CHAPTER 2
TUNABLE EXTERNAL-CAVITY DIODE LASERS
2.1 External-cavity diode lasers
The external-cavity diode laser (ECDL) comprises an optical gain media (a laser diode
typically with an antireflection (AR) coating on one facet), optically coupled to the external
cavity that includes a retroreflecting element, and one or more wavelength-selective filters.
The earliest paper cited by many authors is published by Fleming and Mooradian in 1981 [10],
since they were the first to study the spectral properties of ECDL’s in detail. A number of
different types of wavelength-selective elements have been used in the ECDL’s including
diffraction gratings, Fabry-Perot étalons, birefringent filters, etc. The most commonly used
filters are the diffraction gratings. Since the diffraction gratings in the ECDL combine the
functions of the filter and external mirror. A number of ECDL arrangements employing
diffraction gratings have been introduced. Among the most simple arrangements is a design
similar to the one described by Ricci et al. [11]. In this design, the laser emitted from an ARcoated front facet of a LD is collimated and hits a grating under Littrow angle. The light
diffracted in the first order is reflected back into the LD, whereas the light diffracted in the
zeroth order is coupled out and can be used for the experiments. A second concept is an
external cavity in a grazing-incident configuration similar to the design from Littman and
Metcalf [12]. They used the dye cell as the gain medium. The output beam of a LD is directed
on to a grating. The first diffraction order of the grating hits a reflecting element, e. g. mirror,
and reflected back on to the grating. The laser resonate cavity is set up between the back facet
of the LD and the reflecting element. Basic designs of the Littrow and Littman-Metcalf
ECDL’s are schematically shown in Fig. 2.1. The optical feedback of the grating or the
reflecting element results in single longitudinal mode operating of the diode [10]. The
arrangements reduce the critical dependence of the output frequency on the laser current and
temperature, and allows coarse wavelength tuning over the wide gain bandwidth of the LD
through the rotation of the grating or the reflecting element.
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AR
Diffraction Grating
LD
Lens
Output
(a)
AR
Diffraction Grating
LD
Lens
Retroreflecting
element
Output
(b)
Fig. 2.1 ECDL of (a) Littrow (b) Littman-Metcalf type mounting. LD: laser
diode; AR: anti-reflection coating
The basic model for an external-cavity laser is based on a 3-mirror compound cavity with
amplitude reflectivity r1 at the left facet of the LD, r2 at the right facet of the LD and rext at the
external reflector. The effective reflectance of the extended cavity is given by [13]
reff (ν ) =
r2 + rext (ν ) exp(i 2πντ ext )
1 + r2 rext (ν ) exp(i 2πτ ext )
(2.1)
τext is the round-trip time of extended cavity section. In any external-cavity design, one should
try to maximize the external feedback strength and wavelength selectivity of the cavity.
Strong external feedback is desirable to avoid a number of undesirable phenomena such as
tuning nonlinearities and axial mode instabilities caused by the LD-cavity étalon effect.
Excellent optical performance can be obtained by an excellent AR coating applied to the LD.
However, even with a high quality facet coating, effects of the residual diode cavity
resonances are still observable and are sometimes the cause of non-ideal behavior.
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2.2 Wavelength tuning mechanisms
A number of different types of wavelength-selective elements have been used to tune the
ECDL’s. The ideal wavelength filter for an ECDL has a bandwidth that is less than the axial
mode spacing of the cavity and has no insertion loss at its peak. The filters can be grouped
according to whether they are actuated mechanically (e.g., have moving parts) or
electronically (no moving parts).
2.2.1 Mechanical tuning
2.2.1.1 Diffraction gratings
The grating-tuned ECDL is the most commonly reported type of ECDL [14-19]. The laser
wavelength is tuned as the grating rotates in the “standard” Littrow configuration. Grating
tuning for broad range of several tens nanometers and as high as 100 nm are possible,
depending on wavelength and diode material family. In the Littman-Metcalf configuration,
widely wavelength tuning is achieved by rotating the optical feedback end mirror [20-22].
With simple grating or mirror feedback, an ECDL will usually tune by hopping from one
longitudinal mode to the next over the laser’s entire gain bandwidth. To deal with this issue, a
mechanical approach by mounting the feedback element on a pivot arm is well-known [23-27].
The idea is based on scanning the cavity length and the grating feedback angle simultaneously.
The geometric requirements for a widely mode-hop-free tunable Littman-Metcalf cavity are
shown in Fig. 2.2 [28].
