Fiber optic intrusion sensor investigation

Abstract

A novel type of fiber optic intrusion sensor has been proposed and investigated theoretically and experimentally. The sensor is configured as a coherent optical time domain reflectometer (OTDR) with a highly coherent laser as the light source. An external modulator gates repetitive optical pulses into the fiber. This type of sensor can provide long range coverage and distributed capability with high sensitivity utilizing the coherent interference of Rayleigh backscattering or repetitive low-reflectance mirrors in a conventional single mode optical fiber. The performance of the proposed fiber optic intrusion sensor has been analyzed based upon a statistical model for the backscattering and reflection process. Cases with and without polarization discrimination in the optical receiver are considered. The model is used to predict the probability of false alarm and the probability of miss of an intruder, as a function of achievable operating range and range resolution. The maximum operating ranges of 27.5 km and 23.5 km are predicted for 10$sp{-3}$ and 10$sp{-4}$ missed intruder rates, respectively, with averaging over 10 laser pulses. The allowable frequency drift of the laser has been determined for each configuration. The mirror type sensors require small frequency drift, in which far less than 1 kHz of frequency drift over a measurement interval are required with 100 m spatial resolution, 10$sp{-3}$ mirror reflectance and 10 mW optical power coupling to the sensing fiber. Rayleigh backscattering sensors require several kHz of frequency drift without polarization discrimination or several tens of kHz with polarization discrimination. A preliminary experiment has been carried out for the proposed fiber optic intrusion sensor configuration with Rayleigh backscattering without polarization discrimination.