The Study Of Propagation Properties Of Fiber Bragg Grating Couplers

Modern communication network, the electro-optical network, its inherently bottleneck of electronic is one of the key factors which confines its development. The key of this difficult problem is the transformation between optical- and electrical-field. Now a new network, which no longer need optical-to-electronic conversion is so-called all optical network. All-optical switcher is the key device of implementing all-optical switching. The directional coupler has stimulated a great deal of researches and interests because of its potential application to all-optical switching. It can use one optical wave to control and modulate another optical wave by nonlinear interacting inside the waveguide overlapping filed.The optical fiber Bragg grating make all optical communications and all optical sensor possible. It has many advantages, such as small and flexible, anti-corrosion, low insertion loss, anti-electromagnet disturbance, high sensitivity, good stability, etc. It is widely used in wavelength division multiplexing (WDM), channel equalization, dispersion managements and also has important application value on special domains such as space industries and power equipment.The main research work and results are as follow:At first, this paper introduces the significance, process and application prospect of coupler and fiber Bragg grating research in all-optical network, especially introduces the gap soliton theory in fiber Bragg grating and several principles of optical fiber Bragg grating couplers, and analyzes their advantages and disadvantages.In chapter 2, the mathematical model is been studied and deduced for fiber Bragg grating in terms of the coupled mode theory. At the same time, we have studied the propagation property of light wave in fiber Bragg grating. Based on the mathematical model, the numerical computation for reflective spectrum is conducted. The curves and its change direction, as well as the relationship between second order dispersion and detuning frequency have been analyzed.In chapter 3, two-core optical fiber grating coupler is introduced in detail. And this paper has analyzed impact of bandgap structure on the distribution of reflectivity and transmissivity and the relationship between the bandgap structure and the phase shift between two fiber cores. By selecting appropriate grating length to keep the pulses propagate periodically inside coupler when it is far away from bandgap, energy properties in two-core in phase and out-of-phase optical fiber grating coupler are compared. When the frequency is in the bandgap, the energy of forward and backward waves in two cores decay along with the distance, it shows that the energy can be totally reflected if the length of grating is long enough, energy of backward waves are equal to forward waves in the same location of two cores when out-of-phase, and the energy is centered on the coupling arm. In the edge of the bandgap, the energy of forward and backward waves are periodic when out-of- phase.When the frequency is far away form the bandgap, since the influence of backward modes can almost be ignored, the periodic in the coupler is same as that in fiber optic coupler, and it have good switching characteristics.In chapter 4, the transmission of soliton pulses in two structures of couplers (three-core parallel equidistant arrangement fiber couplers and three-core equilateral- triangle arrangement) have been investigated, and the bandgap is analyzed. Depending on the edge of bandgap, there are different states of slow light. The research shows that the phase shift not only affects the width of bandgap, but also changes the number of slow-light modes. For the in-phase three-core coupler, the research suggests that the pulse can be totally reflected in the input core if the frequency in the bandgap. When the frequency is far away form the bandgap, it is same as that in fiber optical coupler.