The (AlGaIn)(AsSb) semiconductor materials system has been shown to be ideally suited to realize optically pumped
Vertical External Cavity Surface Emitting Lasers (VECSELs) for the 2-3 μm wavelength range. For a 2 μm emitting
single-chip VECSEL with a ternary GaInSb quantum well active region, employing a linear planar-concave resonator
geometry, a maximum continuous wave (cw) output power of 4.2 W at 20°C heatsink temperature (6 W at 0°C) has been
achieved. Standard fiber-coupled 980 nm diode lasers have been used for optical barrier pumping. Employing a
W-shaped resonator with two optically pumped gain chips acting as planar folding mirrors, a maximum cw output power
of 5.5 W has been achieved at a heatsink temperature of 20°C, increasing to 10.4 W when reducing the heatsink
temperature to -10°C. The resonator versatility of a VECSEL also allows the insertion of additional optical elements into
the optical cavity for wavelength selection and linewidth control. Employing a V-shaped folded cavity with the gain chip
acting as the planar end mirror, single-mode operation with a linewidth <100 kHz at 1 W optical output power has been
achieved at a lasing wavelength of 2.05 μm.
In this paper we report on the development of narrow-linewidth vertical-external-cavity surface-emitting laser
(VECSEL) at a wavelength of >2 μm.
Starting from a laboratory setup, we designed a highly stable VECSEL module machined from a solid block of
aluminum. For linewidth precise measurements, heterodyne beatnote measurements were employed. For this firstgeneration
module a linewidth of 9 kHz was achieved when actively stabilizing the laser wavelength, whereas without
stabilization the linewidth amounted to 45 kHz at an output power of 100 mW, both data referring to a 100-μs sampling
time.
To further increase the output power, a second-generation module was fabricated, for which the on-chip mode diameter
was increased. This allowed operation at a larger pump-spot diameter and still maintaining TEM00 operation, while
increasing the maximum pump power and hence the output power. This module yielded an output power above 1 W in
single-mode operation at a linewidth of 60 kHz (100 μs sampling time) without active wavelength stabilization.
Modehop-free single-mode operation could be maintained for more than 18 hours.
This new multiple-Watt, narrow-linewidth VECSEL module is apt for plane-to-ground communications without the
necessity of amplifiers.
In recent years, optically pumped semiconductor disk lasers (OPSDLs) have attracted increasing interest due to their
capability of delivering simultaneously high output power and excellent beam quality. Here we report on group-III-Sbbased
OPSDLs allowing to cover the wavelength range around and above 2 μm. First the current state-of-the-art and
recent progress for OPSDLs emitting in the 2.0-to-2.3 μm spectral range is presented, which includes power scaling
through the use of multiple gain elements and as well as spectral tuning and line width narrowing, exploiting in both
cases the versatility of the external cavity concept. Then, results on III-Sb-based OPSDLs emitting at 2.8 μm with a cw
output power of up to 0.12 W and a peak output power in pulsed mode of >0.5 W, both data referring to roomtemperature
operation, are presented. In both cases, the active region of the OPSDL chip consists of compressively
strained GaInAsSb quantum well (QW) layers embedded between AlGaAsSb barrier and pump-light-absorbing layers.
The emission wavelength is controlled by adjusting the composition of the quaternary QW material. The active region is
grown on top of an epitaxial GaSb/AlAsSb Bragg mirror. For efficient heat extraction, SiC intra-cavity heat spreaders
were bonded to the surface of the cleaved laser chips. An N-shaped resonator with one OPSDL chip acting as an end
mirror and the second OPSDL chip as a folding mirror was used for power scaling, while a V-shaped resonator
configuration with a birefringent tuner inserted into the collimated beam path of the resonator was employed for
wavelength tuning. Optical pumping was achieved by standard fiber-coupled diode laser modules emitting at 980 nm.
We report on the concept, realization and performance data of infrared semiconductor laser modules serving as compact
and robust laser sources for a Directed Infrared Countermeasures (DIRCM) system. While the 2-2.5 μm atmospheric
transmission window is covered by a GaSb-based optically pumped semiconductor disk laser (OPSDL), delivering a
continuous-wave (cw) or temporally modulated output of ≥ 1 W with a high beam quality (M2 < 3), an external cavity
(EC) quantum cascade (QC) laser module is used to cover the 4.5-5 μm spectral range. The EC-QC laser concept allows
efficient spectral beam combining of the output of several QC laser located side-by-side on the same semiconductor chip,
while preserving the high-quality output beam of a single emitter. Both the OPSDL and the EC-QC laser have been integrated
into rugged laser modules, comprising also all necessary power supply and control electronics, ready for use in
field trials.
Many applications exist for high performance lasers in the short-wave, mid-infrared spectral regime between 1.9 and
2.5μm - from long-range communications systems through to remote atmospheric gas sensing and pollution monitoring.
