In this contribution, we demonstrate that spectral beam combining in an external cavity (EC), a technique which has been
applied previously to shorter wavelength diode laser bars [1], is also applicable to mid-infrared QC lasers. Within this
concept, the output of multiple emitters from a 4.6 μm emitting QC laser chip is combined in a single, collinear beam.
The average power of an EC-QC laser module realized that way surpasses the output of a corresponding single emitter
by more than a factor of 4. Furthermore, the EC-concept allows a certain degree of wavelength tuning during operation.
The EC, consisting of a collimating lens, a grating and a partially reflecting outcoupling mirror, forces each laser to emit
at a unique wavelength defined by its offset relative to the main optical axis. The EC approach further ensures the
collinear directional and spatial overlap of the individual QC laser output beams forming a single combined output beam.
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.
We report on the characteristics of (AlGaIn)(AsSb)-based optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) emitting at wavelengths around 2.35 μm. For barrier-pumped VECSELs mounted substrate-side down without substrate thinning, typical room temperature cw output powers of 2 mW were achieved, limited by premature thermal rollover due to strong device overheating. The thermal impedance of the VECSEL semiconductor chip could be considerably reduced by bonding an intra-cavity polycrystalline CVD diamond heat spreader to the top surface of the chip. This way, at -18°C a maximum cw output power of 0.6 W and a slope efficiency of 10% were obtained for a multiple transverse mode output beam limited by the available pump power rather than by thermal rollover. Optimising the resonator for TEM00 mode operation (M2≈1.1), an output power exceeding 0.4 W was achieved. To reduce the large quantum deficit of more than 50% inherent to barrier-pumped (1.06 μm pump wavelength) GaSb-based VECSELs which emit at wavelengths above 2 μm, we realized a first in-well pumped (AlGaIn)(AsSb) VECSEL where the pump light is absorbed directly in the quantum wells, with the amount of absorbed light enhanced by a higher order microcavity resonance. Using a pump wavelength of 1.94 μm, the quantum deficit is reduced to only 18% and an output power of 5mW, limited by the available pump power, and a slope efficiency of 10% were achieved. Further optimisation of the pump optics is expected to result in a significant increase in device performance.
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