Drastic reduction of thermally induced depolarization in CaF₂ crystals with [111] orientation.

The key importance of the sign of the stress-optic anisotropy ratio for reducing thermally induced depolarization in cubic crystals with 432, 4¯3m and m3m symmetry is addressed. A simple method for measuring the stress-optic anisotropy ratio (including its sign) was proposed and verified in CaF<₂ and TGG crystals by experiment. The ratio at room temperature for the wavelength 1076 nm was measured to be -0.47 and + 2.25, respectively. In crystals with a negative value of this parameter thermally induced depolarization may be reduced significantly by choosing crystal orientation. In a CaF₂ crystal with the [111] orientation a 20-fold reduction of thermally induced depolarization as compared to the [001] orientation was obtained in experiment, which is very promising for using CaF₂ as an active element in high-average-power lasers.