOA 2) is only used around
transparency to pass the light to the output.
The phase shift coefficients a, b and c could be measured in the same way as in the
HR-filter. The difference is, however, that these coefficients are not measured at the central
wavelength of the filter but at fixed chosen wavelengths resulting in an extra offset value
which will be discussed later.
Within the LR-filter, 7 out of 8 PHMs worked properly and one PHM had a high
leakage current as discussed above. The a, b and c coefficients have been determined at
different wavelengths between 1640nm and 1780nm. The average linear phase shift term, b,
including the statistical error is given in Fig. 4-9b. The average b values correspond to the
founded b values in the HR-filter. The quadratic phase shift term, a, was in all cases
between -0.04rad/V2 and +0.04rad/V2 as in the HR-filter.
During the calibration each calibrated PHM is tuned to its 0 phase position
(Δi(Vi)=aVi2+bVi+c=0) after its calibration. This tunes the filter towards the calibration
wavelength. Since the coefficients are now also determined at wavelengths in between the
LR filter transmission maxima the offset values, c are to be interpreted somewhat different
from those determined at the passbands of the HR filter. The c values are equal to the
phases needed to compensate the linear increasing/decreasing phase change nessesary to
tune the filter away from the static central wavelength. By analysing the c values found, it
70
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
is possible to calculate back the static central wavelength and the FSR of the filter. In Fig.
4-11a the c values that have been determined using the PHM calibration method described
above, are given for the LR-filter at different wavelengths. For the c-values at each
wavelength it is possible to determine a slope (rad/PHM). This represents the linear
increasing/decreasing phase change Δ (4-8) necessarry to tune the filter to that
wavelength. The slopes that have been found for each wavelength are presented in Fig.
4-11b. For the central wavelength of the filter it is known that the c values are ideally all
0rad which means Δ=0. In Fig. 4-11b the wavelength for which Δ=0 indicates the central
wavelength of the filter and it was determined to be 1726.6nm. Furthermore it is known
that the filter is tuned over one FSR if Δ=2π. With a linear fit (neglecting the wavelength
dependency of the FSR) on the datapoints in Fig. 4-11b it was possible to estimate the FSR
and it was found to be 220nm.
2.0
15
b
1640
1660
1680
1700
1720
1740
1760
1780
10
5
Slope (Δφ) [rad/PHM]
phase offset c (rad)
a
0
-5
-10
1.0
0.0
-1.0
-2.0
-3.0
1
2
3
4
5
6
7
8
1640
PHM number
1660
1680
1700
1720
1740
1760
1780
Wavelength [nm]
Fig. 4-11 (a) Determined c values for each PHM in the LR-filter for different wavelengths. (b)
Wavelength dependent slopes of the determined c values over the PHMs and a linear fit to determine
the FSR.
4.7 Tuning results
When the wavelength dependent phase shift coefficients are determined for each PHM
as well as the central wavelength(s) of the passband(s) and the wavelength dependent FSR,
all information is available for the filters to be tuned in a predictable way. The tuning is
always performed relative to the nearest calibration point (for the HR-filter the passband
wavelengths and for the LR-filter the chosen calibration wavelengths). The extra optical
phase Δ can then be determined with (4-8) using a linear fitted value of the wavelength
dependent FSR ΔλFSR(λ). (The 2π modulus of this phase can be used.) The voltage
necessary for each PHM to realize the required phase change in each PHM can be
calculated with (4-10) using the calibrated coefficients for each PHM.
71
Chapter 4
80
1790
Measured wavelength [nm]
Power [nW/nm]
a
60
40
20
1770
b
1750
1730
1710
1690
1670
1650
1630
0
1685
1690
1695
1700
1705
1710
1630 1650 1670 1690 1710 1730 1750 1770 1790
1715
Target Wavelength [nm]
Wavelength [nm]
0.6
0.20
d
0.6
0.10
FWHM [nm]
Detuning [nm]
0.15
c
0.05
0.00
-0.05
0.5
0.5
0.4
-0.10
0.4
-0.15
-0.20
0.3
1630 1650 1670 1690 1710 1730 1750 1770 1790
1630 1650 1670 1690 1710 1730 1750 1770 1790
Target Wavelength [nm]
Target Wavelength [nm]
Fig. 4-12 (a) Spectral response HR-filter for three tuning wavelengths, 1700nm (solid), 1698nm
(dashed) and 1696nm (dotted) (b) Tuning accuracy HR-filter between 1630nm and 1790nm. The
measured central wavelength of the selected passband is given with respect to the target wavelength
(c) Detuning between measured central wavelength and target wavelength. (d) Measured FWHM of
the passband.
4.7.1 HR-filter
In Fig. 4-12a the spectrum between 1685nm and 1715nm is given for the HR-filter
using the ASE from one of the SOAs as a light source. This 30nm bandwidth covers 3 FSR
resulting in 3 passbands within this spectrum. The filter is tuned from 1700nm (solid)
towards 1698nm (dashed) and 1696nm (dotted). All passbands of the filter shift together
with respect to the FSR of the filter. In this case Δ=-(2/5)π for tuning from 1700nm to
1698 and Δ=-(4/5)π for tuning from 1700nm to 1696nm.
The performance of the filter is determined by the detuning of the filter with respect to
the target wavelength and the FWHM of the filter. These quantities have been measured
72
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
over 160nm between 1630nm and 1790nm in 0.1nm steps while tuning the HR-filter over
this wavelength region and using the ASE from the SOA as a light source. The measured
central wavelength of the used passband and the FWHM of the passband are recorded. In
Fig. 4-12 the measured central wavelength as well as the detuning with respect to the target
wavelength and the FWHM of the passband is given. It can be seen from Fig. 4-12 that the
filter can be tuned in a predictable way over the full 160nm with an accuracy of ±0.1nm
(1% of the FSR) and a FWHM between 0.4nm and 0.5nm. The detuning presented in Fig.
4-12c shows a periodical behavior with a periodicity of the FSR of the filter. This indicates
a small error (approximately 1%) in the determination of Δ. This error is most probably
from neglecting wavelength dependency (λFSR and bi) within one FSR.
The calibration of the phase modulators and the response of the tunable filter over time
is very stable. We did not observe any notable change in the filter tuning performance after
using the filter with the same calibration data for 4 months.
4.7.2 LR-filter
The LR-filter is again less straightforward to characterize. The spectral response has
been measured using the ASE from one SOA as light source which is sent through the
LR-filter and afterwards amplified by the second SOA. To minimize spectral deformation
this second SOA has been biased around threshold. Due to the wavelength dependent gain
in the SOA (Chapter 3)[69] this spectral deformation could not completely be prevented.
The transmission spectrum through the filter was corrected for the ASE spectrum from the
second SOA. The signal from the second SOA was measured with the first SOA switched
off and this spectrum was then subtracted from the spectrum with both SOAs switched on.
In Fig. 4-13a the resulting spectra are presented for three tuning wavelengths 1700nm,
1720nm and 1740nm of the LR-filter. (Note that the ASE from the output SOA is
dependent on the input signal and more intense without input signal. Subtracting this
spectrum thus resulted in a negative power spectrum). The performance of the filter has
been measured between 1670nm and 1770nm by measuring the detuning and the FWHM of
the spectra. The detuning is defined as the difference between the peak wavelength in the
spectrum and target wavelength. The FWHM is measured at half the intensity difference
between the peak wavelength and the lowest valley in the spectrum. It can be seen from
Fig. 4-13 that the LR-filter can be tuned in a predictable way with an accuracy of ±9nm
(4% of the FSR) at the edge of the spectrum and less than ±6nm (2.7% of the FSR) in the
central part of the spectrum. The FWHM fluctuates between 15nm and 27nm where 25nm
was designed. This narrowing of the measured FWHM is due to the non-uniform input
spectrum and the spectral deformation in the output amplifier.
73
Chapter 4
30
1770
Measured Wavelength [nm]
a
Power [nW/nm]
20
10
0
-10
-20
b
1750
1730
1710
1690
1670
1670
1690
1710
1730
1750
1770
1670
1690
Wavelength [nm]
20
1730
1750
1770
40
c
15
d
35
10
FWHM [nm]
Detuning [nm]
1710
Target Wavelength [nm]
5
0
-5
30
25
20
-10
15
-15
-20
10
1670
1690
1710
1730
1750
1770
1670
Target Wavelength [nm]
1690
1710
1730
1750
1770
Target Wavelength [nm]
Fig. 4-13 Spectral response LR-filter for three tuning wavelengths, 1700nm (solid black), 1720nm
(black dotted) and 1740nm (solid gray) (b) Tuning accuracy LR-filter between 1670nm and 1770nm.
The measured central wavelength of the passband is given with respect to the target wavelength (c)
detuning between measured central wavelength and target wavelength (d) Measured FWHM of the
passband.
4.7.3 Filter Tuning speed
To get a clear indication of the tuning speed of the filters a measurement was made of
the time dependent transmitted optical power from an external laser through the HR filter
when switching the filter between two wavelengths. Due to the fact that we did not have a
suitable external light source available in the 1700nm wavelength region and the ASE from
the QD amplifier was not strong enough we performed this switching measurement with an
external single mode laser at 1597.9nm. The 1597.9nm is chosen because this wavelength
is at a higher order passband of the HR-filter when the filter is tuned to 1700nm. The laser
light is coupled into a monitor output of the HR-filter with a lensed fiber and collected with
another lensed fiber from a monitor output at the other side of the HR-filter. The
74
Tunable wavelength filters in the 1.6 to 1.8 µm wavelength region
transmitted light was detected with an amplified photo-detector with 3ns rise time. The
filter is switched between 1700nm and 1745nm to pass or reject the 1597.9nm laser light
through the filter. The 10%-90% rise- and fall-time are measured to be 100ns and 80ns
respectively. These values are significantly longer than the response time of a single PHM
in a Mach-Zehnder interferometer of less than 1ns [83]. The measured 100ns response time
is therefore limited by the electronics. First of all there is a limitation in the control
electronics which has a limited rise time determined by the slew rate of 0.25V/ns. Since the
largest voltage swing in the switching of the wavelength is approximately 4 Volt this means
approximately 16ns. Apparently the additional capacities of cabling (1 meter) and in the
PCB are the main reason for the lower than expected switching speed. In a commercial
product this might be prevented by combining the control electronics with the optical chip
in a single package to avoid capacities from cabling.
4.8 Conclusion
In this work we have presented two monolithically integrated electro-optically tunable
filters in the 1700nm wavelength region. The filters are designed to be used in a
monolithically integrated tunable laser source in this wavelength region. The filters have
been fabricated within a QD active/passive InAs/InGaAsP/InP integration technology used
at COBRA. We demonstrate that this integration technology designed for the use at
1550nm wavelength, can also be used successfully in the 1600nm to 1800nm wavelength
region without a large penalty in performance.
The high resolution filter is an AWG filter with 5mm long PHMs in the 28 arms of the
AWG. The low resolution filter consists of an MMI-tree based filter with 5mm long PHM
in the 8 arms of the filter. To our knowledge this is the first time such an MMI-tree based
filter has been used. The phase shift efficiency of the PHM has been determined for a wide
wavelength region between 1630nm and 1800nm. The linear phase shift efficiency is found
to be wavelength dependent and varying between 0.33 rad/Vmm at 1633nm to
0.26rad/Vmm at 1796nm. A quadratic term in the phase shift efficiency was observed but
this only had a minor influence on the voltage to phase relation in the PHMs. The phase
offset was determined to be less than 2.2µm in all waveguides, indicating an error in the
optical length of the arms of less than 0.04%.
Both filters could be tuned in a predictable way. The HR-filter was demonstrated to be
tunable over 160nm from 1630nm to 1790nm with an accuracy of ±0.1nm (1% of the FSR)
and a FWHM between 0.4nm and 0.5nm. The LR-filter was demonstrated to be tunable
over 100nm from 1670nm and 1770nm with an accuracy of ±9nm (4% of the FSR) and a
FWHM between 15nm and 27nm. These tuning range demonstrations were limited by the
ASE from the QD-amplifiers used as a light source at 1700nm. In principle the filters can
be tuned over a larger wavelength region. Both filters satisfy the requirements for the filters
to be used in the integrated tunable laser source.
75
5 Tunable quantum-dot ring laser
simulations
Abstract – In this chapter we present the simulation results of the complete tunable
laser. The traveling wave model is based on the quantum-dot rate-equation model presented
in chapter 3. With these simulations we can estimate the laser linewidth, the output power,
the spectral suppression and the switching time.
5.1 Introduction
In this thesis the investigation and development of a monolithically integrated tunable
laser is described. This ring laser basically consists of intra cavity quantum-dot (QD)
amplifiers and intra cavity tunable filters as presented in chapter 1. The QD amplifiers and
the tunable filters have been extensively studied, characterized and the results are presented
in chapter 3 and 4 respectively. To predict the behavior of the tunable laser which uses
these tunable filters and QD amplifiers, a simulation model is developed describing the
laser performance in time. The model that has been developed is presented in this chapter.
It is a bidirectional time domain model for the complete tunable laser system. The
dimensions of the different components in the model correspond to the dimensions in the
designed and realized laser system.
Important laser parameters we wished to extract from the simulations are the spectral
suppression in the laser spectrum, the clock-wise (CLW) versus counter-clock-wise
(CCLW) directional suppression, the full-width-half-maximum (FWHM) of the laser peak,
the output power of the laser peak and the switching time. The starting point for the
simulation of the QD amplifiers is the previously developed coupled rate equation system
describing the change in carrier concentrations in the different energy states. This rate
equation model is extended to also describe the photon density in both the clock and
counter clock wise operating directions in the ring laser. For the model of the laser system a
segmented time domain model is used to describe the laser in a discrete number of
segments which can be evaluated separately.
The simulation results presented in this chapter are based on the QD gain model and
the parameters determined from the small signal gain analysis presented in chapter 3 in
combination with the simulations on the filter characteristics presented in chapter 4. The
results presented here are predictions and are not fitted to the measured laser results
77
Chapter 5
presented in chapter 6. A comparison between the simulated results and the measured
results will be given in chapter 6.
In the following section first the amplifier model is described starting with the coupled
rate equations used for the QD amplifiers followed by the segmented model of the laser
system. In section 5.3 the simulations are described. This chapter ends with a conclusion on
the simulation results.
5.2 Laser model
The model that was developed can be described in two main parts. The first part is the
rate equation based model describing the change in time of the carrier concentrations and
photon concentrations in the QD amplifiers. The second part describes the model for the
complete laser system in segments where each segment represents a passive waveguide,
active waveguide (amplifier), tunable filter, or multimode interference (MMI) coupler.
Such models are commonly described as travelling wave models.
5.2.1 QD amplifier rate equation model
The QD amplifier model is an extension of the QD rate equation model presented
in chapter 3 which describes the small signal gain as a function of the carrier concentrations
in the separate confinement heterostructure (SCH) layer, the wetting layer (WL), the exited
state (ES) and the ground state (GS). This model has been extended with the rate equations
describing the photon densities for photons [25] propagating in both the CLW and CCLW
directions in an amplifier.
dN s I N s N s N q
 


