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Thermally induced beam distortions in laser ceramics with arbitrary grain size have been investigated. A model for the spatial spectral density of dielectric permeability variations was suggested. The average scattered intensity was calculated using the small disturbances method (theory of Rayleigh–Debye scattering). The direction pattern of the scattered field and the extinction coefficient have been obtained. The scattered power and the depolarization ratio fit with the corresponding values obtained in the geometric optics approximation in the range of its validity.

Faraday isolators play a key role in the operation of large-scale gravitational-wave detectors. Second-generation gravitational-wave interferometers such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo will use high-average-power cw lasers (up to 200 W) requiring specially designed Faraday isolators that are immune to the effects resulting from the laser beam absorption–degraded isolation ratio, thermal lensing, and thermally induced beam steering. In this paper, we present a comprehensive study of Faraday isolators designed specifically for high-performance operation in high-power gravitational-wave interferometers.

Two alternative methods for determining phase distortions of radiation in the heated active element of a high-power Nd:glass laser are compared: direct phase measurement with the Mach–Zehnder interferometer and phase front reconstruction from measured distributions of depolarization factor. It is shown that the latter method can quite quickly and accurately determine the wavefront distortions of laser radiation with minimal interference to the optical scheme of the laser.

A cryogenic Faraday isolator with disc-shaped magneto-optical element is described. Depolarization of laser radiation caused by transverse inhomogeneity of Verdet constant has been measured for the first time. The decrease of the thermally induced effects caused by heat removal from the face-end of the magneto-optical element of terbium gallium garnet with sapphire and aluminum-yttrium garnet discs is investigated. Characteristics of the cryogenic Faraday isolator at high laser power above 1 kW have been investigated experimentally and the possibility of its use at power as high as 6 kW is demonstrated. The reduction of thermo-optical constants Q and P by a factor of 5.7 and 6.8, respectively, with cooling from 300 K to 80 K has been measured for TGG crystal.

We analyzed a number of optical media, such as GGG, Nd:YAG, Yb:YAG, fused silica, CaF_2, Yb:CaF_2, and CdMnTe, that have not been used, to our knowledge, in the cryogenic Faraday isolator (FI) before. The temperature dependence of the Verdet constant and thermo-optical constants was experimentally investigated for λ=1.07?μm. We calculated the magneto-optical figure-of-merit and assessed the feasibility of using FI media with multikilowatt average laser power.

We have investigated the thermally induced depolarization degree in long-rod cubic crystals of classes 23 and m3 as a function of the heated region and the probe beam radii as well as on crystal orientation. We extended the photoelastic anisotropy parameter and introduced a second anisotropy parameter. Three new specific crystal orientations were defined. General theorems on specific orientations were proved. The best and the worst orientations were determined. They are close or equal to the specific ones in most cases.

It is shown that a uniaxial crystal (cut along the optical axis) placed inside a telescope may compensate for thermally induced birefringence in laser-active elements. Depolarization was reduced in an experiment by an order of magnitude.

Light absorption in optical elements of Faraday rotators results in a nonuniform cross-sectional temperature distribution that leads to depolarization of laser radiation and, consequently, limits the isolation ratio of optical Faraday isolators. We show experimentally that the influence of the temperature dependence of the Verdet constant on the isolation ratio is negligibly small when compared with the influence of the photoelastic effect. We also present two novel methods of optical isolation that significantly reduce the depolarization caused by the photoelastic effect and increase the isolation ratio by two orders of magnitude in comparison with the conventional method. Our results confirm the possibility of magneto-optical glass-based Faraday isolators with isolation ratios of 30 dB for average laser powers of hundreds of watts.