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.
© 2011 OSA
High average power lasers are widely used in various industrial and scientific applications including laser processing , extreme ultra-violet generation , inertial fusion energy (IFE) . With so many applications for such lasers, increased laser power demands such as large mode area photonic crystal fiber (LMA-PCF) Yb fiber lasers [4–6], Nd:YAG zigzag slab lasers  and Yb thin disk lasers  are necessary. The Faraday element is a key optical element for the isolation of laser amplifiers and birefringence compensation in two-pass high-power laser systems. Minimum requirements for a Faraday rotation element are a high Verdet constant, size scalability, excellent optical quality, high laser-induced-damage threshold and a high thermal strength. Tb-doped phosphate and silicate glass  have often been used in large diameter glass laser systems, due to superior size scalability. However, this amorphous glass material cannot be used in high average power lasers because of its low thermal conductivity.
Single crystal TGG (terbium gallium garnet, Tb3Ga5O12) grown by the Czochralski method has frequently been used as a Faraday element for high-average pulse YAG laser systems, because of its excellent optical quality and high thermal stability. Unfortunately, single crystals take significant time to grow, and producing large crystals remains difficult. The development of ceramic TGG is an important solution for Faraday elements of high average power lasers and IFE drivers. Ceramic crystals and crystal materials have been developed using established laser material technology. For ceramic crystals, high average power operation is possible due to the single crystal structure and high thermal conductivity, and significant high average output data has been reported. Ceramic TGG was first reported in 2003 , and theoretical analyses have since been performed . Faraday effects are not observed for ceramic TGG because of their inferior optical quality. High quality ceramic crystals then became obtainable from the advancement in ceramic crystal processing technology [11–13]. Yasuhara et al. first observed a Faraday effect for ceramic TGG in a liquid nitrogen atmosphere, and reported its Verdet constant . Ceramic TGG is attractive for practical applications because of its potential for mass production and low fabrication cost and time. Mechanical properties of such ceramics are generally better than those of single crystals because polycrystalline ceramics are aggregates of crystalline grains. External forces are better dissipated in ceramics because of the random grain orientation. The fracture toughness of ceramic TGG is therefore greater than that of single crystal TGG. The thermal fracture limit of ceramic TGG has also been shown to be greater than that of single crystals because of the internal stress distributed by the crystal grain boundaries .
In this letter, we report the optical properties (laser induced bulk damage threshold and optical scattering properties), Faraday effect and Verdet constant of ceramic TGG at room temperature.
2. Experimental and results
2.1 Optical properties
Transparent polycrystalline TGG was fabricated by slip casting and the vacuum sintering method. A scanning electron microscopy (SEM) image showing the microstructure of a typical polycrystalline TGG sample is shown in Fig. 1 . A uniform grain size distribution ranging from 0.3 to 3 microns was observed, and crystal grain boundaries were observed upon thermal etching. The TGG grain size was smaller than that of YAG ceramics (~3-5 μm). TGG ceramic grains were randomly oriented, so thermal fracture power was eased because of the internal stress distributed by the crystal grain boundaries. Wave-front distortion and transmission spectra were measured to evaluate the optical quality of the TGG sample. Optical distortion was measured at 632 nm using an interferometer (Zygo, Ltd., USA).
Figure 2 shows the wave-front distortion of non-doped YAG and TGG ceramics (Konoshima Chemical Co., Ltd., 3 mm length and 10 × 10 mm cross-section). The wave-front distortion of the YAG and TGG ceramic samples was λ/10 and λ/12, respectively, and the optical homogeneity of the two samples was similar. Transmission spectra of 10 mm thick single crystal and ceramic TGG samples were measured with a spectrophotometer (U4100, Hitachi Ltd., Japan), and are shown in Fig. 3 . These spectra were almost equivalent above 600 nm in wavelength. The TGG absorption band (7F6 –5D4) was observed at 480 nm. Figure 3 demonstrates the transparency of the TGG ceramic at 1 μm, thus the material is applicable in Nd:YAG and Yb fiber laser systems.
Light scattering of the single crystal and ceramic YGG samples were measured using an integrating sphere, and the measured wavelengths of incident light were 632 and 1064 nm, respectively. Sample dimensions were 20 mm in length and 5 × 5 mm cross-section, and incident and exit surfaces of the samples were polished under identical conditions. Twice reflecting fused silica glass surfaces were used as the reference. The reflectivity of the two fused-silica glass surfaces at 632 and 1064 nm were 1.2 × 10−3 and 1.1 × 10−3, respectively. Table 1 shows the experimental results of optical scattering for TGG ceramic and single crystal samples at these two wavelengths. At 632 nm, the optical scattering of the TGG ceramic sample (2 × 10−3 /cm) was twice that of the TGG single crystal sample. The optical scattering of the TGG ceramic (5 × 10−4 /cm) was similar to the TGG single crystal at 1064 nm wavelength. For the short-wavelength laser, an increase in optical scattering was observed with increasing grain diameter. Grain diameter had little effect at 1064 nm wavelength, and similar optical scattering values were apparent for the ceramic and single crystal samples. The TGG ceramic could possibly be used in Nd:YAG and Yb fiber lasers in the near-infrared region, or in Er lasers for communication purposes.
