Fiber Bragg Gratings

Optical fiber grating has been developed quickly and widely used in optical fiber communication and optical sensor field as a kind of optical fiber passive devices in recent years. Optical fiber grating sensors based on optical fiber grating technology have become the focus in sensors study field. Fiber Bragg Grating (FBG) sensor is one of the current heat research fields in the optical fiber grating sensor technique fields. It has good potentials that include distributing sensing, anti-electromagnetic interference, high precision and good stability, and so on. Theoretical analyses of FBG are presented in this paper, including the coupled-mode equations of fiber grating, the propagation characteristics of light wave in FBG and long period fiber gratings(LPFG), the physics characteristics of FBG and LPFG, optical fiber grating sensor mechanism, the cross sensitivity between strain and temperature,and so on. The research work in the dissertation is important to understand the physics mechanism of optical fiber grating and to develop new fiber grating sensors.Firstly, the recent developments in the area of optical fiber grating sensors are reviewed. The mechanism of photo-induced change of refractive index is introduced briefly. Various fabrication methods of fiber gratings are described systematically. The advantages and disadvantages of each method are analyzed and compared. The coupled mode equations are built by means of expanding perfect waveguide modes. According to the theory of coupling, the light-wave travel law in the fiber grating has been studied, the even cycle transmission response characters of FBG and LPFG are deduced, numeral calculations of the reflection spectrum of FBG and LPFG are made, and the relationship between the refractive index modulation depth, length L and the characteristics of fiber Bragg grating have been analyzed.Secondly, based on analysis for the characteristics of fiber Bragg grating Fabry-Perot(F-P) cavity, a physics model is presented for fiber Bragg grating F-P filter. With the method of transfer matrix, the spectrum characteristics of the F-P filter are analyzed and discussed in detail, and the effects of Bragg grating length, refractive index modulated depth and F-P cavity length on the characteristics of transmission and reflection of F-P filter are given. According to condition of phase resonance of fiber grating F-P cavity, the relationship between fiber grating period and cavity length for fiber grating F-P cavity has been deduced. At the same time, theoretical analyses and numeral calculations of the reflection spectrum are made using multilayer film method for dual wavelength Bragg fiber grating. According to the method of multilayer film, the relationships between the reflective spectrum characteristics of dual wavelength fiber Bragg grating and the structural parameters are studied. And characteristic of relative between two fiber Bragg gratings is analyzed and discussed, which provide a basis for the resolve of the cross sensitivity problem between temperature and strain.After that, the temperature and strain sensing mechanisms of FBG and LPFG are studied systematically. The strain and temperature sensing models of fiber Bragg grating are presented. By analyzed the effects of fiber's spring, cycle flex, calorescence and the heat expand, the strain sensitivity and temperature sensitivity of FBG are obtained. Furthermore, the strain and temperature sensitivities of FBG and LPFG are analyzed in detail. The results show that the resonant wavelength of the FBG and LPFG shifts linearly with temperature or axial strain, and the temperature and strain sensitivities of LPFG not only depend on the core parameters and gratings period, but also depend on the cladding parameters. Quantitative analysis shows that the temperature and strain sensitivities or shift direction of resonant wavelength for LPFG are dependent on fiber types and coupling mode orders.Finally, cross sensitivity problem is studied. The influences on the measurement results caused by the strain-temperature cross sensitivity of FBG are analyzed. In addition, various schemes of simultaneously measuring strain and temperature with fiber grating sensors are described. The advantages and disadvantages of the schemes are compared.