Crystalline whispering gallery mode stabilized ring-laser

Whispering gallery mode (WGM) resonators have found multiple important applications due to their compact sizes and high intrinsic quality (Q) factors. The Q value in these resonators is mainly limited by the surface quality and the intrinsic material absorption. We report on crystalline WGM resonators with very low intrinsic material absorption made out of CaF₂, where the required surface quality was achieved by diamond turning and polishing. Such a millimeter size resonator is used to stabilize a ring laser, which consists of a semiconductor optical amplifier (SOA) as a gain medium, an isolator, to prevent lasing in the counter-clockwise direction, and a CaF₂ resonator coupled via two prisms. The system sustains single mode lasing. In order to study the linewidth two different methods are presented.