Faraday isolator based on crystalline silicon for 2-µm laser radiation.

The magneto-optical properties of single-crystal silicon were investigated as a function of wavelength and temperature. A bulk free-space traditional Faraday isolator for the radiation with a wavelength of 1940 nm (magnetic field ∼2.8 Т) was implemented. The negative value of the piezo-optical anisotropy ratio of the used material allowed for the development of a Faraday isolator with compensation of thermally induced depolarization without a reciprocal rotator. The potential of single-crystal silicon as a magneto-optical material for Faraday isolators operating at room as well as at cryogenic temperatures in high-power laser radiation was considered. It was shown that single-crystal silicon is highly promising for the development of Faraday devices, including ones for next-generation laser interferometers aimed at detecting gravitational waves.

Thermally induced depolarization in a silicon (c-Si) single crystal.

Single-crystal silicon is one of the most promising materials for producing test masses for the new generation of laser interferometers intended for the detection of gravitational waves. We studied the thermally induced depolarization of radiation in single-crystal silicon with [001] orientation at a wavelength of 1940 nm at room temperature. The value of the piezo-optical anisotropy ratio was found to be ξ = -0.63 ± 0.05 for one manufacturer and ξ = 0.62 ± 0.05 for another. The physical reason for the different signs was not established, and the search for this reason will continue. For the single-crystal silicon with the negative piezo-optical anisotropy ratio, the directions of the special orientations [C] and [P] for which thermally induced depolarization and thermal lens astigmatism vanish, respectively, were determined. When ξ is close to -0.5, the special orientations are close to [111], making this orientation preferable to the other traditionally used orientation, [001].

Thermo-optical properties of terbium sesquioxide (Tb2O3) ceramics at room temperature.

Thermo-optical properties of several (Tb1-xYx)2O3 ceramic samples were investigated in this Letter. The linear absorption and thermal conductivity coefficients, as well as the power dependence of thermally induced phase and polarization distortions of laser radiation, were measured. In addition, the effective thermo-optical constants Peff and Qeff were estimated. Thermo-optical properties of the studied ceramics were compared with those of the widely used terbium gallium garnet. It was shown that the material under consideration is highly promising for Faraday isolators operating at high average power laser radiation.

Thermo-Optical Studies of Laser Ceramics.

A cycle of works on manufacturing and studying laser and magnetooptical ceramics with a focus on their thermo-optical characteristics performed by the research team is analyzed. Original results that have not been published before such as measurements of the Verdet constant in the Zr:TAG, Re:MgAl2O4, and ZnAl2O4 ceramics are also presented.

Thermal lens astigmatism in glass and in cubic crystals with [001] orientation.

The thermal lens was investigated in CaF2 and Tb3Ga5O12 cubic crystals with [001] crystallographic axes orientation and in magneto-optical glass MOG103 by the method of phase-shifting interferometry. It was demonstrated experimentally that the thermal lens has astigmatism determined by the incident radiation polarization state and by the optical anisotropy parameter ξ. A method of ξ determination by measuring thermal lens astigmatism in cubic crystals with [001] orientation was proposed and verified. It was shown that thermally induced depolarization and the amplitude of phase astigmatism depend on the position of the crystal with [001] orientation in antiphase.

Cryogenically cooled CeF3 crystal as media for high-power magneto-optical devices.

The thermally induced depolarization and Verdet constant of CeF3 crystals-their most important characteristics-have been studied in the 79-293 K temperature range. It has been found that thermal effects reduce substantially upon cooling down to 79 K and the Verdet constant grows in inverse proportion to the temperature. It was shown that CeF3 crystals are not inferior to TGG as a medium for Faraday isolators, including cryogenic ones.

Faraday rotator based on TSAG crystal with <001> orientation.

A Faraday isolator (FI) for high-power lasers with kilowatt-level average power and 1-µm wavelength was demonstrated using a terbium scandium aluminum garnet (TSAG) with its crystal axis aligned in the <001> direction. Furthermore, no compensation scheme for thermally induced depolarization in a magnetic field was used. An isolation ratio of 35.4 dB (depolarization ratio γ of 2.9 × 10-4) was experimentally observed at a maximum laser power of 1470 W. This result for room-temperature FIs is the best reported, and provides a simple, practical solution for achieving optical isolation in high-power laser systems.

