High-Resolution Two-Degree-of-Freedom Displacement Measurement Based on Fiber Fabry–Perot Interferometer and Pound–Drever–Hall Technique

Measurement of two-degree-of-freedom (2-DoF) displacement, 1-DoF for linear displacement and 1-DoF for angular displacement, is highly demanded in the fields of semiconductor manufacturing and structural health monitoring (SHM). In this article, a 2-DoF displacement sensor based on fiber Bragg grating (FBG)-based fiber Fabry–Perot interferometers (FFPIs) and Pound–Drever–Hall (PDH) technique is proposed and experimentally demonstrated. The 2-DoF displacement sensor is formed by a pair of identical FFPIs fixed on two metal clamps in parallel. Due to the inherently narrow resonant bandwidth of FFPI and high demodulation resolution of PDH technique, the proposed 2-DoF displacement sensor is capable of high-resolution displacement measurement. Measurement resolutions of 0.01 nm for linear displacement and 0.49 nrad for angular displacement are achieved in the experiment. For displacement demodulation, the demodulated values and the preset values are matched well, illustrating the good 2-DoF displacement sensing ability of the proposed 2-DoF displacement sensor.