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-microm 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 degrees 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/ degrees 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.