Optical absorption of neutron-irradiated silica fibers

Silica-based optical fibers capable of transmitting light in the ultraviolet, visible and infrared regions of the electromagnetic spectrum are gaining widespread use in fusion reactor diagnostic systems. To assess radiation damage in these optical materials, the optical absorption has been measured at room temperature in the interval 250-2000 nm of neutron-irradiated silica fibers containing low or high hydroxyl concentrations. Fiber irradiations were done in either low (1.4 x 1021 n/m2) or high (1.1 x 1023 n/m2) neutron fluence (E>0.1 MeV) regions of the Los Alamos Spallation Radiation Effects Facility. Attenuation in high-fluence irradiated fibers exceeds 104 dB/km for wavelengths less than about 700 nm, presumably due to radiation-induced absorption of peroxy radical, non-bridging oxygen hole and E'-type centers. Even the low fluence exposures induced considerable fiber absorption in this spectral region. High-OH content fibers exhibit significant radiation-induced absorption above 1500 nm, in addition to the intrinsic absorption due to OH stretching modes. (orig.)