Hoshiteru Nozawa

Cryogenic temperature characteristics of Verdet constant on terbium gallium garnet ceramics

As the first demonstration of Faraday effect in a TGG ceramics, its Verdet constant at 1053 nm is evaluated to be 36.4 rad/Tm at room temperature which is same as that of the single crystal. In addition, the temperature dependence of Verdet constant is obtained experimentally. At liquid helium temperature, it is 87 times greater than that at room temperature.

Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator

The optical properties, Faraday effect and Verdet constant of ceramic terbium gallium garnet (TGG) have been measured at 1064 nm, and were found to be similar to those of single crystal TGG at room temperature. Observed optical characteristics, laser induced bulk-damage threshold and optical scattering properties of ceramic TGG were compared with those of single crystal TGG. Ceramic TGG is a promising Faraday material for high-average-power YAG lasers, Yb fiber lasers and high-peak power glass lasers for inertial fusion energy drivers.

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.

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.

Development of a Magnetic Refrigerator for Hydrogen Liquefaction

For hydrogen infrastructure aiming at a fuel cell society, development of hydrogen liquefaction technology is an urgent necessity. Magnetic refrigeration is a promising method since it can most closely follow the Carnot cycle. This paper reports the design and construction of a magnetic refrigerator being developed and a cooling cycle simulation, which estimate its cooling efficiency and cooling power.

Performance of ceramic magnetic regenerator materials for sub-2 K cryocoolers

A new ceramic magnetic regenerator material GdVO/sub 4/ has been fabricated and tested. The new regenerator material has an antiferromagnetic transition at 2.2 K with a large heat capacity more than 0.8 J/cm/sup 3/K. The cooling tests with a 0.1 W-class GM cryocooler have been done with /sup 3/He gas as a working fluid. The 2nd regenerator consisted of Pb, HoCu/sub 2/, Gd/sub 2/OS, GdAlO/sub 3/ and GdVO/sub 4/. The configuration was adjusted by partially replacing the GdAlO/sub 3/ with the GdVO/sub 4/. A cooling capacity at 1.8 K for the 2nd regenerator was increased by about 10% and the minimum cooling temperature was obtainable at 1.4 K. The cooling test results show that the high heat capacity magnetic materials are useful to provide both the higher cooling capacity and the lower minimum temperature, but more precise adjustments are needed to optimize the configuration of the regenerator.

Temperature dependence of thermal expansion coefficient in ceramic and single crystal TGG

The temperature dependence of thermal expansion coefficient in the ceramic TGG was evaluated by using the Fizou interferometer method. It was corresponding to the of single crystal from 295 K to 86 K in our experiment.

300K-7.8K Temperature Dependence of the Verdet Constant of Terbium Gallium Garnet Ceramic

As the first demonstration of Faraday effect in a TGG ceramics, its Verdet constant at 1053nm was evaluated to be 1453 rad/Tm at 7.8 K which is 40 times greater than that at 300 K.