Investigation of self-induced depolarization of laser radiation in terbium gallium garnet

Absorption of laser radiation in magnetooptical materials results in a temperature gradient which induces depolarization due to both the temperature dependence of the Verdet constant and the photoelastic effect of thermal strains. This results in a limitation of the isolation ratio of Faraday isolators in high average power lasers. Here, we derive expressions for the isolation ratio as a function of beam power, beam radius, angle between incident polarization and crystal axis, and characteristics of the magnetooptical material. The theoretical results are compared with experiments for a terbium gallium garnet crystal. Our results allow us to choose the optimal parameters to maximize the isolation ratio and to compare different materials from this point of view.