Dominik Martin

We present here an approach to flattening the lateral thermal lens that develops in kW-class semiconductor laser bars consisting of multiple broad-area emitters and is responsible for limiting the power, conversion efficiency and beam quality of the bars. Simulation results show that by increasing heat at the edge-emitters of the bars, thermal lens can be diminished. With an experiment on single-emitters, we demonstrated that the required heat in the edge-emitters can be achieved by shortening their pump length. Following these studies, we fabricated 1-cm wide bars containing 37 emitters, each with a 186- $\mu $ m-wide stripe and a 4-mm-long resonator. In contrast to the baseline bars with equally and fully pumped emitters, the best-designed bars with an optimum (shorter) pump length only in the edge-emitters yield a 5%-points higher conversion efficiency, and an improved beam quality, narrowing the lateral beam divergence at 95% power content by over 10% at 0.8 kW power.

Current progress in the scaling of continuous wave optical output power and conversion efficiency of broad-area GaAs-based edge emitters, broad-area lasers (BALs), operating in the 900…1000 nm wavelength range is presented. Device research and engineering efforts have ensured that BALs remain the most efficient of all light sources, so that in the past 10 years, power conversion efficiency at 20 W continuous wave (CW) output power from BA lasers with a 90…100 μm wide stripe has increased 1.5-fold to 57% (via epitaxial layer design developments), whilst peak CW power per single emitter has increased around 3-fold to 70 W (via scaling of device size), with further scaling underway, for example via use of multi-junction designs. However, the peak achievable CW power conversion efficiency and CW specific output power (defined here as peak output power from a 100 μm stripe diode lasers with a single p-n junction) has changed remarkably little, remaining around 70% and 25 W, respectively, for the past decade. Fortunately, research to understand the limits to peak efficiency and specific output power has also shown progress. Specifically, recent studies indicate that spatial non-uniformity in optical field and temperature play a major role in limiting both power and conversion efficiency. Technological efforts motivated by these discoveries to flatten lateral and longitudinal temperature profiles have successfully increased both power and efficiency. In addition, epitaxial layer designs with very high modal gain successfully reduce threshold current and increase slope at 25°C to values comparable to those observed at 200 K, offering a path toward the 80% conversion efficiency range currently seen only at these cryogenic temperatures. Overall, whilst operating efficiency and power continue to scale rapidly, a technological path for increased specific power and peak efficiency is also emerging.

We present single emitter laser diodes with high optical output power ( P _out ), conversion efficiency (η _E ), and lateral beam quality in quasi-continuous-wave (QCW) and continuous-wave (CW) operations enabled by using very wide stripe width (ranging from 400 to 1500 µm) and laterally structured p-side contact to prevent higher order and unwanted ring modes. We show that the maximum QCW P _out increases for wider stripe and P _out of ~ 290 W (limited by facet failure) is obtained at η _E = 60% for 1500 µm stripes using 500 µs pulse width and 10 Hz repetition-rate at a heatsink temperature of T _HS = 25 °C. In contrast, the maximum CW P _out of 71 W (limited by the available cooling of the test set-up) at η _E = 59% is obtained for 1000 µm stripes, with lateral-beam-parameter-product ( BPP _Lat. ) < 75 mm∙mrad, which is suitable for coupling into 1 mm core 0.15 NA fiber.

Local temperature non-uniformity is a critical limit to power in large-area semiconductor lasers, playing a larger role than the conversion efficiency and temperature sensitivity in the most efficient modern devices. For the specific case of kilowatt-level edge-emitting diode laser bars, we demonstrate that laterally re-distributing current locally within each emitter using a customized micro-structuring of the electrical contact can flatten the thermal profile, based on the thermal design using COMSOL. The concept is demonstrated experimentally in adapted bars that contain eight broad-area emitters, each having wide (∼1100 μm) stripes and a 4 mm long resonator. Each emitter in the laser bar has an electrical contact layer that is electrically structured into parallel sub-contacts using implantation, essential to prevent lateral lasing, implemented here with a period of 29 μm. The width of the individual sub-contacts is narrowed in the device center and then monotonically increased toward the emitter edges to collect a higher proportion of heat at the edges, for a fivefold reduction in the thermal lens, despite a significant (20%) overall increase in electrical and thermal resistance. Two performance benefits are observed. First, the slope varies more slowly with average temperature, recovering (here) around half of the efficiency penalty from in-stripe temperature non-uniformity, and hence increasing the power conversion efficiency at 800 W optical output power by around 5%. Second, the lateral far field (95% power content) is narrowed by around 2° at 800 W optical output power, corresponding to a reduction of around half of the thermal contribution.