A White-Light Interferometry for the Measurement of High-Finesse Fiber Optic EFPI Sensors

A white-light interferometry based on the Fourier transform method is proposed to measure the optical path difference (OPD) of the high-finesse fiber optic extrinsic Fabry–Pérot interferometric (EFPI) sensor. The Fourier spectrum of the transmission spectrum signal consists of multiple frequency components because of the multiple-beam interference occurring in the high-finesse EFPI sensor. The high-order frequency component of the Fourier spectrum can be extracted by the Fourier transform method to recover the OPD in order to overcome the spectrum overlapping, which happens when the OPD of the EFPI sensor is short. In the experiment, a high-finesse fiber optic EFPI sensor with the cavity length of 120 m is measured, and the second-order frequency component was extracted to recover the cavity length. The standard deviation of the measurement results was 9.132 nm. The measurement range of the Fourier transform method was effectively extended to the short OPD.