Page 306 - Tunable Lasers Handbook

266 Norman P. Barnes

LiCaAlF, and LiSrAlF, are birefringent materials with relatively low refrac-
tive indices. Refractive indices have been measured for LiCaAlF, at nine wave-
lengths in the visible and near infrared [39]. Ordinary and extraordinary refrac-
tive indices at laser wavelengths are 1.390 and 1.389, respectively. leaving a
difference in the refractive indices of only 0.0013. Variation of the refractive
indices with temperature for LiCaAlF, is negative and relatively small, -4.2 x
10-6/K and -4.6 x 10-6/K for the ordinary and extraordinary waves, respectively.
This small variation of the refractive indices with temperature tends to minimize
the thermally induced focusing. In essence, the negative variation of the refrac-
tive index with temperature tends to compensate for the positive variation of the
refractive index caused by the stress optic effect. In most oxide materials, these
two effects are both positive. which tends to exacerbate the thermal focusing
problem.
Absorption spectra of LiCaAlF, and LiSrAIF, are quite similar. Similari-
ties are expected since the exchange of Sr for Ca is a relatively minor substitu-
tion. Both laser materials exhibit the double-peaked absorption spectra charac-
teristic of Cr [40]. Because both materials are uniaxial, absorption spectra are
recorded for both the x and cs polarizations. At room temperature, the absorp-
tion peaks for the x polarization are approximately at 0.425 and 0.628 pm.
Absorption peaks for the cs polarization are approximately at 0.423 and 0.622
pm. The long-wavelength peak is stronger for the x polarization, and the short
wavelength peak is stronger for the cs polarization. Linewidth of the short wave-
length peak is about 0.064 pm, and the linewidth of the long wavelength peak is
about 0.093 pm. With the concentrations available with LiCaAIF,. typical
absorption coefficients can be on the order of a few hundred per meter. In com-
bination, large absorption coefficients and wide spectral bandwidths leads to
efficient flashlamp pumping. Absorption spectra for Cr:LiSrA1F6 are quite simi-
lar to absorption spectra for Cr:LiCaA1F6. Peaks occur at nearly the same wave-
lengths and the relative strengths of the peaks are also similar. However, the
absolute strengths for Cr:LiSrA1F6 are roughly twice as strong as the strengths
of Cr:LiSrA1F6. Absorption spectra are shown in Fig. 21 and 22 for these two
laser materials.
The upper laser level lifetimes of LiCaAlF, and LiSrAlF, are sufficiently
long to allow flashlamp pumping. Lifetime has been measured as a function of
Cr concentration in LiCaAlF, for concentrations exceeding 0.05. Up to this con-
centration, the lifetime was virtually independent of the concentration. Lifetime
has also been measured as a function of the temperature for both laser materials
[40]. Lifetimes of these laser materials are shown as a function of temperature in
Fig. 23. Cr:LiSrA1F6 has a lifetime of 67 ps, which is independent of tempera-
ture up to 300 K. In contrast, the upper laser level lifetime of Cr:LiCaA1F6 is
independent of temperature to only about 100 K. Above this temperature. the
lifetime decreases slowly, dropping from 215 ps at low temperatures to 172 ps at
room temperature. This decrease in the lifetime is attributed to a dynamic effect
of the crystal field on the transition probability.