YLF rods thermal lens astigmatism

In this paper we discuss interesting experimental results for YLF crystal: high anisotropy and high astigmatism of thermal lens in a cutted YLF rods. The first effect is more or less predictable because YLF is uniaxial crystal. But high astigmatism of thermal lens is unexpected and is not explained in literature so far. Hence we showed that thermal lens astigmatism can be explained by photoelastic effect, which is usually forgotten. To support this idea we developed computer code for calculation of thermal lens in uniaxial crystals and took into account effects of refractive index change, thermal expansion and photoelastic effect. From the results we have 3 constants that determine photoelastic part of thermal lens for extraordinary polarization and 2 for ordinary polarization.