,
dt
e  s  sr  qe
dN q
dt

Ns
s

Nq
N
  eESn   qr
n
ESn
(5-1)

Nq
 qe

Nq
 c0
 1  PESn Gn ,
(5-2)
n
dN ESn N q Gn 1  PESn  N GSn 1  PESn  N ESn N ESn N ESn





1  PGSn 
dt
 c0
 eGSn
r
 e ESn
d0

c
nr
 g mn SCLWm  SCCLWm ,
m
n=0,1,…,N-1
ES
78
(5-3)
Tunable quantum-dot ring laser simulations
dN GSn N ESn

1  PGSn   N GSn  N GSn 1  PESn 
dt
d0
r
 eGSn

c
 g mnGS SCLWm  SCCLWm ,
nr m

(5-4)
n=0,1,…,N-1

dS CLWm
N
S
c
 m 
g mn ES  g mnGS S CLWm  CLWm ,

r
p
dt
nr n
(5-5)
m=0,1,…,M-1


dS CCLWm
N
S
c
 m 
,
 g mnES  g mnGS SCCLWm  CCLWm
r
p
dt
nr n
(5-6)
m=0,1,…,M-1
The rate equation system consist of a series of equations (5-1)-(5-4) which represent
the total number of carriers in the SCH reservoir Ns, the total number of carriers in the WL
reservoir Nq, and two series of N rate-equations which represent the carriers in the ES of the
n-th dot group NESn and the carriers in the GS of the n-th dot group NGSn. These equations
are in detail described in chapter 3. The difference between these equations and those in
chapter 3 is the loss term which has been added representing the carrier loss in the ES and
the GS caused by stimulated emission (the last negative sum terms in equations (5-3) and
(5-4) ). The equations (5-5) and (5-6) represent the two series of M rate-equation which
represent the photons in the CLW direction SCLWm of the m-th optical mode and the photons
in the CCLW direction SCCLWm of the m-th optical mode. Note that these optical modes are
not the longitudinal modes of the laser cavity but the discrete spectral regions that represent
a group of laser modes that experience the same gain. In these equation Γ is the optical
confinement; nr the effective index in the waveguide; β the spontaneous emission coupling
coefficient; Nm the total number of carriers in the ES and GS which contribute to
spontaneous emission to the m-th mode and τp=nr/(cαi) the photon lifetime in the cavity.
The gain from the different dot groups to the different spectral regions gmnES and gmnGS are
as described in chapter 3. All used parameters are given in Table 5-1.
79
Chapter 5
Table 5-1 Used parameters values in the rate equation model
Simulation parameter
FWHM of inhomogeneous broadening
Effective density of states in the WL
FWHM of Homogeneous broadening
Degeneracy of the GS
Degeneracy of the ES
Relaxation time from SCH to WL
Capture time from WL to ES
Capture time from ES to GS
Carrier escape time from WL to SCH
Carrier recombination time in SCH
Carrier recombination time in WL
Carrier recombination time in ES and GS
Quantum-dot density
Number of QD layers
Total active layer thickness
Total quantum-dot area
Total wetting layer area
Refractive index
Optical confinement factor
Transition matrix elements
Internal modal loss
Temperature
Spontaneous emission coupling coefficient
Value:
Γ0 = 41 meV
ρWLeff = 2.4·1011 cm-2 (T = 288 K)
2ħΓcv = 40 meV
µGS = 2
µES = 4
7 ns < τs < 20 ns
τc0 = 1 ps
τd0 = 1 ps
τqe = 3 ns
τsr = 4.5 ns
2 ps < τqr < 100 ps
τr = 1 ns
ND = 3.1·1010 cm-2
NW = 5
Hact = 200 nm
AD = 7·10-4 cm2
AWL = 7·10-4 cm2
nr = 3.261
Γ = 0.34
|PσGS,ES|2 = 2.70·m0·EGS,ES kgeV
αi = 10 cm-1
T = 288 K
β = 1·10-4
Reference:
[12]
[12]
[12]
[25]
[25]
[12]
[2]
[63]
5.2.2 Segmented ring laser model
The tunable ring laser simulation is based on the rate equations presented above
describing the change in the carrier concentrations and photon concentrations in the QD
material in time. To be able to simulate the complete tunable laser including passive
waveguides, filters, multi-mode interferometer (MMI) output coupler, loop mirror and QDamplifiers the ring laser is divided in a discrete number of segments. Each segment
represents a small section of the laser system. The arrayed waveguide grating (AWG) filter
and the MMI filter are represented by single segments with a wavelength dependent loss
component in it. The filter characteristics are calculated in advanced with an S-matrix
oriented CAD-tool [42] as presented in chapter 4. The MMI coupler is represented by a
segment that splits the light from one side into the two outputs at the other side. Each
segment represents a physical length in the amplifiers and the passive waveguides that
corresponds to the path length the light travels in a time interval Δt in that component. This
means that the number of segments is dependent on the time interval Δt used when the total
length of the amplifier sections and passive waveguides is fixed. The physical length of
each segment represents can be calculated with: Lele=cΔt/ng where ng is the group index in
80
Tunable quantum-dot ring laser simulations
the active or passive waveguide. A schematic picture of the laser simulation is given in Fig.
5-1.
MMI filter
Amp 1
AWG filter
Amp 2
cw +ccw
50/50
Passive
MMI coupler
Amp 3
Output
MMI loop
Fig. 5-1 Diagram of the segmented ring laser model. The model consists of four different segments:
passive waveguides, active waveguides (amplifiers), wavelength filters (AWG or MMI filter) and an
MMI coupler. Each segment is indicated with a different grayscale. The black arrows represent the
light propagation directions. Both counter propagating directions are simulated simultaneously. Note
that in this diagram only a limited number of segments is presented. The simulation model uses more
segments as indicated in section 3.
The behavior of the laser has been simulating with an time stepping algorithm. Within
each iteration step first each segment is evaluated over a time interval Δt. This means for
the amplifiers that all coupled QD-rate equations including the N rate equations for ES and
GS and the M rate equations for the CLW and CCLW operating mode are integrated over
the time period Δt. For the passive waveguides this means an optical loss in both CLW and
CCLW operating direction that is the same for all laser mode groups. For the filters it
means a wavelength dependent loss, that is a loss that is different for the different laser
mode groups in both CLW and CCLW operating direction. For the MMI filter it means a
50% splitting over the two different outputs opposite to the input. After the evaluation of
each segment the output light in both directions is passed to the next segment to both sides
(dependent on CLW or CCLW direction) and used as input light for the next segment in the
following iteration step.
The model only describes the amplitude of the optical signal and does not consider the
phase of the light. The mode frequencies of the laser are therefore considered to be constant
and self-phase modulation effects are thus ignored. Not considering the phase in the model
means that phase dependency in the loop mirror and its effect on the mode structure of the
laser cannot be resolved. This simplification is applied to reduce calculation time. Since we
are not looking to accurately simulate fast dynamics in the cavity but are more interested in
steady state like solution of the problem, the use of this approximation is reasonable.
81
Chapter 5
For the simulation of the ring laser the values for a number of parameters have to be
chosen. The first parameter is the time step Δt over which each segment is evaluated during
one complete time step calculation. This time step has to be chosen sufficiently small such
that the integration in time is stable and converges. The second and third parameters are the
total dot size range and the total wavelength range of the laser mode groups that are going
to be evaluated. The last parameters to be chosen are the number of dot size groups and the
number of different laser mode groups which are evaluated during the simulation have to be
chosen. The chosen numbers for these parameters depend on what has to be simulated.
These numbers as well as the other parameters used in the simulation are given at the
different simulation descriptions.
5.3 Simulations
The simulation of the ring laser was implemented in MATLAB®. The change in time
in carrier and photon concentrations for each segment are calculated with a linear
integration in time using the derivatives calculated with the rate equations. More stable
algorithms such as the fixed step size Runge-Kutta method [17] and the Rosenbrock [56]
method are not used due to a too high increase in calculation time. The stability of the
algorithm is ensured by reducing the integration time step when unrealistic oscillations in
the simulations occur. The calculation time for a simulation run was a serious limitation.
This is in particular due to the large number of coupled rate equations which have to be
evaluated for each active segment. The number of segments in the laser, the size of the time
step and the time over which the simulation needed to be run further determine the
calculation time. The number of rate equations is mainly determined by the number of dot
wavelength groups and the number of laser mode groups. For simulations discussed in
detail in the following sections these parameters were optimized to obtain sufficient detail
on a particular aspect of the laser performance. But even then a single run took from a
couple of days up to weeks. This limited the number of simulations that could be performed
to three different simulations. In the first simulation the laser is simulated over the full
1600nm to 1800nm wavelength range. From this simulation the suppression of the
unwanted passbands of the HR filter can be determined. The spectral resolution in this
simulation is 0.5nm which is an order less than the longitudinal mode spacing. This means
that the spectral shape of the laser peak cannot be resolved from this simulation. In the
second simulation only a small part of the optical spectrum (1nm around 1700nm) is
simulated with a higher resolution of 0.02nm to simulate the spectral behavior of the laser
peak during start-up. In the third and last simulation the tuning of the laser over 0.2nm is
simulated using the small 1nm optical spectrum around 1700nm.
The common parameters for the three simulations of the laser are given in Table 5-2.
The simulation dependent parameters are given in the subsections describing these
simulations.
82
Tunable quantum-dot ring laser simulations
Table 5-2 Common laser system simulation parameters values
Simulation parameter
Length amplifier 1 (ring)
Length amplifier 2 (ring)
Length amplifier 3 (output)
Total length passive waveguides
Length loop mirror
Group index active waveguides
Group index passive waveguides
Passive waveguide losses
Current injection amplifier 1
Current injection amplifier 2
Current injection amplifier 3
Value:
Lact1 = 8mm
Lact1 = 8mm
Lact1 = 8mm
Lpas = 27.5mm
Lloop = 16.3mm
ng,act = 3.75
ng,pas = 3.554
αpas = 3dB/cm
I1 = 0.48A
I2 = 0.48A
I3 = 0.48A
Remark:
3000A/cm2
3000A/cm2
3000A/cm2
5.3.1 Simulation 1: complete output spectrum
In the first simulation the total output spectrum of the laser is simulated. This
simulation is performed to have an indication off the spectral suppression of the unwanted
parts of the spectrum with respect to the laser peak. The range of 61 different dot size
groups covered in the simulation ΔλQD corresponds to a possible spontaneous and
stimulated emission over 300nm wavelength from 1550nm to 1850nm. This wide range is
chosen to cover all carrier losses from the WL to the different dots in the laser system,
which is important for the carrier concentrations around central wavelength region. The
covered optical wavelength range is limited to 200nm from 1600nm to 1800nm divided
over 400 laser mode groups which is the largest number within a practical calculation time.
The spectral resolution is therefore 0.5nm. Reducing the spectral resolution increases the
calculation time and is not necessary to simulate the spectral suppression of the higher
order modes of the tunable high resolution filter in the laser. The used time step Δt=0.5ps is
the maximum time step which can be used. Both high and low resolution filters are tuned to
the middle of the tuning range at 1700nm. The parameters used are given in Table 5-3.
Table 5-3 Laser system simulation parameter values used in the 1st simulation
Simulation parameter
Iteration time step
Number of segments amplifier 1
Number of segments amplifier 2
Number of segments amplifier 3
Number of segments passive waveguides
Number of segments loop mirror
QD wavelength range
Number of QD groups
Spectral range
Number of spectral groups
Value:
Δt = 0.5ps
Nr_amp1 = 200
Nr_amp2 = 200
Nr_amp3 = 200
Nr_pas = 652
Nr_loop = 386
ΔλQD = 300nm
N = 61
Δλopt = 200nm
M = 401
Remark:
0.5nm steps
In Fig. 5-2 the simulated carrier and photon concentrations are given for the first 50ns
after switching on the laser system. No carriers and photons are in the optical amplifiers at
t=0. Fig. 5-2a,b,c present the total number of carriers in the SCH, WL, ES and GS and the
83
Chapter 5
total number of photons in both directions in the ring cavity. From this figure it is clear that
at approximately 3ns from the start the laser begins to oscillate, resulting in a strong
increase in photons in the cavity. At this threshold point the CLW direction of the laser is
suppressed due to the feedback from de CLW direction into the CCLW direction. The
suppression of the CLW operating direction is approximately 20dB with respect to the
CCLW direction. The total output power reaches a stable 10mW after 20ns which can be
seen from Fig. 5-2d. The output spectrum at t=50ns is given in Fig. 5-3e. It contains a
homogeneous spontaneous emission component which has a spectral density less than
-30dBm/nm and stronger peaks on the passband wavelengths of the high resolution AWG
filter. The low resolution MMI filter filters out the peak at the 1700nm passband and
suppresses lasing at all other passbands of the high resolution filter. The suppression
between the peak at 1700nm and all other peaks is more than 30dB.
Carriers WL
1.2E+10
6.0E+07
8.0E+09
4.0E+07
4.0E+09
2.0E+07
Carriers ES
8.0E+07
10
20
30
Time [ns]
40
1.2E+08
1.2E+08
1.0E+08
1.0E+08
8.0E+07
8.0E+07
6.0E+07
6.0E+07
4.0E+07
4.0E+07
2.0E+07
2.0E+07
20
30
40
50
12
d
2.0E+06
1.5E+07
1.5E+06
20dB
1.0E+07
1.0E+06
5.0E+06
5.0E+05
Photons CCLW
2.0E+07
Output Power [mW]
c
2.5E+06
Photons CLW
10
Time [ns]
2.5E+07
10
10
20
30
40
8
6
4
2
0.0E+00
0.0E+00
0
0.0E+00
0
50
3.0E+06
1.4E+08
0.0E+00
0.0E+00
0.0E+00
0
1.6E+08
b
1.4E+08
1.6E+10
Carriers SCH
1.6E+08
1.0E+08
a
Carriers GS
2.0E+10
0
50
0
Time [ns]
10
20
30
40
50
Time [ns]
Fig. 5-2 (a) Total number of carriers in the SCH (black) and WL (gray). (b) Total number of carriers
in all ESs of the dots (black) and in all GSs of the dots (gray). (c) Total number of photons in the ring
