A demodulator circuit for an interferometer type of hydrophone

摘要

The optical signal in a fiber optic interferometer (20) is phase modulated to provide a carrier signal for phase changes caused by changes in a physical parameter such as sound, being sensed. The signal is then demodulated to measure changes in the parameter. A crystal oscillator (52) frequency reference and passive filtering cause the circuit to have low phase noise. Digital phase shifters (96, 98) provide mixer references that have low phase noise and stable gain. The signal processing techniques used are applicable to sensor arrays using a single carrier excitation source, time demultiplexing of individual carrier signals and demodulation of each sensor carrier signal. This demodulator circuit is also applicable to frequency-multiplexed approaches, which utilize direct frequency modulation of the source slightly unbalanced interferometers, and electronic frequency-division multiplexing. The passive homodyne technique allows large linear dynamic range of about 100 dB so that both small and large amplitude signals commonly encountered in acoustic sensing applications may be observed with excellent fidelity. IMAGE