Scaling brilliance of high power laser diodes

Abstract

New direct diode laser systems and fiber lasers require brilliant fiber coupled laser diodes for efficient operation. In the German funded project HEMILAS different laser bar designs are investigated with tailored beam parameter products adapted for efficient fiber coupling. In this paper we demonstrate results on 9xx and 1020nm bars suitable for coupling into 200μm fibers. With special facet technology and optimised epitaxial structure COD-free laser bars were fabricated with maximum efficiency above 66%. For short bars consisting of five 100μm wide emitters 75W CW maximum output power was reached. In QCW-mode up to 140W are demonstrated. The 10% fill factor bars with 4mm cavity are mounted with hard solder. Lifetime tests in long pulse mode with 35W output power exceed 5000 hours of testing without degradation or spontaneous failures. Slow axis divergence stays below 7° up to power levels of 40W and is suitable for simple fiber coupling into 200μm NA 0.22 fibers with SAC and FAC lenses. For fiber coupling based on beam rearrangement with step mirrors, bars with higher fill factor of 50% were fabricated and tested. The 4mm cavity short bars reach efficiencies above 60%. Lifetime tests at accelerated powers were performed. Finally fiber coupling results with output powers of up to 2.4 kW and beam quality of 30 mm mrad are demonstrated.