cavity propagating in the CLW direction (black) and in the CCLW direction (gray). (d) Total output
power.
84
Tunable quantum-dot ring laser simulations
20
10
Power [mW/nm]
0
>30dB
-10
-20
-30
-40
-50
-60
1600
1650
1700
1750
1800
Wavelength [nm]
Fig. 5-3 Output spectrum of the laser system at t=50ns showing the laser peak at 1700nm and the
suppressed other peaks in the spectrum at the passbands of the HR-filter.
5.3.2 Simulation 2: 1nm around laser peak (start-up)
In the second simulation the width of the calculated spectrum is reduced to 1nm
around the expected laser peak at 1700nm. This simulation has a 0.02nm spectral resolution
which is approximately the longitudinal mode spacing of the 43.5mm long ring cavity. The
reduction in the calculated spectrum can only be done if the neglected part of the spectrum
has a negligible influence on the laser peak. The light in this neglected part of the spectrum
is amplified spontaneous emission. The influence of this light on the carrier concentrations
in the ES and GS during laser operation are negligible due to the 20dB less spectral power
in the neglected part of the spectrum compare to the power in the 1nm around the laser peak
in the ring cavity determined from simulation 1. Before the laser reaches threshold, the
neglected spectrum has a small influence on the spectrum around 1700nm due to the
coupling of the carriers in the WL and the SCH layer. This simulation describes the laser
start-up behavior over 50ns starting from off. The parameters used are given in Table 5-4.
Table 5-4 Laser system simulation parameter values used for the 2nd and 3rd simulation
Simulation parameter
Iteration time step
Number of segments amplifier 1
Number of segments amplifier 2
Number of segments amplifier 3
Number of segments passive waveguides
Number of segments loop mirror
QD wavelength range
Number of QD groups
Spectral range
Number of spectral groups
Value:
Δt = 0.2ps
Nr_amp1 = 500
Nr_amp2 = 500
Nr_amp3 = 500
Nr_pas = 1630
Nr_loop = 964
ΔλQD = 300nm
N = 61
Δλopt = 1nm
M = 51
85
Remark:
0.02nm steps
Chapter 5
The time step had to be reduced to 0.2ps in the second and third simulations. The QD
wavelength range ΔλQD is kept at 300nm because of the fact that these dots directly affect
the carrier population in the WL and SCH due to non-radiative losses which in turn affects
on the laser behavior.
3.0E+06
120
12
2.5E+07
2.0E+06
1.5E+07
1.5E+06
20dB
1.0E+07
1.0E+06
Photons CCLW
2.0E+07
5.0E+06
5.0E+05
0.0E+00
Output Power [mW]
10
100
8
80
6
60
4
40
2
20
10
20
30
40
0
0
0.0E+00
0
0
50
10
Time [ns]
20
Time [ns]
40
50
0.20
1700.10
Central Wavelength [nm]
30
c
1700.05
0.15
1700.00
0.10
1699.95
0.05
FWHM [nm]
Photons CLW
2.5E+06
Peak Power [mW/nm]
b
a
0.00
1699.90
0
10
20
30
Time [ns]
40
50
Fig. 5-4 (a) Total number of photons in the ring cavity propagating in the CLW direction (black) and
in the CCLW direction (gray). Note that scale is different for the CLW and the CCLW direction. (b)
Total output power (black) and peak output power (gray) as a function of time during laser start up.
(c) Determined central wavelength (black) and FWHM (gray) of the laser peak.
In Fig. 5-4 the most important simulation results are presented. In Fig. 5-4a the
evolution of the total number of photons in both directions is presented. The laser started to
reach threshold at approximately 3ns from t=0 as in the first simulation. The suppression of
the power in the CLW direction with respect to the power in the CCLW direction is also
comparable to the 20dB in simulation 1. The identical laser starting point of 3ns, the 20dB
CLW suppression and the comparable number of photons in the cavity between the first
and second simulation indicate that the neglecting of the ASE spectrum around the laser
peak was allowed. Fig. 5-4b presents the evaluation of the total output power from the
output amplifier and the spectral peak output power of the main lasing mode. The total
86
Tunable quantum-dot ring laser simulations
output power stabilises at 20ns from the starting point however the peak power of the laser
peak has a longer stabilization time of approximately 40ns. This means that after the 20ns
start-up period most light is confined in the laser peak however in the following 20ns the
laser peak is narrowed resulting in a larger peak power. The narrowing of the laser peak can
also be seen from Fig. 5-4c where the central wavelength and the FWHM of the laser peak
are presented. The FWHM of the laser peak at 50ns from the starting point is 0.09nm (Fig.
5-4c).
5.3.3 Simulation 3: 1nm around laser peak (0.2nm tuning)
In the third simulation the tuning of the laser is simulated. This simulation has as
starting point the carrier and photon concentrations calculated at the end of simulation 2.
The tuning behavior is simulated by shifting the filter characteristics of the AWG filter and
the MMI filter over 0.2nm towards 1700.2nm. This shift is performed at t=0 of this
simulation and is performed instantaneously. This abrupt change in filter characteristics
triggers an oscillation in the simulated results which is unrealistic in the real laser due to the
limited bandwidth of the filter behavior. The oscillations in the simulation are minimized
by taking small 0.2ps time steps. The influence of this abrupt change is however still
visible. In Fig. 5-5a the total number of photons CLW and CCLW in the cavity are given.
These are nearly constant and thus not much affected by the switching of the filter. In Fig.
5-5b the total output power and the peak output power during the 0.2nm switch is
presented. The total output power remains the same, however the peak power has a dip due
to the shift of the central wavelength. From Fig. 5-5c can be seen that this shift in peak
wavelength is performed within 10ns. The narrowing of the laser peak however takes some
more time resulting in an approximately 40ns stabilization time in which the laser peak
narrows back to 0.09nm and a peak power of 100mW/nm. The step like behavior in the
figures originates from the abrupt change in filter transmission at t=0. The steps have a
time period of 0.53ns which corresponds to the roundtrip time in the 43.5mm long cavity.
87
Chapter 5
3.0E+06
2.5E+07
12
120
10
100
8
80
6
60
4
40
Photons CCLW
Photons CLW
2.0E+07
2.0E+06
1.5E+07
1.5E+06
20dB
1.0E+07
1.0E+06
5.0E+06
5.0E+05
20
2
Peak Power [mW/nm]
2.5E+06
Output Power [mW]
a
b
0.0E+00
20
30
40
0
10
Time [ns]
20
40
Time [ns]
0.20
1700.30
Central Wavelength [nm]
30
1700.25
0.15
1700.20
1700.15
0.10
1700.10
1700.05
1700.00
FWHM [nm]
10
0
0
0.0E+00
0
0.05
1699.95
c
0.00
1699.90
0
10
20
Time [ns]
30
40
Fig. 5-5 (a) Total number of photons propagating in the CLW direction (black) and in the CCLW
direction (gray). Note that scale is different for the CLW and the CCLW direction. (b) Total output
power (black) and peak output power (gray) as a function of time during 0.2nm laser wavelength
tuning. (c) Determined central wavelength (black) and FWHM (gray) of the laser peak during 0.2nm
laser wavelength tuning.
These simulations clearly show that the laser in principle can be tuned within 10ns.
The FWHM and the peak power however need some more time to stabilize in the order of
40ns. This 10ns switching can be reduced by decreasing the total length of the ring cavity.
5.4 Conclusion
A model is presented that simulates the tunable laser system presented in this thesis. In
the model the laser cavity is divided of a number of segments in which we are describing
the clock-wise and counter-clock-wise optical modes in the laser system. Each segment
represents a small physical part in the laser system which can be part of the passive
waveguides, filters, MMI couplers and QD amplifiers. The segments representing the QD
88
Tunable quantum-dot ring laser simulations
amplifiers are evaluated with a coupled QD rate equation model describing the change in
carrier and photon concentrations in time.
Simulation results show that the two filters used are able to get a single laser peak in
the spectrum at the target wavelength at 1700nm. The suppression of the rest of the
spectrum was more than 30dB. A detailed simulation on the laser peak showed that the
laser linewidth reduces to 0.09nm FWHM and a peak intensity around 100mW/nm with a
total output power of 10mW. The switching of the laser to another wavelength, in this case
0.2nm shift, can be made within 10ns, however the narrowing of the laser peak to 0.09nm
takes approximately 40ns. This means that the laser can be tuned with 20MHz step
frequency provided the response of the filters on the control signals is significantly faster
than 50ns. This can be increased to 100MHz steps however this reduces the laser linewidth
quality from 0.09nm to approximately 0.15nm FWHM.
89
6 Integrated tunable quantum-dot laser in
the 1.7 µm wavelength region
Abstract – In this chapter we present the design and characterization of a
monolithically integrated tunable laser for optical coherence tomography in medicine. The
demonstrated tuning range between 1685nm and 1745nm opens new possibilities for
optical coherence tomography due to the deeper imaging depth in this wavelength range
compared with systems working in the 800nm, 1000nm or 1300nm wavelength regions. We
demonstrate a tuning over the 60nm with a 0.11nm effective linewidth and an
approximately 0.1mW output power. Scanning capabilities of the laser are demonstrated in
a free space Michelson interferometer setup where the laser is scanned over the 60nm in
4000 steps with a 500Hz scan frequency. Switching between two wavelengths within this
60nm range is demonstrated to be possible within 500ns.
6.1 Introduction
The demand of continuously tunable lasers is currently growing quickly owing to the
rapid increase in the use of light sources in measurement systems. Examples are the use of
tunable laser sources in medicine such as optical coherence tomography (OCT) for
ophthalmology [23] and spectral imaging in dermatology and hematology [86]. Outside
medicine, there are applications in measurement systems such as gas sensing [79] and fiber
Bragg strain sensing [38]. The integration of these tunable laser sources, hybrid or
monolithically, has a large influence on the compactness, the power efficiency and the
robustness of the laser compared to bulk lasers. This enables the introduction of these types
of lasers on a large scale in for example medical measurement systems which are to be
operated by non- laser specialists.
The medical imaging technique of OCT in particular can benefit from these types of
tunable lasers. OCT is an imaging technique using a Michelson interferometer where the
coherence of the light that is reflected back from an object (sample) is analyzed and used to
obtain information in the object. This imaging technique can either be carried out with a
broadband light source in combination with a moving reference mirror or with a tunable
laser source in combination with a fixed reference mirror [23]. A drawback in the use of a
broadband light source is the relatively slow moving mirror in the setup, limiting the scan
speed. Furthermore in these measurements the tissue under test is continuously illuminated
91
Chapter 6
with light from the complete spectrum. This means that a relatively large amount of optical
power needs to be directed onto the tissue in order to have the best spectral reflection
signals. Obviously there is an absolute limit on this power level in medical imaging. In the
case of a tunable laser, the amount of optical power on the tissue can be reduced due to the
spectral distribution in time.
The integration of a tunable laser source is however not as straightforward as the
realization of a broadband light source, which can be a superluminescent light emitting
diode (SLED). The most commonly used monolithically integrated tunable laser sources
are distributed Bragg reflector (DBR) type of tunable laser [19]. In these lasers the
wavelength can be selected using fixed DBRs and electro-optical tunable DBRs. A
combination of DBRs can be used to tune over a large bandwidth using the Vernier
principle. Another option is the digital supermode (DS)-DBR laser that uses a tunable DBR
in combined with a relatively broadband digitally chirped grating to realize a widely
tunable laser [76]. Quickly continuous tuning over many tens of nanometers is however a
requirement for OCT. This is more difficult to achieve due to the Vernier-like effect and the
current control in the DBRs used in such devices. A second drawback of the tuning
technique through current injection in the DBR mirrors is that heat is generated in the
waveguide thus changing its temperature. The amount of heat depends on the amount of
current and so on the detuning of the DBR. Since the relaxation of the temperature to
equilibrium typically takes milliseconds in these devices, tunable lasers with DBR type of
filters can have a stabilization time in the millisecond region for some of the wavelength
tuning steps. Furthermore the injected carriers in the DBR have an effect on the total
roundtrip gain in the laser [16].
Another type of tunable laser sources, more suitable for OCT, is the hybrid integrated
micro-electromechanical-system (MEMS) controlled tunable lasers further discussed in
[45]. In these laser systems a MEMS mirror is used to scan the laser beam in microseconds
over a grating which selects the feedback wavelength and so the laser wavelength.
In this work we present the design, fabrication and characterization of an InP-based
monolithically integrated tunable laser source for OCT in the 1700nm wavelength region.
A major advantage of this laser source in comparison to existing tunable laser sources is the
tunability in the 1700nm wavelength region [40]. In this wavelength region the absorption
in human tissue due to water is lower in comparison to light at 1550nm. Light scattering is
usually dominant in biological samples and Rayleigh scattering will be significantly lower
around 1700nm than at e.g. 1300nm or 800nm, which are more commonly used wavelength
ranges. These effects will lead to an increase in the imaging depth. Furthermore in principle
we expect such a tunable laser source to be capable of scanning over more than 100nm with
a repetition rate in the 20kHz to 50kHz range. This expectation is based on the use of
integrated waveguide electro-optical phase modulators to control the emission wavelength.
Such modulators can easily achieve a modulation signal bandwidth of 1 GHz [83] and have
been demonstrated up to tens of GHz [10][85]. Such fast scanning is necessary to obtain
92
