A novel long-range perimeter security sensor based on hybrid michelson and Mach-Zehnder interferometers

Abstract

Over the past 25 years a number of fiber optic sensors have been developed to address fence and buried perimeters, and pipeline security. Today, sensors that locate targets along the length of the fiber sensor dominate the long-range perimeter market. There are a number of fiber optic sensors that locate targets including sensors based on interferometry and C-OTDR (Coherent Optical Time Domain Reflectometry). In general, existing interferometric techniques infer the location of a disturbance based on the magnitude of the interfering signals, as opposed to the actual phase differences, and critically suffer from polarization induced fading. A novel technology is developed, as discussed in Optellios' earlier patent, which measures the actual phase difference of the interferometric signal. As a result, this technology is more accurate and precise for locating a disturbance, works well with any magnitude of disturbance, and does not critically depend on polarization of the interfering signals. The technology uses a hybrid Michelson and Mach-Zehnder interferometer architecture that shares the same two sensing fibers. The laser light is frequency modulated, and the In-phase and Quadrature phase responses of the sensor are measured to extract the phase difference of the interfering signals. A disturbance of the sensor cable causes the phase difference of the sensor to change. This phase change is measured from each end of the fiber sensor, and the time delay between the two phase signals is used to locate the disturbance along the length of the sensor cable. The Michelson interferometer is terminated in Faraday Rotational Mirrors to avoid the issues relating to polarization induced fading. Fundamentals of this novel technology will be presented along with its relative performance and merits compared to other interferometric technologies. This technology will be further compared with C-OTDR technology, and experimental data will be discussed.