Page 168 - High Power Laser Handbook

136 Diode Lasers High-Power Diode Laser Arrays 137

Conduction sheet

Isolation foil
N-soldering contact

N-isolation foil
DL-chip
FAC-lens
Microchannel heat sink
FAC-carrier

Figure 6.4 Components of a mini-/micro-channel heat sink, including a fast-
axis collimation lens.

has been developed to the point that the thermal impedance is signifi-
cantly lower than that of conventional single-bar platforms (typical
values are 0.25 to 0.35°C/W, depending on the flow rate), which
enables either increased power per diode laser bar or a longer lifetime
of a diode laser at the same power level. To take full advantage of the
improved cooling, the expected usage time of all water-cooled heat
sinks must exceed the lifetime of the semiconductor material. The
minichannel heat sinks most commonly employed by high-power laser
diode suppliers are made out of copper because of its high thermal
conductivity. The heat sink typically serves as the anode of the diode
(without the thick Au plating required with alternative heat sink mate-
rials, such as Si, which are nonconductive). The dimension of the chan-
nel structure in the copper heat sink is typically in about 300 mm (which
is 10 times or more than is employed in Si microchannel designs). As a
result, the active copper heat sink can be operated at a pressure drop of
approximately 15 psi with a 30-mm particle filter (compared with a
greater than 45 psi with a 5-mm filter for Si microchannel heat sinks).
Although the copper minichannel technology offers clear benefits
to users, early adopters have suffered from uneven reliability due to
long-term corrosion effects. Detailed design optimization of the inter-
nal structures and advances in heat-sink fabrication and die-assembly
processes have made today’s devices capable of continuous opera-
tion well in excess of 10,000 hours, thus meeting the reliability require-
ments of most industrial applications.
6.3.1 Water Guidelines for Minichannel Heat Sinks 2
As mentioned earlier, the water specification for vertical stacks, hori-
zontal stacks, or any other assembly in a pump cavity depends on the