Tunable quantum-dot ring laser simulations
OCT images quickly for patient comfort and to reduce imaging artifacts caused by patient
movement.
Another significant advantage of the laser presented here is that it can be fabricated
within the standard active-passive integration technology used at COBRA [7][60], by
modifying only the gain layerstack to go from 1550nm to 1700nm [54].
To be competitive or better than existing tunable lasers our aim was to obtain a) a
tuning bandwidth of ideally more than 100nm to get to a 10µm depth resolution (13µm in
vacuum); b) a laser linewidth less than 0.07nm to get a coherence length of over 5mm
(6.5mm in vacuum); c) a spectral step size less than 0.1nm to reconstruct an image of at
least 11mm in depth (14 in vacuum) and d) an output power of approximately 1mW.
In section 6.2 the design of the tunable laser system is discussed. In section 6.3 the
characterization results of the laser system are presented where we demonstrate that the
laser can be tuned over sixty nanometers. In section 6.4 we demonstrate that the developed
laser is suitable for OCT measurements by testing the laser in a free space Michelson
interferometer setup, the first step towards an OCT setup. Section 6.5 consists of an
extensive conclusion where the performance of the laser is discussed as well as options for
improvements.
6.2 Design and fabrication
The designed InP-based laser system is based on a ring laser structure. The advantage
of a ring laser above a linear laser with cleaved mirrors is that there is more freedom in
design parameters like cavity length and output coupling. The ring basically consists of two
8mm long intra cavity quantum-dot (QD) amplifiers (Chapter 3)[69] and two intra cavity
tunable filters (Chapter 4). A schematic picture of the ring laser system is presented in Fig.
6-1. The special QD amplifiers are used to generate and amplify light in the 1700nm
wavelength region [54]. For the filters, tunable arrayed waveguide grating filters have been
chosen that include electro-optic phase modulators. The two electro-optically tunable filters
combined are used to select the wavelength in the laser cavity. Simulations on the laser
(Chapter 1)[68] show that an intra cavity filter with a parabolic filter shape with a
full-width-half-maximum (FWHM) less than 0.5nm and a free spectral range (FSR) larger
than 200nm should be sufficient to fulfill the laser requirements. For the design of this laser
a combination of a high-resolution (HR) filter and a low-resolution (LR) filter is chosen to
minimize the number of arms in both waveguide arrays. For the HR-filter an AWG type
filter is used and for the LR-filter an MMI-tree type of filter is used. The LR-filter
suppresses the transmission of the unwanted orders of transmission of the HR AWG filter.
5mm long phase modulators (PHMs) are placed in the arms of both filters to make the
filters tunable. The PHMs are voltage controlled and their low power consumption
combined with the speed attainable are the main reasons for selecting this type of tunable
filter. The combination of the two filters can be used to tune the filter combination over
93
Chapter 6
more than 200nm within the 1600nm to 1800nm wavelength range. Further details and
results on these filters are presented in Chapter 4.
The total ring length is approximately 43.5mm. Part of the light in the ring cavity is
coupled out with a 50% 2x2 multimode interference (MMI) coupler. Light coupled out
from the clockwise (CLW) directional operation of the laser is fed back into counter
clockwise (CCLW) directional operation of the laser with a MMI loop mirror [81]. The
total length of the MMI loop mirror is designed to be 16.26mm. This length was chosen to
make that if there is an internal reflection in the ring cavity itself, the mode structure
originating from that reflection and the feedback would be of the same order as the free
spectral range of the ring. The light which is coupled out of the ring in the 2x2 MMI
coupler originating from the CCLW direction is led through an 8mm long QD output
amplifier to boost the output signal.
The high resolution tunable AWG filter (Chapter 4) has monitor outputs which couple
out a fraction of the light in the laser cavity due to the position of these waveguides on a
higher order focal point of the free propagation regions (FPR).The output waveguide of the
output amplifier and of the monitor outputs exit the chip under an angle of 7 degrees
relative to the normal of the output facet to minimize facet reflections. All waveguides are
low contrast shallow etched (100nm into the film layer) waveguides to minimize
waveguide losses.
Tunable high resolution filter
Loop
mirror
Tunable low
resolution filter
QD amplifier
QD amplifier
coupler
QD amplifier
output
Fig. 6-1 Schematic picture of the laser system. The ring cavity is folded to reduce the chip size.
The complete mask layout of the laser is presented in Fig. 6-2. In the center of the
mask the AWG-type HR-filter is located including the twenty-eight 5mm-long PHMs.
Directly below this HR-filter the MMI-tree-type LR-filter is located including 8 PHMs. The
two 8mm long QD ring amplifiers and the 8mm long output amplifier are located below
these two filters. On the mask the 170µm wide, 8mm long gold contacts on these amplifiers
can clearly be seen. Each of the PHM in the filters is connected to one of the bond pads.
Around the PHMs in the filters, 20 extra PHMs and waveguides are located to get a
uniform structured area necessary for polyimide planarization as discussed in (Chapter 2
94
Tunable quantum-dot ring laser simulations
and 4). Five of these test PHMs are also connected to bond pads. Extra monitor outputs on
the free propagation region (FPR) of the HR AWG filter are included in order to be able to
calibrate the filter using ASE from a QD amplifier. On both FPRs of the HR filter, two of
the monitor outputs are positioned at a higher diffraction order of the filter in order to
monitor the light in the cavity during laser operation. The MMI loop mirror is located on
the left hand side of the mask and is connected to one of the outputs of the MMI output
coupler.
Fig. 6-2 Mask design of the complete integrated tunable laser system. Size 10x6 mm
The laser chip is fabricated within the generic active-passive integration technology
used at COBRA [7][60] using only shallow ridge waveguides. The chip specific design and
fabrication considerations are presented in Chapters 2 and 4.
6.3 Laser characterization
The tunable laser system has been glued on a copper mount. The temperature of this
oxygen free copper mount was controlled using water cooling to a temperature of
13 Celsius. The heat that is to be removed is solely generated in the quantum-dot
amplifiers. More active temperature control is not necessary since the current injection in
the amplifiers is kept constant and the reverse bias currents through the electro-optical
phase modulators (PHM) of the tunable filters are several orders of magnitude lower. The
PHMs are contacted via bond wires to a printed circuit board (PCB). On the PCB high
bandwidth (1GHz) multi-pin connectors, each connecting eight voltage signals, are
positioned to connect the laser to the electronics. The PCB is also mounted on the copper
chuck. This method avoids the use of multi-probes directly on the fragile chip. The current
95
Chapter 6
to the QD-amplifiers is provided via probe needles on the p-contact pads of the amplifier.
The light emitted by the laser system is collected with a lensed fiber, either from a monitor
output or from the output amplifier and analyzed with a spectrum analyzer with a 0.05nm
resolution (YOKOGAWA AQ6375).
To be able to tune the laser in 1000 steps (0.1nm steps) over 100nm with a scan speed
of 20kHz the laser needs to be tuned within 50ns. For the PHM in the arms of the filter this
means a tuning should take place in less than 50ns, preferably in the 1-10ns range to leave
some time for the selected laser mode to stabilize. The PHMs themselves have been
demonstrated to be capable to switch in the 1-10ns range [83]. More difficult is the control
electronics which is required to apply the reverse bias voltage on all PHMs in parallel. In
principle the voltage step size used to scan the filters with small wavelength steps (e.g.
0.1nm for HR-filter) is in the order of 40mV for the central PHMs and at most 0.6V for the
outermost PHM (Chapter 4). A large step in voltage is only necessary when a PHM setting
needs to cross 2π rad and the phase is truncated to 0 rad. This means for the electrical
control a step of approximately 4V which requires a higher slew rate than the smaller
voltage steps. These 2π steps fortunately occur for each PHM at a different time in the scan.
If this large voltage step on one PHM is slightly slower than the other PHM, it has a minor
influence on the switching speed.
For the electrical control of these 38 PHM in the tunable filters, 100MHz analog
waveform generators that have been developed for this application are used [70]. These
13-bit resolution wave generators have a voltage range between -10V and +10V (we will
only use the -10V to 0V range for the PHMs). A waveform pattern can be uploaded into a
4096 word memory for each wave generator. A common clock and common trigger signal
can be used to step through each waveform pattern for the PHMs at the same time. The
minimum step size is 10ns and settling time of 4ns between +0.5V and -0.5V (40ns
between +5V and -5V) at the output of the electronics.
We expect a total optical loss inside the laser cavity of the passive components in the
order of 25-30dB. This means that relatively high current densities will be used in the
amplifiers (4 kA/cm2 and above). This in turn means that the peak gain wavelength of the
QD amplifiers will not shift much at and above threshold. We therefore start by looking at
P-I curves from the laser with the filter set near the peak wavelength of the gain. The
threshold of the laser has been measured by connecting both ring amplifiers together to a
single current source. The optical output power is measured as a function of input current
between 0A and 3A.
The optical output power is measured on both monitor outputs of the laser to compare
CLW and CCLW operation in the laser cavity. In Fig. 6-3 the P-I curve for the monitor
outputs is given for the laser operation at 1715nm, both filters are tuned to 1715nm. From
this P-I curve it can clearly be seen that the laser threshold is at 1500mA. Above 2A
injection current the laser appears to start operating unidirectionally and switches between
CLW or CCLW when the current increases [61]. Measurements at a number of different
tuning wavelengths and at 2A injection current show that the output power from the CLW
96
Tunable quantum-dot ring laser simulations
direction monitor output was approximately 2.3dB higher than the CCLW monitor output.
From this observation it follows that the suppression of the CLW direction with the loop
mirror is not visible in this current range. This results in an unpredictable switching
between CLW and CCLW operation above 2A injection current. The suppression of the
CLW direction is expected to work in case of a higher feedback signal from CLW into
CCLW direction with the loop mirror. This can either by reducing the losses in the
feedback loop or by increasing the light intensity in the ring laser. This has not been further
explored yet.
P-I curves at other wavelengths show similar behavior with a threshold current
between 1500mA and 1750mA. However unidirectional operation is not always observed.
Because of the unpredictable operation direction above 2A we chose to use a 1A current
through each ring amplifiers in the rest of the measurements presented in this work. This
puts an upper limit on the gain in the amplifiers and therefore also the tuning range of the
laser and the output power. When the output amplifier is used, a current of 700mA is
injected in the output amplifier. Higher currents through this amplifier did not increase the
effective optical power in the laser peak, only the broadband amplified spontaneous
emission (ASE).
Monitor output power [µW]
1.4
CCLW
1.2
CLW
Sum
1.0
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
2000
2500
3000
Total Injection Current [mA]
Fig. 6-3 P-I curve of the ring laser at 1715nm. Both ring amplifiers are connected to the same current
source.
6.3.1 Laser tuning – influence of the gain spectrum
At first the tuning behavior of the ring cavity has been studied by measuring the
CCLW operation from the monitor output. Both ring amplifiers are biased at 1A forward
current. For the tuning of the filter the calibration files of the filters (Chapter 4) have been
used (without any modification or optimization) to calculate the setting points of each PHM
in the filters for each desired wavelength. The output spectrum is recorded with a 0.05nm
resolution spectrum analyzer. The laser output spectrum and power has been studied at
97
Chapter 6
tuning wavelengths between 1670nm and 1770nm in 1nm steps around the approximate
gain peak of the QD gain spectrum at 1715nm. Between 1702nm and 1733nm we could
clearly see a single laser peak with a detuning between +0.1nm and +0.3nm from the target
wavelength and a FWHM less than 0.15nm. An example of output spectrum at 1715nm is
depicted in Fig. 6-4 (black curve). Outside this 31nm wavelength region there is on both
sides of the spectrum a region (the regions 1695-1701nm and 1734-1745nm) where the
laser wavelength sometimes jumped away from the set wavelength value. The laser started
to operate close to the target wavelength or at a wavelength approximately 10nm from the
target wavelength (next passband of the high resolution filter). An example of this can be
seen in Fig. 6-4 for the target wavelength 1742nm (light gray curve), most output power is
in a mode at 1732nm. In some cases the laser started to work on two passband wavelengths
of the HR filter, as can be seen in Fig. 6-4 for the target wavelength 1696nm (dark gray
curve). Outside the 1695nm-1745nm wavelength region the laser did not reach the lasing
threshold. A closer look at the spectra showed that in the 1695-1701nm region the laser
tended to operate 10nm towards the longer wavelengths and in the 1734-1745nm region the
laser tended to operate 10nm towards the shorter wavelength region. This clearly indicated
that the passband of the high resolution filter 10nm towards the peak in the gain spectrum
was not sufficiently suppressed by the low resolution filter. Outside the 1695-1745nm
