for Custom appl icat ions Special connectors
curamik’s ilasco heatsink uses an Aln layer to isolate the copper microchannel structure from the electrical circuit. the laser diode bar is shown in red Image courtesy of Curamik
alloys or diamond, and new designs such as double-sided cooling and impingement coolers.
For example, Jenoptik Laser in Germany
has succeeded in doing something that looks like a really simple idea, but in practice is extraordinarily difficult to implement – cooling both sides of the laser diode bar. Most laser diode cooling technologies cool the laser chip only from one side – the p-side – which is located directly above the microchannels. The n-side is usually left uncooled, with wire bonds or thin copper sheets used as n-contacts. ‘Water cooling both sides of the laser is usually only done for record attempts to determine the maximum optical output power of diode laser bars,’ says Leers. ‘These packages often only have a lifetime of a few hours, because the laser cannot endure the resultant mechanical and thermal stresses. Conductive cooling both sides of a laser diode bar can significantly improve the cooling performance but there are considerable challenges to overcome.’ Jenoptik’s Ekkehard Werner and Petra
Hennig believe they have found the solution. ‘Our CN package cools the laser from both sides, passively, without the need for microchannels and their associated problems,’ says Hennig. ‘This may seem like a simple idea, but the mechanical stresses experienced by the laser on each side are not equal. We had to find the right solder, the right pressure, adjust the geometry of the structure and change the surface quality of the heatsink. It has taken several years of tweaking, but we are now ready to go to market.’ Jenoptik claims that the performance of its
two-sided passive technology is comparable with actively-cooled microchannel technology. Its thermal resistance has been reduced from 0.6K/W to 0.3K/W and this has enabled a laser optical efficiency increase of more than five per
www.electrooptics.com
cent when compared with standard passively cooled heat sinks. Jenoptik has patented this approach, so
other companies have had to come up with other solutions to the problem. This can lead to some innovative thinking.
For example, UK company Versarien is commercialising a technology developed at the University of Liverpool – copper foam. Copper foam can be manufactured using the lost carbonate sintering (LCS) technology developed by Professor Yuyuan Zhao at the University of Liverpool. In this process, copper powder and carbonate powder are mixed together, compacted and sintered. As the carbonate powder starts to decompose is it removed and leaves behind an open cell network of holes. The size of the holes and the
conductive cooling both sides
of a laser diode bar can significantly improve the cooling performance
porosity of the foam can be controlled by using the right particle size of carbonate powder. ‘Forcing a liquid coolant through the porous copper can take away a large amount of heat,’ says Zhao. ‘We have shown that LCS copper has a superior performance in this respect to many current products.’ A foam structure will inherently have
a greater surface area than a conventional microchannel cooler and so has the potential to extract more heat. But turning this idea into a commercial product is, as always, not easy. But Will Battrick, technical director of
Versarien, is optimistic. ‘We hope to have product on the market by the end of the year,’ he says. ‘We are developing copper- tungsten alloy foams with coefficients of thermal expansion (CTE) that are matched to semiconductors, such as GaAs used in laser diodes. While this alloy does not have the
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