Tm,Ho co-doped disordered calcium niobium gallium garnet (CNGG) crystals are investigated as a novel gain medium for mode-locked lasers near 2 μm. With a GaSb-based semiconductor saturable absorber mirror (SESAM) and chirped mirrors for dispersion compensation such a laser is mode-locked at a repetition rate of 89.3 MHz. For a 5% output coupler, a maximum average output power of 157 mW is obtained with a pulse duration of 170 fs (28-nm broad spectrum centered at 2.075 μm, leading to a time-bandwidth product of 0.331). With a 0.5% output coupler, 73-fs pulses are generated at 2.061 μm with a spectral width of 62 nm (time-bandwidth product of 0.320) and an average output power of 36 mW.
Mode-locked lasers emitting ultrashort pulses in the 2-μm spectral range at high (100-MHz) repetition rates offer unique opportunities for time-resolved molecular spectroscopy and are interesting as pump/seed sources for parametric frequency down-conversion and as seeders of ultrafast regenerative laser amplifiers. Passively mode-locked lasers based on Tm3+- and Ho3+-doped bulk solid-state materials have been under development for about a decade. In 2009 we demonstrated the first steady-state operation of such a Tm:KLu(WO4)2 laser using a single-walled carbon nanotube (SWCNT) saturable absorber (SA), generating 10-ps pulses at 1.95 μm. In 2012 this laser produced 141-fs pulses at 2.037 μm. More recently, the study of numerous active media with different SAs resulted in the generation of sub-100-fs (sub-10-optical-cycle) pulses. Materials with broad and smooth spectral gain profile were selected, naturally emitting above 2 μm to avoid water vapor absorption/dispersion effects, including anisotropic materials, strong crystal-field distortion in hosts that do not contain rare-earths, crystals with structural or compositional (i.e. mixed compounds) disorder that exhibit inhomogeneous line broadening, mixed laser ceramics, and Tm,Ho-codoping of ordered and disordered crystals and ceramics. A broad absorption band in semiconducting SWCNTs spans from 1.6 to 2.1-μm whereas the absorption of graphene extends into the mid-IR and scales for multilayers, increasing the modulation depth. Compared to GaSb-based semiconductor SA mirrors (SESAMs), the carbon nanostructures exhibit broader spectral response and can be fabricated by simpler and inexpensive techniques. Chirped mirrors were implemented for groupvelocity dispersion compensation, to generate the shortest pulses, down to 52 fs at 2.015 μm.
The recent advances in the development of Holmium monoclinic double tungstate thin-disk lasers are reviewed. The thin-disk is based on a 250-μm-thick 3 at. % Ho:KY(WO4)2 active layer grown on a (010)-oriented KY(WO4)2 substrate. When pumped by a Tm-fiber laser at 1960 nm with a single-bounce pump geometry, the continuous-wave Ho:KY(WO4)2 thin-disk laser generates an output power of 1.01 W at 2057 nm corresponding to a slope efficiency η of 60% and a laser threshold of only 0.15 W. The thin-disk laser is passively Q-switched with a GaSb-based quantum-well semiconductor saturable absorber mirror. In this regime, it generates an average output power of 0.551 W at ~2056 nm with η = 44%. The best pulse characteristics are 4.1 μJ / 201 ns at a repetition rate of 135 kHz. The laser performance, beam quality and thermo-optic aberrations of such lasers are strongly affected by the Ho3+ doping concentration. For the 3 at.% Ho3+-doped thin-disk, the thermal lens is negative (the sensitivity factors for the two principal meridional planes are -1.7 and -0.6 m-1/W) and astigmatic. The Ho:KY(WO4)2 epitaxial structures are promising as active elements in mode-locked thin-disk lasers at ~2060 nm.
We review the development of the first GaSb-based passively mode-locked VECSEL generating sub-picosecond pulses
at 2 μm wavelength range. The general goal of this development was to leverage the unique features of the mode-locked
VECSELs (i.e. high-average power, sub-ps operation, high repetition rate, low-noise properties) to the 2-3 μm
wavelengths. Such lasers could have a significant impact on the development of practical ultrafast systems required for
frequency-combs, time-resolved molecular spectroscopy, THz generation, or as seeders for optical amplifiers and mid-IR
supercontinuum sources. By using semiconductor gain mirrors and saturable absorber mirrors incorporating
InGaSb/GaSb quantum wells, we have been able to demonstrate a VECSEL producing near transform-limited 384 fs
pulses at a wavelength of 1950 nm. Important part of this development has been focused on understanding the ultrafast
absorption recovery dynamics of the SESAM. An interesting observation is that the absorption recovery time of asgrown
InGaSb SESAMs is within ps range and is not much affected by a change of the growth parameters.