wavelength region the ring laser did not reach the lasing threshold resulting in only ASE at
the output.
-15
1715
Power [dBm/nm]
-20
1696
-25
1742
-30
-35
-40
-45
-50
-55
-60
1685
1695
1705
1715
1725
1735
1745
Wavelength [nm]
Fig. 6-4 Laser output spectra for three different target wavelengths in which the central wavelength of
both filters are tuned to the target wavelengths: 1715nm (black), 1696nm (dark gray) and 1742nm
(light gray).
From the laser behavior presented above it can be concluded that the unwanted
passbands of the high resolution filter are not sufficiently suppressed in wavelength regions
where gain spectrum of the amplifiers is strongly wavelength dependent. In these initial
measurements both filters were tuned to get the central wavelength of both filters at the
target wavelength. This results in an equal suppression of both neighbor passbands,
98
Tunable quantum-dot ring laser simulations
+/-10nm from the desired passband of the HR-filter. The gain in the QD amplifiers can
however have a strong slope over this 20nm resulting in a larger roundtrip gain on the side
of the peak wavelength of the QD amplifier in compare to the other side. A schematic
representation of this situation is given in Fig. 6-5a.
a
Original filter tuning
b
After re-calibration LR-filter
λ
λ
Fig. 6-5 Schematic representation of the three elements in the ring laser that
determine the wavelength dependent gain; the HR-filter, the LR-filter and the
QD gain profile. (a) Original situation where both filters are tuned to the
central wavelength resulting in a higher roundtrip gain at the rightmost
passband of the HR filter. (b) Situation where the LR-filter is shifted away
from the peak in the QD gain resulting in a peak in the roundtrip gain at the
central wavelength.
 HR-filter
…… LR-filter
- - - QD-gain
 G-round
To compensate for this asymmetric gain profile around the target wavelength, the LR
filter can be shifted a fraction away from the peak in the gain spectrum. A schematic
representation of this situation is given in Fig. 6-5b. This increases the loss towards the
peak in the gain spectrum, suppressing the unwanted lasing in the neighbor passband of the
HR-filter, 10nm towards the gain peak. Optimizing the laser output by a simple retuning of
the MMI filter as indicated in Fig. 6-5b did not improve the suppression of the lasing in
unwanted modes in all cases. Nearly complete suppression of these modes could however
be obtained by using a kind of recalibration procedure of the LR-filter. In this procedure the
individual PHMs of the LR-filter are scanned over the voltage range necessary to reach 0 to
2π phase shifting. During this scan, the suppression of the two laser peaks at 10nm from the
target wavelength is measured relative to the laser peak at the target wavelength. The
voltage at which the suppression is the highest is stored and directly applied for each
scanned PHM. The extracted voltage array containing the voltages for each PHM to get the
highest suppression of the neighbor pass-bands of the HR-filter for one wavelength is
stored. This procedure was executed each 5nm over the tuning range of the laser. After the
execution of the procedure over the complete spectrum, the phase settings for intermediate
wavelengths could be determined by interpolation.
99
Chapter 6
Using the new calibration data of the LR-filter the spectral higher order mode
suppression has been measured between 1670nm and 1750nm in 1nm steps. The results for
the mode obtained are given in Fig. 6-6. From this figure it can be seen that that in the 1685
to 1745nm wavelength range the suppression in most cases is better than 25dB. Only
around 1727nm we could still observe a second laser peak around 1717 in the laser output
spectrum. In the measurements presented in Fig. 6-6 this unwanted laser peak was still
15dB lower than the peak at the target wavelength 1727nm.
Suppression (∆) [dB]
-10
-15
-20
-25
-30
-35
-40
1685
1695
1705
1715
1725
1735
1745
Target Wavelength [nm]
Fig. 6-6 Contrast between laser peak at target wavelength and power in the next passband of HR-filter
between 1685nm and 1745nm in 1nm steps.
6.3.2 Laser tuning – influence of the cavity mode structure
An extensive characterization of the laser performance has been executed over the
complete tuning range of 1685nm to 1745nm. The laser is tuned over this range in 4000
steps (0.015nm steps). An overview of the measurement results is presented in Fig. 6-7.
Except from a small wavelength region between 1726nm and 1727nm the laser system was
lasing between 1686nm and 1745nm (Fig. 6-7a/b). The detuning with respect to the target
wavelength was in all cases between -0.2nm and +0.2nm (Fig. 6-7c) and the FWHM of the
laser peak between 0.05nm and 0.30nm (Fig. 6-7d). Between 1726nm and 1727nm the
lasing wavelength could so far not be guaranteed to work only on the target wavelength. In
this small range a second laser peak appears at 10nm from the target wavelength resulting
in dual wavelength operation.
Remarkable in the measurements presented above are the 0.1nm variation band in the
detuning, the large 0.25nm fluctuation in the FWHM and the fluctuation in the measured
peak power. A closer look at the output spectrum when the laser is tuned over 1nm
bandwidth gave more information about the origin of these fluctuations. In Fig. 6-8a the
peak wavelength of the laser peak is presented with respect to the target wavelength. From
this figure it can clearly be seen that the peak wavelength jumps with approximately 0.1nm
100
Tunable quantum-dot ring laser simulations
steps through the spectrum while tuning the laser with constant steps. The spectra of a
series of measurements between 1715.420nm and 1716.00 are given in Fig. 6-8b. Also
from these spectra it can clearly be seen that the ring cavity has a preferred operating
wavelength at 1715.5nm and 1715.6nm. Tuning the laser in between these two wavelengths
results in a combination of these two preferred wavelengths. The FWHM will thus be
higher and the peak power will be proportionally lower. This behavior explains the
fluctuations in Fig. 6-7.
0.30
5.0
Peak power [mW/nm]
Total Power [mW]
a
0.25
0.20
0.15
0.10
0.05
0.00
b
4.0
3.0
2.0
1.0
0.0
1685
1695
1705
1715
1725
1735
1685
1745
1695
Target Wavelength [nm]
1725
1735
1745
d
c
1735
0.2
1725
0.1
1715
0.0
1705
-0.1
1695
-0.2
1685
-0.3
1695
1705
1715
1725
1735
FWHM [nm]
0.4
Detuning (•) [nm]
Central wavelength [nm]
1715
0.5
0.3
1745
1685
1705
Target Wavelength [nm]
0.3
0.2
0.1
0.0
resolution limit
1685
1745
Target Wavelength [nm]
1695
1705
1715
1725
1735
1745
Target Wavelength [nm]
Fig. 6-7 Laser system performance when the laser is tuned between 1685nm and 1745nm in 0.015nm
steps. Each circle represents an individual measurement of the laser output spectrum with a different
target wavelength. (a) Total output power of the laser peak (b) Peak power density of the laser peak
(c) Measured central wavelength of the laser peak and the detuning with respect to the target
wavelength (d) FWHM of the laser peak.
These 0.1nm spectral jumps indicate a cavity within the ring structure or some
feedback into the cavity. A laser cavity with an 8.1mm long roundtrip results in an extra
mode structure with a 0.1nm spacing. The most probable locations from which reflections
in the ring laser system can be expected are the isolation sections at both sides of the
amplifiers and PHMs. A possible explanation of the 0.1nm mode structure is that a
101
Chapter 6
combination of a reflection on both sides of the 8mm long amplifiers, resulting in a 0.05nm
mode spacing, in combination with the 0.02nm mode spacing from the 43.5mm long ring
cavity. A combination of this 0.05nm and 0.02nm could result in the 0.1nm mode structure.
This has however not yet been explored.
1.0
1716.0
b
1715.8
0.8
Power [mW/nm]
Peak wavelength ( ) [nm]
a
1715.6
1715.4
1715.2
0.4
0.2
1715.0
1715.0
0.6
0.0
1715.2
1715.4
1715.6
1715.8
1716.0
1715.2 1715.3 1715.4 1715.5 1715.6 1715.7 1715.8
Target Wavelength [nm]
Target Wavelength [nm]
Fig. 6-8 (a) Measured peak wavelength of the laser with respect to the target wavelength for a small
target wavelength region between 1715nm and 1716nm in 0.015nm steps. (b) Measured laser spectra
for tuning between 1715.42nm and 1715.00nm in 0.15nm steps showing the 0.1nm mode structure in
the laser performance.
6.3.3 Laser tuning speed
As an indication of the attainable tuning speed of the laser, the switching behavior of
the laser has been measured between two wavelengths separated by tens of nanometers.
When the laser is scanned using small wavelength steps (<0.1nm) it can in principle switch
faster than when switching between two widely separated wavelengths. This is due to the
fact that there is a higher light intensity in cavity modes close to the lasing wavelength
compared to those a couple of nm away from the laser peak. This reduces the build-up time
for the lasing close to the original laser peak. The tuning speed over a very small
wavelength region could not be directly measured; the measurement we performed gives a
lower limit on the attainable scanning speed. To measure the switching over a couple of
nm, the output from the laser is passed through a free space interference band-pass filter,
filtering out one of the wavelengths and passing through the other wavelength. The light
through the filter is collected on an amplified InGaAs photodiode with a rise time of 80ns
(3MHz bandwidth) and then recorded using a 1GHz bandwidth oscilloscope. The laser is
switched at a 1kHz repetition rate between 1700nm and 1745nm with the band-pass filter at
1700nm. The 10%-90% rise- and fall-times are measured to be 490ns and 380ns
respectively. This is mainly the time the laser needs to build up the laser peak at the new
wavelength. The switching time of the HR intra-cavity filter is approximately 100ns
102
Tunable quantum-dot ring laser simulations
(Chapter 4) and it is limited mainly by the speed of the electrical control of the phase
modulators. Therefore the influence on the switching speed of the filter on the measured
switching time of the laser is limited. The difference between the expected 50ns switching
(Chapter 1)[68] and the measured 490ns can be attributed to the fact the laser could not be
operated further above its threshold. The unsaturated gain of the amplifiers is only just
above total round trip loss which in turn leads to a long build-up time.
6.4 Laser coherence length and effective linewidth
The performance of the tunable laser with respect to its use as a source for OCT has
been studied using a free space Michelson interferometer setup. This is a first step towards
using the tunable laser in an OCT system and can be used to determine the effective
linewidth of the laser. A schematic picture of the free space Michelson interferometer setup
is given in Fig. 6-9. The tunable laser, controlled by the control electronics, is used to scan
the laser over 60nm in 4000steps with a 500Hz scan rate in order to have 500ns per step.
The light from the laser is collected with a lensed fiber and coupled into the free space
Michelson interferometer setup with a microscope objective. In the cubical beam splitter
the light is equally separated in two arms, one towards a fixed mirror and the other to a
movable mirror. After reflection on the two mirrors the light is again combined and
collected with a microscope objective into a single mode fiber. The collected light is
measured with a p-doped InGaAs detector with a 60MHz bandwidth and traced on a digital
oscilloscope (8 bit resolution) with a 10ns time between samples. This 10ns sampling
results in 50 samples per wavelength. For each wavelength an average over the 30 central
samples (nos. 15-45) is used to reduce the influence of the switching dynamics of the laser
during tuning of the filters.
The start of the laser scan is indicated on the oscilloscope with a trigger signal from
the control electronics.
103
Chapter 6
trigger
control
electronics
p-InGaAs
detector
Scope
Tunable
laser
- 60nm 4000 steps
- 500ns steps
- 500Hz
Moving mirror
Fixed mirror
Fig. 6-9 Schematic picture of the tunable laser in the free space Michelson interferometer setup. The
used fibers and lenses are not shown.
The recorded trace on the oscilloscope contains the information on the reflection on
the moving mirror with respect to the fixed mirror. The difference in distance of the two
mirrors with respect to the beam splitter results in a modulation in the spectral domain. In
Fig. 6-10a the recorded trace of one spectral scan is given in which the moving mirror is
located 1mm further from the beam splitter than the fixed mirror. The information in the
trace has been extracted with an Inverse Nonlinear Extended Discrete Fourier Transform
(INEDFT) [43]. In this INEDFT the spectral trace is Fourier transformed using the
previously measured wavelengths at each wavelength step. The instable wavelength region
between 1725nm and 1727nm is excluded from the Fourier transformation. The final time
trace is an average over 20 calculated time traces. The time trace is transformed to a spatial
trace using the speed of light. The spatial scale is divided by two to take into account the
double path length between mirror and beam splitter. The resulting spatial trace is given in
Fig. 6-10b. The peak at 1mm originates from the reflection on the moving mirror. The
x-axis represents the location of the reflection with respect to the location of the fixed
mirror.
104
Tunable quantum-dot ring laser simulations
1695
1705
1715
1725
1735
1745
Voltgage [V]
0.07
a
0.06
0.05
0.04
0.03
0.02
0.01
0
Reflected Power (normalized)
1.2
Wavelength [nm]
1685
b
1.0
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
2000
0
Time [us]
1
2
3
4
5
6
7
8
9
10
Reflection position [mm]
Fig. 6-10 (a) A single recorded spectral trace on the oscilloscope while scanning the tunable laser
between 1685nm and 1745nm in 4ms and a relative difference of 1mm between reference mirror and
moving mirror. (b) Reconstructed special trace from the measured spectral trace. The peak at 1mm
corresponds to the 1mm relative distance between the fixed reference mirror and moving mirror.
The effective linewidth of the tunable laser over the complete wavelength tuning range
can be determined using the free space Michelson interferometer setup. The peak intensity
of the reflected signal is dependent on the relative path length difference between the
reflection and the fixed reference mirror. This exponential intensity decay R, can be
described as a function of imaging depth z [52]
  2 2  z  2 
Rz   exp 
  
