Research On High Spatial Resolution And High Accuracy Distributed Sensing Of FBG-FP Array

With its unique advantages,optical fiber sensing technology has become a widely used technology in the health monitoring technology of intelligent structure.For major damage types such as large structures,internal cracks in composite materials,and corrosion of metal structures,due to its characteristics of strong concealment,complex mechanism of structural failure,and difficulty in determining the degree of structural damage,the distributed sensing detection system with ultra-high spatial resolution,large multiplexing capacity and high accuracy is required.This paper will use fiber Bragg grating Fabry Perot(FBG-FP)array to build sensing network system with the ultrashort and ultraweak reflectivity FBGs in ultrashort intraval.And this system is based on optical frequency-domain reflectometry.Through the research of demodulation principle,demodulation algorithm and experimental verification,we have realized a new distributed optical fiber sensing method and new technology with super high spatial resolution,large capacity and high accuracy.The main contents are as follows:(1)Research on the sensing principle and the multiplexing capacity of the FBGFP array.The coupling mode equation of fiber Bragg grating is established.On this basis,the spectral mathematical expression of FBG-FP is derived,and the sensing mechanism of temperature and strain is analyzed.The multiplexing capacity intensive FBG-FP sensor array is mainly limited to the multireflection effect and the spectralshadowing effect,numerical study on effect of the two kinds of effects,that reduce the reflectivity can weaken the above two effects,further put forward by using grating sensor array interval length is not less than the gate length and the center wavelength of the random distribution to suppress multipath reflection effect the wavelength and spectral shadow effect,random distribution no bad effect on the sensor.(2)Research on the distributed sensing demodulation system of dense FBG-FP array.A distributed demodulation system based on dense FBG-FP array is proposed.On the one hand,we study the location method of the sensing unit with high spatial resolution.Through calibrating the nonlinear tuning effects of the tunable laser,the ultra-high spatial resolution within 82?m is achieved in the sensing distance of 50 m.By calculating the equivalent optical frequency tuning rate and the time-wavelength conversion axis,the positioning stability and wavelength demodulation accuracy of the system can be promoted.On the other hand,we study the wavelength demodulation method of the sensing unit,derive the mathematical express of FBG-FP spectral reconstruction,and the dense FBG-FP array distributed demodulation algorithm is proposed.(3)Research on crack tip detection.Temperature experiments on a single mode fiber multiplexing of 8557 gratings in the dense FBG-FP array,in which the FBG length was 400?m,the interval was 440?m,and the reflectivity of sensor was about-42 dB.The spatial resolution have reached to 840?m,and the demodulation accuracy of temperature was less than 0.65?.Without sacrificing the spatial resolution,the dense FBG-FP array can solve the problem that the demodulation precision is degraded due to the wide spectrum width of the ultra-short grating array of the same size,and the non-uniform strain field of the structural specimen cracks can be detected.