Research On Integrated Silicon Multimode Optical Power Splitters Based On Multimode Interference Couplers

The on-chip mode multiplexing system carries the transmitted signals in multiple eigenmodes,which can effectively increase the system transmission bandwidth and has the advantages of small size and dense integration.Building on-chip mode multiplexing system requires a lot of basic devices.Among them,multi-mode power splitters can realize the multi-port distribution of power to multiple modes at the same time which have very high requirements for port power consistency,bandwidth characteristics,and low loss characteristics.This paper proposes two different structures of silicon-based integrated multi-mode optical power splitters and corresponding optimization design methods based on the spectroscopic principle of multi-mode interference couplers.We manufactured and tested the designed device.We alos discusse and analyze the device structure,optimization methods and fabrication errors.The main contents of this paper are as follows:(1)We summarize the research progress of several silicon waveguide coupler structures and the problems in the realization of multi-mode power splliter.We analyze the design principles and solutions of existing multimode power splitters.We analyze the mode fields of each order of the silicon-based waveguide and introduce the working principle of the multimode interference coupler.We introduce the manufacturing process and experimental test methods of silicon-based devices.(2)We propose a design model and optimization method for N-mode 3 d B power splitter based on the guided mode excitation coefficient method.A guided mode excitation coefficient method is proposed for the phase error introduced by the excitation of higher-order guided modes.The objective is to maximize the sum of the excitation coefficients of low-order guided modes by carefully selecting structural parameters which will limit the excitation of the high-order guided mode and reduce the phase error.The design process of the device structure and guided mode excitation coefficient method is described.The average excess loss of TE₀is less than 0.44 d B,and the average excess loss of TE₁ is less than 0.76 d B.The method can be used to design 3 d B power splitters for more modes,and the average excess loss for TE₂ obtained by simulation is less than 0.9 d B.In addition,the optimization effect of guided mode excitation coefficient method,value conditions,and fabrication tolerance of the device are discussed and analyzed.(3)We propose a dual-mode 3 d B power splliter and design method based on a multi-segment multi-mode waveguide structure.Based on the spectroscopic principle of the symmetric interference MMI,a multi-segment multi-mode waveguide structure is proposed to reduce the difference between the self-mirror effect of the fundamental mode and the higher-order mode.The width of the multi-segment multi-mode waveguide is optimized by the particle swarm algorithm to adjust the beat length of the fundamental and higher-order modes which will reduce the difference in self-image effect.The optimization design process is described,including the definition of optimization goals,setting the structural parameter range,etc.The average excess loss of TE₀ is less than 0.9 d B,the average excess loss of TE₁ is less than 0.82 d B and the average crosstalk is less than–19 d B over a wide 60 nm spectral range.In addition,the device structure improvement,the optimal solution of the particle swarm optimization algorithm,and the device's fabrication error are analyzed and explained in detail.At the same time,the proposed structure and design method can design a higher-order mode3 d B power splliter.The designed TE₂ 3 d B power splliter has an average excess loss of 0.62d B,and the average crosstalk is lower than–18 d B.Compared with the existing design schemes,the two multi-mode 3 d B power splliters proposed in this paper have simple device structure,small excess loss,low crosstalk,and good fabrication tolerance.The proposed guided mode excitation coefficient method can be used to design 3 d B power splliter supportting more modes.The final device length of dual mode 3d B power divider based on particle swarm algorithm is only 12.1?m,which has the characteristics of ultra-compact size for dense system integration.