Pressure induced changes in optical pathlength are being utilized to realize a hydrophone consisting of glass fibers in a Mach-Zehnder interferometric arrangement. In our experimental setup we use single-mode fiber optical power dividers and a simple control system that maintains the polarization of both light signals so that interference with constant contrast is obtained after superposition. We use a modified phase-tracking detection system to maintain maximum interferometer sensitivity and show a design for a sensor-body with high acoustical sensitivity.
E. O. Schulz-DuBois,
"Feasibility Of Fiber Optical Hydrophone", Proc. SPIE 0369, Max Born Centenary Conf, (16 June 1983); doi: 10.1117/12.934401; https://doi.org/10.1117/12.934401