<title>Suppression of self-induced depolarization of laser radiation in Faraday isolators</title>

Abstract

A lot of laser applications require propagation of extremely high average power radiation through Faraday isolators. Although many investigations are devoted to the thermal lens effect, polarization contamination has not been discussed in detail. However, the depolarization can significantly limit of the isolation ratio. The physical reason for the self-induced depolarization is absorption of laser radiation. It results in spatial nonuniform distribution of temperature giving rise to two effects which reduce the isolation ratio: the temperature dependence of Verdet constant and birefringence due to the photoelastic effect of thermal strains. It is shown (theoretically and experimentally) that the depolarization ratio is sum of two terms which represent these two effects and the last phenomena is more efficient. In order to suppress the self-induced depolarization two novel optical schemes were suggested and realized in experiment. The idea of compensating depolarization is to replace one 45°F araday rotator by two 22.5° rotators and a reciprocal optical element between them. The polarization distortions which a beam undertakes while passing the first rotator will be partially compensated in the second rotator. Both schemes allow to increase isolation ratio up to 100 times in comparison to the traditional scheme. Different ways of design of Faraday isolator for 1kW average power with isolation ratio about 30 dB are discussed.

Effect of terbium gallium garnet crystal orientation on the isolation ratio of a Faraday isolator at high average power

“…The values of depolarization ratios at orientations ͓001͔ ͑␤ ϭ 0͒ and ͓111͔ ͑␣ ϭ 45°, ␤ ϭ 54.8°͒ almost coincide, differing by only 1%. At the ͓001͔ orientation, minimization of depolarization requires a fine mutual alignment of magneto-optic elements with respect to a horizontal angle ͑the crystallographic axes of two elements must be parallel to each other 16,17 ͒. The ͓111͔ orientation does not have this disadvantage.…”

“…17 In conclusion, we summarize the main results obtained during our study. First, for an arbitrary orientation of a cubic crystal, we obtained analytical expressions for self-induced depolarization for the traditional FI design and for the recently developed FIQR design 16,17 and also for self-induced depolarization without any magnetic field. For all three cases, we have shown that, for a given beam shape, the depolarization is determined only by parameters and p for any orientation.…”

“…This theoretical prediction was confirmed experimentally. 16,18 For crystals with ͓001͔ orientation ͑and also for glass-based magneto-optic elements͒, a figure of merit that characterizes the magneto-optic medium was defined as ϭ V͞␣Q, where Q is a thermo-optic constant that characterizes the thermophotoelastic effect for the components of rod configuration:…”