Page 265 - Tunable Lasers Handbook

6 Transition Metal Solid-state lasers 225
laser pumping is used rather than flashlamp pumping. Slope efficiency also
depends on the losses, including excited state absorption.
Energy per pulse depends on the pump source and the effective stimulated
emission cross section. A high energy per pulse usually favors flashlamp pump-
ing over laser pumping, primarily because of the higher optical pump energies
available. An effective stimulated emission cross section limits the amount of
energy per pulse that can be extracted from a single device. If the effective stim-
ulated emission cross section is high, the resulting high gain will promote ASE,
as mentioned earlier. In essence. a photon emitted because of natural sponta-
neous emission will cause the emission of several other photons before it can
escape from the laser material. Because both the amount of fluorescence and the
gain increase as the stored energy increases. ASE rapidly becomes a limiting
mechanism in high-energy-per-pulse or high gain applications. Thus. a high
energy per pulse favors moderate effective stimulated emission cross sections
when long optical pump pulses are used.
Average power limitations are limited by the thermal. optical, and mechani-
cal propenies of the laser material. Ultimately, the average power is limited by
thermally induced fracture in the laser material. To mitigate this effect, a laser
material should be durable and have a high thermal conductivity. Such properties
are discussed for the laser materials appearing in the following sections. Before
the laser material fails because of thermally induced fracture. thermally induced
lensing and thermally induced birefringence tend to degrade beam quality. An
analysis of these problems is available but is beyond the scope of this chapter.

2. TRANSITION METAL AND LANTHANIDE SERIES LASERS

In transition metal lasers, electrons in the 3d subshell participate in the las-
ing process. Transition metal atoms that have demonstrated laser action reside in
the fourth POW of the periodic table of the elements. Electronic configurations of
these atoms, derived from quantum mechanics, are shown in the Fig. 1. The first
two shells, consisting of the 1s subshell as well as the 2s and 2p subshells, are
completely filled. In this notation. the first digit is the radial quantum number
and the letter represents the angular quantum number; s representing 0. p repre-
senting I, d representing 2. f representing 3, and so forth. As electrons continue
to be added, the first two subshells of the third shell, the 3s and 3p subshells, are
filled. In the free atom configuration, the next two electrons are added to the 4s
fourth subshell. After this, the 3d subshell begins to fill. When the transition
metal atom is put into the laser material. the 4s electrons and possibly one or
more of the 3d electrons are used to fomi the chemical bonds associated with the
laser material. This leaves the remaining 3d electrons exposed to the electric
forces of the neighboring atoms. that is. the crystal field associated with the laser
material. As such. the 3d electrons are strongly affected by the crystal field.