High Repetition Frequency Mode-locked Semiconductor Disk Laser
Yanrong Song
, Peng Zhang
, Jinrong Tian
, Zhigang Zhang
, Hark Hoe Tan
and C. Jagadish
College of Applied Sciences, Beijing University of Technology, Beijing, 100124, P.R. China
School of Electronics Engineering and Computer Science, Peking University, Beijing, 100871, P.R. China
Department of Electronic Materials Engineering, Research School of Physics and Engineering,
The Australian National University, Canberra, ACT 0200, Australia
Keywords: Lasers, Diode-pumped, Ultrafast Lasers, Semiconductor Lasers.
Abstract: A compact passively mode-locked semiconductor disk laser with a high repetition frequency of 3GHz is
demonstrated. 4.9ps pulse duration and 30mW average output power are obtained with 1.4W of 808nm
incident pump power. The gain chip consists of 16 compressively strained InGaAs symmetrical step
quantum wells in the active region.
High repetition frequency pulse trains are suitable
for a wide variety of applications, such as optical
clocking (Miller, 2000), frequency conversion
(Lecomte et al., 2005), high-speed electro-optic
sampling (Weingarten et al., 1988), time-resolved
spectroscopy (Bartels et al., 1999), and the primary
light source for telecommunication systems
(Ramaswami and Sivarajan, 1998). For a long time,
mode-locked solid-state lasers and edge-emitting
semiconductor lasers were the dominators in the
field of multi-gigahertz picosecond pulse resources.
In recent years, passively mode-locked
semiconductor disk lasers (SDLs, also known as
optically pumped vertical-external-cavity surface-
emitting lasers), have been demonstrated capable to
generate multi-gigahertz pulses with high average
output power and good beam quality (Hoogland et
al., 2000); (Häring et al., 2002); (Aschwanden et al.,
2005); (Keller and Tropper, 2006). By comparison,
SDLs are cost-effective hence attractive for their
potential substitutes of mode-locked solid-state
lasers or edge-emitting semiconductor lasers in some
applications (Gherman et al., 2004); (Dupriez et al.,
2206); (Mihoubi et al., 2008).
After the first report of a passively mode-locked
SDL (Hoogland et al., 2000), performance of the
mode-locked SDL has been improved significantly:
pulse duration to 190fs at 3GHz repetition rate and
5mW output power by a fast SESAM (Klopp et al.,
2009); repetition rate to 50GHz with 3.3ps pulse
duration and 100mW output power by a low
saturation fluence quantum dot SESAM (Lorenser et
al., 2006); and average output power to 2.1W at
4GHz repetition rate and 4.7ps pulse duration by the
substrate-removing method (Aschwanden et al.,
2005). However, the lasing wavelength of a SDL
(GaAs based) redshifts at a rate of about 0.3nm/K
with increasing temperature because of the intrinsic
characteristic of the QWs (Tropper et al., 2004), and
this problem has not been solved so far. In this
paper, we demonstrate a picosecond passively mode-
locked SDL at multi-gigahertz repetition rate. The
gain chip is without any post processing (such as
substrate-removing and heatspreader-bonding), the
fabrication is quite simple and the configuration is
compact. Temperature stability of the laser is
achieved using the symmetrical step QWs in the
active region.
Figure 1 shows the gain chip used in the
experiments. It contained three main components:
the Bragg reflector, the gain region, and the
confinement window layer. The Bragg reflector was
grown on a GaAs substrate and comprised 27
GaAs/AlAs layer pairs designed to act as a high
reflection mirror (calculated reflectivity > 99.95%).
The gain region was grown by standard metal-
organic chemical vapor deposition (MOCVD)
method which consisted of 14 InGaAs quantum
Song Y., Zhang P., Tian J., Zhang Z., Hoe Tan H. and Jagadish C..
High Repetition Frequency Mode-locked Semiconductor Disk Laser.
DOI: 10.5220/0004022903610364
In Proceedings of the International Conference on Data Communication Networking, e-Business and Optical Communication Systems (OPTICS-2012),
pages 361-364
ISBN: 978-989-8565-23-5
2012 SCITEPRESS (Science and Technology Publications, Lda.)
wells(QW) surrounded by GaAsP barrier layers for a
total thickness of 7λ/2, which allows an efficient
absorption of the pump radiation. Finally, a window
layer of AlAs was grown and the full structure was
completed by a thin InGaP capping layer (not
appeared in fig.1). The AlAs window layer
prevented the excited carriers escaping to the
surface. This surface barrier layer was transparent
for the pump wavelength.
Figure 1: The scheme of the gain structure.
The passively mode-locked SDL with the V-
shaped cavity setup is shown in Fig. 2. A
2mm×2mm unprocessed gain chip cleaved off the
wafer is directly mounted to a copper heat sink and
is temperature controlled by a Peltier cooler. The
back plate of the entire device is water-cooled. A
fiber-coupled 808nm pump radiation is collimated
and focused on the semiconductor disk at an angle of
about 45 degree, and the diameter of the pump spot
is approximately 100μm. The gain chip is used as a
folding mirror, and the laser cavity is ended by a
low-temperature grown SESAM and a highly
reflective plane-concave output coupler (OC). The
folded angle of the cavity is smaller than 10
reduce the astigmatism.
