Page 301 - High Power Laser Handbook

268 So l i d - S t at e La s e r s Heat-Capacity Lasers 269

High-power Deformable
diode arrays (20) mirror
Polarization
filter

3+
Ceramic YAG:Nd Tip/tilt
laser gain media (5)
mirror
“slab”
Adaptive optics
system
Figure 11.1 Heat-capacity laser architecture in use at Lawrence Livermore
National Laboratory.

The HCL’s basic building block is the laser gain module, which
consists of a single slab pumped by four high-powered diode arrays,
two on either side of the slab. Each diode array pumps the slab’s adja-
cent face at a defined angle, providing uniform pump light intensity
across the entire slab face. In this particular example, the laser gain
media is a transparent ceramic Nd:YAG slab that is edge cladded
with cobalt-doped gadolinium gallium garnet (GGG) to suppress
amplified spontaneous emission. Figure 11.1 shows five gain mod-
ules, interlocked to form a compact cavity from which energy is
extracted as a free-running resonator. An intracavity adaptive optic
(AO) system, consisting of a wavefront sensor, a deformable mirror, a
tip-tilt mirror, and a controller, maintains wavefront phase unifor-
mity. The output laser beam wavelength for this HCL configuration is
1064 nanometers, and the diode light pump wavelength is 808 nm.
Two critical hardware components make up the HCL’s gain
module. The first is the high-powered diode arrays, which are used
to pump the laser gain media. Each diode array comprises hundreds
of relatively small, but very high-power, diode bars, which are carefully
aligned and precision assembled to form a homogeneous diode array.
Figure 11.2 shows a state-of-the-art high-powered diode array manu-
5
factured by Simmtec, Inc. This diode array comprises 560 individual
diode bars (seven rows of eight 10-bar tiles per row) and is capable of
producing conservatively 84 kW of peak power. Electrical-to-optical
conversion efficiency for a diode array is approximately 40 to 50 percent,
giving rise to a significant cooling requirement to dissipate the waste
heat generated (a cooling water flow rate of approximately 10 gallons
per minute per diode array). In addition, the temperature of the cool-
ing water must be maintained to within a few degrees to ensure that
the wavelength of light being emitted by the diode bars is centered on
the optimum absorption wavelength of the laser gain media. As a
frame of reference, each high-powered diode array is about the size of
a small loaf of bread, yet weighs about twice that of a standard
bowling ball.