Hydrogen diffusion and ortho–para conversion in absorption and Raman spectra of germanosilicate optical fibers hydrogen-loaded at 150–170 MPa

IR absorption and spontaneous Raman scattering spectra of germanosilicate optical fibers loaded with molecular hydrogen at pressures of 150–170 MPa, as well as the change of these spectra during hydrogen out-diffusion from the fibers are investigated. Purely rotational transitions of molecular hydrogen in the Raman spectra of optical fibers are observed for the first time. Changes in spectral characteristics of the fiber Bragg gratings subjected to hydrogen processing are analyzed. It is found that after loading at so high pressures a decrease in hydrogen concentration in the fiber core exhibits two different stages, the initial stage being faster as compared with that for the fibers loaded at 10–15 MPa. To explain this phenomenon, the influence of gradient of internal stress caused by hydrogen dissolved in the glass network as well as the ortho–para hydrogen conversion in germanosilicate glass are considered. An increase in solubility of molecular hydrogen in the glass network subjected to UV irradiation is revealed.