Research On Spread Spectrum Technology In Distributed Optical Fiber Ranging System

In recent years,Optical Time Domain reflectometry(OTDR)has been widely used in the field of distributed fiber optic sensing,to measure parameters such as distance,temperature,strain,and vibration.However,the use of single pulse detection in OTDR results in a contradictory relationship between measurement distance and spatial resolution.In order to solve the above problems,the paper studies spread spectrum coding technology and differential phase shift keying(DPSK)modulation technology and applies them to distributed fiber optic ranging systems,making the system have the advantages of lower requirements for input signal signal-to-noise ratio and stronger anti-interference ability.The main content of the paper is as follows.Firstly,analyze the current development status of distributed fiber optic sensing technology,and focus on introducing the OTDR technology commonly used in fiber optic ranging,and analyze its existing problems.Afterwards,the advantages of spread spectrum technology in OTDR systems will be elaborated,laying a theoretical foundation for the following text.The characteristics of pseudo random sequence commonly used in spread spectrum technology are analyzed to provide theoretical guidance for the ranging system to select appropriate spread spectrum sequence.Secondly,in order to realize DPSK modulation and demodulation with broadband light source,a demodulation system based on Mach Zehnder(MZ)interference structure is adopted.The demodulation principle is studied in this paper,and the demodulation function is verified by building a demodulation experimental platform.To achieve optical domain spread spectrum and despreading,an optical domain multiplier is proposed by combining Mach Zehnder interferometer and dual Mach Zehnder intensity modulator,which realizes the same or and exclusive or operations.A distributed fiber optic ranging system based on MZ interference structure is formed by combining the optical domain multiplier with the DPSK demodulation system.Then,in order to verify the feasibility of the system,a simulation model of distributed optical fiber ranging system based on MZ interference structure is built.The simulation results show that it can overcome the contradiction between the measurement distance and spatial resolution,and can improve the system’s signal-to-noise ratio and spatial resolution by selecting different pseudo random sequence and different pseudo random sequence modulation rates.In order to further improve the spatial resolution of the system,reduce hardware costs and time costs,chip subdivision method and related triangle prediction method have been proposed.Finally,a distributed fiber optic ranging system experimental prototype based on MZ interference structure was built,and different spread spectrum sequences were generated through FPGA for comparative experiments.The experimental results were consistent with the simulation results.Experimental verification was conducted on the rate improvement method,chip subdivision method,and related triangle prediction method,respectively.The experimental results of the related triangle prediction method showed that its spatial resolution reached the centimeter level,which improved the measurement accuracy by one order of magnitude compared to the chip subdivision method and reduced the number of subdivision times.This not only saves measurement time but also improves measurement efficiency,providing a reference scheme for further improving the performance of distributed fiber optic ranging systems.