Astigmatic thermal lensing due to surface bulging in Yb:KYW laser crystals

One of the sources of thermal lensing in crystals is surface bulging that results from an inhomogeneous temperature distribution. We investigate a thermal lens caused by surface bulging in an end-pumped Yb:KYW crystal, which serves as a gain medium in lasers and optical amplifiers. The surface profile of the pumped crystal is measured using a Fizeau-type interferometer and compared with a numerical simulation using a finite element method. The study reveals that due to anisotropic thermal expansion, the surface shape of the Yb:KYW crystal is anisotropic and the profile of the expansion is transversely displaced with respect to the pump beam profile which generates the temperature distribution. The observed surface bulging gives rise to aberrations and deflection of the transmitted beam. It was found that the surface bulging introduces astigmatism that is significantly larger than previously estimated [Appl. Opt. 56, 3857 (2017) [CrossRef] ]. Our results allow the evaluation of the bulging contribution with improved accuracy. We show that these effects can be significant in certain designs of amplifiers and lasers.