Page 169 - High Power Laser Handbook

138 Diode Lasers High-Power Diode Laser Arrays 139

Min. Max.
Resistivity Resistivity pH Level Expected Life
Vertical 200 kΩ-cm 500 kΩ-cm 6–7 > 10,000 hr
stacks (pitch < 2 mm)
Horizontal 50 kΩ-cm (pitch 150 kΩ-cm 6–7 > 20,000 hr
or vertical > 5 mm)
stacks 20 kΩ-cm (pitch
> 10 mm)

Table 6.1 Water Specifications for Actively Cooled Heat Sinks

pitch or the actual separation l between the heat sinks through the
s
water. The distance is described as the length of the water along a
dielectric passage (not including conducting spacers or manifolds).
In a standard vertical stack (with a pitch of 1.8 mm), the distance
l equals 0.7 mm. The maximum recommended water resistivity is
s
500 kΩ-cm (see Table 6.1). The desired pH level of greater than 6 can
be reached with a mixed-bed deionization cartridge. In a vertical
stack with spacers (plastic inserts in the water passage), the distance
l is increased to the distance l , which includes the spacer thickness.
s
sp
Therefore, the water resistivity can be reduced by the ratio of l /l sp
s
until reaching a value of 100 kΩ-cm. No deionization cartridge is
needed in this case. The same holds for all horizontal stacks with a
typical distance l of more than 10 mm. The increased distance for
s
horizontal stacks enables reduced water specifications and increased
reliability in terms of the heat sink’s expected lifetime. Horizontal
stacks can be arranged with integrated optics to achieve vertically
stacked beams with even higher brightness due to the increased fill
factor and greater reliability than vertical stacks (see Chap. 7).

6.3.2 Expansion-Matched Microchannel Heat Sinks
The diode laser bar can be mounted directly with indium to the copper
heat sink, or a CuW submount can be used to enable a hard solder
(AuSn). Both solutions will have an electrical potential in the water and
will need to follow the water specifications of Table 6.1. A new expan-
sion-matched mini-channel heat sink avoids the electrical potential in
the water and can therefore use any kind of coolant. The heat sink con-
sists of a copper-AlN sandwich. The top and bottom copper layers are
connected with an electrical feedthrough that is isolated from the cen-
ter cooling structure made of copper. The center part is isolated by two
AlN layers from the top and bottom layers. By adjusting the copper
thickness on top to about 80 mm, the expansion is matched to the coef-
ficient of GaAs (6.5 10-6/°C), as the coefficient of AlN is about 4.5
10-6/°C and copper is about 16 10-6/°C. The top copper layer is
designed to accommodate the anode as well as the cathode for the laser