Fiber Optic Sensing System for Disturbance Location Detection With Extended Measurement Range Based on Dual Sagnac Interferometer

In this article, a simple, low-cost, all-fiber, and full-passive optical sensing setup for disturbance location detection based on a dual Sagnac interferometer and time domain multiplexing (TDM) scheme is further explored in order to extend the measurement range and utilize the entire length of the sensing fiber for disturbance measurement. In addition to measuring the ratio of two signals obtained in dual loop Sagnac configuration, the study conducted in this work shows that by measuring also the phase difference between these signals, the length of the effective measurement range can be doubled. The optimization of the proposed setup in terms of the sensing fiber length is discussed and the setup characterization under different test conditions is performed. The obtained results show that a measurement error of less than 1% of the full-scale measurement range can be achieved. Furthermore, using a simple experiment, the detection of multipoint disturbances occurring simultaneously at different frequencies is observed and discussed.