Evaluation of fast-neutron radiation effects in optical fibers for fusion reactor diagnostics

A simple CCD-camera spectrometer was deployed at the Los Alamos Spallation Radiation Effects Facility to characterize fast neutron irradiation effects in several silica-based optical fibers over the wavelength range ~450-1100 nn. The experimental arrangement allowed optical loss spectra to be developed from remotely recovered frame grabs at various times during irradiation without necessary resort to “cut back” methods. Data recorded for a pure-silica-core/F-doped-silica-clad fiber displayed a peculiar artifact which is described and mathematically modelled in terms of leaky modes propagating in an optical cladding which is substantially less susceptible to radiation-induced optical attenuation than is the core. Evidence from optical time-domain reflectometry supports the postulate that mode leakage into the cladding may be due to light scattering from the tracks of ions displaced by the 14-MeV neutrons. These results suggest that fibers with fluorine doping in the core, as well as in the cladding, would be relatively resistant to radiation-induced attenuation in the UV-visible spectral region