Page 303 - High Power Laser Handbook

272 So l i d - S t at e La s e r s Heat-Capacity Lasers 273

Figure 11.6 Energized diode arrays “pumping” a transparent ceramic laser
gain medium.

In a typical HCL operation with the architecture as described ear-
lier, at 25-kW average laser output power, 200-Hz pulse repetition
rate, and 10 percent duty cycle for the high-powered diode arrays,
the transparent ceramic laser gain media’s bulk temperature will
increase from room temperature to approximately 130°C in about
10 seconds of continuous lasing operation when working in its heat-
capacity mode (i.e., no active cooling of the laser gain media during
lasing). Although transparent ceramics have demonstrated no crack-
ing or physical damage at temperatures of 200°C, standard engineering
practice dictates a slab change-out after a 10-s run. This change-out
allows material stresses developed at this elevated slab temperature
to remain well within reasonable values for reliable operation.
After lasing for 10 s, the transparent ceramic can either be cooled
in situ or moved off-line to be aggressively cooled. Several methods
for slab cooling have been demonstrated with good success. For
example, cold plates are moved to within a few thousandths of an
inch of the slab faces to provide a conduction path for the heat out of
the slab; cold gas is flown across each slab face; or a fine mist that
absorbs the heat is applied to the slab face surface. The requisite