Moshe Levy

Effect of compressive and tensile strain on the performance of 808 nm QW high power laser diodes

The effect of compressive and tensile strain of Quantum Wells (QWs) on the gain and transparency current density of high power laser diodes was studied. Material composition of InGaAlAs/AlGaAs and InGaAsP/InGaP was utilized for the study of compressive and tensile strain QWs, respectively. Variation in the strain degree was achieved by changing the In and P mole fraction accordingly. We found that the transparency current densities of compressively strained QWs decrease from 117 to 100 A/cm2 as a function of strain. The transparency current in tensile strained QWs decrease from 140 to 130 A/cm2 as the strain is increased. The material gain of compressively strained QWs is almost insensitive to the variation of strain degree (~1000 cm-1), while for tensile strained QWs the material gain increases from 1000 cm-1 to 1250 cm-1 when the tensile strain is increased. In spite of the higher transparency densities the gain achieved at maximum strain is larger for tensile strained QW laser. This result is explained by the strain influence on the electron-hole recombination strengths. Consequently the effect of strain on the performance of High Power QCW and CW laser bars was also investigated. The threshold current of bars with compressively strained QWs is decreased to 8.5 A and the external differential efficiency is increased to 1.0 W/A as a function of strain. On the other hand, as the tensile strain in the QW is increased the threshold current reduces to 10 A and the slope efficiency increases to 1.2 W/A. As a result, tensile strain QWs bars are more efficient at high power operation.

Development of asymmetric epitaxial structures for 65% efficiency laser diodes in the 9xx-nm range

High-power single emitters have recently become a viable alternative to laser diode bars for fiber pumping applications. Single emitters offer a tenfold increase in brightness over bars, and can be optically combined to scale the power towards 100 W with high brightness. Wall-plug efficiencies >60% are needed to warrant the use of fiber-coupled single emitters in fiber laser systems, which requires careful minimization of the optical loss, electrical resistance and operating voltage of the emitters. Epitaxial wafer design necessarily involves multiple trade-offs, since doping concentrations have opposing effects on the electrical resistance and optical losses. In this paper, we report asymmetric epitaxial waveguide designs for high-efficiency laser operation at 9xx nm. We present a simulation study of the influence of design parameters such as the number of quantum wells, doping profiles, and overlap integral of each epilayer. We also show that by introducing an auxiliary waveguide into the lower cladding, we can control the overlap of the optical mode with the doping profiles - as well as the vertical far-field - without compromising the electrical resistance. The optimized structures were grown and devices fabricated, with optical losses reduced to 0.5 cm-1, and resistivity to 6.5 Ohm×sq.cm. An optical power of 10 W with >60% efficiency was achieved from 100 μm stripe emitters.

Space-grade reliability of 808nm QCW laser diode arrays (LDAs) delivering over 20 billion shots

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 ESAs 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.

High-power single emitters for fiber laser pumping across 8xx nm-9xx nm wavelength bands

Fiber lasers require diode laser pumps of ever increasing power and brightness delivered via low-NA multimode fiber. The preferred fiber pump construction is based on the combination of the outputs of several diode lasers (single emitters) in single-strand multimode delivery fiber with NA ≤ 0.15. We report on the recent advances in the reliability and output power of single emitters at several wavelengths from 800 nm to 980 nm, with an emitting aperture optimized for coupling into 105 μm core fiber. By the use of long laser cavities and low-loss epitaxal design, we have achieved single emitter powers close to 20 W. Lifetest data from these emitters under high optical load are presented and analyzed using a reliability model.

