Study On The Characteristics Of Photonic Quasicrystals And Design Of Si Wire Interleaver

Optical devices are the basis for the optical communications system. The performance of optical devices will directly affect the performance of optical communications system. As the development of optical communications system, optical devices have been clearly shown the trend of integrated and micro-nano structured.So far, the solutions of the passive optical devices with integrated and micro-nano structured are: photonic crystal devices and silicon-based photonic integrated circuits, etc.Photonic crystal is an artificial crystal with periodic arrangement of different dielectric constant. If we found an appropriate medium and periodic structure parameter, there will be a photonic band gap existed. Photon whose frequency is the gap can not propagate in photonic crystal. So photonic crystal has a unique ability to control light propagate. In recent years, photonic quasicrystal as a special class of photonic crystal has received an increasing amount of attention. Compared with the photonic crystal, photonic quasicrystal has distinct advantages. It shows a quasiperiodicity which exhibits high-order rotational and mirror symmetry, but no translational periodicity. So the frequency band features of photonic quasicrystal will not be affected by the the impact of the direction of incident light. And a photonic band gap can exist easier. It is noteworthy that the size of photonic quasicrystal can be achieved in nano-or micro-order of magnitude.This offers the possibility of optical devices with integrated and micro-nano structured. Another approach for optical devices with integrated and micro-nano structured is to silicon-based photonic integrated circuits. Silicon is "transparent" under the wavelength of optical communications which is near 1.31μm and 1.55μm. As a large refractive index difference between silicon and cladding (SiO2), Si photonic wire (micro-nano size waveguide) can form a strong limit-type waveguide structure. Because of the strong limit-type waveguide structure, the loss will be less when photons turn around at small radius of curvature. So Si photonic wire can achieve small-size and compact optical waveguide devices. More importantly, the technology of Si wire is fully compatible with mature silicon-based CMOS technology. For the reason, it will be easy to implement large-scale production. So silicon-based photonic integrated circuits will be one approach to make optical devices with integrated and micro-nano structured.The main work of this paper is to design 2D photonic quasicrystal, photonic quasicrystal fiber and Si wire interleaver under the trend of optical devices with integrated and micro-nano structured. Our aim is to provide a possibility that the basic research of these three kinds of devices would move the process forward of optical devices with integrated and micro-nano structured.First, we have used rigorous coupled wave analysis (RCWA) and finite difference time domain (FDTD) method to calculate the frequency band features of Stampfli and Penrose 12-fold photonic quasicrystal. A photonic band gap of these two photonic quasicrystals could be found the wavelength of optical communications in the specific structure parameters. And we found that photonic band gap will be not changed under different direction of incident light. So we confirmed that the frequency band features of photonic quasicrystal could be calculated by the transmission spectrum along high-symmetry directions.After the analysis of frequency band features of photonic quasicrystal, we started to study the constituent parts making the photonic quasicrystal. First, we have found the constituent parts making the photonic quasicrystal. Then we constructed new translational periodic structures by arranging the constituent parts. We could also constructe new structures when only the constituent parts are left. Finally, we calculated the frequency band features of these new structures. The purpose of doing this is to analyse the influence of the optical characteristics of photonic quasicrystal by there constituent parts. From the results, we found that some special constituent parts could determine optical characteristics of photonic quasicrystal. And we show that new structures, obtained by arranging some of the quasicrystal's constituent parts in a translational symmetry pattern or only the constituent parts are left, can reproduce the same optical characteristics of photonic quasicrystal. These special constituent parts must follow the condition: large coverage areas, using the same basic cells of the quasicrystal, possessing the same short-range rotational symmetry of the quasicrystal, large unit area.When we have understood the frequency band features and the mechanism of photonic band gap of photonic quasicrystal clearly, we started to study index guiding photonic quasicrystal fiber by the plane-wave expansion method in a frame of super-cell description. We found that the cladding mode of photonic quasicrystal has its maximum intensity along the center of the fiber. So the effective refractive index of the cladding will be the same when the size of the cross section of photonic quasicrystal fiber changed. This phenomenon will induce that the V parameter of photonic quasicrystal fiber could be calculated, similar to index guiding photonic crystal fiber.Then we havc tried to find the single-mode operation condition of Stampfli 12-fold, 6-fold, 8-fold and 10-fold index guiding photonic quasicrystal fiber by methods: V parameter calculation and looking for a sudden increase in the extension of the second-order mode of the fiber. We have demonstrated that V parameter defined as a useful tool for identifying single-mode operation regime can be utilized only for smoothly changing photonic quasicrystal fiber which do not have isolated high-refractive regions around the core, like Stampfli 12-fold and 6-fold index guiding photonic quasicrystal fiber. On the other hand, if such high-refractive regions do exist in the photonic quasicrystal fiber, such as 8-fold and 10-fold index guiding photonic quasicrystal fiber, the V parameter approach looses its validity and only a mode area analysis can identify the threshold value of for single-mode behavior of the fiber.We have analysised the endlessly single mode and almost zero ultraflattened dispersion characteristics of Stampfli 12-fold photonic quasicrystal fiber. We found that a single mode is allowed for all wavelength band when the ratio of diameter of air holes and lattice constant d/a is less than 0.515, which is the cutoff ratio for the endlessly single mode operation. The cutoff ratio of 12-fold photonic quasicrystal fiber is larger than a triangular lattice and a square lattice photonic crystal fiber. 12-fold photonic quasicrystal fiber can also has almost zero ultraflattened dispersion, -0.5 to 0.5 ps /nm? km, in the range of wavelength (455nm) from 1.320μm to 1.775μm, which is wider than a triangular lattice photonic crystal fiber.Finally, we have proposed a cross-ring Mach-Zehnder interferometer (CR-MZI) Si photonic wire interleaver. We first detailed analysed the filter response property of the Si photonic wire MZI and ring-resonator(RR)structure by optical waveguide theory. From the filter response property of MZI and RR, we pointed out replacing the input terminal of MZI with a cross-ring(CR), an "8" shaped ring resonator (RR) to form the proposed CR-MZI interleaver. This special way of comprised of MZI and CR can be achieved as an interleaver. As the cross-ring serves as both a splitter and a delay line, the proposed interleaver is more compact and easier to design. So CR-MZI interleaver has greater potential for applications in Si-based integrated optical circuit. In order to design a Si wire interleaver with flat-top spectral response and higer transmission efficiency, it's necessary to optimize the CR-MZI interleaver structure.We have calculated the length of 3dB directional coupler (DC) and the length of RR DC. We have also designed and optimized the structure of an "8" shaped CR. In this way, we have established the structure parameters of CR-MZI Interleaver. Combined with the transmission spectrum of the Si wire CR-MZI which is fabricated in Institute of Microelectronics in Singapore, we considered the influence by the loss in CR, angle of the 3dB DC and the loss by the transmission and coupling in RR DC. Following the influence, we have modified CR-MZI Interleaver structure parameters. In this way, we could use the principle and structure parameters of CR-MZI Interleaver to explain the results of the experiment.In summary, 2D photonic quasicrystal, photonic quasicrystal fiber and Si wire interleaver have been studied, designed, analysed in this thesis. From the results, we could see that these three kinds of devices could satisfy the requirements for optical devices with integrated and micro-nano structured.