Next Generation Heat Sinks for High-power Diode Laser Bars

As for other electronic or optoelectronic devices, the package of a high-power laser bar has to provide the following basic features: 1) Mechanical stability for mounting and handling; 2) Electrical contacting of the n-side and the p-side of the device; and 3) Cooling to remove the waste heat generated by the diode laser. Given the large power turnover and comparably small size of a diode laser bar, the cooling capabilities of the package are of great importance. With a typical efficiency of 60 percent, a laser bar producing an optical output power of up to 100 W simultaneously generates 80 W of waste heat. Taking into consideration a typical size of 10 times 1.2 mm² to 10 times 1.5 mm², this results in an extremely high heat-flux density of about 500-600 W/cm . To maintain its high efficiency and a long lifetime, the operating temperature of the laser bar should be kept as low as possible, typically below 60degC/140degF. This requires very effective cooling, resulting in the fact that the cooling aspect dominates the package design and material choice. Therefore, the standard heat sink material in nearly all commercially available diode laser packages is copper owing to its excellent thermal conductivity (390-400 W/mK), its good mechanical machining properties, and its comparably low price. As a consequence of the increasing output power, heat sinks with a higher thermal conductivity are desired as well. Therefore in parallel new high performance materials like diamond composites will be integrated in the heat sinks. Next generation heat sinks made out of diamond composite materials combined with copper in a sandwich structure have been designed at Fraunhofer ILT. As a first step heat sinks without water-cooling, so-called conductive cooled heat sinks, are fabricated and compared with available standard copper heat sinks. The second step is that composite materials will also be used for water-cooled heat sinks to realize higher output power of the diode laser bar. In parallel erosion and corrosion effects will be minimized. Particle image velocimetry and CFD simulation are helpful tools to locate critical areas in the cooling structure like turbulences or dead water areas. Together with expansion matching these steps will help to increase the lifetime of high power diode laser bars.