Real-time location of multiple time-varying strain disturbances, acting over a 40-km fiber section, using a novel dual-Sagnac interferometer

Updated results using a novel sensing architecture based on a Sagnac interf erometer are presented and, for the first time, real-time separation and po sitioning of multiple disturbances has been realized. A 40-km long dual-Sag nac sensor was formed by spectral slicing of light from a single, broad-ban d erbium-doped-fiber super-luminescent source and wavelength division multi plexed (WDM) routing around the loop to form an inherently low loss system. Independent active phase biasing of each Sagnac was employed, allowing the use of a single optical detector. The effects of residual optical cross ta lk between the two Sagnacs has been accurately modeled, allowing resulting errors to be corrected. The new system has capability for narrow-band fast Fourier transform (FFT) analysis of detected disturbance signals, and hence their separation in the frequency domain. For audio-frequency excitation, an average positional resolution of 100 m over a 40-km length was achieved with a postdetection signal processing bandwidth of 8 Hz.