Radiation hardening of pure-silica-core optical fibers by ultra-high-dose γ-ray pre-irradiation

Induced optical absorption in three SiO2-core/SiO2:F-clad fibers (two high-OH, one low-OH) and one SiO2:Ge-core fiber was investigated in situ in the range 415--1100 nm during repeated 60Co γ-ray exposures to doses >1 MGy(Si). Injected visible light (∼5 μW) was maintained during the first and second irradiations at ∼27 degree C and an intervening period out of the source; an additional irradiation and subsequent out-of-source period were monitored with the light off. Bleaching effects due to radiation alone, light alone, and the synergism of light plus irradiation were independently observed. All visible-range damage in the pure-silica-core fibers in excess of a few tens of dB/km was deduced to arise from activation of precursor states including chloride impurities and process-induced defects. These extrinsic color centers were eventually eradicated and the fibers permanently rad hardened after a cumulative radiation dose of 8.3x106 Gy and a visible light fluence of ∼1.4 mW h. The hardening mechanism is here ascribed to radio- or photostimulated complexing of the extrinsic centers with radiolytically derived interstitial oxygens