Effect of full compensation of thermally induced depolarization in two nonidentical laser elements.

Thermally induced depolarization of radiation introduced by a system of two optical elements separated by a quartz rotator has been analyzed. The conditions of full compensation of thermally induced depolarization for two nonidentical optical elements have been found. The model experiment has demonstrated that full compensation in two optical elements of different materials is possible without a quartz rotator between them.

Terbium gallium garnet ceramic Faraday rotator for high-power laser application.

A terbium gallium garnet (TGG) ceramic Faraday rotator (FR) with an isolation ratio of 33 dB was demonstrated at a laser radiation power of 257 W. This FR can be equipped with a large optical aperture by using ceramics technology to prevent laser damage at high-energy pulse operation. The thermal lens of a 257 W laser with a beam diameter of 2.6 mm had a focal length of 9.5 m, which is easily corrected using a spherical lens to suppress the undesirable effects of thermal lensing. The rotation angle of the FR was stabilized by water cooling. The results indicate that the TGG-ceramic-based FR is suitable for high-energy laser systems with high repetition rates.

Study of the thermo-optical constants of Yb doped Y2O3, Lu2O3 and Sc2O3 ceramic materials.

Thermally induced depolarization and thermal lens of three Konoshima Chemical Co. laser-ceramics samples Yb(3+):Lu(2)O(3)(C(Yb) ≈ 1.8 at.%), Yb(3+):Y(2)O(3)(C(Yb) ≈ 1.8 at.%), and Yb(3+):Sc(2)O(3) (C(Yb) ≈ 2.5 at.%) were measured in experiment at different pump power. The results allowed us to estimate the thermal conductivity of the investigated ceramic samples and compare their thermo-optical properties. The thermo-optical constants P and Q and its sign measured for these materials at the first time.

Self-compensation of thermally induced depolarization in CaF2 and definite cubic single crystals.

Compensation of thermally induced depolarization in laser active elements at small birefringence without additional phase elements was proposed and observed experimentally. Requirements to the crystals were formulated. An order of magnitude reduction of depolarization degree was obtained experimentally. A further modification of the scheme was developed.

Drastic reduction of thermally induced depolarization in CaF2 crystals with [111] orientation.

The key importance of the sign of the stress-optic anisotropy ratio for reducing thermally induced depolarization in cubic crystals with 432, 4¯3m and m3m symmetry is addressed. A simple method for measuring the stress-optic anisotropy ratio (including its sign) was proposed and verified in CaF<2 and TGG crystals by experiment. The ratio at room temperature for the wavelength 1076 nm was measured to be -0.47 and + 2.25, respectively. In crystals with a negative value of this parameter thermally induced depolarization may be reduced significantly by choosing crystal orientation. In a CaF2 crystal with the [111] orientation a 20-fold reduction of thermally induced depolarization as compared to the [001] orientation was obtained in experiment, which is very promising for using CaF2 as an active element in high-average-power lasers.

Compensation of thermally induced depolarization in Faraday isolators for high average power lasers.

A compensation scheme for thermally induced birefringence in Faraday isolators is proposed. With the use of this scheme a 36-fold increase of the isolation degree was attained in experiment. A comparative analysis of the considered scheme and the earlier Faraday isolator schemes with high average radiation power is performed. A method for optimizing the earlier Faraday isolator scheme with birefringence compensation is developed.

Experimental study of thermal lens features in laser ceramics.

Thermal lens measurements were made by means of a high-accuracy phase shift interferometer that combines a lambda/1000 sensitivity and 10 microm transverse resolution. The effect of random small-scale modulation in thermally induced phase distortion predicted earlier was proved experimentally. The statistical parameters of modulation were measured depending on heating power for two different ceramic samples. The experimental data agree well with results of numerical simulation.