Polarization anisotropy of thermal lens in Yb:KY(WO₄)₂ laser crystal under high-power diode pumping

Thermal lensing was studied in an N_g-cut monoclinic Yb:KY(WO₄)₂ (Yb:KYW) laser crystal under high-power diode pumping at ∼980  nm, for the two principal light polarizations, E||N_m and E||N_p. For both polarizations, the thermal lens (TL) was positive. It was found that operation of an Yb:KYW laser with the E||N_p polarization corresponds to a weaker TL that results in a better quality of the laser beam and superior power scaling capabilities. The sensitivity factors of the TL in this case were M=0.37 and 0.57  m^-1/W for the mg and pg principal meridional planes, respectively, and the astigmatism degree was as low as S/M=35% (for a pump beam radius of w_p=175  μm). The photoelastic terms, fractional heat loading, and generalized thermo-optic coefficients were evaluated.