Fiber optic interferometric sensors can be extremely sensitive. A sensor that operates in the 1 to 10K radian regime has been developed at the Naval Postgraduate School. Present demodulators operate over a 10 microradian to .1 radian range. Therefore a different approach to demodulation is required for this sensor. A fringe rate scheme is described here. The output of an optical shaft encoder driven by a pendulum resembles the fringe pattern produced by a fiber optic interferometric sensor in response to a sinusoidal perturbation. The HEDS 6000 shaft encoder used in this project provides both in-phase and quadrature waveforms --both are required for demodulation. This simulates the two waveforms provided by an interferometer with a 3X3 coupler. The well behaved outputs of the shaft encoder were used to test the fringe rate demodulator circuit. The cicuitry that reconstructs the analog signal from the in-phase and quadrature signals must do two things: convert the interferometric fringe rate to a proportional voltage level and determine whether this level should remain positive or be inverted. A frequency-to-voltage converter chip (SK9209) accomplishes the first task. Since the chip is based on a charge pump/RC integrator there are some trade-offs for noise vs. slew-rate. A possible solution is a digital frequency-to-voltage circuit, i.e. a digital counter and a D/A converter. The up/down decision can be made by determining which waveform (in-phase or quadrature) is leading. This is accomplished by a JK flip-flop and edge detector circuitry. The flip-flop output controls a switch which is the converting element in a voltage follower/inverting amplifier. The output of the circit is a waveform which indicates particle velocity.
C. M. Crooker,
S. L. Garrett,
"Fringe Rate Demodulator For Fiber Optic Interferometric Sensors", Proc. SPIE 0838, Fiber Optic and Laser Sensors V, (23 March 1988); doi: 10.1117/12.942528; https://doi.org/10.1117/12.942528