Low-NA fiber laser pumps powered by high-brightness single emitters

Fiber laser manufacturers demand high-brightness laser diode pumps delivering optical pump energy in both a compact fiber core and narrow angular content. A pump delivery fiber of a 105 μm core and 0.22 numerical aperture (NA) is typically used, where the fiber NA is under-filled to ease the launch of laser diode emission into the fiber and make the fiber tolerant to bending. At SCD, we have developed high-brightness NEON multi-emitter fiber-coupled pump modules that deliver 50 W output from a 105 μm, 0.15 NA fiber enabling low-NA power delivery to a customer’s fiber laser network. Brightness-enhanced single emitters are engineered with ultra-low divergence for compatibility with the low-NA delivery fiber, with the latest emitters delivering 14 W with 95% of the slow-axis energy contained within an NA of 0.09. The reduced slow-axis divergence is achieved with an optimized epitaxial design, where the peak optical intensity is reduced to both lessen filamentation within the laser cavity and reduce the power density on the output facet thus increasing the emitter reliability. The low mode filling of the fiber allows it to be coiled with diameters down to 70 mm at full operating power despite the small NA and further eliminates the need for mode-stripping at fiber combiners and splices downstream from our pump modules. 50W fiber pump products at 915, 950 and 975 nm wavelengths are presented, including a wavelengthstabilized version at 976 nm.

High-brightness 800nm fiber-coupled laser diodes

Fiber-coupled laser diodes have become essential sources for fiber laser pumping and direct energy applications. Single emitters offer reliable multi-watt output power from a 100 m lateral emission aperture. By their combination and fiber coupling, pump powers up to 100 W can be achieved from a low-NA fiber pigtail. Whilst in the 9xx nm spectral range the single emitter technology is very mature with <10W output per chip, at 800nm the reliable output power from a single emitter is limited to 4 W – 5 W. Consequently, commercially available fiber coupled modules only deliver 5W – 15W at around 800nm, almost an order of magnitude down from the 9xx range pumps. To bridge this gap, we report our advancement in the brightness and reliability of 800nm single emitters. By optimizing the wafer structure, laser cavity and facet passivation process we have demonstrated QCW device operation up to 19W limited by catastrophic optical damage to the 100 μm aperture. In CW operation, the devices reach 14 W output followed by a reversible thermal rollover and a complete device shutdown at high currents, with the performance fully rebounded after cooling. We also report the beam properties of our 800nm single emitters and provide a comparative analysis with the 9xx nm single emitter family. Pump modules integrating several of these emitters with a 105 μm / 0.15 NA delivery fiber reach 35W in CW at 808 nm. We discuss the key opto-mechanical parameters that will enable further brightness scaling of multi-emitter pump modules.

Multi-spectral investigation of bulk and facet failures in high-power single emitters at 980 nm

Reliable single emitters delivering >10W in the 9xx nm spectral range, are common building blocks for fiber laser pumps. As facet passivation techniques can suppress or delay catastrophic optical mirror damage (COMD) extending emitter reliability into hundreds of thousands of hours, other, less dominant, failure modes such as intra-chip catastrophic optical bulk damage (COBD) become apparent. Based on our failure statistics in high current operation, only ~52% of all failures can be attributed to COMD. Imaging through a window opened in the metallization on the substrate (n) side of a p-side down mounted emitter provides valuable insight into both COMD and COBD failure mechanisms. We developed a laser ablation process to define a window on the n-side of an InGaAs/AlGaAs 980nm single emitter that is overlaid on the pumped 90μm stripe on the p-side. The ablation process is compatible with the chip wire-bonding, enabling the device to be operated at high currents with high injection uniformity. We analyzed both COMD and COBD failed emitters in the electroluminescence and mid-IR domains supported by FIB/SEM observation. The ablated devices revealed branching dark line patterns, with a line origin either at the facet center (COMD case) or near the stripe edge away from the facet (COBD case). In both cases, the branching direction is always toward the rear facet (against the photon density gradient), with SEM images revealing a disordered active layer structure. Absorption levels between 0.22eV – 0.55eV were observed in disordered regions by FT-IR spectroscopy. Temperature mapping of a single emitter in the MWIR domain was performed using an InSb detector. We also report an electroluminescence study of a single emitter just before and after failure.