Mirror
Normal to grating
θd
Diode laser
and lens
l2
θi
l1
Grating
L
Pivot point
Fig. 2.2 Geometric requirements for a widely mode-hop-free tunable ECDL [28]
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The pivot point links the source, grating and mirror such that adjusting the angle of the mirror
about this point meets the requirements for single mode tuning without mode hops. When the
adjustment is done correctly, the physical length of the cavity and the selected wavelength are
changed simultaneously. The requirement for position of the pivot point is that the optical
path from emission point to the pivot point must balance the path from the mirror to the pivot
point. The geometric relation of l1, the cavity length from source to diffraction grating, l2, the
cavity length from grating to feedback mirror, l, distance between the pivot point and grating
and the grating pitch a is thus given as
(l1+ l2)/λ=l/a
(2.2)
The continuous tuning without mode-hopping requires delicate adjustments and a high
mechanical stability. The mode-hop-free tuning range is quite sensitive to the precise location
of the rotation axis. The task then becomes setting the pivot point correctly. In any real ECDL
there will be some error in positioning the pivot point. Assume the laser is positioned at the
origin with the output in the positive x direction. The optimum pivot point (x,y)=(0, -l⋅cotθ) of
Littrow configuration is derived by equating the modal and the grating tuning rates as a
function of the rotation angle [29-31]. As an example, the continuous tuning range as a
function of x and y pivot-point error with respect to the optimum pivot point is plotted in Fig.
2.3, assuming a 1200-line/mm grating, a center wavelength λ=780 nm and a 5-cm cavity
Continuous tuning range (THz)
length.
40
35
30
25
20
15
10
5
-0.01
-0.005
0
0.005
Position error of pivot point (x/m)
(a)
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0.01
Continuous tuning range (THz)
40
35
30
25
20
15
10
5
-0.095
-0.0948
-0.0946
-0.0944
-0.0942
-0.094
Position error of pivot point (y/m)
(b)
Fig. 2.3 The continuous tuning range as a function of (a) x and (b) y pivot-point error
with respect to the optimum pivot point
mirror
X2
X0
A
X1
grating
(a)
mirror
mirror
grating
grating
A
A
(b) X0=X1=X2=0
(c) X1=0 and X0+X2=0
Fig. 2.4 Optimum pivot points of ECDL in the Littman-Metcalf configuration
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The tolerance to pivot error in the y direction is much restrictive than in the x direction. To
obtain a continuous tuning range of 10 THz (20 nm), the pivot point must be placed within a
region roughly 4 mm long in the x direction and 50 µm wide in the y direction.
For Littman-Metcalf configurations, Liu and Littman [32] first pointed out that a suitable
choice exists for the axis of rotation of the mirror so that the cavity mode scan will exactly
match the feedback wavelength scan. There are two possibilities for the optimum pivot point
[33]. The configurations are illustrated in Fig. 2.4, where Xj are perpendicular distances from
point A to the surfaces of the element. A is the pivot point of rotation. A change of 2π in the
round trip phase results in one complete step of the cavity mode structure relative to the
feedback frequency. Thus the change in this phase is angle scanned over the specified range
should be a small fraction of 2π to ensure continued single-mode operation. The configuration
tolerance is plotted in Fig. 2.5, assuming an 1800-line/mm grating, a center wavelength
λ=780 nm and a conventional phase tolerance of 2π/10.
Tolerance of pivot point (mm)
10
8
6
4
2
0
20
40
60
80
100
120
Continuous tuning range (GHz)
Fig. 2.5 The tolerance of pivot point position (Littman-Metcalf configuration)
The X-axis in Fig. 2.5 is the synchronous scan range and Y-axis is the tolerance of pivot point
position. For scan range of 10 GHz, the tolerance of pivot point position is 3 mm. For scan
range of 100 GHz, the tolerance of pivot point position is restricted to only 0.3 mm.
2.2.1.2 Fabry-Perot étlons or interference filters
Instead of the diffraction gratings Fabry-Perot étlons can be inserted into the coupled
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external cavity as the wavelength-selected elements. The Fabry-Perot étalon has periodic
transmission peaks at wavelength that satisfy the relation
2nd cos θ = mλ
(2.3)
Where d is the mirror spacing, n is the refractive index of the space between the mirrors, θ is
the angle of incidence, and m is an integer. Tuning can be accomplished by changing the
mirror separation or by varying the angle of incidence. C. Voumard [34] has demonstrated an
external-cavity-controlled GaAlAs diode laser containing two glass-plate Fabry-Peror étalons
as the dispersive elements. The angles of the two Fabry-Perot étalons can be adjusted
independently. The emission spectrum was reduced to one single-axial mode of the
external-cavity without decrease of output power. M. J. Chawki et al. [35] demonstrated an
all-fiber semiconductor ring laser with a fiber Fabry-Perot (FFP) filter. Wavelength tuning
was possible by changing the voltage of the FFP thus scanning the center frequency of the
FFP over the period modes of the semiconductor FP cavity.
