Study On Grating Coupler Based On SOI Structure

Because of its wide bandwidth,high stability and low cost,the integrated photonic chip is widely used in important fields such as large-capacity communication,optical signal processing,avionics system and so on.Waveguide grating coupler is an important part of photonic integrated chip.It is used to couple optical signal into or out of photonic chip.Its coupling efficiency and bandwidth will directly affect the performance of photonic integrated chip.At the beginning of the development of integrated photonics,people began to study the waveguide grating coupler.People early used the lithography production,but limited by lithography process.It occured to simple structure and poor performance.Driven by the CMOS and micro-nano processing technology,a variety of new grating coupler continued to emerge,and the performance was improving.Based on SOI structure,the research is focusing on grating coupling device.Firstly,we introduced the coupling principle and analysis theory of grating coupler.It used the method of numerical simulation for uniform grating coupler,linear linearly front-end varying chirp-apodized grating coupler,ascending cosine front-end varying chirp-apodized grating coupler and Fibonacci sequence-changed front-end varying chirp-apodized grating coupler and analyzed the influence of effective refractive index on the coupling efficiency and bandwidth.The finite-difference time-domain(FDTD)algorithm is used to simulate the coupling process.In this paper,the SOI-based grating coupler is studied by combining theoretical analysis with numerical simulation.The research results are as follows:1.We use the theoretical analysis and numerical simulation to compared the refractive index distribution of the effects of the step length on the linearly varying chirp-apodized grating coupler and the different coefficients of ascending cosine grating coupler.By analyzing the effects of the refractive index distribution on the coupling efficiency and bandwidth,it established a complete mathematical model to obtain the optimal solution.The coupling efficiency of the linearly varying chirp-apodized grating coupler is 72.46%,and over 90% bandwidth of 127 nm,over 80% bandwidth of 187 nm.The coupling efficiency of the ascending cosine front-end varying chirp-apodized grating coupler can reach 72.92%,and over 90% bandwidth of 124 nm,over 80% bandwidth of 184 nm.2.It proposed the Fibonacci sequence-changed front-end varying chirp-apodized grating coupler.Under the same process conditions,the coupling efficiency of the grating coupler can be increased to 72.56%,and over 90% bandwidth of 139 nm,over 80% bandwidth of 193 nm.The over 90% bandwidth of Fibonacci varying chirp-apodized grating coupler compared to the linearly varying chirp-apodized grating coupler was boosted by 12 nm.The over 90% bandwidth of Fibonacci grating coupler compared to the ascending cosine front-end varying chirp-apodized grating coupler was boosted by 15 nm.In addition,the chirp-apodized grating coupler proposed in this paper is based on a commercial SOI wafer,which top layer of silicon was designed of 250 nm,and the fabrication process is compatible with the existing photonic integrated chip processing technology.3.The grating coupler with reflector is designed to improve the coupling efficiency and bandwidth of grating coupler.The coupling efficiency of the Fibonacci sequence-changed front-end varying chirpapodized grating coupler with Ag mirror reflector is 97.68% with over 90% bandwidth of 182 nm and over 80% bandwidth of 230 nm.The performance is better than the linearly front-end varying chirpapodized grating coupler and the ascending cosine front-end varying chirp-apodized grating coupler.The coupling efficiency of the Fibonacci sequence-changed front-end varying chirp-apodized grating coupler with three distributed bragg reflectors is 97.9%.