Thermally induced depolarization in TGG ceramics

Fabrication and characterization of a gadolinium-doped terbium gallium garnet crystal with an enhanced Verdet constant

Magneto-optical materials based on Tb₃Ga₅O₁₂ (TGG) crystals offer promising opportunities when used in modern Faraday isolators (FIs). Gadolinium (Gd)-doped TGG (Gd:TGG) crystals with high transmittance were prepared using the Czochralski (Cz) method. The as-prepared crystal presented a high optical transmittance of up to 80.0% in the range from 520 nm to 1520 nm. The Verdet constants of the as-grown crystal at 405.0, 533.0, 632.0, and 1064.0 nm were -583.0, -230.1, -150.3, and -50.3 rad m^-1 T^-1, respectively. The measured value at 1064 nm was about 1.3 times stronger than that of TGG. Thus, it could minimize the strength of the magnet of Faraday rotators. These excellent characteristics make Gd:TGG single crystal an attractive magneto-optical material.

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.

Magneto-optic pyrochlore ceramics of Tb2Hf2O7 for Faraday rotator

In the first demonstration of the Faraday effect in an optical-grade Tb₂Hf₂O₇ (THO) ceramic, its optical properties and the Verdet constant's wavelength dependence were evaluated at room temperature. The transmittance loss of this material was equally low as for the terbium gallium garnet, a well-known magneto-optic material. The Verdet constant at 1064 nm was 50.4 rad/Tm, which was 1.4 times greater than that of the terbium gallium garnet. THO ceramics show great potential for use in Faraday devices in near-infrared laser applications.

TSAG-based Faraday isolator with depolarization compensation using a counterrotation scheme

A Faraday isolator (FI) with depolarization compensation using a counterrotation scheme has been realized in experiment for the first time, to the best of our knowledge. It is based on terbium scandium aluminum garnet crystals with negative optical anisotropy parameters. An order of magnitude advantage over the traditional FI scheme is achieved in this case. An isolation ratio of 35.7 dB at a radiation power of 1440 W has been obtained. According to the numerical estimates, an isolation ratio of 30 dB can be provided up to a power of 5.5 kW.

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.

High magneto-optical characteristics of Holmium-doped terbium gallium garnet crystal

Magneto-optical characteristics of a new magneto-active material, (Tb_(1-x)Ho_x)₃Ga₅O₁₂ crystal, have been grown by the Czochralski (Cz) method. A high value of the Verdet constant was obtained at room temperature-namely, 214.9 and 77.8  rad·m^-1 T^-1 for 632.8 and 1064 nm, respectively. The Verdet constant of the Ho-doped terbium gallium garnet crystal at 1064 nm is about 2 times higher than that of terbium gallium garnet crystal. High value of magneto-optical figure-of-merit makes it an attractive next-generation magneto-optics material for high-power Faraday isolators.

Wavelength dependence of magneto-optic properties of terbium gallium garnet ceramics

The wavelength dependence of magneto-optic properties of TGG ceramics, including the Verdet constant, has been investigated experimentally. The previously obtained Verdet constant of 36.4 rad/Tm for 1064 nm wavelength and 139.6 rad/Tm for 633 nm are in good agreement with presented white light measurements . The comparison with previously reported Verdet constant and absorption coefficient values for TGG single crystal has shown very similar results. These results lead to the conclusion that TGG ceramics is a very good alternative to TGG single crystal and is a powerful approach for realizing large-aperture optical isolators which are required in high-average-power laser systems.

Improving characteristics of Faraday isolators based on TAG ceramics by cerium doping

A Faraday isolator (FI) based on a new magneto-optical medium-Ce:TAG ceramics-has been fabricated and studied in experiments. Compensation of thermally induced depolarization in the FI increases the isolation ratio from 31 to 39 dB at the laser power of 300 W. Estimates predict stable operation of the device with kilowatt laser power.

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.

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.

High-power Faraday isolators based on TAG ceramics

The Faraday isolator based on a new magneto-optical medium--TAG (terbium aluminum garnet) ceramics was implemented and investigated experimentally. The magneto-optical element was temperature-stabilized using water cooling. The device provides a stable isolation ratio of 38 dB at 300 W laser power. Estimates show high performance of the device at a kilowatt laser power.

Temperature dependence of thermo-optic effects of single-crystal and ceramic TGG

The temperature dependence of the thermo-optic effects in single crystal and ceramic TGG were evaluated by using the Fizou interferometer method. The temperature dependence of the refractive index and thermal expansion are significantly improved at low temperature for both ceramics and single crystals. Our estimation using a figure of merit indicated that a TGG ceramics cooled to liquid nitrogen temperature can reduce thermal wave-front distortion by a factor of up to 4.7 with respect to that at 300 K, and can reduce thermal birefringence effects by up to a factor of 12 with respect to those at 300 K.