Page 203 - High Power Laser Handbook

172 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 173

stress fracture ultimately limits the power density attainable for any
pumping and cooling geometry. However, often well before powers
reach the level at which thermal fracture is a risk, thermo-optic wave-
front distortions can severely limit the beam quality (BQ) of the
extracting laser beam. This section describes how the need to mini-
mize thermal aberrations drives the selection of geometries for pumping,
cooling, and laser extraction.

7.3.1 Pump Sources
The lasing process unavoidably generates waste heat because of the
energy difference between the pump and emission photons. This heat
is deposited throughout the volume of the lasing material in propor-
tion to the amount of pump light absorbed locally. Any nonunifor-
mity in the profile of absorbed pump light across the laser clear
aperture will translate into nonuniformities in heat deposition and
development of thermal gradients that can aberrate the laser beam.
Hence, a primary design consideration for high-power SSLs is to
ensure that the material volume is pumped as close to uniform as
possible across the extracting beam aperture. A second key consider-
ation is to minimize the heat generation per emitted photon—that is,
to pump the material with a photon as close in wavelength as possi-
ble to the emission wavelength. In this section, we discuss how these
considerations affect selection of an appropriate pump source and
conditioning optics.

Lamp Pumping
In 1960, Ted Maiman at Hughes Research Lab demonstrated the first
laser, using a cheap and simple photographic flash lamp to pump a
19
solid-state ruby crystal. Although ruby was soon supplanted with
more efficient and higher power Nd-doped materials, CW arc lamps
and pulsed flash lamps filled with noble gases remained the predom-
inant pump sources for SSLs until the development of high-power
diodes in the 1990s. Nevertheless, lamp pumping severely limits the
performance of high-power SSLs, and its use today is confined to
either low-end, multimode lasers in the less than ~100 W range or to
low-repetition-rate, high-pulse-energy lasers, in which the cost of
sufficient diode pumps is prohibitive (including, interestingly
enough, the multibillion-dollar NIF laser [Chap. 14]).
The primary disadvantage of lamp pump sources is their broad-
band emission spectrum, which spans the entire visible range from
the ultraviolet (UV) to the near infrared (IR) (Fig. 7.5). For compari-
son, the absorption spectrum of Nd:YAG is also shown in Fig. 7.5.
Only the small fraction of lamp power that coincides with an Nd
absorption feature can be absorbed and be converted to laser light;
the remaining power is simply wasted (Fig. 7.5, shaded regions).
Regardless of the SSL gain material, this waste severely limits the