Consequences of radiation effects on pure-silica-core optical fibers used for Raman-scattering-based temperature measurements

Two types of pure-silica-core fibers (one low-OH, Al-jacketed, one medium-OH, polyimide jacketed) suitable for use as sensing fibers for Raman-scattering-based temperature measurements in nuclear environments have been subjected to gamma and fission reactor irradiation tests. Spectral attenuation measurements were performed between 500 and 1500 nm with samples kept at room temperature, 80 and 300°C. The Al-jacketed fiber was developed for use under ionizing radiation and showed lower loss compared with the polyimide-jacketed fiber at room temperature under gamma irradiation. Both fiber types showed similar spectra at room temperature with the main part of the loss originating from a band tail extending from the ultraviolet. Thermal bleaching of the radiation-induced defects was found to be effective in both fiber types. At 80°C the loss in both fibers was compounded of a band at 625 nm together with the band tail from the ultraviolet, which now had a strength several times lower compared with room-temperature irradiations. At 300°C, both fibers exhibited similar low-loss spectra, except for the band at 625 nm which reached levels of approximately ∼2000 dB/km at an accumulated dose of 2.8×10⁴ Gy(SiO₂). In light of the experimental spectral findings, selection of suitable Raman-distributed temperature sensors for nuclear plants can be made.