Phase-modulated fiber optic sensors have been shown to possess high sensitivities for the measurement of strain, temperature, vibration, pressure and other parameters.1 Fabry-Perot (FP) sensors that are based on multiple beam interference eliminate the need for a reference arm and do not require sophisticated stabilization techniques as in the case of Mach-Zehnder and Michelson interferometers.2 Several techniques to create intrinsic optical fiber Fabry-Perot interferometers have been described in the past.3-5 In a recent paper, we described an optical fiber extrinsic FP interferometer and used it as a sensor of microdisplacements and thermally- induced strain.6 The fiber interferometer was classified as extrinsic because the FP cavity was an air-gap between two fiber ends and the sensor output was immune to perturbations in the input/output fiber.
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