Fiber optic acoustic hydrophone with double Mach–Zehnder interferometers for optical path length compensation

We report on the development of a fiber optic acoustic hydrophone which consists of a sensing Mach–Zehnder (MZ) interferometer and a compensating MZ interferometer for optical path length compensation. This double-interferometer configuration has the following advantages: the hydrophone is a true heterodyne device; a laser source with a short coherence length can be used; the sensing interferometer is completely passive; the compensating interferometer can be located near the signal processing electronics, far away from the sensing interferometer; a conventional modulation analyzer can be used to demodulate the optical phase shift, which greatly simplifies the demodulation electronics. The performance of the hydrophone is evaluated experimentally by immersing the sensing interferometer in a water tank to detect underwater acoustic signals generated by an acoustic wave projector. Experimental results show that over the frequency range of 5 to 20 kHz, the hydrophone has an almost flat response with an average normalized phase sensitivity of −322.3 dB re 1/μPa and an average pressure sensitivity of −153.7 dB re rad/μPa. These performance figures are better than those obtained from a commercial piezoelectric hydrophone. Furthermore, we have also demonstrated that with improved signal processing techniques, the normalized phase sensitivity of the hydrophone increases to −313 dB re 1/μPa, and the pressure sensitivity increases to −136.9 dB re rad/μPa. These results indicate that the present design offers equal or better performance in terms of sensitivity over its counterparts employing conventional Mach–Zehner configurations.