High brightness laser diode arrays are increasingly found in defense applications either as efficient optical pumps or as
direct energy sources. In many instances, duty cycles of 10- 20 % are required, together with precise optical
collimation. System requirements are not always compatible with the use of microchannel based cooling,
notwithstanding their remarkable efficiency. Simpler but effective solutions, which will not involve high fluid pressure
drops as well as deionized water, are needed. The designer is faced with a number of challenges: effective heat
removal, minimization of the built- in and operational stresses as well as precise and accurate fast axis collimation. In
this article, we report on a novel laser diode array which includes an integral tap water cooling system. Robustness is
achieved by all around hard solder bonding of passivated 940nm laser bars. Far field mapping of the beam, after
accurate fast axis collimation will be presented. It will be shown that the design of water cooling channels , proper
selection of package materials, careful design of fatigue sensitive parts and active collimation technique allow for
long life time and reliability, while not compromising the laser diode array efficiency, optical power density
,brightness and compactness. Main performance characteristics are 150W/bar peak optical power, 10% duty cycle and
more than 50% wall plug efficiency with less than 1° fast axis divergence. Lifetime of 0.5 Gshots with less than 10%
power degradation has been proved. Additionally, the devices have successfully survived harsh environmental
conditions such as thermal cycling of the coolant temperature and mechanical shocks.
High Power Laser Diode Arrays developed and produced at SCD-SemiConductor Devices support a number of
advanced defence and space programs. High efficiency and unsurpassed reliability at high operating temperatures are
mandatory features for those applications. We report lifetime results of high power bar stacks, operating in QCW mode
that rely on a field-proven design comprising Al-free wafer material technology and hard soldering robust packaging. A
variety of packaging platforms have been implemented and tested at very harsh environmental conditions.
Results include a long operational lifetime study totaling 20 billion pulses monitored in the course of several years for
808 nm QCW bar stacks.. Additionally, we report results of demanding lifetime tests for space qualification performed
on these stacks at different levels of current load in a unique combination with operational temperature cycles in the
range of -10 ÷60 °C.
Novel solutions for highly reliable water cooled devices designed for operation in long pulses at different levels of PRF,
are also discussed. The cooling efficiency of microchannel coolers is preserved while reliability is improved.
Space missions are probably the most demanding environment for laser diodes. A comprehensive study on the reliability
of commercially available laser diodes arrays (LDA), with the objective of bar stacks for ESA's BepiColombo Laser
Altimeter mission to the planet Mercury was performed. We report the best results of lifetime tests performed on SCD
808 nm QCW stacks at different levels of current load in a unique combination with operational temperature cycles in
the range of -10°C to 60 °C. Based on a field-proven design that includes Al-free wafer material and a robust packaging
solution, these arrays exhibit long operational lifetime of up to 20 billion pulses monitored in the course of several years.
Zero failures and stable performance of these QCW arrays were demonstrated in severe environmental conditions
reflecting both, military and space applications. In order to achieve maximum device efficiency at different operational
conditions of the base temperature and current, an optimum combination of the wafer structure and bar design is
required. We demonstrate different types of QCW stacks delivering peak power of up to 1 kW with a usable range of
50-55% wall plug efficiency at base temperatures up to 60 °C.
808 nm, QCW laser bars delivering peak power higher than 150 Watts were developed. The optimization of the tensile
strain in the QW structure, the design configuration of the laser cavity together with an improved packaging technology
lead to more than 55% wall plug efficiency when assembled as stacks. Due to the high characteristic temperature (T<sub>0</sub>,
T<sub>1</sub>) values and high efficiency, the output power of these devices is almost insensitive to elevated heat sink
temperatures. In addition, a collimation technique which significantly improves the beam quality of the laser stacks was
developed. The active collimation method is flexible and control over the level of collimation is achievable. The use of
this collimation technique alongside with high quality micro lenses allows for a reduction of the fast axis divergence to
values as low as 3 mrad with minimal power losses. An automatic process control was developed allowing for the
efficient attachment of the collimating micro lenses in a highly reproducible fashion. The combination of the collimation
technique with a reliable mounting and stacking technology supports the serial manufacturing of devices delivering 1
kW peak power in QCW operation. These QCW collimated diode laser stacks demonstrate stable operation and high
reliability in the course of more than 6*10<sup>8</sup>shots at 2% duty cycle. Another important advantage of the collimated
stacks is their capability to withstand severe environmental conditions, maintaining high beam quality and performance.
Antimonide Based Compound Semiconductors (ABCS) and a new family of advanced analogue and digital silicon read-out integrated circuits form the basis of the SCD 3<sup>rd</sup> generation detector program, which builds on the firm platform of SCDs existing InSb-FPA technology. In order to cover the MWIR atmospheric window, we recently proposed the epitaxial alloys: InAs<sub>1-y</sub> Sb<sub>y</sub> on GaSb with 0.07 < <i>y</i> < 0.11 and In<sub>1-z</sub>Al<sub>z</sub> Sb on InSb with 0 < <i>z</i> < 0.03. In this paper we focus on the results of some of our recent work on epitaxial In<sub>1-z</sub>Al<sub>z</sub> Sb grown on InSb by Molecular Beam Epitxay (MBE). In epitaxial InSb (<sub>z</sub> = 0), we demonstrate the performance of Focal Plane Arrays (FPAs) with a format of 320x256 pixels, at focal plane temperatures between 77K and 100K. An operability has been achieved which is in excess of 99.5%, with a Residual Non-Uniformity (RNU) <i>at 95K</i> of less than 0.03% (standard deviation/dynamic range). Moreover, after a two point Non-Uniformity Correction (NUC) has been applied at 95K, the RNU remains below ~0.1% at all focal plane temperatures down to 85K and up to 100K without the need to apply any further correction. This is a major improvement in both the temperature of operation and the temperature stability compared with implanted diodes made from bulk material. We also demonstrate rapid progress in the development of low current epitaxial InAlSb photodiodes with high uniformity and low dark current that offer a range of cut-off wavelengths shorter than in InSb. Preliminary results are presented on FPAs with a cut-off wavelength in the range λ<sub>C</sub>~5μ.
We propose that the antimonide family of semiconductors should be considered in some cases as a serious alternative to Mercury Cadmium Telluride (MCT) for the active region of next generation IR detectors, based on epitaxial materials. Among the alloys, epitaxial InAs<sub>1-y</sub>Sb<sub>y</sub> on GaSb with 0.07 < y < 0.11 and In<sub>1-z</sub>Al<sub>z</sub>Sb on InSb with 0 < z < 0.03 together span important regions of the MWIR atmospheric window, yet exhibit strains of less than 0.15%. Both InSb and GaSb are binary substrates available in high quality. The sensitivity of bandgap to composition in In<sub>1-z</sub>Al<sub>z</sub>Sb is similar to that in MCT. However, in InAs<sub>1-y</sub>Sb<sub>y</sub> this sensitivity is more than halved. In growth from the gas phase, the constraints on temperature stability are about 3 - 5 times lower than in MCT. Together, these characteristics make it easier to achieve high uniformity, particularly in InAs<sub>1-y</sub>Sb<sub>y</sub>. Finally, high quality superlattices based on InAs/Ga<sub>1-x</sub>In<sub>x</sub>Sb can be grown by lattice matching to GaSb. This epitaxial material is emerging as an attractive alternative to MCT with a high degree of spatial uniformity and with an ability to span cut-off wavelengths from 3-20m in a single material system.