Laser-power dependence of absorption changes in Ge-doped SiO2 glass induced by a KrF excimer laser

The generation mechanism of the absorption changes, which cause a photorefractive change through the Kramers-Kronig relation in Ge-doped SiO2 glass, has not been clarified yet. In the present paper, we examined the laser-power dependence of the absorption changes around 5 eV, induced by a KrF excimer laser. The induced absorption around 5 eV is composed of three different components, centering at 4.50, 5.08, and 5.80 eV. The increasing behavior of each absorption component depends strongly on the energy density. The three absorption components reach different saturation levels, depending on the energy density. Furthermore, the absorption induced by a high-power KrF excimer laser is bleached by a laser, the energy density of which is about one-twentieth of the inducing laser. Combining the results of mathematical analysis, it was found that a two-photon process and a one-photon process are, respectively, involved with the induction and the bleach of each absorption. It was also found that the precursor defect, which causes the absorption change, is of an oxygen-deficient type.