Faraday Michelson interferometers for signal demodulation of fiber-optic sensors

We developed an all-fiber-based optical demodulator for the signal interrogation of low-coherence fiber-optic Fabray-Perot interferometric sensors. The optical demodulator consists of a Michelson interferometer implemented by using a 3x3 fiber coupler and two fiber-coupled Faraday reflectors with tunable fiber delay lines. The demodulator's output contains two optical interference signals with a constant phase shift and the output shows no sensitivity to the polarization variation in the light source or fibers. A digital phase recovery algorithm is used to extract the measurand information from the phase-shifted signals at high accuracy, high dynamic range and high stability. The optical demodulator has been applied to the measurement of highfrequency vibrations and strains using fiber-optic sensors.