A symmetric analogue demodulator for optical fiber interferometric sensors

Describes a form of passive homodyne demodulation for recovering a signal of interest from a Mach-Zender optical fiber interferometric sensor. The interferometer uses a 2×2 optical fiber coupler at the input, a push-pull arrangement to stretch the fibers comprising the two legs of the interferometer in a different manner, and a 3×3 optical fiber coupler at the output, generating three phase-modulated outputs. The demodulator uses all three outputs in a symmetric manner to reconstruct the signal of interest. A dynamic range of 115 dB in a 1-Hz bandwidth has been measured for a circuit implementing this scheme. The minimum detectable signal at a frequency of 600 Hz is 220 μrad/√Hz. The maximum acceptable signal at 600 Hz is 140 rad, increasing linearly with decreasing frequency. A schematic diagram of an implementation of the symmetric demodulation technique in analog electronics is presented