A brief history of kilowatt-class diode-laser bars

An overview is presented on the developments in semiconductor design and technology needed to reliably and reproducibly realize 1-cm GaAs-based diode laser bars with optical output in the kilowatt class, summarizing studies performed at the Ferdinand-Braun-Institut and elsewhere. In the more than ten years since the first demonstration in 2007, the operating conversion efficiency of kilowatt-class 1-cm laser bars has improved from 35% to over 65% and lateral far field has reduced from < 20° to below 9° (at 95% power) in quasi-continuous-wave testing. Greater than a kilowatt output is also demonstrated in continuous wave testing with ever-increasing efficiencies and brightness. In addition, when tailored for cryogenic operation, peak power increases to at least 2000 W and operating efficiency to above 70%. The highest performance is seen in the 940 nm to 980 nm wavelength range, with kilowatt operation also confirmed in the 780 nm to 900 nm range. We review the diode laser design and technological steps that were needed to enable steady performance progress, including both epitaxial design development (for low optical losses and reduced power saturation) and advances in 1-cm bar and packaging technology (for low stress, low electrical and thermal resistance). We also review status on efforts to increase power in other wavelength ranges, and discuss the path to further performance improvements.