Terbium Aluminium Garnet Ceramics for Ultrahigh Power Laser Isolators

Terbium gallium garnet (TGG) single-crystals and ceramics are widely used for high power laser isolators. The average laser power, however, limited by thermally-induced effects. For example, the thermal-birefringence-induced depolarization in TGG ceramics has been investigated experimentally [1]. The extinction ratio and beam quality after passing through the Faraday isolator are degraded significantly by the thermal birefringence and the thermal wave front distortion of using a Faraday medium under kW-level high-average-power laser radiation. The magnitude of the thermal effects depends on the thermal properties of the Faraday materials. In particular, 1) Verdet constant, 2) thermal conductivity, 3) absorption coefficient at the laser wavelength, 4) thermo-optic coefficient (dn/dT), and 5) linear thermal expansion coefficient are essential characteristics of the thermal effect.