We review recent results concerning the development of dilute nitride based semiconductor disk lasers. We have
demonstrated over 7.4 W of output power at the second harmonic wavelength (around 590 nm) using a β-BBO crystal.
Over 10 W has been demonstrated at ~1.2 μm, and multi-watt output power has been achieved at 589 nm with narrow
linewidth (δν < 20 MHz).
Dilute nitride semiconductor disk lasers offer a convenient way of producing 570-650 nm radiation required in medicine
and life science. These lasers can produce multi-watt powers with narrow spectra and compact footprints similar to solid
state lasers. The advantage of using semiconductor gain materials is their ability to reach wavelengths that are not
attainable by traditional solid state lasers. Other advantages include a wide tuning range and the possibility for electrical
modulation. Here we demonstrate a narrow band (<30 MHz) yellow (589 nm) disk laser with 2.7 W output power. The
gain mirror of the laser is optically pumped with an 808 nm diode laser. The emission wavelength of the laser can be
tuned over several nanometers by tilting the filter inside the laser cavity.
We demonstrate a dilute nitride (GaInAsN) based gain mirror capable of meeting the wavelength and linewidth
requirements for laser guide stars. The mirror was grown by molecular beam epitaxy on a GaAs(100) substrate. The heat
generated during laser operation was extracted from the active region with a wedged intracavity CVD diamond. An
intracavity birefringent filter was employed for wavelength selection and a YAG etalon for linewidth narrowing. The
laser radiation was intra-cavity frequency doubled to achieve emission at 589 nm. The frequency-doubled semiconductor
disk laser emitted a narrow linewidth beam (~20 MHz) at 589 nm. In a free-running mode, the laser emitted more than
6W of yellow-orange light with a maximum conversion efficiency of 15.5%.
Mid-infrared semiconductor laser are highly attractive sources for environmental monitoring since the spectral
fingerprints of many environmentally important gases are located in the 2-3.3 μm wavelength regime accessible by
gallium-antimonide technology. Here an electrically-pumped vertical-external-cavity surface-emitting laser (EP-VECSEL)
was realized at 2.34 μm wavelength, using a gain mirror based on the GaSb material system. The gain mirror
was grown by molecular beam epitaxy on an n-type GaSb substrate and it included a distributed Bragg reflector made of
24-pairs of AlAsSb/GaSb layers, and a gain region with 5 GaInAsSb quantum wells placed in a 3-λ thick micro-cavity.
A structured buried tunnel junction (BTJ) with subsequent overgrowth was used in order to obtain efficient current
confinement, reduced optical losses and increased electrical conductivity. Different components were tested with
aperture sizes varying from 30 μm to 90 μm. Pulsed lasing was obtained with all tested components at 15 °C mount
temperature. We obtained a maximum peak power of 1.5 mW at wavelength of 2.34 μm.
We report a GaInNAs/GaAs-based disk laser producing 7 W output power at 1180 nm wavelength at a temperature of
15 °C. The laser generated more than 5 W of output power when it was forced to operate with a narrow spectrum at 1178
nm. The gain mirror was grown using a molecular beam epitaxy reactor and it comprised 10 GaInNAs QWs and a 25.5-
pair GaAs/AlAs distributed Bragg reflector.
We review recent results concerning the development of GaSb-based heterostructures for semiconductor disk lasers. We
focus on fabrication and design details of gain and semiconductor saturable absorber mirrors used to demonstrate disk
lasers exhibiting high output power, broad tunability, and short pulse generation. We demonstrate a 2 μm gain structure
with 15 InGaSb quantum wells emitting more than 4 W of output power at 15°C. Almost 1W output power was
measured at an elevated temperature of 50°C. A tuning range of more than 150 nm was achieved by employing a gain
mirror comprising quantum wells with different widths to provide broadband gain. Ultra-short pulse generation based on
synchronous mode-locking and a preliminary demonstration of passively mode-locked semiconductor disk lasers based
on GaSb saturable absorber mirrors are also discussed.