 8 ln 2  d  
(6-1)
where d is the maximum scan depth, d=λ2/(4Δλ) in which Δλ is the wavelength sampling
interval in the scan (in this case 0.015nm) and ω is the ratio of the spectral resolution
(FWHM), δλ to the wavelength sampling interval ω=δλ/Δλ. The spectral resolution can also
be called as the effective linewidth of the tunable laser over the complete wavelength
tuning range.
To determine this effective linewidth a series of 29 measurements has been performed
in which the relative path length difference of the moving mirror is stepwise increased with
0.25mm. For each path length the spectrum is recorded on the oscilloscope over 20 scans.
The recorded spectra are translated to the spatial domain with the INEDFT and averaged in
the spatial domain over the 20 scans. The exponential decay function (6-1) is fitted on the
peak intensities of the reflections. The spatial domain traces as well as the fitted curve are
presented in Fig. 6-11. The effective linewidth δλ of the tunable laser is determined to be
0.11nm which corresponds to the average measured FWHM of the laser peak presented in
Fig. 6-7d (an average over all 4000 measured FWHM gave 0.11nm).
105
Chapter 6
The axial resolution of an OCT image δL is determined by the scan range Δλspan=60nm
of laser and can be calculated with:
L 
2 ln 2


 02
span
(6-2)
where λ0=1715nm is the central wavelength in the wavelength span. The axial resolution
for this tunable laser in an OCT system is 21.6µm in vacuum.
Reflected Power (normalized)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
7
8
9
10
Reflection position (mm)
Fig. 6-11 Series of reconstructed spatial traces in which the moving mirror is positioned at relative
distances between 0.25mm and 7mm (0.25mm steps). The black dashed overlapping curve is a fit
over the peaks of the reconstructed traces showing the expected exponential decay in reflected peak
power.
6.5 Conclusion
In this chapter we have presented the fabrication of a monolithically integrated tunable
laser source in the 1700nm wavelength region. This laser is meant to be used for optical
coherence tomography in medical applications. Important requirements on this laser for use
in medical applications are as follows: 1) An output wavelength range around 1700nm to
reduce absorption due to water in the human tissue and a reduction in Rayleigh scattering;
2) tuning bandwidth of ideally more than 100nm which mostly defines the image depth
resolution; 3) Laser linewidth less than 0.07nm to get a coherence length of at least 6mm
106
Tunable quantum-dot ring laser simulations
necessary for the imaging depth; 4) scan rate of at least 20kHz for patient comfort and to
reduce imaging artifacts caused by patient movement; and 5) output power of 1mW. Other
advantages of a monolithically integrated laser source are the miniaturization compared to
the current expensive bulky laser systems. This miniaturization can reduce the cost and the
power consumption of the system. Furthermore, due to the use of voltage controlled
electro-optically PHMs the calibration of the filters remains stable over a long period as
discussed in (Chapter 4). The arbitrary sequence in which the filters can be tuned makes it
possible to scan the laser also linear in the frequency domain which is preferable in
real-time OCT measurements due to the reduced calculation time in Fourier transformations
on data with a linear frequency scale.
If we compare the laser presented above with these requirements we can state that we
made a large step towards the realization of the desired tunable laser. We were able to
realize a monolithically integrated tunable laser source in the 1700nm wavelength region
due to the integration of special designed QD-amplifiers into the active-passive integration
technology. The fabrication of such a relatively large and complex chip is more sensitive to
fatal defects in the wafer and the processing due to the large chip size.
The performance of the laser presented in this work approaches or satisfies most of the
required performance specifications, while others still need some improvements to reach
these requirements. We will summarize the results, compare them with the requirements
and discuss if and how these requirements can be obtained.
6.5.1 Tuning bandwidth
The measured tuning bandwidth of the laser system was 60nm where more than
100nm is ideal. This results in a maximum depth resolution of 22µm (in vacuum). The
limitation on the measured bandwidth is imposed by the limitation of the roundtrip gain in
the ring laser cavity. This roundtrip gain was limited by the fact that laser starts switching
directions at higher current levels due to the fact that the provision for making the ring
uni-directional did not function. The tuning range of the filters is far wider than the tuning
bandwidth of the laser and did not limit the laser tuning bandwidth. Increasing the roundtrip
gain, especially at the edge of spectrum, will directly improve the tuning bandwidth. This
can be done in several ways; reducing the passive length of the ring laser cavity, reducing
the waveguide losses in the passive waveguides, increasing the length of the gain sections
or the gain per unit length and flattening of the gain spectrum. The first three solutions are a
matter of optimization of the design. For example the passive length can be reduced by
rotating the HR-filter 180 degrees which reduces the cavity by approximately 6mm. This
will introduce other problems, such as the location of the bond pads and the polyimide
planarization in the PHM regions as discussed in (Chapter 2 and 4). The passive waveguide
losses can be reduced by optimizing the layerstack for 1700nm wavelength, however this
will make the required fabrication technology move away from the generic integration
technology. Increasing the gain per unit length in these QD-amplifiers is less
107
Chapter 6
straightforward. This increase in gain has however been demonstrated in a QD layerstack
by growing quantum-dots on quantum wells which increases the quantum-dot density [41]
and consequently the modal gain. Flattening of the gain spectrum in these QD-amplifiers
can in principle be done during the growth process by introducing a chirped central
wavelength over different QD layers. This however also reduces the maximum gain, which
can again reduce the tuning bandwidth. Another possibility which would require additional
research, is the use of strained Quantum Wells (QW) instead of quantum-dots. The
advantage of QW above QD is the larger gain per unit length, however the gain bandwidth
is normally much more limited to approximately 40nm. The gain bandwidth can in
principle be increased by designing each QW to have a different peak wavelength, also
called chirped QWs [44].
6.5.2 Laser linewidth
The effective linewidth of the laser is measured to be 0.1nm where 0.07nm was
required to get a coherence length >6mm. From Fig. 6-7d we could see that the FWHM of
the laser peak can be less than the 0.07nm and is often 0.05nm or less. The effective
linewidth is however limited by what looks like mode hopping and dual mode operation at
the 0.1nm mode spacing observed in the laser. This mode pattern causes a broadening of
the laser FWHM while tuning in between two modes Fig. 6-8b. The origin of this 0.1nm
cavity mode spacing has not yet been identified. A possible explanation of the 0.1nm
spacing can be the combination of the 0.02nm longitudinal mode spacing of the total ring
cavity together with the 0.05nm etalon transmission peak spacing caused by reflections
from the amplifier ends.
6.5.3 Scan rate
The maximum scan rate is determined by the switching time between two wavelength
settings and the number of wavelength steps over the tuning range of the laser. The
switching time between two wavelengths has been measured to be 500ns. Assuming the
ideal 0.05nm wavelength steps are used this implies 600µs for a 60nm scan. This 1.67kHz
scan rate is just over one order of magnitude less than the desired 20kHz scan rate. To
increase the scan speed we have to focus on the reduction of the switching time. First of all
the step to step switching of the wavelength during a scan is probably faster. This is due to
the lower suppression at 0.05nm from the starting wavelength than at 45nm as has been
done during the measurement. The neighboring cavity modes will already be at a higher
power level than cavity modes much further away. The switching time is mainly
determined by the roundtrip gain, necessary to build up the laser peak. Increasing the
roundtrip gain will reduce the switching time. Options to increase the gain have been
presented above. We have to take into account that the non-uniform gain over the tuning
bandwidth also introduces a non-uniform maximum tuning speed over the tuning
108
Tunable quantum-dot ring laser simulations
bandwidth. In most tunable lasers it is not possible to utilize this fact due to the fixed linear
or sinusoidal wavelength tuning mechanism. However with the laser presented each
wavelength step is individually controlled. This means that the scan speed can be changed
over the tuning range. Furthermore, the suppression of the clock wise direction will also
increase the small signal gain for the mode building up and so reduce the switching time. A
decrease in the ring cavity length reduces the roundtrip time and therefore the switching
time.
6.5.4 Output power
The measured 0.05-0.15mW output power is one order of magnitude lower than the
preferred 1mW output power. The output power can be increased by (approximately a
factor two) suppressing the clock-wise direction. The rest should be done by increasing the
roundtrip gain.
6.5.5 Overall conclusion
A monolithically integrated continuously tunable laser for OCT fulfilling all requirements
stated above can be realized in the InP-based active-passive integration technology
combined with QD active areas. The tunable laser system presented in this work does not
fulfill all requirements but neither do its limitations appear unsolvable. Most improvements
can be made by increasing the roundtrip gain, making the ring unidirectional and solving
the issue of the 0.1nm interference pattern. Underneath the main conclusions of this thesis
are stated.

