Optimization And Design Of Fiber Gratings

The applications of fiber gratings are more and more popular in optical communication and optical fiber sensors because of its advantages of small cubage, low cost, good compatibility to fiber systems, low insertion loss, simple fabrication and so on. They can be used to make fiber laser, semiconductor laser and ring laser etc with accurate wavelength and power. Fiber gratings can be used in fiber sensor systems as temperature sensors, strain sensors, pressure sensors, refractive index sensors, filtering, devices of dispersion compensation, gain flattening oferbium-doped fiber amplifiers and so on.The optical properties of the optical fiber core would be changedeternally when it is exposed to UV light. Using this photosensitive property, optical fiber grating is made to have a periodical index change along the fiber core. When light is transmitted through such kind of waveguide, it experiences periodical phase changes. In future the object of fiber communication is to establish large capacity and high-speed system and the object of fiber sensor is to establish smart material, smart structure and smart skin (3S) sensor system. For the above reasons, fiber gratings with better performance are demanded. The fiber grating's structure have a lot of kinds, each has its own characteristics. So, for the fiber gratings designer, it is difficult to forecast the characteristics of a fiber grating with the arbitrary structure. On the basis of theoretic model of fiber gratings, optimizing the parameters of fiber grating and analyzing the performance of fiber grating is a significant work.The main work of this thesis focuses on characteristic analysis and optimized design of fiber gratings.Firstly, we summarize the applications of fiber gratings in the fields of fiber communication system and sensor systems.Secondly, we deduced wave-guide modes of step-index fiber by use of optical fiber wave-guide theory. We deduced strictly couple mode theory of fiber gratings using weak wave-guide approximation and introduce the transfer matrix method of fiber gratings.Thirdly, by using the couple mode functions and transfer matrix method, we analyzed the characteristics of fiber gratings when the grating parameters, such as index change, grating period, grating length, apodization function etc., have different values.The sidelobes in the reflectivity spectrum of common fiber Bragg grating will greatly deteriorate the filtering properties. Optical apodization can remove these sidelobes. Numerical results show that apodization can effectively suppress the sidelobes in the reflectivity spectrum. The reflectivity in the stop band can reach 100%, and the light frequencies outside the stop band will not be affected by the stop band and will be transmitted out the fiber gratings. By analyzing and contrasting the reflectivity spectrum obtained under different apodization functions, we find these laws that guide the characters of apodization:For a certain apodization function, the band width, steepness of the band edges and the extent of the sidelobe suppression are all related to the parameters of the apodization function. As a matter of fact, sidelobe suppression will deteriorate the band width and the steepness of the band edges. So the parameters of the apodization functions have to be carefully selected before an apodized FBG is fabricated. Partially apodized FBGs have a wider bandwidth and steeper band edges than the fully apodized FBGs.By inserting one or more phase shifts along the fiber grating, one or more narrow transmission peaks would be created in the stop band of the reflectivity spectrum. Using the transmission matrix approach, this thesis analyzed the main characters of the phase-shifted fiber grating. Study results show that:The numbers of the transmission peaks created in the stop band is equal to those of the phase shifts inserted in the grating. The size and the bandwidth of the transmission peak are determined by the location of the phase shift in the grating. When the shift becomes closer to the central of the grating, the size of the transmission peak becomes bigger and the bandwidth of the transmission peak becomes narrower. The location of the transmission peak in the stop band is determined by the quantity ofthe phase shift. When (?) <π/2, the wavelength of the transmission peak islonger than the Bragg wavelength. When (?) >π/2, the wavelength of the transmission peak is shorter than the Bragg wavelength. When the quantity of the phase shift becomes closer to (?)/2, the transmission peak becomes closer to the Bragg wavelength.These theoretical results will guide our application and fabrication of the phase-shifted fiber grating. Numerical analysis on dispersion compensation characteristic of linearly chirper fiber gratings was carried out and the main parameters were optimized.