Theoretical Models And Optimal Designs Of Silicon-based Nonlinear Devices

After the first demonstration of the laser,nonlinear optics,which studies the interactions between the laser and the matter,has been developed rapidly.Owing to the ultrafast response speed and the unique properties of energy conversion and transformation,the nonlinear optics,with the ability of light controlling light,has grown into a mature and widely used academic discipline and played indispensable roles in the field of communication,sensing,computing,imaging,etc.In recent years,the photonic integration technology is aggressively developing,in order to accommodate the “faster,smaller,smarter,greener” trend of the big data era,the traditional research of nonlinear optics based on bulky material and optical fiber is gradually expanding to the integrated chips.Integrated devices allow for a tight confinement of optical field to a sub-micron region,and thus theoretically enhance the light-matter interactions and improve the nonlinear efficiency and minimize the device size.Although many new integrated platforms and novel device structures have been proposed and demonstrated,the efficiencies of the on-chip nonlinear devices and systems are lower than that of optical fibers,and thus cannot meet the practical requirements.It is highly related to the absent of on-chip nonlinear models and design criterions.In this dissertation,considering various nonlinear effects and wide range of applications,we take the silicon as the research platform,establish complete nonlinear models that include light,electric and heat for the most commonly used non-interferometric and interferometric devices(i.e.straight waveguide and microring resonator).Furthermore,we propose simple and reliable quality factors to offer guidances for efficient device optimizing and novel integrated platforms exploring,based on the complicated models.Finally,the two types of devices are applied in multi-channel all-optical signal processing.The main research contents of this dissertation can be summarized as follows:(1)The structural features,classification and theoretical basis of straight waveguide and microring resonator are studied.Using the derivation method of wave optics and the numerical simulation methods,the eigenmodes supported by the straight waveguide and the optical field transmission characteristics of the corresponding modes are solved.Based on the most commonly used steady-state solution method,the transfer function,the optical field distribution and the important characteristic parameters of the microring resonator are derived under the linear state.Based on the nonlinear Schrodinger equation,the common nonlinear effects and the commonly used nonlinear models of the two types of devices are analyzed.(2)The nonlinear Schrodinger equation is generalized to the silicon-based platform,and the nonlinear models of the straight waveguide and microring resonator are improved and experimentally verified.Two-photon absorption,free carrier absorption and dispersion,as well as thermo-optic effect are considered into the nonlinear Schrodinger equation and the nonlinear model of the straight waveguide.On the other hand,an equivalent straight waveguide model is established for the microring resonator to study the nonlinear processes involving multi-beam interference.A silicon-based microring resonator is fabricated,and the four-wave mixing is used as an example for experimental verification.The results show that the relative error between the simulation based on the improved model and the experimental results is less than 0.55 d B,which is 20.68 d B lower than that of the commonly used model.(3)The design methods on optimizing the nonlinear efficiency of straight waveguide and microring resonator are proposed,and the theoretical maximum efficiencies are presented.According to the improved model,the influences of various nonlinear effects on the two types of devices are quantitatively analyzed.Through reasonable approximation,the analytical solutions of the nonlinear models are derived.On this basis,the relationship between the highest nonlinear efficiency and the geometric parameter of a straight waveguide are obtained,as well as the corresponding optimal waveguide length.It can be concluded that there is no absolute optimal ring length and coupling coefficient for microring resonators,and the nonlinear efficiency of the reported microring resonators cannot reach the theoretical resonance enhancement of an equal length straight waveguide.The highest nonlinear efficiency of a microring resonator is theoretically confirmed to be identical with that of a straight waveguide,under the same integrated platform.(4)The applications of the integrated nonlinear devices in multi-channel all-optical parallel signal processing are investigated.Based on the straight waveguide and the microring resonator,two high-efficiency,low-crosstalk dual-mode signal processors are designed for the mode division multiplexing system.Through the comprehensive optimization on both the transversal and longitudinal dimensions of the straight waveguide,the simultaneous control of intra-mode and inter-mode nonlinear effects is achieved.Dual-mode wavelength conversion with efficiencies higher than-23.15 d B and crosstalks lower than-19.27 d B is achieved in the experiment.Compared with the literatures,the efficiency is improved by 5d B.In addition,a multi-mode signal processing scheme based on the microring resonator is proposed and implemented.Compared with the straight waveguide scheme,it avoids linear and nonlinear inter-mode crosstalks.The measured dual-mode wavelength conversion efficiencies are higher than-28.69 d B,and the crosstalks are lower than-35 d B.