An ultrasonic fiber-optic hydrophone incorporating a push-pull transducer in a Sagnac interferometer

A theoretical and experimental analysis of a push-pull acoustic transducer incorporated in a Sagnac interferometer is presented. The transducer is optimized for detection of ultrasonic signals in the frequency range 0.4-1.5 MHz. Two different configurations are investigated. In one, the sensing fiber forming the transducer constitutes the total loop length, and in the second an additional delay coil is included in the middle section of the Sagnac loop. In the latter we demonstrate experimentally noise-equivalent pressures of 36-43 dB re. 1 μPa/Hz^1/2 in the frequency range 0.4-1.0 MHz. An approximate theoretical model is presented, and we obtain reasonable agreement between theory and experiment for both the frequency-dependent noise equivalent pressure and the directional responsivity of this push-pull acoustic fiber-optic sensor