However, a simple, efficient laser source offering the desired performance characteristics and flexibility has not been
available. In the last few years considerable progress has been made in the development of optically-pumped
(AlGaIn)(AsSb) quantum well semiconductor disk lasers emitting in the 2.Xμm mid-infrared spectral region -
continuous-wave and pulsed-pumped output power levels now exceed 6W and 16W respectively. Furthermore, singlefrequency
operation with linewidths <4MHz and broad tunability of up to 170nm have also been demonstrated, all at
near-diffraction-limited beam quality. Such performance metrics are only possible through the very best materials
growth, a sound understanding of the design principles of these highly multi-layered devices and, importantly, the
application of effective thermal management.
A novel active region concept for GaSb-based optically pumped mid-infrared vertical external cavity surface emitting
lasers (VECSELs, also referred to as optically pumped semiconductor disk lasers - OPSDLs) is presented. The concept is
based on GaxIn1-xAsySb1-y type-I quantum wells (QWs) embedded between AlAs0.08Sb0.92 barrier layers designed for
optical in-well pumping where the pump absorption at pump wavelengths between 1 μm and 2 μm takes place
exclusively in the active QWs. This concept provides several advantages such as a high modal gain, the suppression of
thermal leakage currents, and an improved thermal conductivity of the active region compared to a conventional
GaInAsSb/AlGaAsSb active region design. Using the novel design approach an in-well pumped VECSEL emitting at
2.24 μm has been realized, yielding at a heat sink temperature of 20°C in continuous-wave operation a power slope
efficiency of more than 32% and an absorption of the 1.96 μm pump light of more than 50% without pump recycling,
These data constitute a significant improvement in device performance compared to previously reported data on in-well
pumped GaSb-based VECSELs.
We report on the development and characteristics of infrared semiconductor lasers as compact and robust light sources
for Directed Infrared Countermeasures (DIRCM). The short-wavelength side of the 2-5 μm wavelength band of interest
can be covered by GaSb-based optically pumped semiconductor disk lasers (OPSDLs), delivering a continuous-wave
(cw) or temporally modulated multiple-Watt output with a high beam quality (M2<3). For the 3.7-5 μm wavelength
range InP-based quantum cascade (QC) lasers are the best suited semiconductor laser source, delivering several hundreds
of mW of average output power in a nearly diffraction limited output beam (M2<2). Further up-scaling of the output
power can be achieved for OPSDLs by intra-cavity coupling of several semiconductor chips as gain elements in a
multiple-disk cavity arrangement. For a 2.3 µm emitting dual-disk OPSDL, a doubling of the maximum roomtemperature
output power compared to that of a comparable single-chip OPSDL has been demonstrated. For QC lasers
power scaling by beam-quality-preserving beam combining has been demonstrated via polarization coupling of the
output beams of two individual QC lasers, yielding a coupling efficiency of 82%.
We report the realization of GaSb-based optically pumped vertical-external-cavity surface-emitting lasers (VECSELs)
emitting at 2.25 μm which are capable of multiple-Watt output power. VECSEL structures were grown on GaSb-substrates
by molecular beam epitaxy. SiC heat spreaders were capillary bonded onto the surface of the VECSEL chip in
order to facilitate efficient heat removal. A continuous-wave output power of more than 3.4 W was recorded at a heat
sink temperature of -10 °C. At room temperature (20 °C) we still obtained more than 1.6 W output power. A beam
propagation factor in the range of M2≤5 was measured at maximum output power. In adjusting the fundamental mode
diameter on the VECSEL chip to the pump spot diameter the beam quality could be further improved resulting in a beam
propagation factor of M2~1.5. Furthermore, initial results on a GaSb-based dual-chip VECSEL are reported, capable of
delivering a maximum output power of 3.3 W for a heat sink temperature of 20 °C and an emission wavelength of
2.25 μm.
We report on recent advances in the performance of GaSb-based optically pumped semiconductor disk lasers
(OPSDLs), emitting in the 2.0 - 2.3 μm wavelength range. Both barrier pumped OPSDL (using 980 nm laser
diodes as pump source) and in-well pumped OPSDL (using 1.96 μm pump radiation) have been fabricated
and characterized. Using alternative SiC or diamond intracavity heatspreader, multiple-watt CW-output
powers have been achieved (e.g. >3W at 2.3 μm and >5W at 2.0 μm), with power efficiencies in the range of
18 % - 25 %. For an optimised resonator setup, the beam profile is close to the diffraction limit with M2
values around 1.2; and even for the highest power levels, M2 is in the range of 2-5.
We report on recent advances in the performance of GaSb-based vertical-external-cavity surface-emitting lasers (VECSELs) emitting at wavelengths around 2.3 μm. Barrier-pumped VECSELs have been fabricated and analysed, which incorporates a diamond intra-cavity heat spreader as an efficient means for heat extraction from the active region. A maximum output power of 1.5 W has been achieved at a heat sink temperature of -20°C and still more than 1 W at 10°C. Two VECSEL structures with different spectral offsets ▵λ between the microcavity resonance and the maximum of the gain spectrum have been examined in particular. The results clearly show that the proper choice of this offset ▵λ is critical for an optimization of the high-power performance of the VECSEL, e.g. maximizing the output power before thermal rollover.
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