Acoustic resonance sensing of hydrogen diffusion in standard single-mode optical fibers

This work investigates hydrogen diffusion in standard single-mode optical fiber (SMF) through its effect on the resonant frequencies of Transverse Acoustic Mode Resonances (TAMRs), classified into radial R_(0,<i>m</i>) and torsional-radial TR_(2,<i>m</i>) acoustic modes of angular order 2. These modes are generated via electrostriction using a pulsed laser, which induces Forward Stimulated Brillouin Scattering (FSBS). In our specific arrangement, a continuous-wave laser probes the TR acoustic modes taking advantage of the polarization modulation produced by the acoustic field of these modes. By exposing the fiber to hydrogen pressure of 50 bar, a positive frequency shift in the TAMRs was observed, demonstrating their sensitivity to hydrogen permeation. This response enables direct monitoring of hydrogen diffusion within the fiber by tracking the resonant frequency shifts of the TAMRs, paving the way for the development of more accurate and robust hydrogen sensors.