Research On Modulation And Demodulation Method Of BOTDR System Based On Spread Spectrum Technology

With the acceleration of industrialization and informatization,more and more super projects have emerged,such as dams,bridges,tracks,etc.The safety and health of these projects are extremely important for the future strategic development of society.Because Brillouin scattering is sensitive to both temperature and strain,the Brillouin Optical Time Domain Reflection(BOTDR)system has been widely used in various large-scale projects since it was proposed.Therefore,it is of great significance to research BOTDR system.On the basis of traditional BOTDR system,a BOTDR system based on spread spectrum technology is proposed which is combined with spread spectrum modulation amplification technology and optical domain demodulation technology,and the specific contents are as follows:Firstly,the development and application of distributed optical fiber sensing based on Brillouin scattering are introduced,and the principle and detection technology of Brillouin scattering are analyzed.The principle and characteristics of the spread spectrum technology are elaborated,and the advantages of the spread spectrum technology are clarified.Next,the principle of spread spectrum modulation amplification in BOTDR system is studied.The problem of the imbalance of the Erbium-doped fiber amplifier is optimized by adding a pre-laser,and the optimized Erbium-doped fiber amplifier is simulated in OptiSystem.The principle of optical domain demodulation is analyzed,and a dual-pass unbalanced Mach-Zehnder interferometer is actually designed.An optical time domain reflection system based on a dual-pass unbalanced Mach-Zehnder interferometer is established in the OptiSystem,and the simulation experiments of location and spatial resolution are carried out respectively.Thirdly,combined the optimized Erbium-doped fiber amplifier and the dual-pass unbalanced Mach-Zehnder interferometer,the BOTDR system based on spread spectrum technology is built.The generation principle and related characteristics of m sequence,Gold sequence and Kasami sequence are studied.The positioning experiments are carried out with three sequences,and the performances of them are analyzed.Finally,with m sequence as spread spectrum sequence,a BOTDR system is built.The system uses m sequence as the spread spectrum modulation sequence,and the positioning experiment,the spatial resolution experiment and the temperature sensing experiment are carried out respectively.