γ and fission-reactor radiation effects on the visible-range transparency of aluminum-jacketed, all-silica optical fibers

Four aluminum-jacketed, fluorine-doped silica clad optical fibers with silica core materials fabricated by differing technologies were subjected to sequential 60Co-γ ray and fission-reactor irradiations (at ∼20 and 40 °C, respectively), an intervening isothermal anneal (∼20 °C), and a final isochronal anneal (to 600 °C) while monitoring the radiation-induced absorption spectra in the range ∼400–1000 nm. The two low-OH/low-chloride core fibers (one of which was doped with 0.5 mass % fluorine) both developed bands at 660 and 760 nm which exceeded 10 000 dB/km for doses in the range ∼102–106 Gy(Si); however, these bands declined to <1000 dB/km by the end of the γ-irradiation phase [12 MGy(Si) at 5.6 Gy(Si)/s]. All fibers displayed an ‘‘UV band tail,’’ which was the strongest in the high-chloride core fiber, as well as bands in the range ∼600–630 nm generally attributed to nonbridging-oxygen hole centers (NBOHCs). During the γ-irradiation phase the strengths of the NBOHC bands proved to be strongly dependent on the method of core material manufacture. Contrary to previous results for acrylate-jacketed fibers, no substantial bleaching of the UV-tail or NBOHC bands took place during γ irradiation despite the continuous propagation of white light powers ∼5–50 μW. The incremental induced absorption spectra consequent to the reactor-irradiation [∼4 MGy(Si) γ-ray dose at 70 Gy(Si)/s, plus a fluence of ≳2.8-MeV neutrons ∼2×1016 cm−2] were much less sensitive to fiber core material. The prospects for developing rad hard optical fibers for fusion reactor diagnostics are discussed in light of these findings.