The curved OC determines a small mode area on
the SESAM, such that the saturation energy of the
absorber is lower than the saturation energy of the
gain medium, as required for pulse formation
(Paschotta et al., 2002. In the experiment, the total
cavity length is approximately same as the radius of
curvature of the OC to force the laser to operate at
the edge of the stable region. The lengths of OC arm
and SESAM arm are about nine tenths and one tenth
of the total cavity length, respectively, to produce a
laser spot about 100μm on the gain chip for
matching the pump spot. The ratio of the area
between the spot on the gain chip and on the
SESAM is about 25. With a fixed optimum laser
spot on the gain chip, we adjust the lengths of the
SESAM arm and the OC arm to obtain a stable
continuous-wave (CW) mode-locking operation.
We use an Infiniium 54833A oscilloscope
(Agilent Technologies) to monitor the mode-locked
pulse trains. The output beam is sent to a FR-103XL
autocorrelator (Femtochrome Research) to obtain the
autocorrelation trace. Meanwhile, a fast 818-BB-35F
InGaAs PIN photoelectric detector (Newport) with
10GHz bandwidth and <35ps rise time and a 40GHz
E4447A spectrum analyzer (Agilent Technologies)
are used to receive the optical pulses and obtain the
radio-frequency (RF) spectrum.
Figure 2: Cavity setup used for the passively mode-locked
Figure 3: Measured and sech
fitted autocorrelation trace
(a) and RF spectrum (b) of the mode-locked pulses. The
inset in (a) shows the optical spectrum and the inset in (b)
shows the RF spectrum on a 5GHz span. The radius of
curvature of the OC is 100mm.
Firstly, a plane-concave mirror with a radius of
curvature of 100mm is used as the OC. The
measured and sech
fitted autocorrelation trace of
the mode-locked pulse trains are shown in Fig. 3(a).
The sech
fit indicates that the ideal width (full
width at half maximum, FWHM) of the pulses is
5.0ps, which is about 4.2 times of the Fourier
transform limited sech
pulse duration of 1.2ps,
according to the inserted optical spectrum with 0.96
nm width. It is known that for a passively mode-
locked SDL, the presence of chirp in the pulses is
inevitable because of the saturation effect (also
known as phase modulation effect) of the gain
medium and the SESAM. With some dispersion-
controlled elements such as an etalon, the above
phase modulation effect can be counteracted and the
transform limited pulse trains can be produced
(Aschwanden et al., 2005).
Fig. 3(b) shows the RF spectrum of the laser.
The frequency span is 180MHz and the resolution
bandwidth (RBW) is 1.8MHz. The inset shows the
frequency spectrum on a 5GHz span. As shown in
Fig. 3(b), the fundamental frequency is 1.5GHz,
corresponding to the round-trip repetition rate of
100mm cavity. The RF spectrum trace is free from
sidebands down to the level of -35dBc,
demonstrating that the SDL exhibits stable CW
mode-locking with no Q-switching instabilities.
Figure 4: Measured and sech
fitted autocorrelation trace
(a) and RF spectrum (b) of the mode-locked pulses. The
inset in (a) shows the optical spectrum and the inset in (b)
shows the RF spectrum on a 12GHz span. The radius of
curvature of the OC is 50mm.
When the 100mm radius of curvature of OC is
replaced by a 50mm radius of curvature OC, the
autocorrelation trace and the RF spectrum of the
mode-locked laser are shown in Fig. 4(a) and (b). It
can be seen from Fig. 4(a) that the sech
fitted pulse
duration is 4.9ps, which is about 2.3 times of the
Fourier transform limited sech
pulse width of 2.1ps,
according to the inserted 0.77nm width optical
spectrum. Fig. 4(b) shows a 3GHz RF of the mode-
locked pulses, and this is consistent with the round-
trip repetition rate of 50mm cavity. The RF spectrum
trace is free from sidebands down to the level of -
45dBc, again demonstrating no Q-switching
instabilities in the CW mode-locked SDL.
Figure 5 shows the average output power versus
pump power under 3
C heat sink temperature. The
repetition is 3GHz, and the laser operates in a
circular TEM
mode. The maximum average output
power of 30mW is reached with 1.4W pump power;
the slope efficiency (SE) and the optical-to-optical
conversion efficiency are about 3.1% and 2%,
respectively. Limitations of the output power and SE
are mainly result from the thermal problem. We
soldered the gain chip to a heat-spreader to remove
the thermal and then the output laser could be higher.
Figure 5: Average output power of the mode-locked laser
at 3GHz repetition rate versus pump power at a
temperature of 3
We have demonstrated a compact passively mode-
locked SDL. The CW mode- locking operation at
3GHz repetition rate, 4.9ps pulse duration and
30mW average output power are obtained using a
low-temperature grown SESAM. The laser can be
used as gigahertz picosecond pulse source in many
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