Page 308 - Tunable Lasers Handbook

268 Norman P. Barnes

250
. - Model
e Cr: LiCaAlFs
Cr: LiSrAlFs
2 *0° e
v
a,
.E 150-
c
=
a,
-I

I I
0 100 200 0
Temperature (K)
FIGURE 23 Upper laser level lifetimes of Cr:LiCaAIF, and Cr:LiSrAIF,. (courtesy of s. A.
Payne. Lawrence Livermore National Laboratory.)

Ground state and excited state absorption both exist for Cr:LiCaAlF, and
Cr:LiSrAlF,. Ground state absorption for these laser materials can be observed
in Fig. 21. Ground state absorption can seriously affect the laser performance for
wavelengths shorter than 0.75 pm. Single-pass absorption depends, of course, on
the length of the laser rod and concentration of Cr. However, even at low con-
centrations, absorptions of 0.05 have been observed at 0.725 pm. At long wave-
lengths, ground absorption becomes an increasingly smaller effect. Because
Cr:LiCaAlF, emission peaks at shorter wavelengths, ground state absorption is a
more serious effect for this laser material. Excited state absorption can occur
between the 3T, and 4Tl manifolds. Because more than one manifold is desig-
nated as the 4Tl-manifold, a further designation is given. On the Tanabe-Sugano
diagram, the manifold that originates at the origin is designated the 3T,a mani-
fold, and the manifold that originates at an EIB ratio of about 15 is designated
the 4T,b manifold. In Cr:LiCaAlF,, the excited state absorption between the 4T2
and the Tlb manifold peaks around 0.5 1 pm and stretches across much the visi-
ble [37]. Excited state absorption in this region is non-negligible and will limit
the level of inversion if flashlamp pumping is employed. However, if laser or
laser diode array pumping is utilized, this problem can be mitigated. Excited
state absorption between the 4T2 and the AT,a manifolds occurs at wavelengths at
which lasing can occur. Excited state absorption has been measured in
Cr:LiCaA1F6 for wavelengths longer than 1 .O pm. Experimental difficulties
made measurements at shorter wavelengths difficult. Through extrapolation of
the measured data, it was determined that the peak of this absorption occurred at
0.997 pm and the linewidth was estimated to be 0.243 pm. An excited state
absorption cross section at the peak was inferred to be at 0.17 x 10-24 m2. Peak
effective stimulated emission cross section is 1.3 x 10-14 m2. Thus, near the peak
emission wavelengths, excited state absorption is a small effect.