Study On Locating Technology Of Distributed Optic-fiber Intrusion Early Warning System Applying Sub-loop Fiber

At present there are lots of important fields suffer the illegal intrusion threaten during using period every minute, such as airport, prison, oil house, power plant, long distance oil/gas pipelines and high voltage transmission facility etc. The safety precaution methods which have been fixed fall behind and cannot fit the requirement of intrusion early alarm and localization in new environment. Moreover electromagnetic distributed sensor is too costly and cannot realize localization, so a new kind sensing system is needed so much. Optical fiber is sensing and transporting at the same time, meanwhile it is cheap, anticorrosion, non-maintaining and transporting dozens of kilometers without repeater. It almost cannot be influenced by any environment factors, so it becomes the hot research subject for long distance intrusion early alarm and localization monitoring.But the sensing structure of distributed optical fiber based on Sagnac interferometer cannot fit the true intrusion acts. The location demodulation technology doesn't work on the characteristics of low frequency intrusion. So it needs to study new distributed fiber structure and localization algorithm for intrusion monitoring which has better universality.Directed towards the limits of Sagnac interferometer in localization intrusion, the research has been done as following:①Analysis the intrusion early alarm and localization theory for distributed optical fiber based on Sagnac interferometer, then research the characteristics of its structure and find the problems;②To overcome the flaws of traditional Sagnac interferometer, it proves the local structure of the interferometer. The delay fiber ring is coupled on annular sensing fiber with a coupler, and the continuous broadband laser source is changed to adjusted pulse laser source. So the improving structure could induce a series of pulse interference lights which separated in time domain together, and these pulse signals separate from each other and correlate with each other as well. Choosing the coupled apparatus with special phase biased, the phase variance of interference lights presents obvious linearity, and this characteristic makes signal simplify and process easy. After configuration the suitable structure of distributed fiber intrusion localization system, it extracts the output signal of sensing system. With different times of travelling in delay fiber, the power of interference lights which effected by intrusion acts are different.③Starting at optical wave guide theory, using mathematic physics formula derivation and theory analysis, mathematic model location expression of intrusion position happens on sensing fiber is deduced. It analyzes the influence of related devices parameters to location accuracy and designs every device of system, asking them interaction very well and accomplishing the system's function. Trough mathematic simulation, the theory localization accuracy of that system is discussed.④Then through software simulation analysis, it derives that when monitoring distance is 40km the monitor system and signal processing method can realize the localizing accuracy of 20m theoretically. It constructs experiment platform in laboratory and derives the localization accuracy almost 100m with 10km total distance. This result proves that this kind of fiber distributed intrusion early warning system could carry out the long distance intrusion early warning and localization monitoring.