Pressure gradient, pressure, and accelerometer hydrophones have been constructed and tested. These hydrophones were based on single mode fiber optic directional couplers made by drawing and fusing. All couplers were fabricated out of commercial fiber optic cable using an electric furnace. The couplers were potted in several kinds of silicone rubber in a number of different sizes and configurations. Acoustically produced stress occurring near the coupler fusion joint causes the output of the coupler to change. These optical power level changes were remotely monitored by photodiodes. Gradient hydrophones, constructed by the described techniques, were shown to exhibit very high sensitivities, large linear dynamic range, and deep directional nulls with no observable hydrostatic pressure response. Pressure type hydrophones have a fairly flat response down to 10 Hertz with sensitivities better than -160 db rel 1V perp. Pa. By mass loading the silicone rubber membranes which pot the couplers, accelerometer-type hydrophones can be physically very small in size and have very low frequency resonances. This paper discusses the very inexpensive methods used to construct variable coupler hydrophones. A noise analysis shows that the sensor's projected self-noise is below sea state zero noise.
D. W. Gerdt,
N. T. Richardson,
"Fiber Optic Hydrophones Based On Variable Ratio Couplers", Proc. SPIE 1169, Fiber Optic and Laser Sensors VII, (13 February 1990); doi: 10.1117/12.963039; https://doi.org/10.1117/12.963039