Measurement of optical homogeneity of ZnGeP₂crystal using a 2.02 μm single-longitudinal-mode Tm:LuAG ring laser

We demonstrate a single-longitudinal-mode Tm:LuAG unidirectional ring laser based on the Faraday effect and its application in measuring the optical homogeneity of ZnGeP2 (ZGP) crystal. The maximum single-longitudinalmode power was 511 mW at 2022.4 nm, with a slope efficiency of 9.7%, and the M2 factors were 1.13 and 1.23 in the x and y directions, respectively. The laser was s polarized with a polarization degree of 30 dB. Also, the optical homogeneity of ZGP crystal was measured by the Mach-Zehnder interference method, which adopts the single-longitudinal-mode laser as the monochrome source. According to the interference fringes, the quantitative calculation or qualitative analysis of the optical homogeneity of ZGP crystal was achieved, which could quickly and conveniently judge the quality of ZGP crystal, and provided high-quality crystals for high-power mid-infrared lasers. To our knowledge, it is the first report of the optical homogeneity measurement of ZGP crystal by a 2 μm single-longitudinal-mode laser.