We report an essential progress towards the development of efficient GaInNAs-based semiconductor disk lasers
operating at 1220 nm spectral range. The gain mirrors were fabricated by molecular beam epitaxy using a radio
frequency plasma source for incorporating the nitrogen. The typical structure consisted of a 30-pair GaAs/AlAs
distributed Bragg reflector and 10 GaInNAs quantum wells with relatively low content of nitrogen. The growth
parameters and the composition of the structures have been optimized to reduce the detrimental effect of nitrogen on the
emission efficiency. We have achieved a maximum output power of 3.5 W and a differential efficiency of 20%.
Owing to their good beam quality and high output power, near-infrared semiconductor disk lasers provide an attractive
opportunity for visible light generation via frequency conversion. The typical cavity arrangement of a semiconductor
disk laser, consisting of a semiconductor multiple quantum well gain mirror and one or more external mirror, offers a
convenient configuration for intracavity frequency doubling. Recent progress in the disk laser development has led to
demonstrations of multi-watt green-blue-yellow sources. These achievements have been enabled by the possibility to
integrate high performance InGaAs/GaAs gain media and Al(Ga)As/GaAs Bragg reflectors operating in the 940-1160
nm wavelength range. In order to achieve ~620 nm red emission, a laser emitting near the fundamental wavelength of
1240 nm is needed. To achieve this spectral range we have developed GaInNAs/GaAs gain mirrors and we have
achieved 1 W of output power at 617 nm by frequency doubling in a BBO crystal. This is to our knowledge the highest
power reported to date for intracavity doubled disk laser based on dilute nitride gain material.
Semiconductor Disk Lasers (SDLs) are compact lasers suitable for watt to multi-watt direct generation in the 670-
2350nm waveband and frequency-doubled operation in the ultraviolet and visible regions. This is, however, critically
dependent on the thermal management strategy used as, in this type of laser, the pump is absorbed over micrometer
lengths and the gain and loss are temperature sensitive. In this paper, we compare the two heat dissipation techniques that
have been successfully deployed to-date: the "thin device" approach where the semiconductor active mirror is bonded
onto a heatsink and its substrate subsequently removed, and the "heatspreader" technique where a high thermal
conductivity platelet is directly bonded onto the active part of the unprocessed epilayer. We show that for SDLs emitting
at 1060nm with pump spots of ~80µm diameter, the heatspreader approach outperforms the thin-device alternative, with
the best results being obtained with a diamond heatspreader. Indeed, the thermal resistances are measured to be 4.9, 10.4
and 13.0 K/W for diamond-bonded, SiC-bonded and flip-chip devices respectively. It is also observed, as expected, that
the thermal management strategy indirectly affects the optimum output coupling and thus the overall performance of
We present new approaches for power scaling and tunability in semiconductor disk lasers. The novel concepts allow for
reduced thermal load of the gain material, increasing the threshold of rollover and extending the capability for boosting
the output power without significant degradation in the beam quality. The proposed technique for power scaling of
optically-pumped semiconductor disk lasers is based on the multiple gain scheme. The method allows for significant
power improvement while preserving good beam quality. Total power of over 8 W was achieved in dual-gain
configuration, while one-gain lasers could produce separately about 4 W, limited by the thermal rollover of the output
characteristics. The results show that reduced thermal load to a gain element in a dual-gain cavity allows extending the
range of usable pump powers boosting the laser output.
Tunable Sb-based semiconductor disk laser operating at 2-&mgr;m is demonstrated with nearly 100 nm operation range. The
maximum output is 210 mW and the 3dB tuning range spans from 1946 to 1997 nm. The wavelength tuning is based on
an intracavity birefringent filter. The potential of semiconductor disk lasers for high repetition rate ultrashort pulse
generation using harmonic mode-locking is also discussed. We report on optically-pumped vertical-external-cavity
surface-emitting lasers passively mode-locked with a semiconductor saturable-absorber mirror. The potential of
harmonic mode-locking in producing pulse trains at multigigahertz repetition rates has been explored. The results present
first systematic study of multiple pulse formation in passively mode-locked VECSELs.