An interference filter is a multilayer thin-film device. It can be treated as a Fabry-Perot
étalon with a small thickness. The interference filter is tuned by tilting it in the incident beam.
P. Zorabedian and W. R. Trutna, Jr. demonstrated an interference-filter-tuned ECDL, and
compared the angular alignment tolerance and the tuning range with a conventional
grating-tuned ECDL [36]. They concluded that the interference-filter laser had a 260 fold
greater alignment tolerance and nearly the same tuning range as the grating laser. Gratings can
be used in tandem with Fabry-Perot étalons to tune ECDL’s as demonstrated by Olsson and
Ziel [1]. In their approach, the grating is illuminated with a broad beam and provides most of
the spectral selectivity. An étalon improves the stability of single-mode operation. They
divided the tuning of an ECDL into a coarse, medium, and fine tuning regime. Coarse tuning
was obtained by rotating the grating reflector and selecting the internal mode (longitudinal
modes of the solitary laser without the external cavity) which is closest to the desired
wavelength. The medium tuning was achieved by adjusting the intracavity étalon in
combination with a fine rotation of the grating. Fine tuning is done by fine adjustments to the
external cavity length.
2.2.2 Electronically tuning
Mechanical controlled wavelength tuning filters require mechanical movement of
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relatively bulky components. It is desirable to be able to tune the laser frequency
electronically by varying the driving voltage applied to the tuning element only. The
electronic tuning provides rapid voltage-controlled cavity length scan and hence output
wavelength tuning. Birefringent filters [37-41], acousto-optic (AO) filters [42-44] and
microelectromechanical systems (MEMS) [45, 46] have been used for electronic wavelength
tuning. Electronically tuned birefringent filters can be realized using electro-optic effect,
either in bulk crystals or in birefringent lithium niobate waveguides [37-41]. The liquid
crystal cells can also be used. The birefringent filters using liquid crystals will be discussed in
details in the next chapter.
2.2.2.1 Birefringent filters tuning ⎯ electro-optic crystals
The electro-optic crystal (EOC) provides rapid voltage-controlled cavity-length scans and
hence output frequency tuning. Several groups of researchers have successfully built ECDL’s
with intracavity EOC. J.-P. Goedgebuer et al. [37] reported a tunable ECDL using a bulk
LiNbO3 crystal inside the cavity as the wavelength selective element. Wavelength tuning was
achieved by varying the optical delay introduced by the LiNbO3 tuner. The LiNbO3 crystal
forming the tuner is 70 mm long. Its half-wave voltage is Vπ=2400 V at the wavelength of
1500 nm. A tuning rate of 1 GHz/V over a tuning range of about 4 nm was obtained. B. Boggs
et al. [38] has built a simple electro-optically activated ECDL with an intracavity lithium
tantalite EOC (Fig. 2.6). The half-wave voltage is Vπ=475 V. Continuous tuning over
approximately 3 GHz for 5-cm optical-path-length external cavity and tuning as fast as 23
GHz/µs over gigahertz frequency ranges was demonstrated.
EOC
Grating
Diode
laser
θ
Lens
Fig. 2.6 ECDL tuned with an intracavity EOC [38]. EOC: electro-optic crystal
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L. Ménager et al. [39] has demonstrated a cavity contains a LiNbO3 electro-optic 25° prism
for synchronous tuning the cavity length and the grating’s incident angle. A mode-hop-free
tuning of 12 GHz with high linearity and reproducibility for a 4-kV voltage variation was
reported. L. Levin [40] designed a Littrow type ECDL with a 1~1.51-mm-thick intracavity
LiTaO3 crystal. He increased the sensitivities of the frequency change-voltage ratio and the
frequency-tuning interval by making the crystal thinner. The change in cavity length that is
imposed by the EOC is inversely proportional to the crystal thickness. A mode-hop-free
single-mode tuning range of 50 GHz and tuning speeds of 1.5 GHz/µs is demonstrated. An
electro-optical scanner consisting of a LiNbO3 crystal with a series of domain inverted
triangular prisms used in a Littrow-type ECDL was reported by M. Laschek et al. [41]. With
the configuration, a wavelength tuning of more than 1 nm was realized. The tuning coefficient
is 2 nm/kV.
