Coherence Multiplexing of Distributed Sensors Based on Pairs of Fiber Bragg Gratings of Low Reflectivity

A coherence multiplexing scheme for distributed sensors based on fiber Bragg grating (FBG) pairs is introduced. Each pair of identical FBGs forms a Fabry-Perot (FP) interferometer (FPI) and induces an additional optical path difference (OPD) that is proportional to the center-to-center interval between the two FBGs (one for sensing and the other for reference). The interference intensity reaches its maximum when the FPI-induced OPD is compensated by scanning one arm of a Michelson interferometer to a certain position. The variation of the measurand induces a mismatch between the central reflection wavelengths of two FBGs and consequently reduces interference intensity. To separate the interferometric signals for demultiplexing, the intervals between the two FBGs are preset to different values for different sensors. In order to improve the multiplexing ability of the system and reduce the crosstalk among the sensors, we use gratings of low reflectivity. Temperature sensing is demonstrated to show the high sensitivity (-1.92%/degC) and low crosstalk of our distributed sensing system.