Determination of the thermal lens in solid-state lasers with stable cavities

A useful method to determine the thermal lens in a thermally loaded material inside a laser resonator under operation is presented. The laser beam emitted is focused with a lens placed exactly at the distance of its focal length behind the output mirror. For stable spherical resonators this focusing is not astigmatic even for an astigmatic cavity. This fact is used to experimentally determine the thermal lens including the beam quality factor (beam propagation factor) M/sup 2/ of the laser beam. Measurements are presented for an end-pumped rod and a longitudinally pumped slab laser serving as an example for an astigmatic cavity.< >