Diffractive-optic unstable resonator for low-gain lasers

A technique for designing unstable resonators for low gain lasers has been developed. The technique features a Gaussian reflectivity mirror (GRM) output coupler whose central reflectivity can be much higher than the conventional 1/m² and allows the production of an output beam which does not have a large depression in the center. It uses a phase structure on the end mirror to alter the Gaussian mode inside the cavity in such a way that, after passing through the high-reflectivity Gaussian output coupler, it will result in a flat output profile.