It is demonstrated that the standard integration technology at COBRA originally
developed for telecommunication wavelengths can be used up to 1800nm.
In this standard integration technology quantum-dots with an emission wavelength

around 1700nm can be used to generate and amplify light around this wavelength.

These quantum-dots can amplify light over a wavelength region of at least 150nm
wide and can be used for tunable laser purposes over at least 100nm.
The tunable filters presented in this thesis are able to tune over a large wavelength

region, demonstrated up to 180nm wide. The maximum tuning range of the filters is
only limited by the operating range of the waveguide structure.

A tunable laser as presented in this thesis can be fabricated with an emission linewidth
<0.07nm over the full wavelength range. The bandwidth of the high resolution filter
presented in this thesis is sufficiently narrow enough to obtain this linewidth.

The scan speed of the tunable laser presented in this thesis has a theoretical upper limit
of 50ns per wavelength step. (10ns if a narrow linewidth is not required). Based on
1000 wavelength steps over 100nm this means a maximum achievable scan rate of
20kHz.
109
Chapter 6

The output power of the laser as presented in this thesis can be increased by a factor of
approximately two by forcing the laser to be unidirectional. Increasing the output
power further can only be done by increasing the roundtrip gain. This can be achieved
either due to a higher gain per unit length in the amplifiers using the newly developed
high density quantum dot structure or by reducing the waveguide losses in the passive
waveguides by optimizing the waveguide dimensions to reduce the overlap with
p-doped layers in the semiconductor layerstack.
110
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119
List of abbreviations
As
Arsenide
ASE
Amplified Spontaneous Emission
AWG
Arrayed Waveguide Grating
BHF
Buffered Hydrofluoric Acid
COBRA
COmmunication technologies Basic Research and Applications
CLW
Clock Wise
CW
Continuous Wave
CCLW
Counter Clock Wise
DBR
Distributed Bragg Reflector
ES
Exited State
FD-OCT
Frequency Domain Optical Coherence Tomography
FPR
Free Propagation Regions
FSR
Free Spectral Range
FWHM
Full-Width-Half-Maximum
Ga
Gallium
GS
Ground State
HR
High Resolution
In
Indium
INEDFT
Inverse Nonlinear Extended Discrete Fourier Transform
KCN
Potassium Cyanide
LR
Low Resolution
MEMS
Micro-ElectroMechanical-Systems
121
ML
Mono Layer
MMI
Multi-Mode Interferometer
MOVPE
Metal-Organic Vapor-Phase Epitaxy
n.i.d.
not intentionally doped
OCT
Optical Coherence Tomography
P
Phosphide
PCB
Printed Circuit Board
PL
Photoluminescence
PHM
Phase Modulator
QD
Quantum-dot
QW
Quantum Well
RIE
Reactive-Ion Etching
SCH
Separate Confinement Heterostructure
SE
Spontaneous Emission
SEM
Scanning Electron Microscope
SLED
Superluminescent Light Emitting Diode
SG-DBR
Segmented Distributed Bragg Reflector
Si
Silicon
SOA
Semiconductor Optical Amplifier
TE
Transverse Electric
TD-OCT
Time Domain Optical Coherence Tomography
WDM
Waveguide-Division Multiplexing
WL
Wetting Layer
122
Summary
Integrated tunable quantum-dot laser
for optical coherence tomography
in the 1.7µm wavelength region
In this thesis the results are presented that were obtained in the development of a novel
integrated semiconductor laser light source suitable for the three-dimensional (3D) imaging
technique of Optical Coherence Tomography (OCT) in biological tissues. The laser
development was driven by new possibilities and requirements in OCT imaging which
could be seen to be realizable through use of the latest developments in optical integration
technology on indium phoshide (InP) and capabilities of quantum-dot (QD) based
integrated optical amplifiers. It has led to a fully integrated continuously tunable laser
source in the 1600nm to 1800nm wavelength region on a single InP chip.
For a particular version of OCT that allows for 3D imaging within a fraction of
second, there are two main requirements. The first is that the laser must be able to scan over
a wavelength range of at least 100nm to obtain sufficient depth resolution. The second main
requirement is that the laser must be able to scan within several tens of microseconds over
the whole tuning range repeatedly to obtain the imaging speed. To enable an increase in the
OCT imaging depth into the tissue the development was aimed at a laser source in the
1600nm to 1800nm wavelength region where Rayleigh scattering is reduced.
The goal was to realize a laser design based on a ring laser cavity with intra cavity
quantum-dot amplifiers and tunable filters within the generic active-passive integration
technology of COBRA. It is demonstrated that this integration technology designed for the
use in the 1550nm wavelength region can also be used in the 1700nm region without a
large penalty in performance. The quantum-dot amplifiers used are specially designed to
generate and amplify light over a wide bandwidth centred at 1700 nm. To tune the
wavelength of the ring laser the tunable filters are included. These electro-optical tunable
wavelength filters are based on arrayed waveguide gratings (AWGs) with electro-optically
phase modulators in the arms of the AWGs. This allows the control of the filter tuning over
a bandwidth of over 200 nm and provides the possibility of tuning with the same speed for
any wavelength step within its range.
123
The research consisted of three major parts. The first is the realization,
characterization and modelling of the QD amplifiers realised on a fully active wafer. It was
shown that the quantum-dot amplifiers are suitable for the task and the dependency of the
gain spectrum on current density has been modelled successfully. The shift of over 100 nm
observed in the gain spectrum with changing current has been explained satisfactorily using
a coupled rate equation system and has revealed the role of the level structure in the dots
and the coupling of those levels to a common charge carrier reservoir.
In the second part the focus was on the design, fabrication and characterization of the
electro-optically tunable filters. A combination of two filters is chosen to fulfil the
requirements for the laser. The first filter is a high resolution filter with a 0.5nm full-widthhalf-maximum of the passband and a 10nm free spectral range. The second filter, a low
resolution filter, is used to suppress unwanted passbands of the high resolution filter and
has a 29nm full-width-half-maximum of the passband and a 210nm free spectral range. The
lay-out of the filters was optimised for fabrication, the performance of the filters was tested
and the results compared well with the design values. A dedicated measurement setup and
electronics to control the 40 electro-optic phase modulators in the filters and laser chips was
realised to accommodate the many connections between the electronics and the InP chip.
The automatic calibration systems, control principles for the filter tuning and related
software were developed. Tuning over more than 100nm is demonstrated with an accuracy
of 0.1nm wavelength accuracy (1% of the free spectral range) for the high resolution filter
and 9nm wavelength accuracy (4% of the free spectral range) for the low resolution filter.
The switching of the high resolution filter was demonstrated with a 10%-90% rise and fall
time within 100ns.
The third part is the design, modelling, fabrication and characterization of the
complete tunable quantum-dot laser with integration of active and passive devices on a
single InP chip. To simulate the tuning capabilities of the laser, the rate equation model of
the quantum-dot amplifier was extended to a laser model. The whole laser system was
realised on a 6 x 10 mm2 chip and tested. It is one of the most extensive integrated optical
circuits realised in active/passive technology in Europe. The laser was demonstrated to be
tunable over 60nm between 1685nm and 1745nm with an 0.2nm wavelength accuracy and
an effective linewidth of 0.11nm. The output power of the laser was approximately 0.1mW.
Switching between two wavelengths in this 60nm was possible with a 500ns 10%-90% rise
and fall time.
Finally the tunable laser has been used in a free space Michelson interferometer setup.
This is the first step towards an OCT measurement. The measurement of the mirror position
in one of the interferometer arm was demonstrated with a 500Hz scan rate and 4000
wavelength sampling points over the 60nm scan.
The performance of the tunable laser presented in this thesis represents a significant
step towards the requirements of a OCT measurement system. The results did not raise any
fundamental limitations in the laser concept which would limit its development to a market
competitive product.
124
List of publications
International Journals

B.W. Tilma, Y. Jiao, J. Kotani, E. Smalbrugge, H.P.M.M. Ambrosius, P.J.
Thijs, X.J.M. Leijtens, R. Nötzel, M.K. Smit, and E.A.J.M. Bente,
“Integrated tunable quantum-dot laser for optical coherence tomography in
the 1.7µm wavelength region,” submitted to IEEE J. Quantum Electron.

B.W. Tilma. Y. Jiao, P.J. van Veldhoven, E. Smalbrugge, H.P.M.M.
Ambrosius, P.J. Thijs, X.J.M. Leijtens, R. Nötzel, M.K. Smit, and E.A.J.M.
Bente, “InP-based monolithically integrated tunable wavelength filters in the
1.6µm to 1.8µm wavelength region for tunable laser purposes” submitted to
IEEE J. Lightwave Technol.

