Research On OFDR System Based On Microwave Frequency Sweeping

Distributed optical fiber sensing technology is a kind of sensing system which uses optical fibers as signal transmission.It can be used to measure the strain,temperature,vibration,etc.of objects,with high sensitivity,good safety,anti-corrosion,anti-electromagnetic interference and other advantages are widely used in various fields,such as pipeline leak detection,building structure health monitoring,landslide monitoring,power system detection,etc.Optical frequency domain reflection technology is a new type of distributed optical fiber sensing technology.In addition to the advantages of ordinary distributed optical fiber sensor,it has obvious advantages in sensitivity,signal-to-noise ratio and spatial resolution,and it can realize short-distance and ultra-high spatial resolution detection of external parameter changes such as strain,temperature and three-dimensional shape perception of object,so it has high research value for military,medical and other applications.At present,there are still many difficulties in optical frequency domain reflection technology,which requires high linearity of frequency sweeping laser in the system,leading to a monopoly trend in the domestic and foreign markets.Mature optical frequency domain reflectometers are expensive,so a lot of research is still needed on this technology.This paper proposes an optical frequency domain reflection system based on microwave frequency sweeping,which is using microwave frequency sweeping modulating linear continuous frequency sweeping light to replace the traditional frequency sweeping laser,reduces the system cost,solves the nonlinearity of the frequency sweeping laser,and enhances the system performance.In addition,the determinants affecting system performance are analyzed.The main research work is as follows:(1)An optical frequency domain reflection OFDR system based on microwave frequency sweeping is proposed.The principle of the coherent detection structure used in the OFDR system is theoretically analyzed.At the same time,several methods of generating frequency sweeping light are explained,and the external modulation method used in this article is discussed,the principle of using IQ modulator to modulate the frequency-stabilized laser to produce linear frequency sweeping light is analyzed,and the key factors affecting the system performance are analyzed.In addition,the traditional cross-correlation demodulation method used by OFDR system is introduced,and the principle of OFDR system measuring temperature strain is explained.(2)The principle of OFDR system simulation based on microwave frequency sweeping is introduced,the system simulation model is established,and the differential FFT demodulation algorithm is proposed.The simulation analysis and data demodulation are completed.The simulation demodulation results obtained by the differential FFT demodulation method and the traditional cross-correlation demodulation method are compared,and realize the accurate positioning of the changes of the parameters to be measured.The spatial resolution of the simulation system is tested by two demodulation methods.The spatial resolution of 0.08 m and 0.23 m is achieved on the 100 m sensing fiber,respectively.(3)An optical frequency domain reflection system based on microwave frequency sweeping is designed.The main components in the system,including laser,external modulator,microwave signal generator,optical amplifier,etc.,are analyzed and selected.The experimental platform of the system is built with the selected components.(4)Based on the traditional cross-correlation demodulation algorithm,a data demodulation program of zero-filling cross-correlation is written.The functions of some selected key devices are verified,and the frequency sweep range of the system is determined.Using the built-up microwave frequency sweeping OFDR system,the temperature positioning experiment is carried out,and the data of part of optical fibers at 50?,60?,70? and 80?were collected,respectively.The traditional cross-correlation algorithm and the zero-filled cross-correlation algorithm are used separately to demodulate,with four different window functions and different window widths at the same time.The advantages and disadvantages of the two kinds of algorithms are compared,and the temperature positioning function of the system is verified.The spatial resolution of 3.6m is achieved on the 1km sensing fiber.