Thermal lensing in Yb:KLu(WO4)2 crystals cut along the optical indicatrix axes

Thermal lensing is studied in Yb:KLu(WO4)2 crystals cut along the three optical indicatrix axes, Np, Nm and Ng. The cut along the Ng-axis is the only one that provides a pure positive thermal lens with the sensitivity factors M = 3.5 and 2.8 m−1 W−1 (wp = 100 μm) for the mg- and pg-planes, respectively, and a low astigmatism degree, S/M < 20%. Both Np-cut and Nm-cut crystals produce different-signed lens with at least six-times stronger astigmatism. This behavior is related to the anisotropy of the photo-elastic effect. A CW Yb:KLu(WO4)2 microchip laser delivering a maximum output power of 992 mW at 1051 nm with a slope efficiency of 40%, and possessing low spatial distortions of the laser beam and M2 < 1.1, is realized on the basis of Ng-cut crystal.

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