Distributed fiber-optic intrusion sensor system

A distributed sensor system for detecting and locating intruders based on the phase-sensitive optical-time-domain reflectometer (φ-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-Pe´rot laser with a narrow (≈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 φ-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 φ-OTDR traces were observed. In field tests, people walking on the ground above a buried fiber cable induced phase shifts of several-π radians.