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FBG-FP阵列的高空间分辨率高精度分布式传感研究
Research on High Spatial Resolution and High Accuracy Distributed Sensing of FBG-FP Array
【作者】 王凡；
【导师】 周祖德；
【作者基本信息】 武汉理工大学 ， 信息与通信工程， 2018， 硕士
【摘要】 光纤传感技术以其独特的优势,成为目前智能结构健康监测技术中研究较为广泛的技术。针对大型结构、复合材料内部裂纹、金属结构腐蚀等主要损伤类型,由于其具有隐蔽性强、结构失效机理复杂、结构破坏程度难以判断等特点,需进行超高空间分辨率、复用容量大、精度高的传感检测。本文采用间距极小的超短弱反射的光纤光栅(Fiber Bragg Grating,FBG)构筑的光纤光栅法布里珀罗(Fiber Bragg Grating Fabry-Perot,FBG-FP)阵列搭建传感网络,基于光频域反射技术搭建传感光路,通过对解调原理、解调算法和实验验证等相关问题的研究,实现一种具有超高空间分辨率、超大容量、高精度的全分布式光纤传感新方法与新技术。主要研究内容如下:(1)FBG-FP阵列的传感机理与复用容量研究。以FBG的耦合模式方程为基础推导FBG-FP的光谱数学表达式,并分析其温度和应变的传感机理。数值模拟多重反射效应和光谱阴影效应对FBG-FP传感阵列的复用极限的制约,证明降低反射率可抑制上述两种效应,并进一步提出采用光栅间隔不小于栅长和中心波长随机分布的传感阵列可分别抑制多径反射效应和光谱阴影效应,其中波长随机分布对传感没有坏的影响。(2)FBG-FP阵列的分布式传感解调系统的研究。提出基于光频域反射(Optical Frequency-domain Reflectometry,OFDR)技术的FBG-FP阵列的分布式解调系统。一方面研究传感单元高空间分辨率的定位方法,通过对可调谐光源的非线性调谐效应进行补偿,在50m的传感距离内实现82μm内的超高空间分辨率;通过计算等效光频域调谐速率和可调谐光源的时间波长转换轴,提高系统的定位稳定度和波长解调精度。另一方面研究传感单元的波长解调方法,推导FBG-FP光谱重构的数学表达式,提出FBG-FP阵列的分布式传感解调算法。(3)裂纹尖端检测。温度实验测试系统解调性能,实现8557个长度为400μm、间隔为440μm、反射率约为-42dB的FBG构成的超短弱反射的FBG-FP阵列传感,传感解调空间分辨率达到840μm,温度解调精度小于0.65℃。在不牺牲空间分辨率的情况下能解决同尺寸超短光栅阵列由于谱宽太宽导致解调精度下降的问题,并对结构试件裂纹的非均匀应变场实现检测。
【Abstract】 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 82μm 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 400μm,the interval was 440μm,and the reflectivity of sensor was about-42 dB.The spatial resolution have reached to 840μm,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.