Rayleigh scatterings in fibers result from the density fluctuation associated refractive index fluctuation. When the fiber is subjected to acoustic wave, the local refractive index will be modulated as function of time, which can be detected in the form of time domain or frequency domain reflectometry in the fiber. Based on this principle, distributed acoustics sensor can be realized for frequency range of 1Hz-MHz, and the spatial resolution varies from cm to meters. The sensing range requirement varies from meters to kilometres for structural monitoring of pipelines to aerospace, aircraft and etc., especially for internal damages detection to identify early failure and to prevent disaster event.
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