Measurement of the birefringence variation induced by dihydrogen diffusion into a polarization-maintaining fiber

We report continuous measurements of the transmission spectrum of a fiber loop mirror interferometer composed of a Panda-type polarization-maintaining (PM) optical fiber during the diffusion of dihydrogen (H₂) gas into the fiber. Birefringence variation is measured through the wavelength shift of the interferometer spectrum when the PM fiber is inserted into a gas chamber with H₂ concentration from 1.5 to 3.5 vol.% at 75 bar and 70°C. The measurements correlated with simulation results of H₂ diffusion into the fiber lead to a birefringence variation of -4.25 × 10^-8 per mol m^-3 of H₂ concentration in the fiber, with a birefringence variation as low as -9.9×10^-8 induced by 0.031 μmol m^-1 of H₂ dissolved in the single-mode silica fiber (for 1.5 vol.%). These results highlight a modification of the strain distribution in the PM fiber, induced by H₂ diffusion, leading to a variation of the birefringence that could deteriorate the performances of fiber devices or improve H₂ gas sensors.