Page 195 - High Power Laser Handbook

164 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 165

and optical conditioning to couple the pump photons to the gain
material. Due to their optically pumped nature, HAP SSLs essentially
function as brightness enhancers—that is, they convert low-spatial-
brightness pump photons into an output beam with improved beam
quality (BQ), but with lower total power due to imperfect efficiency.
The overriding consideration that drives HAP SSL designs is minimi-
zation of the output beam’s thermo-optic distortion so as to maximize
the brightness increase (where brightness is loosely defined as the ratio
of power to BQ ). Finally, many SSL materials exhibit relatively long
2
upper-state lifetimes or broad-gain bandwidths compared with other
types of lasers. This allows SSLs to act as energy-storage devices, in that
the energy accumulated during a long optical pumping cycle can be
released very quickly in the form of a short, high peak-power pulse.
This chapter discusses considerations that typically drive the
selection of the laser gain material, pump source, pump delivery
optics, and the geometries for both heat removal and optical extrac-
tion. The chapter is intended to serve as a brief prelude and introduc-
tion to Chaps. 8–14, which describe some of the most successful
state-of-the-art high-power SSL architectures. More general back-
ground for the design and engineering of solid-state lasers can be
found in the classic textbook by Koechner. 4

7.2 Laser Gain Materials
All SSL materials consist of an optically transparent host doped with
active ions that absorb pump light and emit laser light. Since the
invention of the laser, an enormous body of research has accumulated
on various combinations of lasant:host materials optimized for par-
5
ticular features or applications. We confine this chapter to a discus-
sion of specific laser materials and properties that are most relevant
for peak and average power scaling, along with the basic concepts
underlying SSL laser emission.

7.2.1 Cross Section and Lifetime
The probability of an active ion absorbing or emitting a photon is pro-
portional to its transition cross section σ. The cross section represents
the gain per unit length per inversion density ∆N, so that the laser
small-signal gain is g = σ∆N. A high cross section is usually advanta-
0
geous for an SSL, as fewer incident photons are needed to saturate any
given transition, whether during pumping or stimulated emission.
This relaxes the need for high laser intensities and reduces the propen-
sity for optical damage of the material. Moreover, a large laser gain
enables an SSL architecture to be more tolerant to optical losses with-
out substantial sacrifice in efficiency, thus providing design flexibility
for the optical configuration of the extracting beam.
Another key spectroscopic parameter is the fluorescence lifetime τ
for spontaneous decay of the upper laser level via emission of a photon.