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Processing of Ceramics


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are highly expected.

      H. Kim et al. made a slurry with a planetary mill using YAG, Ho2O3 powder, an organic binder, and ion‐exchanged water as a solvent and extruded the slurry at a pressure of 20–35 MPa using a nozzle with a diameter of 50 μm to form a fiber‐like green body [18, 19]. After drying at room temperature, it was calcined at 600 °C to remove organic components and then sintered in a vacuum furnace at 1700–1800 °C, followed by annealing at 1400–1500 °C to produce a fiber‐like sintered body. Since the surface of the obtained fiber is uneven, the surface is polished. The outer periphery of the polished fiber is coated with glass powder and then heat‐treated for cladding.

Photos depict SEM micrographs of the surfaces of the polycrystalline YAG fibers with different degrees of surface polishing: (a) shows the surface of the as-sintered fiber and (c) is further polished than (b). Surface roughness values of (a) and (c) are given in root mean square (RMS). (d) SEM micrograph of the cross section of the polycrystalline Ho:YAG fiber before surface polishing.

      Source: Kim et al. [19].

Schematic illustration of output power as a function of input power for the HR + Fresnel configuration during power scaling efforts.

      Source: Kim et al. [19].© 2017, The Optical Society.

      2.3.6 Optically Anisotropic Ceramics

      YAG laser ceramics developed by Ikesue in 1995, and all subsequent laser ceramics are a cubic crystal system. When a polycrystalline ceramic having a cubic crystal structure is synthesized, it basically becomes an optically isotropic body, so that it is possible to oscillate a laser with this material if the optical scattering is low. Cubic materials are not always optically isotropic, and many of the synthesized cubic ceramics contain optical anisotropy such as birefringence. Even now, only a few researchers can synthesize high‐quality ceramic laser materials worldwide.

      Akiyama et al. succeeded in synthesizing Nd:FAP and reported a 15% scattering loss (which is five times larger than the loss in this case) in a microchip‐shaped laser element [21]. This means that it is necessary to significantly improve the optical quality of this material to be applied as a laser gain medium.

Schematic illustration of (a) X-ray diffraction pattern of FAP powder as a raw material for Yb:FAP ceramics and Yb:FAP ceramics. Diffraction from Yb:FAP ceramics were from the surface of 3 mm × 3 mm. (b) Transmission and absorption spectra of Yb:FAP ceramics.