Silicon Subwavelength Multimode Y-junction And Its Mode Manipulation

Silicon on insulator plays an important role in short-range optical interconnection such as data center and on-chip interconnection,since it can be used to manufacture large-scale optoelectronic devices by the mature microelectronic techniques.Mode-division multiplexing could use different eigenmodes of waveguides to separately transmit data,which can avoid the integration of multiple wavelength silicon lasers compared to wavelength-division multiplexing,and thus reduce the cost and power consumption.The topic of the dissertation focuses on silicon multimode Y-junction and its mode manipulation.The purpose is to improve the communication capacity of on-chip optical interconnection and optimize the mode channel more flexibly and efficiently.Silicon multimode Y-junctions based on adiabatic evolution mechanism have been fundamental components for the construction of mode-division multiplexing devices due to their flexible and wavelength independent mode manipulation capabilities.However,the limited feature size achievable by lithography can greatly the adiabatic branching property and result in large excess loss and crosstalk.In this dissertation,we design a new type of multimode Y-junction with excellent performance.Such Y-junction can help to design on-chip devices or circuits with flexible mode manipulation,excellent performance and low power consumption based thermo-optic effects or phase change materials,such as reconfigurable multimode converter,multimode cross-connect router and dual-mode ring topology circuit with multiple optical nodes,which offer important reference values and application potentials for mode-division multiplexing systems.The main contents of the dissertation are as follows:（1）We propose and experimentally demonstrate a subwavelength adiabatic silicon multimode Y-junction with excellent performance.We propose an approximate ideal adiabatic branch structure consisting of a subwavelength chirped circular-hole-based array,which solves the technical problems that conventional Y-junctions are almost unusable due to limited lithography precision.Our fabricated device shows the average excess losses<0.5 d B and average crosstalks<-21.0 d B for subwavelength adiabatic four-mode symmetric Y-junction over the wavelength span from 1.53μm to 1.57μm,and the average excess losses<0.8 d B and average crosstalks<-18.0 d B for the four-mode multiplexer based on cascaded subwavelength adiabatic asymmetric Y-junctions.（2）We propose and experimentally demonstrate two types of reconfigurable silicon multimode mode converter.We propose a a new type of scheme to realize reconfigurable multimode mode converter based on Mach-Zehnder interferometer structure that using parallel-connected and cross-connected subwavelength adiabatic symmetric Y-junctions respectively.Such scheme is simple and could be expanded to include more modes.Both the two types of four-mode converter show the average insertion losses<2.5 d B and average crosstalks<-16.5 d B over the wavelength span from 1.53μm to 1.57μm.Benefited from the cross-connected scheme,the number of heating electrodes can be reduced by half,which has potential advantages of low power consumption.（3）We propose and design a power-efficient reconfigurable four-mode 2×2non-blocking cross-connect router based on Benes network.Power consumption is one of the important indicators of reconfigurable multimode devices,and there are two main sources:the number of optical switches and the power consumption of the optical switches themselves.First,the number of optical switches can be reduced by 15%using a Brute-force Search algorithm,in which the redundant optical switches were replaced by crossing waveguides.Second,we show a power-efficient single-mode 2×2 switch using a routing scheme based on subwavelength adiabatic Y-junctions,in which the path-switching is realized using phase change material Sb₂Se₃.The simulated results show the insertion losses and crosstalks are less than 0.6 and-20.0 d B over the wavelength span from 1.5μm to 1.6μm.（4）We propose and experimentally demonstrate a dual-mode ring topology mode-division multiplexing photonic integrated circuit with multiple optical nodes to realize TE0/TE1 data transmission in either specified clockwise or anticlockwise direction,where subwavelength adiabatic asymmetric Y-junctions are designed to achieve the desired optical ring topology.As proof of concept,we fabricate a dual-mode circuit with four nodes on silicon-on insulator platform,and the measured results show that the average crosstalks<-16.0 d B and average insertion losses<2.35 d B for both TE₀ and TE₁ modes over an observable 60 nm bandwidth centered at 1.55μm.