Analysis of performance index of fiber-optic liquid-level sensor based on an extrinsic Fabry-Perot interferometer

摘要

A novel configuration of fiber-optic liquid-level sensor based on an extrinsic Fabry-Perot cavity is presented and demonstrated in this paper. The main principle of this sensor is that output intensity will vary linearly in a locally special linear region with liquid-level increasing and cavity length decreasing. The experimental results prove that the actual curve of extrinsic Fabry-Perot cavity is approximate cosine curve, and that the peak values of the fringes slowly decrease with increased cavity length. Consequently, cavity length loss influences on performance indexes of extrinsic Fabry-Perot cavity optical fiber sensors operating in linear region such as measurement range, sensitivity, minimum resolution, and linearity. To obtain high finesse and throughout for reducing the complexity of signal processing system, the loss must be kept as small as possible by selecting an appropriate cavity length in view of different sensing surface and reflectivity. Otherwise, the sensor with too small initial cavity length can output more intensity, perform wider range of measurands, possess higher minimum resolution, and is more sensitive, but the linearity becomes worse and the measurement errors are maybe unavoidable to rise. On the contrary, with bigger original cavity length, the things are opposite. Hence, the original cavity length and the end reflectivity must be selected appropriately to meet with the requirements for performance indexes of the liquid-level sensor in realistic circumstance.