A simple fiber optic sensing system based on dual Sagnac interferometer for disturbance location detection

In this paper, a simple sensing system for disturbance location detection is presented. The system utilizes a dual loop Sagnac interferometer, where one of the loops is substantially longer than the other. Both Sagnac loops share a common section of a sensing fiber which is exposed to the sensing field of interest, while a pulse modulated low coherence light source is used for proposed system excitation. Dual loop configuration introduces different propagation delays among pulses launched into the both loops of Sagnac interferometer which allows their temporal separation at the system output. Furthermore, different propagation delays among these split pulses within both Sagnac loops leads also to different phase delays among interfering pulses at the loops output when the sensing fiber is exposed to a time varying disturbance. By measuring the ratio of the pulses amplitudes obtained at system output, the precise location of time varying disturbance can be determined. The entire sensing loop is made of passive optical components which allows for simple remote operation of the proposed system. A very simple signal processing is required to extract the position of the disturbance.