Fiber Bragg gratings in the radiation environment: Change under the influence of radiolytic hydrogen Available to Purchase

The change of the transmission spectra of fiber Bragg gratings written in the optical fibers, whose silica cores are doped with either germanium or nitrogen, is studied experimentally under the influence of gamma-radiation. The transmission spectra in the neighborhood of the resonance (Bragg) wavelengths were regularly recorded “in-situ” in the course of irradiation during 24 days. For this purpose, uncoated gratings were placed in a pool near the spent fuel rods of a nuclear reactor. The fibers with the gratings written in them were in immediate contact with water. The estimated total absorbed radiation dose of the fibers is approximately 5 MGy. Molecular hydrogen, which is produced by radiolysis of water and penetrates into the core of silica fiber, is found to interact with the defects of Ge-doped silica induced by gamma-radiation, thereby causing a strong impact on the parameters of the spectrum of the Bragg gratings. On the contrary, in the case of gratings inscribed in N-doped silica fibers, the hydrogen molecules interact with defects induced in the course of laser UV exposure during the grating writing only. The possible subsequent formation of additional defects in N-doped silica under the influence of gamma-radiation has no substantial impact on the transmission spectra of Bragg gratings, which remained stable. The obtained results suggest that a small amount of molecular hydrogen resided in the fiber core is the main source of radiation instability of Ge-doped fiber Bragg grating sensors in radiation environments. These hydrogen molecules can remain in the Bragg gratings, in particular, after the inscription process in the hydrogen-loaded fibers.