An all fiber interferometric gradient hydrophone with optical path length compensation

Summary form only given. An all fiber optic interferometric gradient hydrophone has been developed to determine the direction of underwater acoustic signal. It consists of a sensing and a compensating Mach-Zehnder (MZ) interferometer cascaded in series. A fiber coupled Acousto-Optic Modulator (AOM) with a center frequency of /spl omega//sub c/=27 MHz is used to shift the optical frequency of the compensating interferometer from /spl omega//sub 0/ to /spl omega//sub 1/. An extra length of fiber (L/sub 0/) is added to the other arm of the compensating interferometer to create significant optical path length mismatch. Two identical fiber coils are used in each arm of the sensing interferometer to form the sensing portion of the gradient hydrophone. An isolated fiber with the same length L/sub 0/ is added in one arm to compensate for the mismatch from the first interferometer. We find that the /spl omega//sub 0/ light wave component in the upper arm, and the /spl omega//sub 1/ component from the lower arm, both of the second interferometer have propagated the same optical path length in the system. When they meet at coupler C/sub 4/, interference occurs and an optical heterodyne signal with a frequency of /spl omega//sub c/ can be received by the photodetector. A modulation analyzer is used to demodulate the phase (gradient) information from the sensor head.