Transparent polycrystalline YAG with nearly the same optical characteristics as those of a single crystal were fabricated by a solid‐state reaction method using high‐purity powders (>99.99 wt% purity). The average grain size and relative density of the 1.1 at.% Nd:YAG ceramics obtained were about 50 μm and 99.98%, respectively. An oscillation experiment was performed on a cw laser by the diode laser excitation system using the fabricated ceramics. The experimental results indicated an oscillation threshold and a slope efficiency of 309 mW and 28%, respectively. These values were equivalent or superior to those of the 0.9 at.% Nd:YAG single crystal fabricated by the Czochralski method.

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▪ Abstract  Yttrium aluminum garnet (YAG) (Y3Al5O12) single crystals doped with active species such as Nd and Yb have been used as laser media in solid-state lasers requiring high energy and excellent beam quality. This is because single crystals have extremely high thermal mechanical properties and optical qualities and because they enable high-efficiency laser oscillation. In 1995 the authors, using polycrystalline Nd:YAG, demonstrated a high-efficiency laser that was comparable to a single-crystal laser. Subsequently, single-longitudinal-mode oscillation, green and blue laser oscillation, and ultrashort-pulse laser oscillation were reported. Using the ceramic powder approach, the authors developed a composite laser element with a complicated structure that could not be produced by crystal growth techniques. This review discusses problems of conventional single-crystal growth, the fabrication and characteristics of ceramic lasers, laser oscillation properties (continuous-wave and pulse operation), light-scattering sources in ceramics, and typical applications of ceramic lasers.

Polycrystalline, transparent YAG (Y3AI5O12) ceramics were fabricated by a solid‐state reaction method using high‐purity Al2O3 and Y2O3 powders. The mixed powder compacts were sintered at 1600° to 1850°C for 5 h under vacuum. Optical transmittance in the region between the ultraviolet and infrared wavelengths for YAG ceramics (1 mm thick) sintered at 1800°C was similar to that for a YAG single crystal.

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For the present study, 1.1‐at.%‐Nd‐doped YAG ceramics with controlled amounts of grain‐boundary phase were fabricated by a solid‐state reaction method using high‐purity powders. The optical scattering loss of the Nd:YAG ceramics, obtained from Fresnels equation, increased simply with increased amounts of grain‐boundary phase. The continuouswave laser output power of the Nd:YAG ceramics clearly was related to the scattering loss coefficients of the specimens that, in turn, were affected by the amount of grain‐boundary phase. Although the scattering loss coefficients of Nd:YAG ceramics with grain‐boundary‐free structure and a lower pore volume (}150 vol ppm) were almost equivalent to those of a 0.9‐at.%‐Nd‐doped YAG single crystal grown by the Czochralski method, the laser output power of the Nd:YAG ceramics exceeded that of the Nd:YAG single crystal with increased exciting power under excitation with an 808 nm diode laser because of the large amount of neodymium additives. Lasing performance was not affected by the existence of grain boundaries in the polycrystalline specimen.