Research On The Application Of Long Period Fiber Grating In Smart Structure

A long-period fiber grating(LPFG) is a periodic refractive index structure on the fiber core whose periodicity is in the range of several hundred micrometers. The transmission spectrum of a typical LPFG consists of a number of rejection bands that arise from light coupling from the fundamental mode to the cladding modes of the fiber. LPFG has many remarkable advantages, such as low insertion loss, low back reflection, easy to fabricate, polarization independence and etc., The LPFG also offers a series of interesting properties for sensing applications, compared with FBG, they are also sensitive to measurands such as strain, curvature, temperature or the external refractive index, so it becomes a most important part of fiber sensors. Meanwhile, most research on it are in the laboratory while a little in application. This dissertation aims to research on the application of LPFG in structural health monitoring, including the vibration monitoring and the civil engineering structure. All these work provide theoretical and practical preparation for promoting the availability of LPFG detection technology. The main achievements are described as follows:(1)The theory analyse of LPFG has been studied, the written technology and the formation mechanism were introduced. Based on the coupling theory and the three-layer fiber model, the transmission speciality of long period fiber grating is analyzed, and the transmission spectrum function, resonance wavelength, effective refractive index of LPFG were deduced, which offer the theoretical foundation of the manufacture and the sensor characteristic of LPFG.(2)The sensing characteristic of LPFG is the precondition to apply in practice. The basical characters of long-period fiber gratting are studied. It includes the temperature, axial strain, bending, refractive index and transverse-load characteristic. The simple theories and experiments related were concerned.(3)Based on the study of micro-bending characteristic of LPFG, it is found that the loss amplitude of one wavelength is linear to the curvature of LPFG. A new scheme to monitor the structure vibration by gluing the LPFG onto the experimental laminate is proposed and realized. (4)The narrow band reflection light characteristic of Bragg grating was used to demodulate LPFG, by detecting the power of certain fixed wavelength transmission light of LPFG, the dynamic signal monoring system is realized based on the micro-bending characteristic of LPFG, and the acquisition of distributed vibration signal was realized preliminary.(5)Based on the vibration monitor of optical fiber grating smart structure, different damage situation are studied through an analysis of an experimental laminate, the variation of wavelet packet energy spectrum used as the damage character is picked up to identify the structual damage status. And the damage diagnosis of the optical fiber grating smart structure were realized.(6)Base on the mechanism of steel corrosion in reinforced concrete, a scheme was proposed to measure the steel corrosion using the refractive index characteristic, and the experimental results was effective.(7)A new technique to measure the corrosion of steel in concrete structures was proposed, it is based on the microbending characteristic of LPFG, and the corrosion sensor designed was produced to monitor the corrosion of steel in concrete.