Page 312 - Tunable Lasers Handbook

272 Norman P. Barnes

lower, which limits the average power available from these laser materials. Some
of the physical properties of these laser materials are listed in Table 5.
Garnet materials are isotropic materials with a relatively high refractive
index. As expected, the refractive index of these materials is higher than that of
YAG. Specifically, the refractive indices of GSGG, YSAG, and GSAG are 1.952,
1.867, and 1.890, respectively. Because these materials are isotropic, laser output
is not polarized in general. If polarizers are included in the laser resonator. depo-
larization losses can be expected at high average powers [47,48]. Depolarization
losses are due to the thermally induced birefringence in normally isotropic mate-
rials and are exacerbated by the reduced thermal conductivity when compared
to YAG.
When Cr is incorporated into these materials, the two absorption bands char-
acteristic of Cr are readily identified. Absorption peaks for the short-wavelength
absorption are approximately 1.5 times stronger than the absorption peaks for the
long wavelength. Spectra of Cr:GSGG and Cr:GSAG appear in Figs. 24 and 25,
respectively. Short-wavelength absorption peaks occur at 0.46 and 0.45 pm:
widths are 0.09 and 0.12 pm for GSGG and GSAG, respectively [49,50]. Long-
wavelength absorption peaks occur at 0.64 and 0.63 pm; widths are 0.12 and 0.09
pm for the same laser materials. These absorption features are strong enough to
produce absorption coefficients on the order of 200 m-1 even with concentra-
tions below 0.01 atomic. Efficient flashlamp pumping is possible with these
materials because of these strong absorption features. However, the shorter
wavelengths, shorter than about 0.4 pm, should be filtered out since these wave-
lengths are absorbed primarily by the laser material itself. Short pump wave-
lengths thus contribute to heating of the laser material while producing little
population in the upper laser level. Worse still is the creation of detrimental
flashlamp-induced loss.

TABLE 5 Physical Properties of Garnets

~~
Parameter YAG GSGG Units
Lattice constants 1201 1256
Density 4550 6439
Heat capacity 620 402
Thermal conductivity 13.0 5.78
Thermal expansion 7.0 7.5
Refractive index 1.8289 1.9518
Refractive index variation 10.4 10.1 10-6K
Optical transparency 0.24-5.5 0.3-6.5 Pm
h1elting point 1940 ~1830 "C