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

Abstract — Absorption of laser radiation in magnetooptical materials results in a temperature gradient which induces depo-larization 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 com-pared 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. Index Terms—Depolarization, Faraday effect, photoelastic ef-fect, Verdet constant