Study On The Optical Properties And Sensing Characteristics Of Torsion Fiber Gratings

Fiber grating have important applications in the optical communication, optical sensing, optical measurement, etc all, especially in the aerospace and bridge monitoring field of structural health monitoring. However, the traditional fiber grating can't work in the strong ultraviolet, strong radiation and high temperature environment for a long time, the reason is grating structure would be affected and even damaged. In order to solve this problem, in recent years, a new type of spiral Chiral Fiber Gratings (CFG) gradually cause the attention of the researchers.This thesis work around twisting fiber gratings, which mainly includes two aspects. One is that the study of twisting optical fiber in the process of production, this fiber can be manufactured by twisting optical fibers or perform with non-circular symmetric cross section at melting state at high speed. Spectral and sensing characteristics of the chiral fiber gratings were analyzed. The other is that the normal optical fiber which did not spinning in the process of production in the application of the twist sensor were discussed.First of all, the working principle of optical fiber grating were expounded. According to the coupled-mode theory, the spectral and coupling characteristics of the double-helix CLPG were analyzed. The relevant transmission spectra were simulated and transmission spectra were tested.On this basis, using the phase matching conditions, the pitch and twist rate correspond to the specific resonant wavelengths were calculated. The relevant transmission spectra were simulated and the influence of the design parameters of gratings on the transmission spectrum were discussed. The strain sensing features of the double-helix CLPG were analyzed. The research showed that their sensing properties were similar to that of the traditional long-period fiber gratings, which means the resonance wavelength would be shifted with the change of the strain, moreover, the wavelength shift is almost linearly with the strain and the strain sensitivity is 0.828pm/??.According to the Jones matrix formulation and transfer matrix method, the relationship between twist and the FBG's PDL response is theoretically analyzed and numerically simulated. By measuring the PDL amplitude changes, the twist sensing can be achieved without strain and temperature confusion. In addition, both twist direction and angle can simultaneously be obtained. The experimental results and numerical simulation have good agreement, the twist sensitivity is 0.955 dB/rad.