Self-Calibrated Phase-Shifted Demodulation of Dual-Cavity Fabry–Perot Dynamic Displacement Sensors

In this work, we propose a novel large range dynamic phase extraction method for fiber optic Fabry–Perot (F-P) displacement sensors. A programmable modulated grating Y-branch laser is employed as the light source to achieve flexible switching between full-spectrum scanning mode and fixed-frequency mode. The cavity length difference of the dual cavity sensor, the dc component, as well as the fringe visibility for phase recovery, is directly measured by white light interferometry. Unlike acoustic vibration measurements (where cavity length changes are within a few microns), larger cavity length variations will result in significant changes in the intensity of reflected light and the dc and ac components of interference fringes. We propose to calibrate the key parameters for phase extraction in real time by tracking the demodulated phase results. Calibration operation will be triggered when the detected phase change exceeds 4π. Thanks to the self-calibration mechanism, it is possible to achieve dynamic displacement monitoring of over 1000 µm, as well as displacement monitoring accompanying vibrations. In addition to acoustic sensing and microvibration sensing applications, the proposed method further expands the dynamic range of phase-shifted demodulation scheme and provides a new solution for all-fiber dynamic displacement measurement in confined spaces.