A three-wavelength method is developed to interrogate an extrinsic Fabry-Pérot interferometer (EFPI) for the measurement of a dynamic measurand. Three signals 120 deg out of phase are created by using three fiber Bragg gratings (FBGs). A phase demodulator, which is the same as that used for demodulating a 3×3 coupler-based interferometer, is used to measure the phase change. A minimum detectable phase of 0.43 μrad/Hz1/2 is achieved.
"Passive interrogation of an extrinsic Fabry-Pérot interferometer using a three-wavelength method," Optical Engineering 48(6), 064401 (1 June 2009). https://doi.org/10.1117/1.3156018