Radiation hardening of pure-silica-core optical fibers: Reduction of induced absorption bands associated with self-trapped holes

Visible/near-IR-range radiation hardening of a pair of low-OH/low-Cl silica-core optical fibers has been accomplished by γ-ray preirradiation at 27 C to a dose of 13 MGy(Si) at a dose rate of 1 Gy(Si)/s in the dark. Reirradiation under identical conditions three months later demonstrated a 25-fold decrease in the initially induced intensities of the prominent radiation-induced bands centered near 660 and 760 nm and comparable decreases in weaker bands at longer and shorter wavelengths. Spectral changes observed in the wavelength regime 400–1000 nm upon quenching the irradiated fibers to 77 K have revealed the likely copresence, even at room temperature, of a previously reported “low temperature infrared absorption” which peaks near 1600 nm. Based on this insight, induced losses at 1550 nm have been extrapolated from the present data. It is argued that self-trapped holes are most likely responsible for most of the metastable induced absorption bands in the range $∼400–2000 nm.$