Distributed optical fiber acoustic sensing method based on dual-chirped pulses and cross-correlation analysis

A fiber-optic distributed acoustic sensing method based on phase-sensitive optical time domain reflectometry combined with dual-chirped pulse and micro-reflective fiber Bragg grating (FBG) is proposed. The sensing fiber consists of a micro-reflective FBG with uniform spatial interval. The micro-reflective FBG help to gain a high signalto-noise ratio light signal, comparing with the Rayleigh backscattering (RBS) light of optical fiber itself. The dualchirped pulses are generated by a time delay, whose corresponding spatial interval approximately equal to twice the spatial interval of adjacent micro-reflective FBG. A beating signal is generated due to the interference of the two identical chirped pulses reflected by the micro-reflective FBG array. Acoustic disturbance between the microreflective fiber gratings will change the phase of the beating signal and the interference waveform will shift. Quantitative measurement can be achieved by directly demodulating the beating signal through using a crosscorrelation algorithm. By using such a method to perform the sensing for the micro-reflective FBG array, distributed quantitative measurement can be realized with only direct detection scheme and simple demodulation algorithm. Experiment are carried out with 2km fiber and PZT vibration simulation and the results verified the effectiveness of our method.