A variable-loop Sagnac interferometer for distributed impact sensing

A variable-loop fiber optic Sagnac interferometer is designed to locate a time-varying disturbance. Phase perturbation on a Sagnac loop produces a nonreciprocal phase signal which contains information of the perturbation location. Both location and amplitude of the perturbation can be determined by combining two phase signals from the variable-loop Sagnac interferometer. Spatial resolution better than 1 m has been demonstrated in a 180 m prototype system.