High-brightness laser diode arrays operating at a duty cycle of 10% -20% are in ever-increasing demand for the optical pumping of solid state lasers and directed energy applications. Under high duty-cycle operation at 10% -20%, passive (conductive) cooling is of limited use, while micro-coolers using de-ionized cooling water can considerably degrade device reliability. When designing and developing actively-cooled collimated laser diode arrays for high duty cycle operation, three main problems should be carefully addressed: an effective local and total heat removal, a minimization of packaging-induced and operational stresses, and high-precision fast axis collimation. In this paper, we present a novel laser diode array incorporating a built-in tap water cooling system, all-hard-solder bonded assembly, facet-passivated high-power 940 nm laser bars and tight fast axis collimation. By employing an appropriate layout of water cooling channels, careful choice of packaging materials, proper design of critical parts, and active optics alignment, we have demonstrated actively-cooled collimated laser diode arrays with extended lifetime and reliability, without compromising their efficiency, optical power density, brightness or compactness. Among the key performance benchmarks achieved are: 150 W/bar optical peak power at 10% duty cycle, >50% wallplug efficiency and <1˚ collimated fast axis divergence. A lifetime of >0.5 Ghots with <2% degradation has been experimentally proven. The laser diode arrays have also been successfully tested under harsh environmental conditions, including thermal cycling between -20°C and 40°C and mechanical shocks at 500g acceleration. The results of both performance and reliability testing bear out the effectiveness and robustness of the manufacturing technology for high duty-cycle laser arrays.Keywords: laser diode array, high power, QCW, high duty cycle, built-in cooling, tap water, hard soldering, fast axis collimation
INTRODUCTIONHigh-brightness laser diode arrays (LDA) operating at a duty cycle of 10-20% are increasingly being used for the optical pumping of solid state lasers and directed energy applications. Under such high duty cycle operation, passive (conductive) cooling is of limited use, while micro-coolers using de-ionized cooling water can considerably degrade the LDA reliability.Conductive cooling presents a simple, robust and cost effective solution for laser diode arrays, allowing them to meet a variety of power density and brightness requirements. However, conductively cooled devices are limited to duty cycles not exceeding 4%, with a practical limit of 2%, and to pulsewidths not exceeding 300 μs -400 μs.Microchannel cooling provides sufficient heat extraction capacity for high-power LDA to be operated in CW mode and is equally suitable for high duty-cycle operation. While offering excellent thermal performance, this approach suffers from several drawbacks. The use of metallic microchannel coolers causes the electrical path to come in direct contact with the coolant. This req...