Page 297 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 257

Wavelength (micrometers)
0.80 0.75 0.70
15 I I I
N -
E
Lo -- Excited-State Absorption
% I ...... Emission Cross Secrion
Cross Section
::., ! :
- 10- ,, .. I 8 ,
.,- k k
X ,... .-: i
c
. ..
(I)
v)
__-- +-'
12000 13000 14000 l! 00
Energy (cm-1)
FIGURE 17 Excited state absorption of Cr:BeAl,O,. (Courtesy of M. L. Shand, Allied Signal
Corporati0n.i
absorption is one of the reasons why this laser operates most efficiently around Ihis
wavelength. On the long-wavelength side, about 0.83 pm. the emission cross sec-
tion and the excited state cross section are equal. Lasing at wavelxgths longer
than this is not possible under these conditions. On the short-wavelength side, the
emission cross section and the excited state absorption cross section again become
equal slightly on the short-wavelength side of the R lines, about 0.68 pm.
Although excited state absorption does not prevent laser operation of the R lines, it
does significantly reduce the laser performance.
Effective stimulated emission cross sections were determined by using the
1McCumber theory for the analysis [25]. At room temperature. the effective stimu-
lated emission cross section at the wavelength of peak gain. about 0.77 pm, was
calculated to be about 0.6 x 10-24 m2. As the operating temperature increases. the
effective stimulated emission cross section increases. nearly linearly. At 200°C.
the effective stimulated emission cross section has increased to about 2.0 x 10-24
rnl. Increases in this parameter result from the increased population of the T,
level. However. the increased effective stimulated emission cross section is balI
zinced by the concomitant decrease in the upper laser level lifetime. For normal
mode operation, the shortening of the upper laser level lifetime is not as serious
as it is for &-switched operation. Excited state absorption of the laser radiation
will have the effect of decreasing the effective emission cross section.
Due io the relatively low effective stimulated emission cross section and
competition from other absorption mechanisms, Cr:BeAl,O, is usually pumped at
high levels. High pump levels are usually achieved by ;sing two flashlamps to
pump a single laser rod. Although high pumping levels cause thermal problems in
many materials, they are compensated to some degree by the excellent thermal
properties of the laser material. However, because of the high pump levels, it
becomes more difficult to achieve good beam quality and narrow spectral band-
width operation at high pi-fs.