ED STEPHENS AND JOHN GOINGS
A microchannel-cooler package under development would eliminate many of the concerns
of older-style microchannel packages and promises a roughly 30% improvement in cooling
ability for laser-diode bars.
Laser manufacturers have recently made significant improvements to the amount of output power that can be produced by high-efficiency laser-diode bars. Optical output powers greater than 120 W continuous wave (CW) from a single diode bar when packaged on a microchannel cooler have been demonstrated. 1 These high powers test the limits of epitaxial structure design, dielectric coating design, and packaging techniques. While epitaxial and dielectric coating designs have made significant advances recently, many suppliers and consumers of high-power bars have not addressed the packaging developments necessary to take advantage of improved bar performance. Fortunately, new diode package designs are being implemented to provide superior technical solutions FIGURE 1. Common types of high-powered diode array packages while maintaining the size and performance characteristics include conductively cooled (upper left), water-cooled (lower left), of traditional diode heat exchangers. and microchannel-cooled (right) heat exchangers.
NORTHROP GRUMMAN
Removing heat capacity. However, the use of MCCs demands a more com-The purpose of packaging a diode bar is to deliver the neces- plex cooling system than traditional air- and water-cooled sary electrical current and remove the waste heat. Depending packages. Part of the additional complexity derives from on the amount of heat to be removed, a variety of packaging the fact that in commercially available MCCs the electrical solutions can be implemented, ranging from very small con- current and the cooling fluid must coexist within the cooler. ductively cooled heat exchangers to water-cooled packages Without proper design of the cooling system, corrosion and and larger microchannel water-cooled arrays (see Fig. 1). erosion will significantly shorten the lifetime of the devices.
Many of these packages have been in use for years and have To minimize this effect, deionized (DI) cooling water must
a long history of proven reliability. However, these pack- be used, coupled with careful selection of the cooling system
ages cannot provide adequate cooling if a device is operat- components. For many industrial users, requiring DI water
ed above its nominal performance limits. As manufacturers has limited the use of MCCs as a practical solution.
develop bars with higher output powers, package improve- By requiring DI water, the water-chiller system not only
ments must keep pace. needs to supply the flow and pressure, but must also actively
The most efficient method of cooling high-power bars has monitor and control the electrical resistivity and pH of the
been to use gold-plated copper heat exchangers with very water. The technical superiority of MCC designs is often com-
small (micro) water channels, called microchannel coolers plicated by the facility and monitoring system requirements
(MCC). This design minimizes the distance between the heat they impose.
source and the coolant, creating maximum cooling efficiency. The reliability of standard microchannel-cooled stacks has also
Commercial MCC designs allow conventional diode bars to been questioned. The MCC packaging scheme can be considered
be driven at higher powers because of this improved cooling less reliable because of the many O-ring seals and weaker elec-
trical contacts involved. Associated packaging failure modes of
ED STEPHENS is director of engineering, RYAN FEELER is diode product standard MCC units include water leaks at the O-rings, corro- specialist, and JOHN GOINGS is North American sales manager for Northrop sion due to water exposure, and erosion due to excessive water Grumman Cutting Edge Optronics, 20 Point West Blvd., St. Charles, MO
63301; e-mail: john. goings@ngc.com. flow, among others. Because of the complexity of microchannel
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