A small aperture wideband ultrasonic optical fiber hydrophone is described. The transduction mechanism is based on the detection of acoustically induced changes in the optical thickness of a 25-µm thick Parylene polymer film acting as a low finesse Fabry Perot (FP) interferometer that is deposited directly onto the end of a single mode optical fiber. The acoustic performance compares favorably with that of PVDF needle and membrane hydrophones with a peak noise-equivalent-pressure (without signal averaging) of 10 kPa over a 25-MHz measurement bandwidth, a wideband response to 20 MHz, and a near omnidirectional performance at 10 MHz. The dynamic range was 60~dB with an upper limit of linear detection of 11 MPa and a temporal stability of <5% over a period of 20 h. The hydrophone can also measure temperature changes with a resolution of 0.065°C, offering the prospect of making simultaneous acoustic pressure and temperature measurements. The transduction parameters of the FP sensing element were measured, yielding an ultrasonic acoustic phase sensitivity of 0.075 rad/MPa and a temperature phase sensitivity of 0.077 rad/°C. The ability to achieve high acoustic sensitivity with small element sizes and to repeatably fabricate rugged sensor downleads using polymer deposition techniques suggests that this type of hydrophone can provide a practical alternative to piezoelectric hydrophone technology.
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