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Absolute phase measurement with extrinsic Fabry-Perot optical fiber sensors

This paper describes the use of the first and second optical return paths in a moderate to high finesse Fabry-Perot sensor to measure the absolute phase in extrinsic Fabry- Perot interferometric (EFPI) sensors. Path-matched differential interferometry (PMDI) using high finesse EFPI sensors, a low finesse Fabry-Perot read-out interferometer, and a broadband light source consisting of amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) is used to illustrate the idea. The first and second multiple paths in the Fabry-Perot read-out sensor are used to provide two distinct path-match conditions from the same scanning Fabry-Perot read-out interferometer. The difference in fringe numbers between the centers of two orders of interference fringe packets formed by the distinct path-match conditions makes possible a simple method of measuring the cavity length of EFPI sensors, which in turn can be used to measure absolute phase and the corresponding strain. Sensor cavity length measurement using the multiple return paths in the high finesse Fabry-Perot sensor is compared with measurements made using the modulation transfer function found using an optical spectrum analyzer. Then the multiple return path technique is then used to make strain measurements on a cantilever beam. Comparisons with resistance strain gate measurements are favorable. Characterization tests indicate that the proposed technique has a cavity length measurement resolution on the order of 1.1. micrometer, which translates to a strain resolution of 28 (mu) (epsilon) for a 4 cm gage length sensor.