Measurement of elastic properties of optical fibers under hydrogen and deuterium diffusion by forward-stimulated Brillouin scattering

The hydrogen and deuterium diffusion process in standard single-mode silica optical fibers was investigated using forward-stimulated Brillouin scattering. The method relies on the sensitivity of the resonance frequency of the transverse acoustic modes of the fiber to changes in the fiber's elastic properties. We developed a pump-and-probe experiment to stimulate and interrogate a large number of transverse acoustic modes in a short section of fiber undergoing gas loading at high pressure, followed by outdiffusion. The shift in resonance frequencies was monitored throughout the process. Thus, the temporal dynamics of gas diffusion were investigated. The changes in the two acoustic velocities -shear and longitudinal- and in Poisson's ratio resulting from gas diffusion were obtained. For an estimated hydrogen concentration of 0.57 mol.%, a change of 0.96 m/s and 2.46 m/s was measured in the shear and longitudinal acoustic velocities, respectively, and a change in the Poisson's ratio of 1.6 × 10^-4.