Page 295 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers

300 Cr Concentration = 2.2 x 1019 ions:cm3
A (0.00063 atomic)

0.40 0.50 0.60 0.70
Wavelength (micrometers)
FIGURE 14 Absorption spectra of Cr:BeAl,O,. (a) a axis absorption. (b) b axis absorption (ci c
axis absorption. (Courtesy of M. L. Shand. Allied Signal Corporation.)

Quantum efficiency of Cr:BeAl,O, is high, about 0.95 ar room temperature
[23]. Quantum efficiency was measured using a sophisticated photoacoustic
technique. which measures the phase shift between a modulated pump source
and the photoacoustic signal. The high quantum efficiency of this laser material
promotes efficient laser operation.
The upper laser le.Jel lifetime of Cr:BeAl,O, is strongly temperature depen-
dent [2 11. Temperature-dependent effects can be successfully modeled by consid-
ering the population of the upper laser manifold to be divided between two mam-
folds, the 2E and the 4Tq. Lifetimes of these two manifolds are 1.51 ms and 6.6 ps.
respectively [20]. In c&parison, the lifetime of the ZT, manifold is assumed to
have an arbitrarily long lifetime. Assuming thermal equilibrium among the vari-
ous manifolds near the 2E level, the fractional population of the various manifolds
can be calculated using Boltzmann‘s statistics. Iff, and fr are the fractional popu-
lations of these two manifolds, the fluorescent lifetime can be approximated as

A plot of this lifetime appears in Fig. 15. Near room temperature, efficient
energy storage under flashlamp pumping is feasible with lifetimes available in
this laser material.
Polarized emission spectra of Cr:BeAl,OS have both R line features and the
vibronic sidebands. Emission spectra are shown in Fig. 16. Vibronic spectra exist
&om the R lines, about 0.68 pm, to beyond 0.82 pm. Of the three emission spectra.
the strongest emission is associated with radiation polarized along the b axis. Conse-
quently, laser rods are usually cut so that the b axis is perpendicular to the axis of the