Ilya Snetkov

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.

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 Y 2 O 3 , Lu 2 O 3 and Sc 2 O 3 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.

Review of Faraday Isolators for Kilowatt Average Power Lasers

Faraday isolators for high average power lasers operating at room temperature are surveyed. Three devices on [001] oriented TGG crystals with the most known optical schemes are considered: traditional scheme, and schemes with compensation of thermally induced depolarization inside magnetic field and outside magnetic field. We report a unique 30-mm-aperture Faraday isolator with thermally induced depolarization compensation inside magnetic field. It provides 33.5-dB isolation ratio at 1.5-kW average laser power.

Drastic reduction of thermally induced depolarization in CaF 2 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.

TGG ceramics based Faraday isolator with external compensation of thermally induced depolarization.

A Faraday isolator with compensation of thermally induced depolarization outside magnetic field was implemented for the first time on TGG ceramics. Stable isolation ratio of 38 dB in steady-state regime at a laser power of 300 W was demonstrated in experiment. Theoretical estimates show a feasibility of a device that would provide an isolation ratio higher than 30 dB up to laser power of 2kW.

Wavelength dependence of Verdet constant of Tb3+:Y2O3 ceramics

Samples of the magneto-active material—Tb3+:Y2O3ceramics with Tb3+ion concentrations of 10%, 20%, 30%, and 100% (Tb2O3)—were prepared and studied. The wavelength dependence of Verdet constant in the 380 nm–1750 nm range was approximated for all investigated ceramic samples and was predicted for a pure Tb2O3material. Tb2O3ceramics demonstrates a more than three times higher Verdet constant in comparison with terbium gallium garnet crystal or ceramics. The linear dependence of the Verdet constant on Tb3+ion concentration in the Tb3+:Y2O3ceramics was demonstrated. The obtained data will be useful for fabricating magneto-optical elements of Faraday devices based on Tb3+:Y2O3 with arbitrary Tb3+ion concentration operating at room temperature in the wavelength range of 380 nm–1750 nm.

Thermo-Optical and Magneto-Optical Characteristics of Terbium Scandium Aluminum Garnet Crystals

Magnetoactive materials are of considerable current interest, primarily for applications in nonreciprocal Faraday devices used for polarization control, optical isolation, optical switching, and modulation. The need for such devices is growing with laser power enhancement. They reduce risk of self-excitation of the amplifiers and optical elements damage and are a handy tool for organizing multipass schemes. However, at high average power of radiation these devices are subject to thermally induced effects that impair their operability and lead to increased losses and to the formation of phase distortions in the transmitted radiation. One of the methods to reduce thermally induced effects is to use in Faraday devices new magnetoactive materials with better thermo-optical properties. This paper is devoted to the study of thermo-optical and magneto-optical characteristics of a unique magnetoactive material-a terbium scandium aluminum garnet (TSAG) crystal. The TSAG has an extraordinary value of optical anisotropy parameter ξ, a Verdet constant 25% higher than the traditionally used terbium gallium garnet crystal and the highest magneto-optical figure-of-merit known in magnetoactive materials at the moment.

Terbium gallium garnet ceramic-based Faraday isolator with compensation of thermally induced depolarization for high-energy pulsed lasers with kilowatt average power

A scalable aperture Faraday isolator for high-energy pulsed lasers with kW-level average power was demonstrated using terbium gallium garnet ceramics with water cooling and compensation of thermally induced depolarization in a magnetic field. An isolation ratio of 35 dB (depolarization ratio γ of 3.4 × 10−4) was experimentally observed at a maximum laser power of 740 W. By using this result, we estimated that this isolator maintains an isolation ratio of 30 dB for laser powers of up to 2.7 kW. Our results provide the solution for achieving optical isolation in high-energy (100 J to kJ) laser systems with a repetition rate greater than 10 Hz.

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.