Page 296 - Tunable Lasers Handbook

256 Norman P. Barnes

2.00
1 .oo
h
v)
5 0.50 - Theory using
.- E
c
a 0.20
Y-
-I
0.10
0.05
100 200 300 400 500
Temperature (K)
FIGURE 1 5 Upper laser level lifetime of Cr:BeAl,O, versus temperature. (courtesy of M. L.
Shand, Allied Signal Corporation.)

h
5 -.
6 0.2
v
a,
c
2
a,
g 0.1
a,
8
E
0
-
3
lL 0.0
12000 13000 14000 15000 16000
Wave Number (cm-1)
FIGURE 1 6 Emission spectra of Cr:BeAl,O,. (Courtesy of h1. L. Shand, Allied Signal Corporation.)

laser rod. Use of this cut produces the highest gain operation of the laser. If the b axis
is perpendicular to the laser rod axis, the laser rod axis could be along either the a or
the c axis. c axis rods are often utilized based on the growth properties of Cr:BeAl,O,.
Although the emission spectra suggest a relatively wide tuning range for
Cr:BeAl,O,, ground state absorption and excited state absorption restrict the tun-
ing range. Ground state absorption affects primarily the short-wavelength opera-
tion of this material [24]. Experimentally, the ground state absorption cross section
varies nearly exponentially with the energy of the transition. At 0.7 pm, the cross
section is a little over 10-25 m2, whereas at 0.8 pm the cross section has decreased
to a little less than 10-29 m2. For wavelengths longer than 0.7 pm, ground state
absorption is a rapidly decreasing effect. Excited state absorption, on the other
hand, affects both the long- and short-wavelength operations of this laser material
[25]. A plot of the excited state cross section appears in Fig. 17. At about 0.77 pm,
the excited state absorption reaches a minimum. A minimum in the excited state