Influence of the photoelastic effect on the thermal lensin a YLF crystal

The thermal lens in an YLF crystal is calculated taking into account the anisotropy of the thermal conductivity, elasticity, linear expansion, and refractive index. It is proved that the experimentally observed strong astigmatism of the thermal lens in YLF can be explained only taking into account the photoelastic effect. It is shown that, in an YLF crystal cut so that the optical axis lies in the plane of the rod face, the contribution to the thermal lens is made only by three photoelastic coefficients for the ordinary wave and only by two coefficients for the extraordinary wave. Using these five photoelastic coefficients as fitting parameters, all the known experimental data are quantitatively interpreted.