Thermal lensing in diode-pumped [001]-cut tetragonal crystals

Thermal lensing is theoretically studied for laser discs from [001]-cut tetragonal crystals under the plane stress approximation, considering anisotropy of their elastic and photo-elastic properties. Analytical description of the stress-induced birefringence is presented. This allows us to derive analytical expressions for the photo-elastic term χ″, “generalized” thermo-optic coefficient (TOC), sensitivity factors of the thermal lens and its astigmatism degree, as well as discuss orientation of the principal meridional planes of the lens. These parameters are determined for the discs from tetragonal scheelite-type molydbate crystals, Nd:PbMoO4, Nd:CaMoO4 and Nd:NaBi(MoO4)2 at ~1 μm. It is shown that the photo-elastic effect plays an important role in the formation of the thermal lens for tetragonal crystals, and χ″ value can be either positive or negative. We also predict theoretically that for anisotropic crystals the orientation of the principal meridional planes of the thermal lens is not linked to the polarization direction.