A study of a fiber optic intrusion sensor

Abstract

Two configurations for a fiber optic sensor for intrusion detection are investigated. In both cases, a ring laser is the light source and an acoustooptic modulator gates a light. The distributed sensor is configured as an optical time domain reflectometer using coherent interference of Rayleigh backscattering in a single mode fiber. The spatial resoluion of distributed sensor corresponds to the modulation pulse width. For a pulse width of 4 usec corresponding to a spatial resolution of 400m, the signal to noise ratio is about 3.3, and the bandwidth is about 30.5 Hz. The multiplexed sensor consists of low reflectance internal mirrors equally spaced in the sensing fiber. The spatial resolution of the multiplexed sensor is the mirror separation. For the case that the mirrors are 25m apart, the signal to noise ratio is about 20.7, and the bandwidth is about 125 Hz. A fabrication technique for an internal mirror using a mechanical splice and a piece of fiber coated at a cleaved end with a TiO2 dielectric film is developed for the desired properties of low reflectance and low insertion loss. The reflectance with a insertion loss of 0.055dB of an internal mirror produced using this technique is about 0.1%. Two configurations for a fiber optic sensor for intrusion detection are investigated. In both cases, a ring laser is the light source and an acoustooptic modulator gates a light. The distributed sensor is configured as an optical time domain reflectometer using coherent interference of Rayleigh backscattering in a single mode fiber. The spatial resoluion of distributed sensor corresponds to the modulation pulse width. For a pulse width of 4 usec corresponding to a spatial resolution of 400m, the signal to noise ratio is about 3.3, and the bandwidth is about 30.5 Hz. The multiplexed sensor consists of low reflectance internal mirrors equally spaced in the sensing fiber. The spatial resolution of the multiplexed sensor is the mirror separation. For the case that the mirrors are 25m apart, the signal to noise ratio is about 20.7, and the bandwidth is about 125 Hz. A fabrication technique for an internal mirror using a mechanical splice and a piece of fiber coated at a cleaved end with a TiO2 dielectric film is developed for the desired properties of low reflectance and low insertion loss. The reflectance with a insertion loss of 0.055dB of an internal mirror produced using this technique is about 0.1%.