Micro-structured lateral thermal lens for increased output power and narrowed beam divergence in kilowatt-class 9xx nm diode laser bars

Abstract

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.