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Description:
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A distributed fiber optic intrusion sensor capable of detecting intruders from the pressure of their weight on the earth's surface was investigated in the laboratory and in field tests . The presence of an intruder above or in proximity to the buried sensor induces a phase shift in light propagating along the fiber which allows for the detection and localization of intrusions . Through the use of an ultra -stable erbium -doped fiber laser and phase sensitive optical time domain reflectometry , disturbances were monitored in long (several km ) lengths of optical fiber . Narrow linewidth and low frequency drift in the laser were achieved through a combination of optical feedback and insulation of the laser cavity against environmental effects . The frequency drift of the laser , characterized using an all -fiber Mach Zehnder interferometer , was found to be less than 1 MHz /min , as required for operation of the intrusion detection system . Intrusions were simulated in a laboratory setting using a piezoelectric transducer to produce a controllable optical phase shift at the 2 km point of a 12 km path length . Interrogation of the distributed sensor was accomplished by repetitively gating light pulses from the stable laser into the sensing fiber . By monitoring the Rayleigh backscattered light with a photodetector and comparing traces with and without an induced phase shift , the phase disturbances were detected and located . Once the feasibility of such a sensor was proven in the laboratory , the experimental set up was transferred to Texas A &M's Riverside Campus . At the test site , approximately 40 meters of fiber optic cable were buried in a triangle perimeter and then spliced into the 12 km path length which was housed inside the test facility . Field tests were conducted producing results comparable to those found in the laboratory . Intrusions over this buried fiber were detectable on the φ -OTDR trace and could be localized to the intrusion point . This type of sensor has the potential benefits of heightened sensitivity , covertness , and greatly reduced cost over the conventional seismic , acoustic , infrared , magnetic , and fiber optic sensors for monitoring long (multi -km ) perimeters . |