Extended-range pseudo-heterodyne demodulation using electrical phase characteristics of all-pass filters

Abstract

This paper describes two demodulation methods that have been developed for extracting large optical phase changes from pseudo-heterodyne modulated optical-fiber sensor signals. Both demodulator approaches use frequency-dependent electrical phase shifts in an electrical all-pass filter, and differ only in how the electrical phase is extracted from the all-pass filter. One approach uses a summing amplifier and a peak detector, while the other uses a phase meter. Both proved capable of detecting optical phase changes greater than 100 radians, which is far in excess of previous pseudo-heterodyne techniques, but this is achieved at the expense of DC measurement capability. Additionally, the peak detector demodulator and phase meter demodulators had minimum detectable phases of 5*10-4 radians and 1*10-5 radians, respectively, over a 2.0 kHz bandwidth.