Previous treatments of fiber-optic hydrophone sensitivity have neglected the effect of dimensional changes of the fiber in response to acoustic pressure. It is shown here that the changes in length actually contribute more to hydrophone sensitivity than do the refractive index changes; the two contributions are of opposite sign. An experiment verifying the sensitivity calculation was performed using 60-kHz pulses from a spherical source.
Elasto-optic properties of materials can be used for modulating the laser beam by the acoustic pressure. The compressibility and the change of index of refraction with pressure of silicon rubber were measured using the technique of interferometry, and the results were applied to determine the voltage sensitivity and minimum detectable pressure of an optical hydrophone. Both values compare favorably with those of a conventional piezoelectric hydrophone.
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