2.2 Faraday effects at 1064 nm wavelength
A schematic of the experimental setup for investigating the Faraday effects of ceramic TGG is shown in Fig. 4 . A continuous wave (CW) laser passed through the sample, which was located between a pair of Glan laser prisms acting as analyzer and polarizer. The transmitted laser power was measured using a power meter. A polarization plane of laser light was rotated by the Faraday effect because of the magnetic field. A LD pumped Nd:YAG CW laser (IRCL-100-1064, Crystal laser) was used as the incident optical source. The laser wavelength was 1064 nm and the maximum output power was 200 mW. The extinction ratio of the prism was about 40 dB. Ceramic TGG (Konoshima Chemical Co., Ltd., 20 mm length, 5 × 5 mm cross-section) and single crystal TGG (Electro-Optics Technology, Inc.) with <111> orientation (20 mm length, 8 mm diameter) were used as the sample Faraday materials. Each sample was clamped in a copper holder, and commercial Faraday rotator magnetic housing was used. In excess of 10000 Gauss Nd-ion boron permanent magnets were used to generate high axially oriented fields in the magnet housing (Electro-Optics Technology, Inc., Model 8R1064).
Faraday effects of the single crystal and ceramic TGG samples were measured, and Fig. 5 shows transmitted laser power as a function of polarizer rotation angle. The rotation angles shown in Fig. 5 are very similar. The Verdet constant of the TGG ceramic was 36-40 radT−1m−1 at room temperature, and was almost the same as that of the single crystal. The maximum extinction ratio in excess of 35 dB for the TGG ceramic was similar to that of the commercial Faraday rotator using the TGG single crystal. For the TGG ceramic, a Verdet constant equal to that of the single crystal with excellent optical quality and high extinction ratio was obtained.
2.3 Laser induced damage threshold
The bulk laser-induced damage threshold was measured using a Q-switched Nd: YAG laser at the fundamental of 1064 nm, in transverse and longitudinal single-mode. The pulse width of the fundamental wave was 4 ns for the short-pulse. To avoid surface damage of the crystals, the 8 mm diameter laser beam was focused to a 5 mm point inside the crystal surface, using a 50 mm focal length lens. The spot size was measured to be approximately 70 μm in diameter at 1/e2 in intensity in air. Damage occurrence was determined from the observation of a white plasma spark, and a scattered He-Ne laser was used to confirm the small damage spot. The focal point was moved N on 1 shot. Fused silica glass was used as the reference. The measured damage threshold of the fused silica glass for a 4 ns pulse was 53 ± 5 J/cm2. The incident laser energy was adjusted using both the half-wave plate and polarizer. The laser energy and pulse waveform were always monitored using a biplanar phototube, which was calibrated to ± 5% accuracy using a calorimeter. Table 2 shows the experimental results of bulk damage threshold for the TGG and YAG ceramics. The measured damage thresholds for the non-doped YAG ceramic and Nd-doped YAG ceramic were 25 ± 3 and 18 ± 2 J/cm2, respectively. These values are half that of fused silica glass because of established processing technology. However, the measured damage threshold from 2.3 to 5.1 J/cm2 for the TGG ceramic was markedly lower than that of the YAG ceramic. A damage threshold of 5 J/cm2 for a TGG single crystal in a commercial Faraday rotator has been reported , and the value depends on the materials properties such as crystal structure, impurities and material defects. The optical damage threshold of the TGG ceramic was similar to the TGG single crystal at 1064 nm wavelength. The laser damage threshold of ceramic materials depends on the scattering defect density. Kamimura et al. reported that the damage threshold of YAG ceramics increased with the decreasing scattering density due to structural defects at grain boundaries , which indicated that a reduction in defect density could enhance the high resistance of TGG ceramics. The damage threshold could be enhanced up to the YAG ceramic level, by improving the processing technology.
Figure 6 shows damage cracks in TGG ceramic and single crystal samples. The single crystal sample showed a pattern of two cracks spreading. It is conceivable that the single crystal was easily distorted in a line perpendicular to the mechanical fragility, which resulted in a weak shearing stress of the crystal structure [18,19]. A regular crack pattern was not observed. Laser damage to the low quality TGG ceramic occurred easily because of the high-density of scattering defects.
Laser induced bulk-damage thresholds and optical scattering properties were measured for ceramic TGG, and values were comparable with those for single crystal TGG. Excellent optical homogeneity was observed using interference techniques. The measured optical scattering at 1 μm wavelength for the ceramic and single crystal TGG samples was similar. The laser induced damage threshold of ceramic TGG was approximately 1/3 to 1/4 of that for ceramic Nd:YAG, which could be enhanced by establishing further processing technology. The Faraday effect and Verdet constant of the TGG ceramic at 1064 nm were similar to those of the single crystal at room temperature. The extinction ratio of the Faraday effect was in excess of 35 dB. These results show that ceramic TGG is a promising Faraday material for high-average-power YAG lasers, Yb fiber lasers and high-peak power glass lasers for IFE drivers.
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