A distributed sensor system for detecting and locating intruders based on the phase-sensitive optical-time-domain reflectometer ( \phi-OTDR) is described. The sensing element is a cabled single-mode telecommunications fiber buried along the monitored perimeter. Light pulses from a continuous-wave Er:fiber Fabry-Pérot laser with a narrow (\approx 3 kHz) instantaneous linewidth and low (few kilohertz per second) frequency drift are injected into one end of the fiber, and the backscattered light is monitored with a photodetector. The effect of phase changes resulting from the pressure of the intruder on the ground immediately above the buried fiber are sensed by subtracting a \phi-OTDR trace from an earlier stored trace. In laboratory tests with fiber on reels, the effects of localized phase perturbations induced by a piezoelectric fiber stretcher on \phi-OTDR traces were observed. In field tests, people walking on the ground above a buried fiber cable induced phase shifts of several-\pi radians.
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