Page 204 - 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

Xenonlamp emission

Wasted flashlamp power

Nd:YAG absorption

300 400 500 600 700 800 900
Wavelength (nm)
Figure 7.5 (top) Emission spectrum of a xenon-filled flash lamp. The shaded
regions of the spectrum represent wasted energy that is not absorbed by
Nd:YAG (bottom).

laser’s efficiency. Even if a lamp photon happens to be at a favorable
wavelength for absorption, it will most likely be at a transition to an
energy level high above the upper laser level, leading to a large quan-
tum defect and thus a large amount of heat deposited in the gain
material for every emitted laser photon. From the standpoint of per-
formance, this excess heat is the primary disadvantage of lamp pump-
ing in comparison to the modern standard of diode pumping.
One final difficulty with lamp pumping is that lamps emit in all
directions, with low spatial brightness, which severely constrains the
geometric choices for optical coupling of the pump source into the
gain medium. The most common choices are either to simply close-
couple the lamp(s) against the gain medium, typically with a reflector
to capture light emitted away from the desired direction, or to embed
both the lamp and the gain medium (typically in the form of a rod) at
the foci of an elliptical reflecting cavity, so that the lamp light is reim-
aged onto the rod. Neither of these geometries is advantageous for
scaling to higher power, because they both constrain the geometries
for laser beam extraction and heat removal.
Diode Pumping
The development of efficient, high-power laser diodes for pumping
SSLs has revolutionized the development of HAP SSLs over the past
15 years. Owing to the importance of diode lasers both as pump
sources and as high-power lasers in their own right, they are dis-
cussed in detail in Chaps. 5 and 6.
Diodes make ideal excitation sources for SSLs. Their emission
spectrum can be engineered through choice of material and epitaxial