Visible/infra-red absorption study in fiber geometry of metastable defect states in high-purity fused silicas

Optical absorption in the range similar to 400-1000 nm has been recorded for pure-silica-core optical fibers during exposure to Co-60 gamma rays at similar to 27C. The two studied core materials were notable for their low hydroxyl and chloride contents; one had 0.5 mass % fluorine doping. A complex spectrum comprising at least three Gaussian bands reached steady-state intensities similar to 10-30 dB/m near 1.8 eV during the early stages of irradiation. This spectrum could be suppressed by either in-situ white-light bleaching or ex-situ thermal annealing at similar to 20C but recovered within similar to 1/2 hour when irradiation was continued in the dark. After long-term exposure (greater than or equal to 10(6) s) at dose rates of 15, 61, and 321 Gy(Si)/min, the sites responsible for these bands were subject to irreversible radiation bleaching according to dose-rate-independent second-order kinetics. Among the models considered for these (apparently fundamental) defects are quenched-in strained-bond sites which serve as shallow traps for holes.