Radiation effects on pure silica and Ge-doped silica core optical fibers and fiber Bragg grating sensors

Fiber Bragg gratings (FBGs) are widely used in high-radiation environments owing to their high sensitivity, stability, and resistance to electromagnetic interference. In this study, pure and Ge-doped silica core fibers were fabricated using chemical vapor deposition. Based on these fibers, two temperature sensors, FBG-Si and FBG-Ge, were developed using femtosecond laser direct writing combined with metalized armoring. The fibers and sensors were exposed to gamma radiation, and their stability, temperature accuracy, and refractive index were systematically evaluated. Electron paramagnetic resonance and radiation-induced loss were used to investigate the effects of gamma radiation on the fiber materials and temperature sensors at the atomic micro-scale. The results showed that the Bragg center wavelength (λB) of the FBGs linearly redshifted with increasing temperature under non-stressed conditions. After gamma irradiation, at a temperature, λB, redshifted further with increasing radiation dose. The FBG-Si sensor exhibited higher stability and smaller temperature errors than FBG-Ge. Both sensors exhibited a decrease in output power after irradiation. The performance degradation of the FBGs after irradiation is attributed to an increase in the number of color centers and defects within the grating, leading to higher transmission losses. As the radiation dose increased, the concentration of the color centers increased, leading to changes in the refractive index of the gratings. This ultimately resulted in a redshift in λB and caused temperature measurement errors.