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Research On Key Technology Of Broadband Dispersionless Slow Light In Periodic Medium

Posted on:2010-02-08Degree:MasterType:Thesis
Country:ChinaCandidate:F H WangFull Text:PDF
GTID:2178360275970317Subject:Communication and Information System
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This dissertation studies the key technology of broadband dispersionless slow light in periodic medium. Three types of novel slow mode waveguides are proposed to realize the low-group-velocity low-dispersion wide-band slow-mode propagation under room temperature, which could lead to a number of important fundamental and technological advances in the field of slow light communication.This dissertation is organized as follows:Chapter 1 first introduces the applications of the slow light technologies and presents fundamental theories. Then we describe the mechanisms to achieve slow light, focusing on the analysis and comparison among EIT, CPO and CRS. Last, we introduce the current development of the slow mode waveguide. These concepts and theories are necessary for the following research work.In Chapter 2, we propose and evaluate a two-dimensional photonic crystal waveguide with two line defects separated by a row of elliptic air holes. By adjusting the structural parameters, we obtain a waveguide with an inflection point on the dispersion curve corresponding to a slow-light mode with reduced distortion. The group velocity, around 0.0018c, and the GVD parameter, less than the order of the magnitude of 105(a/2πc2) both reach minimum. Moreover, we investigate the transversely confined field and high quality factor, which both indicate that the dispersionless slow wave can be generated in our wave guide.In Chapter 3, we take the bandwidth of the waveguide into account, and design two novel air-hole-array strip waveguides, one with elliptic air holes and the other with sandglass-shaped ones, in order to simplify the structures of the devices and obtain the larger delay-bandwidth-product (DBP). The band diagram and FDTD simulation results both prove that the flatband modes with low group velocity and low dispersion can be obtained. As for the waveguide with elliptic air holes, average group index could reach 418, and DPB equals to 0.166. As for the other with sand-glass shaped air holes, average group index equals to 87.7, and DBP reaches 0.186. Each waveguide has its strong point, one can choose the lower group velocity and narrow bandwidth (one with elliptic air holes) or higher group velocity and wide bandwidth (one with sandglass-shaped air holes), according to the application. Moreover, the structures we proposed are compared with that reported in literatures, and show our structures yield a significant increase in DBP.In Chapter 4, we design a novel cascaded line defect photonic crystal waveguide with different cavities on two sides to break the trade-off between group velocity and bandwidth, realizing the slow light propagation with ultra wide band by simulation. First we investigate a simple line defect photonic crystal waveguide with the same cavities, and achieve an extremely flat band, satisfying both the small group velocity and vanishing GVD. Then, by introducing four different cavities on sides with different radii, a novel cascaded waveguide is proposed. There truly exist four close flat bands on the dispersion curves, which could be regarded equivalently as a wide bandwidth. Operating in the band, we can achieve slow light with ultra low group velocity (c/800) and low dispersion. Simulation is carried out to demonstrate the slow light propagation in the novel waveguide. Compared with conventional structures, our design has a breakthrough on the contradiction between low group velocity and wide bandwidth. The effective bandwidth of our structure could reach 4THz. And one could reduce the increment of the neighboring bands by decreasing the difference between the radii of the cavities, or expand the bandwidth by cascading more different cavities. In Chapter 5, we take the fabrication of the waveguide into account, and propose a waveguide with the triangle lattice of air holes, following the idea of the wideband design in the above chapter. First we investigate a commonly used structure with a line defect of the same index as the background in the middle, and find there is a series of close flat bands. However, the shift of the bands caused by the change of the radius of the air hole in the cavity is too tiny to meet the needs of wide band propagation. To solve this problem, we introduce a line defect made of air to enlarge the shift of the bands by reduce the inflection index in the middle of the waveguide. Then we propose a structure with dual cavities, and demonstrate two close and flat bands by the study of the dispersion curve and field distribution of this waveguide. At last, by introducing four different cavities on sides with different radii, a novel cascaded waveguide is proposed. There truly exist four close flat bands, which could be regarded equivalently as a wide bandwidth. The bandwidth of our structure could reach 2.02×10-2. Simulation of the static field distribution is carried out to demonstrate the slow light propagation in the novel waveguide.In Chapter 6, we improve the cascaded waveguide, which is proposed in Chapter 5, by replacing the line defect made of air with that made of SiO2, in order to avoid the dissipation on the third dimension for application. Similar with the analysis in the above two chapters, in this chapter, we first investigate the waveguide with the same cavities, and achieve an extremely flat band, satisfying both the small group velocity and vanishing GVD. Then we propose a structure with dual cavities, and demonstrate two close and flat bands by the study of the dispersion curve and field distribution of this waveguide. At last, by introducing four different cavities on sides with different radii, a novel cascaded waveguide is proposed. There exist four close flat bands. Operating in the band, we can achieve slow light with ultra low group velocity (c/900) and low dispersion. Simulation is carried out to demonstrate the slow light propagation in the novel waveguide. The effective bandwidth of our structure could reach 4THz.In Chapter 7, we propose an optimal approach on the design of the waveguide. By introducing the genetic algorithm into the band solving method, we design an optimal method on working out the structural parameters to obtain the minimal group velocity. The principal of the optimal method is: first set the structural parameters as the specific genes, and then write the evaluation function to calculate the goup velocity, and last control the genetic algorithm to achieve the minimal group velocity and the optimized structure of the device. This highly simplifies the design procedure of the slow mode waveguide.Chapter 8 summarizes the results of this work. Future research topics for the fabrication and optimize of the slow light waveguides are proposed.
Keywords/Search Tags:slow light, group velocity dispersion, delay-bandwidth product, photonic crystals
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