Fringe Rate Demodulator For Fiber Optic Interferometric Sensors

C. M. Crooker; S. L. Garrett

doi:10.1117/12.942528

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23 March 1988 Fringe Rate Demodulator For Fiber Optic Interferometric Sensors

C. M. Crooker, S. L. Garrett

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Proceedings Volume 0838, Fiber Optic and Laser Sensors V; (1988) https://doi.org/10.1117/12.942528
Event: Cambridge Symposium on Fiber Optics and Integrated Optoelectronics, 1987, Cambridge, MA, United States

Abstract

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.

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C. M. Crooker and S. L. Garrett "Fringe Rate Demodulator For Fiber Optic Interferometric Sensors", Proc. SPIE 0838, Fiber Optic and Laser Sensors V, (23 March 1988); https://doi.org/10.1117/12.942528

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