Definition of Thermo-Optical Characteristics of Uniaxial Crystals

Abstract

Thermally induced distortions of laser radiation caused by the photoelastic effect during high-power beam propagation through optical elements cut along the optical axis of uniaxial crystals have been investigated. Thermo-optical characteristics for uniaxial crystals of all three syngony types have been introduced: the optical anisotropy parameter $\xi $ defining the distribution of the axes of thermally induced birefringence over the cross section of the optical element, and the thermo-optical constants $Q$ and $P$ specifying the value of thermally induced depolarization and thermal lens strength, respectively. The introduced thermo-optical characteristics may be used as applicability criteria of various uniaxial crystals for work with high-power laser radiation. The optical anisotropy parameter $\xi $ of trigonal and hexagonal crystals has been found to be equal to unity by virtue of the isotropy of their piezo-optical properties in a plane perpendicular to the optical axis. We have analyzed by way of example thermally induced polarization distortions in a tetragonal DKDP crystal and in a trigonal CeF3 crystal. The distributions and dependences observed in experiment fully agree with the theoretical considerations.