A disadvantage of electro-optic birefringent tuning is that the large voltage required tends
to limit the tuning to significantly less than the full semiconductor gain bandwidth.
2.2.2.2 Acousto-optic tuning
Acousto-optic (AO) filters are an advantageous means for rapid, electronic wavelength
control of ECDL’s. The wavelength range of an AO tunable filter is typically much broader
than the gain bandwidth of an individual diode laser, so there are no wavelength range
limitations imposed by the filter. Narrow bandwidth and high transmission are required for
the AO filters when applied to tuning applications. In the study reported by Coquin and
Cheung [42, 43], a pair of AO devices, an AO tunable filter and an AO modulator, are used as
the wavelength selective element in an ECDL. Tuning is accomplished by varying only the
drive frequency of the AO devices. The AO tunable filter used for the experiments has an
optical bandwidth of 3 nm. The chief drawback of AO tuning is that the filter spectral width is
much greater than the spectral width that can be readily obtained with the diffraction grating.
This means that the AO-tuned ECDL’s must have excellent suppression of LD cavity modes
in order to achieve good tuning fidelity. Use of an AO tunable filter inside a laser cavity
results in a repeated frequency shift on each pass of the optical signal through the filter.
Putting a second AO device with a frequency shift equal and opposite to that of the first one,
makes a stable single-mode operation is possible. A 3-µs tuning rate over an 83 nm tunable
wavelength at λ=1.3 µm was demonstrated. An approach for achieving rapid and
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phase-coherent continuous broadband tuning of a single-mode ECDL using two internal AO
devices was reported by M. Kourogi et al. [44]. Instead of tunable filters, a grating and a pair
of AO modulators were used as wavelength-selective elements (Fig. 2.7). Compared with the
method in which AO filters are used, the approach gives high-resolution wavelength
selectivity and enables single-mode operation of the laser. The AO modulators control the
angle of incident light on a diffraction grating and the effective round-trip optical phase. To
control the round-trip phase, a long delay line to generate a modulation signal of the AO
device. Single-mode operation and electric tuning over 2 nm were achieved. To achieve
mode-hop-free tuning over a broad wavelength range a long optical delay line with a digital
phase-locked oscillator should be implemented in their present design.
Amp
Delay line
Grating
Diode
laser
Lens
AOM1
AOM2
Amp
fm
VCO
Fig. 2.7 ECDL tuned with two acousto-optic devices [44]. AOM: acousto-optic modulator;
Amp: amplifier; VCO: voltage controlled oscillator
2.2.2.3 Microelectromechanical systems tuning
MEMS technology has been shown to be very promising in miniaturizing tunable ECDL’s.
The precision and stable movement of the microactuators enables fine wavelength tuning. The
small size of the micromachined mirrors facilitates fast tuning speed and low power
consumption. The Iolon Inc. [45] has developed a high power, widely tunable micro- ECDL
based on a MEMS electrostatic actuator for telecommunication applications. It was designed
in the Littman-Metcalf configuration. Wavelength tuning was achieved by applying a voltage
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to the MEMS actuator. The actuator is capable of rotating the mirror ±1.4° when a voltage of
up to 140 V is applied to one of two sets of comb-drive elements. And the suspension in the
actuator is designed to allow rotation of the mirror about a pivot point. The MEM-ECDL
provides 7 dBm fiber coupled output power over a 40 nm tuning range, and exhibits all of the
performance characteristics of traditional ECDL’s, in a low cost, 5 mm×5 mm form factor
(Fig. 2.8). Zhang et al. [46] demonstrated a MEMS discrete wavelength tunable laser formed
by integrating a semiconductor laser, a single-mode optical fiber, and a MEMS mirror onto a
single chip. It has overall dimensions of 1.5 mm×1 mm×0.6 mm. A tuning range of 13.5 nm
and sweeping speed of about 1 ms was obtained.
Suspension
Beams
Rotary Comb Drive
Substrate
Attachment
Rotary Motor Frame
Mirror
Lens
Laser
chip
Output Beam
Pivot point
Grating
Fig. 2.8 ECDL based on MEMS electrostatic rotary actuator [45]
The MEMS based ECDL has advantages of wide tuning range, improved repeatability and
stability compared to conventional ECDL’s, and can be integrated in a very small size.
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