Visible/Infra-Red Absorption Study in Fiber Geometry of Metastable Defect States in High-Purity Fused Silicas

Optical absorption in the range from about 400 to 1000 nm has been recorded for pure-silica-core optical fibers during exposure to Co-60 gamma rays at about 27 C. The two studied core materials were notable for their low hydroxyl and chloride contents; one had 0.5 mass percent fluorine doping. A complex spectrum comprising at least three Gaussian bands reached steady-state intensities from about 10 to 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 about 20 C but recovered within about one-half hour when irradiation was continued in the dark. After long-term exposure (at least 10 exp 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. (Author)