Dan Yanson

Terahertz repetition frequencies from harmonic mode-locked monolithic compound-cavity laser diodes

Compound-cavity laser diodes are mode locked at a harmonic of the fundamental round-trip frequency to achieve repetition rates of up to 2.1 THz. The devices are fabricated from GaAs/AlGaAs material at a wavelength of 860 nm and incorporate two gain sections with an etched slot reflector between them, and a saturable absorber section. Autocorrelation studies are used to investigate device behavior for different reflector types and reflectivity. These lasers may find applications in terahertz imaging, medicine, ultrafast optical links, and atmospheric sensing.

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.

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.

Selective quantum dot intermixing for photonic devices

Dielectric-cap-based techniques, originally developed for quantum well intermixing, have been applied to the intermixing of InGaAs/GaAs/AlGaAs QD material with an emission wavelength of 1280 nm. Intermixing was achieved by sputtering a QDI-enhancing cap in some areas, and plasma deposition of silica QDI-suppressing cap in other areas, followed by a high-temperature anneal cycle. Extremely large bandgap blue-shifts of up to 280 nm have been obtained with an anneal temperature of 800 C. Selected intermixing was obtained by varying the coverage density of QDI-enhancing features over that of QDI-suppressing ones.

Self-focused distributed Bragg reflector laser diodes

Summary from only given. We have reported the successful realisation of the type of a curved-grating DBR semiconductor laser that was previously proposed theoretically. Single-frequency generation was observed and far-field measurements showed good agreement with our prediction for lateral beam behaviour. Future studies will concentrate on near-field investigations and improved devices with non-absorbing c-DBR mirrors, which will be achieved using the quantum well intermixing technique. High power operation in CW and Q-switched regimes will then be investigated.

Monolithic integration of InGaAs/InAlGaAs-based semiconductor optical amplifiers and 10 Gb/s broadband electro-absorption modulators using quantum well intermixing technology

Mach-Zehnder phase modulators fabricated in LiNbO/sub 3/ (LN) dominate the market for data modulators at 10 Gb/s. Although LN modulators display a high-power handling capability with controlled chirp over a large optical bandwidth (i.e., full C-band), they are relatively large and expensive to manufacture when compared to semiconductor devices. By contrast, InP-based electro-absorption modulators (EAMs) are much more compact; however they have a limited bandwidth (5-10 nm) over which chirp is in the correct range to allow > 80 km reach. This paper presents the broadband electro-absorption modulator (BEAM) chip concept, which consists of monolithically integrating a series of EAMs using quantum well intermixing with each one tuned to give the correct chirp over a certain wavelength range. The total bandwidth of the device is thus extended to cover the whole C-band. In addition, a semiconductor optical amplifier (SOA) is serially integrated with the EAMs to recover the insertion loss due to fiber coupling and the series of EAMs. A unique quantum well intermixing (QWI) process is the method employed to achieve the monolithic integration of the multiple sections required for the BEAM chip. Key results reported here are open 10 Gb/s eye diagrams of the integrated EAMs along with data from the demo BEAM chips showing the achievement of >10 dB extinction ratio across the C-band, with >35 km reach.

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.