A two-loop Sagnac interferometer is proposed for fiber-optic distributed sensing. The location of a disturbance can be determined from the two output phase signals of the two Sagnac loops, and the amplitude of the disturbance can be obtained by integration of the phase signal. The polarization-maintaining fiber (PMF) is spliced between two single mode fibers (SMFs) to form the SMF-PMF-SMF (SPS) fiber structure. A prototype. We demonstrated a fiber optic distributed acoustic sensor based on a double Sagnac interferometer, using two wavelengths separated by CWDM modules.
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