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OFDR with double interrogation for dynamic quasi-distributed sensing.

A method for phase sensitive quasi-distributed vibration and acoustical sensing is presented. The method is based on double optical frequency domain reflectometry interrogation of a sensing fiber with an array of discrete weak reflectors. Two replicas of the interrogation signal are launched into the sensing fiber. The time delay between the replicas is equal to the roundtrip time between two consecutive reflectors. Each peak in the spectrum of the returning signal is made from a coherent addition of the reflections of two consecutive reflectors. Its magnitude is highly sensitive to the optical phase in the fiber segment between the reflectors. The system was used to detect and locate the fall of a paperclip from height of 40 cm onto a sandbox where a 15 cm segment of the fiber was buried. In a different experiment the system successfully detected and located minute vibrations at 440 Hz that were induced by touching the fiber with a tuning fork.