Fiber Fizeau interferometer for remote passive sensing

Fizeau sensors constitute a large proportion of the fiber optic interferometric type sensors in use today. These include EFPI, FFPI, certain MEMS devices and in-line fiber intrinsic dual-reflector type sensors. The vast majority of the published literature covering these sensor types models them with a "2-beam" interferometer approximation, and implement interrogation approaches considering the same. Analysis performed and results presented show that the 2-beam model is not sufficient when reflection coefficients exceed 1% and traditional quadrature interrogation can result in linearity or distortion errors roughly in directly proportion to the reflectivity coefficients of the Fizeau sensor. A 4-beam multi-path interferometer model is developed and exercised to demonstrate this problem. Further this model shows that the "errors" in comparison to an ideal 2-beam interferometer model are symmetric across the unit circle and suggests that linear interrogation may be accomplished if orthonormal sample sets over the entire unit circle are used to replace the traditional (simple) quadrature sampling. This is shown to be true in both modeling and lab evaluations. The resulting approach has capabilities of remote, passive sensor operation, high frequency response, large, linear dynamic range and low noise. The interrogation technique demonstrated involves a phase generated carrier with full fringe sampling and quadrature determination which cancels the errors experienced from simple quadrature determination. Such an improvement enables higher reflectivity, higher SNR, high-fidelity fiber Fizeau sensor designs. Applications include embedded sensors, line sensors, or mechanically adapted for acoustic, pressure, vibration, acceleration or seismic sensing.