Elimination of a narrow unstable zone inside the stable region of a two-rod resonator with a thermal birefringence compensator

We propose and numerically investigate a method to remove a narrow unstable zone invaded in the stable region of a two-rod resonator with a thermal birefringence compensator. The method is based on the modification of the two thermal focal lengths of a laser rod for radial and tangential polarizations using an additional lens. This is done by installing the additional lens at the center of the plane-parallel symmetric resonator. We find that the wide stable region without the narrow unstable zone is achieved when the focal length of the additional lens is approximately equal to the half length of the resonator.