Enhanced backscatter and unsaturated blue wavelength shifts in F-doped fused silica optical fibers exposed to extreme neutron radiation damage

Abstract

Amorphous fused silica (a-SiO2) optical fibers, with and without inscribed Bragg gratings, were interrogated using infrared (∼1550 nm) optical backscatter reflectometry during ∼25 days of intense neutron irradiation to a fast (energy > 0.1 MeV) neutron fluence of ∼1021 n/cm2, or ∼1.5 atomic displacements per atom. The reflected light amplitudes in Ge-doped core telecommunications fiber dropped below detection limits (>15 dB attenuation) within 3 days of irradiation (∼1020 fast n/cm2). Amplitudes from a pure silica core, F-doped silica cladding fiber reached equilibrium levels ∼1.5–2 dB higher than pre-irradiation values, whereas Bragg gratings inscribed in the same fiber using a femtosecond laser (point-by-point method) suffered > 45 dB attenuation. Blue wavelength shifts were initially consistent with previous radiation-induced compaction models but increased linearly with increasing neutron fluence (no evidence of saturation) beyond 1020 n/cm2 and exceeded 0.6%, which corresponds to >1,000 °C drift if used to measure temperature changes.