Cooling approaches for high power diode laser bars

The field of applications for diode laser bars is growing continuously. The reasons for this are the increasing width of available wavelengths and optical output power. In parallel to this the packaging requirements for the high power diode laser bars are enlarging and becoming more manifold. Expansion matched, non corrosive, non erosive, low thermal resistance and high thermal conductivity are some of the keywords for the packaging in the near future. Depending on the thermal power density, two different types of heat sinks are used: active water cooled and conductively cooled heat sinks. For applications with less than 60 W thermally dissipated power per laser bar conductively cooled heat sinks are preferred. Pure copper heat sinks with their high thermal conductivity achieve a good thermal performance, but the CTE mismatch between the heat sink and the laser bar and the hence required soft solder technology lead to lifetime limitations. A higher optical output power and a longer lifetime can be realized with AuSn hard solder mounted laser bars on expansion matched heat sinks. The conventional expansion-matched solution uses WCu or A1N submounts requiring an additional mounting step and degrading the thermal performance. In contrast, Fraunhofer ILT has developed conductively cooled heat sinks with integrated expansion matching. In this paper the standard packaging will be compared with the new development at Fraunhofer ILT. The main focus will be on the thermal resistance and corrosive behavior.