Research On The Theory And Dispersion Properties Of Thin-Clad Fiber Gratings

The main content of this thesis is divided into three parts: transfer matrix theory analysis of fiber Bragg gratings, transmission characteristics analysis of uniform fiber Bragg gratings and numerical analysis of dispersion properties of chirped long-period fiber gratings(CLPFGs) based on two fiber analysis models. Chirped fiber gratings offer many advantages,including low insertion loss, compactness, low cost, high dispersion value, thereby paving the way for effective and practical dispersion compensation. The general optical fiber can be viewed as a three-layered medium cylindrical waveguide, but because of its eigen-value equation and the field distribution of the core-mode and cladding-mode based on the three-layered medium model are considerable complex, usually the two-layered medium model is used instead. In the case of thin cladding layer, the difference of the core-mode effective refractive index(RI) produced by the two fiber analysis models is significant. By means of the coupled-mode theory and the transfer matrix method,in combination with the difference of the core-mode RIs based on two fiber analysis models, this thesis mainly study transmission characteristics and dispersion properties of thin-clad fiber gratings. The study in the thesis is categorized as follows:(1)For the purpose of analyzing accurately the transmission characteristics of thin-clad fiber gratings, the contrast between the effective RIs of the core mode in an uniaxial-crystal fiber yielded by two fiber analysis models is demonstrated and analyzed. On its basis, the resonant wavelength and bandwidth of thin-clad fiber Bragg gratings are analyzed respectively. Moreover,we simulate the resonant wavelength and bandwidth of thin-clad long period fiber gratings respectively. According to the obtained data, transmission spectrum,time delay and dispersion of fiber Bragg gratings(FBGs) and long period fiber gratings(LPFGs) based on two fiber analysis models are analyzed. The result shows that the resonant wavelength and bandwidth difference produced by the two fiber analysis models is significant, especially in the case of thin cladding layer, the smaller the cladding radius is, the greater the difference is. Furthermore, the influence of the length of the fiber gratings and cross-coupling coefficient on the dispersion properties of thin-clad fiber gratings is also analyzed. The result shows that long fiber gratings and small cross-coupling coefficient are both effective to increase the dispersion compensation ability of FBGs and LPFGs.( 2) In order to throughly study the dispersion properties of thin-clad fiber gratings,numerically analysis of the dispersion properties of CLPFGs is implemented. Firstly,analysis and contrast on the different dispersion properties of CLPFGs effected by the different cladding radii and cladding mode orders are made. The result shows that the difference of the core-mode effective RIs produced by the two fiber analysis models is significant, especially when the cladding radius and cladding mode order are small. And moreover, for both of thetwo fiber analysis models, it is effective to increase the dispersion by reducing the cladding radius and increasing the cladding mode order. Moreover, the influence of chirping coefficient and cross-coupling coefficient on the dispersion properties of thin-clad CLPFG is also analyzed. The result shows that small chirping coefficient and cross-coupling coefficient both lead to the increase of dispersion compensation ability of CLPFG.