Diamond Cools Diode Lasers

Jörg Schwartz

As the saying goes, diamonds are a laser's best friend -- or something like that. Diamond is a material known to spread heat effectively, with low electrical conductivity. The importance of this precious material is growing further because polycrystalline diamond films can be produced relatively cheaply by chemical vapor deposition.

Researchers have developed a device that uses a diamond film to spread heat from a high-power diode laser more efficiently than traditional methods.

Thermal management is an important factor for high-power diode lasers because the expected lifetime and output wavelength are closely related to temperature. Peltier or copper coolers are typically used to draw off heat, but tend to be inhomogeneous and ineffective. Jörg Bonhaus and Wolfgang R. Fahrner, in Hagen University's electrical engineering department, have been investigating the use of diamond as a better solution for heat transfer in high-power diode lasers.

The objective was to distribute the heat from a relatively small source (the active zones of a laser bar) to a large sink (a copper block cooled by flowing water). Optimization of the laser/cooler geometry was done using a finite element software, showing that increasing the diamond film length and thickness results in a saturation of the maximum temperature and the thermal resistance.

Choosing a high-quality diamond material -- with a measured thermal conductivity of 1800 W/mK at 36 °C -- and optimized dimensions of cooler and spreader in the saturation domain, the overall thermal resistance could be reduced from 0.52 to 0.34 K/W for a typical geometry. Fixing the laser bar onto the cooler without additional solder could lead to further heat reduction.

As a second step, Bonhaus and Fahrner worked toward a homogeneous temperature distribution. In order to be stacked in production, the cooler ideally would have the same width as the laser bar, but a small border remained necessary because of homogeneity constraints.

The researchers tested practical performance using 26 temperature sensors specially placed throughout the emitter of the laser bar, as close as 50 mm to the heat sources. The design is now under test at Jenoptik Laserdiode GmbH, an industrial diode laser manufacturer in Jena, Germany.

Funded by the German government, the research project also was driven and supported by Jenoptik, which is looking for ways to achieve better solid-state laser pumping, since efficiency relies on achieving a precise absorption wavelength such as 808 nm for Nd:YAG crystals.