Dopant, coating, and grating effects in silica optical fibers under extreme neutron irradiation

Radiation effects on fiber Bragg gratings (FBGs) have been studied but data gaps related to the effects of displacement damage resulting from high fast neutron fluence remain. In this work, Type-I and Type-II FBGs inscribed in optical fibers with various core dopants (Ge and F) and fiber coatings (acrylate, polyimide) were monitored in situ during 75 days of neutron irradiation to a peak fast ($>$0.1 $\mathrm{MeV}$) neutron fluence of $3\times 10^{21}$ n${}_{\text{fast}}$/cm${}^2$. The reflected intensity of the Type-I FBGs inscribed in a Ge-doped core fiber decreased by $>30$ dB within 7 h of irradiation ($10^{19}$ n${}_{\text{fast}}$/cm${}^2$), whereas Type-II FBGs inscribed in a pure silica core fiber eventually approached $>$40 dB attenuation after accumulating a fast neutron fluence on the order of $10^{20}$ n${}_{\text{fast}}$/cm${}^2$. Type-II FBGs inscribed in F-doped core fiber improved stability: the attenuation approached an equilibrium value in the range of 10 to 20 dB.