Stable range enhancement in a symmetric confocal two-rod resonator with 90° optical rotator

We propose a method to eliminate the narrow unstable region of a symmetric two-rod resonator with a 90° optical rotator which is used to compensate for thermal birefringence of the laser rod. The narrow unstable region is caused by the difference between the thermal focal lengths of the radial and tangential directions of a laser rod. Using the g-diagram analysis, we find that the narrow unstable region can be removed in the resonator configuration of a symmetric confocal type. We investigate the effect of the distance between two rods of the proposed resonator on the stability and the beam quality.