Elongation of the stable range of a thermal-birefringence compensated two-rod resonator without a discontinuity

A method for the continuous power increase without an abrupt power drop in a thermal-birefringence compensated two-rod resonator with increasing an electric power is proposed and numerically investigated. The abrupt power drop is due to the narrow unstable region invaded in a stable region of a two-rod resonator with a thermal birefringence compensator. It is well known that the stable region of a symmetric two-rod resonator is divided into two stable regions by a narrow unstable region as a thermal-birefringence compensator is installed in the resonator. The narrow unstable zone is generated inside a stable region due to the slight difference of thermal focal lengths of radial (f_r) and tangential (f_φ) direction. This causes unwanted power drop of laser with increasing an electric power. Hence, it is necessary to remove a narrow unstable zone to get a continuous change of laser power by an electric power control. In this study, the removal of the unwanted power drop is accomplished by installing the additional lens at the center of the plane-parallel symmetric resonator. We numerically show that the proposed resonator power can increase without an abrupt power drop when the electric power increases. We theoretically find that to get a wide single stable region without a narrow unstable region, the focal length of the additional lens should be approximately half of the resonator length.