B.W. Tilma, M.S. Tahvili, J. Kotani, R. Nötzel, M.K. Smit and E.A.J.M.
Bente, “Measurement and analysis of optical gain spectra in 1.6 to 1.8 μm
InAs/InP (100) quantum-dot amplifiers”, Optical and Quantum Electron.,
41(10), 735-749, (2010)

M.J.R. Heck, P. Munoz, B.W. Tilma, E.A.J.M. Bente, Y. Barbarin, Y.S. Oei,
R. Nötzel and M.K. Smit, “Design, fabrication and characterization of an InPbased tunable integrated optical pulse shaper”, IEEE J. Quantum Electron.,
44(4), 370-377, (2008)
International Conferences

B.W. Tilma, Y. Jiao, J. Kotani, B. Smalbrugge, H.P.M.M. Ambrosius, P.J.
Thijs, X.J.M. Leijtens, R. Nötzel, M.K. Smit, and E.A.J.M. Bente,
“Integrated Tunable Quantum-dot Laser for Optical Coherence Tomography
in the 1.7µm Wavelength Region,” accepted for CLEO Europe conference
Munich (2011)
125

Y.Jiao, B.W. Tilma, J.Kotani, R. Nötzel, M.K. Smit, E.A.J.M Bente, “An
InAs/InP(100) QD Waveguide Photodetector for OCT application,” accepted
for CLEO Europe conference Munich (2011)

B.W. Tilma, R. Nötzel, M.K. Smit and E.A.J.M. Bente, “Monolithically
Integrated Tunable Laser Source for OCT in the 1.6 to 1.8μm Wavelength
Region”, Proceedings of 18 th Conference on Advanced Laser Technologies
(ALT’10), September 11 -16, 2010, Egmond aan Zee, The Netherlands

E.A.J.M. Bente, M.S. Tahvili, B.W. Tilma, J. Kotani, M.K. Smit and R.
Nötzel, “Modelocked and tunable InAs/InP (100) quantum-dot lasers in the
1.5μm to 1.8μm region”, 12 th International Conference on Transparent
Optical Networks (ICTON), 2010, May 27 – June 01, Munich, Germany. (pp.
1-4). Piscataway: IEEE

B.W. Tilma, M.S. Tahvili, J. Kotani, R. Nötzel, M.K. Smit and E.A.J.M.
Bente, “Analysis of the current dependency of the small signal gain spectrum
in InAs/InP (100) quantum-dot amplifiers”, Proceedings of the 15 th European
Conference on Integrated Optics, ECIO 2010, April 06 – 09, 2010,
Cambridge, United Kingdom. (pp. ThP14-1/2). London: IET. (First prize
poster presentation)

B.W. Tilma, E.A.J.M. Bente, R.Nötzel, J. Kotani, X.J.M. Leijtens and M.K.
Smit, “Integrated tunable optical filters on InP for continuously tunable
lasers” Proceedings of the 22 nd Annual Meeting of the IEEE Photonics
Society, October 04 – 08, 2009, Belek- Antalya, Turkey. (pp WF1-430/431).
Piscataway: IEEE Service Center

B.W. Tilma, M.S. Tahvili, J. Kotani, R.Nötzel, M.K. Smit and E.A.J.M.
Bente, “Observation and modeling of long-wavelength InAs/InP (100)
quantum-dot amplifier small signal gain spectra”, European Semiconductor
Laser Workshop (ESLW), September 25 – 26, 2009, Vienna, Austria.
(pp 6)

B.W. Tilma, E.A.J.M. Bente, X.J.M. Leijtens, R.Nötzel and M.K. Smit,
“Design of an integrated electro-optically tunable filter for tunable laser
purposes”, Proceedings or the 14 th European Conference on Integrated
Optics, ECIO08, June 11 – 13, 2008, Eindhoven, The Netherlands. (pp 181184). Eindhoven: Technische Universiteit Eindhoven
126

B.W. Tilma, E.A.J.M. Bente and M.K. Smit, “Design of an integrated electrooptically tunable filter for tunable laser purposes”, Proceedings of the
ePIXnet Spring School 2008, May 11 – 17, 2008, Elba, Italy. (pp.68)
Local Conferences





B.W. Tilma, Y. Jiao, J. Kotani, X.J.M. Leijtens, R. Nötzel, M.K. Smit and
E.A.J.M. Bente, “Monolithically integrated continuously tunable InP-based
quantum-dot laser source in the 1.6 to 1.8µm wavelength region”,
Proceedings 15 th Annual Symposium of the IEEE Photonics Benelux Chapter,
November 18-19, 2010, Delft, The Netherlands, (pp. 81-84), Delft: Uitgeverij
TNO
B.W. Tilma, X.J.M. Leijtens, M.K. Smit and E.A.J.M. Bente,
“Characterization of integrated electro-optically tunable cascaded filters for
tunable laser purposes”, Proceedings 14 th Annual Symposium of the IEEE
Photonics Benelux Chapter, November 05 – 06, 2009, Brussels, Belgium. (pp.
177-180). Brussels: Brussels University Press
B.W. Tilma, M.S. Tahvili, J. Kotani, R.Nötzel, M.K. Smit and E.A.J.M.
Bente, “Observation and modeling of long-wavelength InAs/InP (100)
quantum-dot amplifier small signal gain spectra”, Proceedings 14 th Annual
Symposium of the IEEE Photonics Benelux Chapter, November 05 – 06, 2009,
Brussels, Belgium. (pp. 169-172). Brussels: Brussels University Press
B.W. Tilma, M.J.R. Heck, E.A.J.M. Bente and M.K. Smit, “Modal gain
measurements in quantum-dot amplifiers in the 1600nm-1800nm wavelength
range”, Proceedings of the 13 th annual symposium of the IEEE/LEOS Benelux
Chapter, November 27 – 28, 2008, Enschede, The Netherlands. (pp. 151-154).
Piscataway: IEEE Service Center
B.W. Tilma, E.A.J.M. Bente and M.K. Smit, “Swept laser source for optical
coherence tomography”, Proceedings Photonica Evenement 2008,
Nieuwegein, The Netherlands
127

M.J.R. Heck, P. Munoz, B.W. Tilma, E.A.J.M. Bente, Y. Barbarin, Y.S. Oei,
R. Nötzel and M.K. Smit, “Design, fabrication and characterization of an InPbased tunable integrated optical pulse shaper”, Proceedings of the 12 th
Annual Symposium of the IEEE/LEOS Benelux Chapter, December 17 – 18,
2007, Brussels, Belgium. (pp 91-94). Brussels: IEEE/LEOS
128
Acknowledgement
In de eerste klas van de middelbare school moest ik mijn motivatie voor het schrijven
van een verslag formuleren. Die luidde als volgt: “Dit verslag schrijf ik, omdat het moet
van mijn leraar.” Voor mij de werkelijke reden, maar niet datgene wat bedoeld werd. Nu
het einde van mijn promotieonderzoek nadert, rijst dezelfde vraag: Waarom schrijf je dit
proefschrift? Het antwoord is eenvoudig: “Omdat het moet om te kunnen promoveren!”
Maar het ligt nu genuanceerder. Ten eerste is het mijn eigen keuze om te promoveren,
en ten tweede is het zonde om vier jaar onderzoeksresultaten in rook te laten opgaan. Maar
bovenal heb ik de afgelopen vier jaar met plezier aan dit onderzoek gewerkt en is die drie
maanden ploeteren aan het proefschrift eigenlijk zo erg niet. Dat is voor een groot deel te
danken aan de mensen om mij heen. Ik zou dan ook graag iedereen willen bedanken die mij
de afgelopen jaren heeft geholpen, in wat voor opzicht dan ook.
Allereerst natuurlijk Erwin, als mijn directe begeleider en copromotor. Ik heb onze
samenwerking als zeer prettig ervaren. Gedegen onderzoek doen moet je leren, en jij bent
een zeer goede leermeester. De inzet waarmee je de totstandkoming en in het bijzonder de
afronding van het proefschrift begeleid hebt, is bewonderenswaardig. Ik vermoed dat jouw
geduld wel eens op de proef werd gesteld door mijn “originele” zinsconstructies.
Daarnaast wil ik Meint, mijn promotor, bedanken voor de fijne samenwerking en het
vertrouwen in het OCT-onderzoek. Je toewijding aan de realisatie van een generieke
integratietechnologie in photonics is geweldig! Ik hoop daar in de toekomst ook nog aan bij
te kunnen dragen of gebruik van te kunnen maken. Ik blijf graag op de hoogte!
I would like to thank Saeed and Yuqing for helping me in different stages of the
research project. Saeed, I have very much appreciated your contribution on the quantumdot measurements and analysis, I hope it will be of use for your research as well.
Yuqing, your help in the past months was amazing! I don’t know if I would have
managed to perform all the measurements on the tunable laser without you. I hope you will
have more nice results from this research project.
Natuurlijk wil ik ook mijn generatiegenoot Milan bedanken. Het was prettig om samen
het promotietraject te doorlopen, ook al werkten we aan totaal verschillende onderwerpen:
een van de kleinste en een van de grootste geïntegreerde lasers die binnen de OED-groep
gemaakt zijn!
129
Xaveer, bedankt voor je hulp bij het ontwerpen van de AWG’s. We hebben daarnaast
heel wat rondjes op de ijsbaan gemaakt, al kon ik je niet altijd bijhouden. Het dagje
schaatsen op de Loosdrechtse Plassen was erg leuk.
Een geïntegreerde laser ontwerpen is een ding, er een maken is een tweede: het vergt
de nodige bekwaamheid in de cleanroom. Ik zou dan ook de technici willen bedanken die
mij dit vakmanschap hebben bijgebracht. Barry, de man met de gouden handen: of het nou
om een plating ging of om bonddraden, het resultaat was altijd geweldig. Tjibbe, Erik-Jan,
Jeroen en Kitty, bedankt voor jullie hulp en inzet om de chips met succes te fabriceren.
Mijn dank gaat ook uit naar Huub, Siang, Hans en Ben: de stille krachten achter veel
successen. Jos, ik heb het één keer aangedurfd om tegen je te schaken, tot die tijd had ik de
illusie dat ik dat wel enigszins kon…. Boudewijn, heel veel succes met je bedrijf. Ik hoop
dat de geïntegreerde optica hiermee vastere voet aan de grond krijgt in Nederland.
I would also like to thank the former OED group members. Martijn, mijn afstuderen
bij jou heeft mijn interesse gewekt voor het onderzoek van de OED-groep, waardoor ik nog
een aantal jaartjes ben blijven hangen. Ling, it was nice sharing our office for two years. I
wish you the very best. Yohan, thank you for introducing me to the Keller group at the
ETH Zürich. Luc, Els, Pietro, Stefano, Jose and Fouad, it was a pleasure to work with you.
Martin, your high level of research is an inspiration to us all. Antonio, Emil, Ray, Jing,
Josselin, Kasia, Stanislaw, Dima, Manuela, Srivathsa and Elton, it was a pleasure to work
with you all. I wish you all the best for your research projects. Lorraine, Robert and Bart,
we had a nice time sharing our office, I wish you all the best in your current jobs.
Het maken van de tunable laser stond niet op zichzelf, maar is uitgevoerd binnen het
grotere IOP-OCT project. Ik wil dan ook de projectpartners van het AMC, TOPCON en
MVM bedanken voor de goede samenwerking. Ik denk dat we samen een mooi stukje
onderzoek hebben afgeleverd. I would like to address special thanks to Richard, René and
Junji from the PSN group for the growth of the wafers and your input in the QD analysis.
Peter Thijs, dankzij de mooie hergroei van verschillende wafers bij Philips MiPlaza moest
zeker tenminste één van de lasers uiteindelijk gaan werken.
Wat had ik verder aan een tunable laser zonder deze elektronisch aan te kunnen
sturen? Fred van Nijmwegen en Gerard Harkema, jullie hebben met de AWG100 units
fantastisch werk afgeleverd. Het blijft een mooi verhaal dat wij van de faculteit Electrical
Engineering aan moeten kloppen bij de faculteit Technische Natuurkunde om elektronica te
laten maken!
Het leven speelt zich gelukkig niet alleen op kantoor af, ik wil dan ook al onze
vrienden bedanken die de laatste jaren met ons zijn opgetrokken. Jullie waren er zowel in
moeilijke als in vreugdevolle tijden. Mijn dank ook aan Monique van der Wijst voor jouw
input bij het ontwerpen van de omslag van dit proefschrift.
130
Stephan en Sonja, Annelies en Loes, jullie waren er altijd voor ons, vooral als we een
paar extra handen konden gebruiken. Catrien, Michel, Ignas, Sierd, Carmela, Jan, Tiny,
Lubbert en Paulus, kom gerust een keer langs in het mooie Zwitserland. Een heel speciale
dank aan hait en mama, zonder jullie was ik hier nooit gekomen, dank voor jullie
vertrouwen en inspanning. Ik hoop dat jullie je een beetje kunnen vinden in de spreuk die ik
van pake en beppe heb meegekregen.
Als laatste natuurlijk Emmy, je hebt mij de afgelopen jaren altijd gesteund in mijn
onderzoek, samen hebben we de juiste balans weten te vinden tussen werken en
ontspannen. Ik heb er erg veel zin in om samen een aantal jaar naar Zwitserland te gaan.
En dan rest me alleen nog een kusje voor Marta!
131
Curriculum vitae
Bonifatius Wilhelmus Tilma (Bauke) was born in Amsterdam, the Netherlands in
1982. In 2003 he received the B.Eng. degree in electronics engineering from Hogeschool
Rens & Rens in Hilversum, the Netherlands. He received the M.Sc. degree in electrical
engineering from the Eindhoven University of Technology in 2006. He performed his
Master thesis research on the full temporal magnitude and phase characterization of
picoseconds laser pulses using spectral interference at the Opto-Electronic Devices group of
this university.
From 2007 to 2011 he has been working towards the Ph.D. degree in the Photonic
Integration group (formerly Opto-Electronic Devices group) of the COBRA Research
Institute at the Eindhoven University of Technology, where he was involved in research on
a monolithically integrated tunable quantum-dot laser source for optical coherence
tomography in the 1.7µm wavelength region.
From summer 2011 he is going to work as a postdoctoral researcher at the
Eidgenössische Technische Hochschule (ETH) in Zürich, Switzerland on electrically
pumped VECSELs and MIXSELS and their modelocking.
133
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