Diffraction Field Of Grating Coupler

With the development of science and technology,electrical interconnection is gradually unable to meet the actual needs for its own limitations.And optical interconnection widely paid attention to studied has become the most potential alternative to electrical interconnection for its advantages such as high bandwidth,low delay,low crosstalk and strong ability to resist electromagnetic interference.Optical waveguide is one of the most important devices in optical interconnection.It is one of the emphases in the research of optical interconnection to realize the efficient and lossless coupling direction of optical waveguide.Grating coupler can be made anywhere on the optical waveguide,which means that it can upload and download signal anywhere in the waveguide.This advantage makes the design of optical path very flexible,so it is one of the most concerned coupling methods.In order to solve the problem of light coupling and steering in optical waveguides,the structure parameters of grating couplers is analyzed and the relationship between them and the performance index of grating couplers is obtained based on the commonly used theory of grating couplers.The common research methods of grating coupler include fundamental electromagnetic theory,analytical method and numerical method.And fundamental electromagnetic theories includes Bragg condition,Bloch theorem,reciprocity theorem and the calculation of grating coupler coupling efficiency.The analytical methods include coupling mode theory,perturbation method,equivalent dielectric film theory,plane wave expansion method and strictly coupled wave theory.Numerical methods include Beam Propagation Method,Eigenmode Expansion Method,Finite Element Method,and Finite Difference Time Domain Method.The Bragg phase matching condition,reciprocity principle,grating coupler coupling efficiency calculation method and FDTD numerical method are mainly used in this paper.Two kinds of grating couplers with different structures applied to Silicon-On-Insulator(SOI)optical waveguides and polysiloxane polymer optical waveguides respectively are designed on the basis of this theory,and their structures are optimized to improve the coupling efficiency.The specific work is as follows:1.The diffraction field simulation and efficiency calculation of grating coupler are realized based on FDTD.In this paper,FDTD algorithm is implemented by MATLAB because FDTD is one of the most commonly used numerical calculation methods to study electromagnetic phenomena and MATLAB has powerful data visualization capabilities.The whole diffraction process of light in grating coupler is simulated and a method for calculating the energy of diffraction light produced by light passing through the grating is proposed,that provides the simulation condition and power calculation method for the study of grating coupler.2.The relationship between grating period,etching depth,duty cycle and diffraction angle of grating coupler is deduced.Generally,the relationship between grating period and diffraction angle can be obtained by using the Bragg condition,but the grating diffraction angle is also related to etching depth and duty cycle.The relationship between grating period,etching depth,duty cycle and diffraction angle of grating coupler is derived with the Eigenmode equation of planar waveguide and the FDTD algorithm is used to verify.According to the formula of diffraction angle,the diffraction angle of grating coupler can be estimated roughly by using the structural parameters of grating(grating period,etching depth,duty cycle),which provides reference for the design of grating coupler with high coupling efficiency.3.A nonuniform grating coupler of SOI optical waveguide is designed.Firstly,the effect of structural parameters(etching depth,grating period,etc.)on coupling efficiency of SOI uniform grating coupler is studied.Based on FDTD algorithm,the grating structure composed of different grating periods(range: 390nm-830nm)and etching depth(10nm-310nm)is traversed,and the coupling efficiency of uniform grating coupler under different structures is obtained.When the period is 630 nm,the duty cycle is 0.5,the etching depth is 190 nm,the coupling efficiency of the uniform grating coupler is the highest,which is 41%.Then,a nonuniform grating coupler of SOI optical waveguide is proposed,in which two grating structures with different periods are etched on the same waveguide,and their different diffraction angles are used to achieve the purpose of focusing.The coupling efficiency is improved by optimizing the structure parameters.The coupling efficiency is 62.37% by FDTD simulation.4.A grating coupler based on polysiloxane polymer waveguide is designed.The coupling efficiency of grating is improved for adding the high refractive index layer covering film on the basis of the structure of uniform grating coupling.Based on FDTD algorithm,the grating structure with the maximum coupling efficiency of grating coupler is obtained(the period is 2.6?m,the etching depth is 5?m).The maximum coupling efficiency is 17.2% by simulation.The structure of two-layer etched grating coupler is designed on this basis and the structure parameters are optimized to improve the coupling efficiency.The simulation results show that the coupling efficiency is 37.4%.