Fiber-optic distributed disturbance sensor based on merged Sagnac interferometers

The fiber-optic distributed disturbance sensor(FDDS) based on merged Sagnac interferometers is proposed and investigated. The disturbance causes a phase modulation and can be detected by the Sagnac interferometer. The proposed sensor includes three Sagnac interferometers merged sharing one broad-band source(BBS). The interference signal is acquired by two photo detectors(PDs) respectively. Since the coherent length of the source is too short for the lightwave propagating along different optical paths to interfere, there are only three paths of the light can interfere. One of the PDs detects the summation of the signals from two Sagnac interferometers which have the same loop length. The other PD measures the interference lightwave from the interferometer comprise of the Faraday rotator mirror (FRM) and the sensing fibers. The disturbance can be located by conducting mathematical operations to the detected signals. The experimental results show that the proposed sensor can locate the disturbance. The maximum location error is 370 m and the average location error is 270 m during 10 times experiments.