For a special issue on the optical applications of translucent ceramics, we describe the challenge of reckless laser oscillation using polycrystalline ceramics with many grain boundaries. The optical properties (scattering) of ceramics can be controlled by the amount of residual pores and the quality of grain boundaries. Unfortunately the latter has not yet been scientifically discussed. The future of ceramics depends on the clarification of the actual characteristics of materials, and understanding the truth of natural science is critical.

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Optical scattering is generally very significant in polycrystalline ceramics. Basically, two types of scattering occur in ceramics: Mie scattering caused by residual pores and Rayleigh scattering at the grain boundary regions due to dislocations. In this article, we discuss the relationship between the scattering and the microstructure of ceramics and describe the current state and the future of optical ceramics including its theory. Based on the expertise obtained in the development of ceramics lasers, we also synthesized advanced ceramic Faraday rotator materials, TYO and YIG ceramics, whose conventional synthesis is considered impossible. Furthermore, the principle for producing cubic and anisotropic crystals by the sintering method and some synthesis examples is also reported.

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Long persistent phosphors of garnet structures were developed in the Ce3+ doped yttrium aluminum gallate system by Cr3+ or Yb3+ codoping. Both Cr3+ and Yb3+ ions play a role of trapping electrons produced by photo-ionization process from excited Ce3+ ions. By taking advantage of cubic system of garnet, these phosphors can be formed into transparent ceramics, which show higher brightness, resulting in longer duration time than powder forms. In optical transmission spectra, the evidence of Yb2+ formation as a result of the electron trapping was clearly observed during the charging process.

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Scintillators are group of materials applied for radiation detection. Among them, heavy inorganic single crystals are used for detection of gamma rays. In order to avoid doping ion segregation or issues related to high melting point (more than 2,500 °C), some researchers have focused on transparent ceramics. In this paper, we review the transparent ceramics scintillators, especially those prepared by spark plasma sintering method.

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It has been more than 20 years since the 1st Nd:Y3Al5O12 (Nd:YAG) polycrystalline ceramic laser was reported, and nowadays, there are a lot of works for transparent ceramics on scintillators, phosphors, faraday rotators, and optical windows not only for laser gain media. A Faraday rotator is one of the key optics element for the isolation and birefringence compensation of laser amplifiers in high-averagepower laser systems and demands high Verdet constant, size scalability, excellent optical quality, high laser damage threshold, and a high thermal conductivity. We report the optical properties, Faraday effect and Verdet constant of ceramic TGG and TAG. Large diameter ceramic TGG (Tb3Ga5O12) have been used as Faraday devices for high-average laser systems. Advanced ceramic TAG (Tb3Al5O12) is a promising Faraday material for high-average-power lasers system.

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To expand the possibility of laser ceramics, we proposed the magnetic orientation control of microdomains in polycrystalline ceramics made of non-isometric crystals. Here the design of the extensive variables in Gibbs energy by inducing 4 f electrons of the rare-earth trivalent into the controlled microdomains enhanced the motive forces and torques under the applied external magnetic field. The orientation control of the microdomains reduced the birefringent scattering loss in the anisotropic ceramics and it enabled them to be transparent. With this technology we achieved anisotropic laser ceramics for the first time in the world. We expect to increase the optical power extracted per unit volume in the gain media using anisotropic laser ceramics.

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Spark plasma sintering (SPS), which can precisely control such sintering conditions as temperature, heating rate, the extent of uniaxial loading, and the holding time under a vacuum, has great potential for fabricating various types of functional materials. In particular, the high densification rates attained by this technique enable suppressed grain growth during sintering and hence fine-grained microstructures. For laser optics, we show that SPS is also effective for fabricating some laser optics including laser materials with fine-grained microstructure, and sapphire/Nd:YAG ceramic composite materials. Concerning these materials, this paper describes the microstructure, optical characteristics, and lasing properties and discusses the future possibility of SPS for fabricating high-average power laser optics.

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We reported the characteristics of optical waveguides that are directly written by femtosecond laser pulses in solid-state laser materials. Double-track structures, fabricated in solid-state laser materials, work as the clad of optical waveguides. When inducing refractive index changes in a Pr:ZBLAN glass with a high repetition rate ( ›25 kHz) in a femtosecond 1,030-nm laser, the heat accumulation effect plays an important role for determining the clad volume. Longer laser pulses can achieve low-loss, higher numerical aperture (NA) waveguides in a Pr:ZBLAN glass. We fabricated laser waveguides in Pr:ZBLAN and Nd:YLF with a 1-kHz femtosecond Ti:sapphire laser to investigate laser oscillation.

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Speckle reduction by angular, wavelength, and polarization diversities is widely used to suppress speckle in laser projectors. Although several methods achieve these three diversities, few have shown a quantitative comparison among these methods. Thus, in this study, we summarize the speckle reduction effect by the wavelength, angular, and polarization diversities introduced by several different methods. Angular diversity is altered by changing either the angular distribution of the illumination light or the speckle measurement condition. Wavelength diversity is introduced using the continuous spectrum of a Xe lamp with bandpass filters or multiple wavelength lasers. We also discussed the dependency between angular and wavelength diversities. Polarization diversity is introduced with an optical fiber to depolarize the laser light and rotating a half-wave plate to temporally change the polarization state. We also investigated the effect of depolarization on a matte screen.

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