Low noise remote optical fiber sound detector

Abstract

An optical system for frequency-modulation heterodyne detection of an acoustic pressure wave signal. An optical beam is directed into a Bragg cell outside of the fluid medium in which acoustic signals are to be detected. The Bragg cell modulates the incident beam such that two beams of different frequency exit the cell. The two beams are directed into an input optical fiber and the resultant combined beam is transmitted over a desired distance to a fiber optic transducer disposed in the fluid medium. The transducer includes two coiled optical fibers, a reference fiber and a signal fiber, each of which has a different sensitivity to incident acoustic pressure wave signals. The transmitted beam is directed from the input optical fiber through a power divider which splits the beam into two equal parts, one part passing through the reference fiber, the other part passing through the signal fiber. A filter in the signal fiber transmits only a fraction of the light at one of the two frequencies. The two parts of the split beam exiting the coiled optical fibers are coupled into another optical fiber and transmitted to a photodetector from which the output signal is processed to indicate the detection of an acoustic pressure wave signal. In a modification of the system, different polarization states are imparted with a polarizer and a half-wave retardation plate to the two beams of different frequency produced by the Bragg cell. The power divider and filter are replaced by a polarization beam splitter and another half-wave plate.