Definition of Thermo-Optical Characteristics of Uniaxial Crystals

摘要

We have studied thermally induced birefringence in uniaxial crystals cut along the optical axis. It has been shown that changes of the refractive index caused by the photoelastic effect in trigonal and hexagonal crystals look like in the medium with isotropic elastic and piezo-optical properties, and in tetragonal crystals like in cubic crystals cut along the crystallographic axis assuming isotropic elasticity. We have introduced thermo-optical characteristics of uniaxial crystals of all three syngony types: the optical anisotropy parameter - 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. These characteristics may be considered as generalization of the corresponding characteristics of isotropic medium or cubic crystals with the elastically isotropic properties.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 defining the distribution of the axes of thermally induced birefringence over the cross section of the optical element, and the thermo-optical constants and 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 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 CeF crystal. The distributions and dependences observed in experiment fully agree with the theoretical considerations.