High-brightness diode pump sources for solid-state and fiber laser pumping across 8xx-9xx nm range

Advanced solid state laser architectures place increasingly demanding requirements on high-brightness, low-cost QCW laser diode pump sources, with custom apertures both for side and end rod pumping configurations. To meet this need, a new series of scalable QCW pump sources at 808nm and 940nm was developed. The stacks, available in multiple output formats, allow for custom aperture filling by varying both the length and quantity of stacked laser bars. For these products, we developed next-generation laser bars based on improved epitaxial wafer designs delivering power densities of 20W/mm of emission aperture. With >200W of peak QCW power available from a full-length 1cm bar, we have demonstrated power scaling to over 2kW in 10-bar stacks with 55% wall plug efficiency. We also present the design and performance of several stack configurations using full-length and reduced-length (mini) bars that demonstrate the versatility of both the bar and packaging designs. We illustrate how the ROBUST HEAD packaging technology developed at SCD is capable of accommodating variable bar length, pitch and quantity for custom rod pumping geometries. The excellent all-around performance of the stacks is supported by reliability data in line with the previously reported 20 Gshot space-grade qualification of SCDs stacks.

Wavelength locking of single emitters and multi-emitter modules: simulation and experiments

Wavelength-stabilized high-brightness single emitters are commonly used in fiber-coupled laser diode modules for pumping Yb-doped lasers at 976 nm, and Nd-doped ones at 808 nm. We investigate the spectral behavior of single emitters under wavelength-selective feedback from a volume Bragg (or hologram) grating (VBG) in a multi-emitter module. By integrating a full VBG model as a multi-layer thin film structure with commercial raytracing software, we simulated wavelength locking conditions as a function of beam divergence and angular alignment tolerances. Good correlation between the simulated VBG feedback strength and experimentally measured locking ranges, in both VBG misalignment angle and laser temperature, is demonstrated. The challenges of assembling multi-emitter modules based on beam-stacked optical architectures are specifically addressed, where the wavelength locking conditions must be achieved simultaneously with high fiber coupling efficiency for each emitter in the module. It is shown that angular misorientation between fast and slow-axis collimating optics can have a dramatic effect on the spectral and power performance of the module. We report the development of our NEON-S wavelength-stabilized fiber laser pump module, which uses a VBG to provide wavelength-selective optical feedback in the collimated portion of the beam. Powered by our purpose-developed high-brightness single emitters, the module delivers 47 W output at 11 A from an 0.15 NA fiber and a 0.3 nm linewidth at 976 nm. Preliminary wavelength-locking results at 808 nm are also presented.

High-brightness power delivery for fiber laser pumping: simulation and measurement of low-NA fiber guiding

Fiber laser manufacturers demand high-brightness laser diode pumps delivering optical pump energy in both a compact fiber core and narrow angular content. A pump delivery fiber of a 105 μm core and 0.22 numerical aperture (NA) is typically used, where the fiber NA is under-filled to ease the launch of laser diode emission into the fiber and make the fiber tolerant to bending. At SCD, we have developed multi-emitter fiber-coupled pump modules that deliver 50 W output from a 105 μm, 0.15 NA fiber at 915, 950 and 976 nm wavelengths enabling low-NA power delivery to a customer’s fiber laser network. In this work, we address the challenges of coupling and propagating high optical powers from laser diode sources in weakly guiding step-index multimode fibers. We present simulations of light propagation inside the low-NA multimode fiber for different launch conditions and fiber bend diameters using a ray-racing tool and demonstrate how these affect the injection of light into cladding-bounded modes. The mode filling at launch and source NA directly limit the bend radius at which the fiber can be coiled. Experimentally, we measure the fiber bend loss using our 50 W fiber-coupled module and establish a critical bend diameter in agreement with our simulation results. We also employ thermal imaging to investigate fiber heating caused by macro-bends and angled cleaving. The low mode filling of the 0.15 NA fiber by our brightness-enhanced laser diodes allows it to be coiled with diameters down to 70 mm at full operating power despite the low NA and further eliminates the need for mode-stripping at fiber combiners and splices downstream from our pump modules.