| Photonic crystal is a kind of periodic and artificial material,which has the characteristics of photonic band gap and photon localization,and is widely used in the field of optical communication.The traditional photonic crystal waveguides are constructed by introducing line defects into photonic crystals,which have backscattering and side-scattering losses,and are susceptible to machining error defects,thus reducing the transmission efficiency of optical waves.Topological photonic crystals have unique unidirectional transmission characteristics and robustness to defects and sharp corners,which provide a powerful means to manipulate light transmission and the interaction between light and matter at the wavelength level.By using topological photonic crystal waveguides instead of the traditional stripe waveguides and line-defect photonic crystal waveguides,efficient optical transmission can be achieved,which makes up for the inherent defects of traditional optical devices with large size and high transmission loss,and improves the integration degree of integrated optical chips.In this paper,a valley photonic crystal waveguide is constructed based on topological photonics theory,and its characteristics are verified theoretically and experimentally.On this basis,a kind of valley photonic crystal Mach-Zehnder type silicon-based electro-optic modulator was designed,and its performance was characterized.The specific research contents of this paper are as follows:(1)The unidirectional propagation characteristics of topological optical waveguide based on the valley hall effect are studied.Firstly,a topological optical waveguide composed of two kinds of valley photonic crystals with different topological indices is designed,and its unidirectional propagation characteristics are analyzed.The optical transmission properties of valley photonic crystal waveguides,line defect photonic crystal waveguide and traditional strip waveguides at sharp cornors are compared.The simulation results show that the wave can pass through the sharp corners almost without loss in the valley photonic crystal waveguide,and the outgoing transmittance is as high as 99%,but the transmission behavior of line defect photonic crystal waveguide and the traditional strip waveguide is poor.The wave suffered severe backscattering and side scattering at the corners,resulting in a serious drop in transmission efficiency.Secondly,the valley photonic crystal waveguide was preliminarily prepared by photolithography and etching technology,and its morphology and optical properties were characterized.The experimental results show that the valley photonic crystal waveguide has good shape.The average corner loss of optical wave in valley photonic crystal bent waveguide is 0.36 d B,while that in line defect photonic crystal bent waveguide is 0.85 d B,which fully shows the excellent unidirectional transmission characteristics and robustness of Valley photonic crystal waveguide.(2)A Mach-Zehnder type silicon-based electro-optic modulator based on valley photonic crystals is designed and its performance is analyzed.Firstly,for the optical structure of the modulator,a valley photonic crystal beam splitter and beam splitter is designed,and its comprehensive performance is analyzed.The results show that the beam splitter and beam combiner based on valley photonic crystal can achieve efficient beam separation and merging in the wavelength range of 133 nm centering on the communication wavelength of 1550 nm,and the efficiency of separating and merging beams is close to 50% and 100%,respectively.Secondly,the electrical structure of the modulator is designed.The structural parameters of the electrode are optimized and the impedance and phase velocity matching are achieved.Finally,the overall modulation performance of the electro-optic modulator is numerically simulated by combining the optical structure with the electrical structure.The results show that the modulator can work in the wavelength range of 1483?1616 nm.At ?1550 nm,the extinction ratio is17.05 d B,the insertion loss is 0.16 d B,the modulation depth is 0.98,and the modulation bandwidth is 126 GHz,and the overall size of the device is only on the micron scale.In summary,the unidirectional propagation characteristics of topological optical waveguides based on the valley hall effect are studied theoretically and experimentally in this thesis,and a valley photonic crystal Machzendel-type electro-optic modulator is designed.The modulator has the characteristics of small volume,high modulation bandwidth and low loss,which provides a research idea for the development of modern optical communication system,optical integration and optical interconnection system. |
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