Effects of Oxygen Pressure on Optical Absorption of YAG

The optical absorption of YAG crystals annealed under oxygen partial pressures of between 10-13.8 and 1 atm was measured. The observed dependence of the optical absorption on the oxygen partial pressure during annealing supports a previous model proposed by Mori, in which excitation-induced transient absorption and 310 nm absorption are ascribed to an F+ center and an Fe2+ impurity, respectively. It was observed that the excitation-induced transient absorption was proportional to the 310 nm absorption. This proportionality suggests that the Fe impurity ions are located near the oxygen vacancies. The excitation-induced stable absorption at 600 nm, which appears to cause the absorption loss during cw laser operation, can be minimized by annealing under an oxygen partial pressure of 10-7 atm at 1400°C.