Page 207 - High Power Laser Handbook

176 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 177

Figure 7.7 Thin-
disk cooling and Flat
extraction wavefront
geometry.

Thin disk

identical optical path length. With such a geometry, the extracting
beam’s wavefront is, to the first order, unaffected by the magnitude of
the thermal gradient; therefore, the architecture can be scaled to high
power (c.f., Chaps. 8 and 9). The same principle underlies the scal-
ability of the thin-disk architecture (Fig. 7.7; also see Chap. 10).
In practice, for geometries such as those shown in Figs. 7.6 and
7.7, the quality of the extracted wavefront is driven by edge effects,
mounting stresses, and uncontrolled nonuniformities in the thermal
gradients. Just as it is critical to minimize nonuniformities in heat
deposition during pumping, it is also important to ensure uniform
heat removal through a spatially uniform, low-thermal impedance
path from the cooled surface to the heat sink. This is relatively
straightforward when the surface is cooled with direct liquid or gas
flow. However, when the surface is conduction cooled, a host of engi-
neering issues must be solved, including avoiding mechanical mount-
ing stresses, CTE matching of the gain material to the substrate,
uniform wetting of solder or other thermal interface materials, and
preventing the extracting laser beam from coupling to the cooling
substrate. Solutions to some of these issues are discussed in the con-
text of specific architectures in Chaps. 8–10.
Finally, one noteworthy exception to these heat-removal consid-
erations are heat capacity lasers, discussed in Chap. 11. These devices
are uncooled and store heat during an operation time that is limited
by the gain material’s heat capacity. In the absence of surface cooling,
the gain material is free from thermal gradients and expands uni-
formly without wavefront distortion. Hence, wavefront aberrations
are driven primarily by nonuniformities in heat generation from
pumping and laser extraction.

7.4 Laser Beam Formation

A low-aberration, high-power laser gain module incorporating favor-
able pumping, cooling, and extraction geometries forms the building
block for any high-brightness SSL system. To generate a high-power
output beam, the gain module(s) must be configured as part of either
a resonant oscillator cavity (stable or unstable) or an amplifier. The
optimum configuration choice is one that efficiently extracts the stored
energy while minimizing losses and accumulated OPD, so as to gener-
ate the highest brightness output beam. This section discusses the