Research On Silicon Waveguide Mode-division （De）Multiplexer Based On Directional Coupler

With the rapid development of science and technology,people’s requirements for information transmission capacity and rate are also increasing.The continuous growth of data traffic in global optical communication has prompted the researchers from various countries to seek new technologies and solutions to increase the capacity of optical fiber communication systems.However,due to the limitation of Shannon limit,the information capacity that single-mode fiber can hold is gradually approaching its theoretical limit.The mode-division multiplexing（MDM）system based on few-mode fiber has received extensive attention and research as the latest solution to solve the capacity limitation of single-mode fiber.MDM technology can transmit spatial modes through independent channels,which has the advantages of large transmission information capacity and low energy consumption.In the MDM communication network and the on-chip optical interconnect system,the mode converters and the mode （de）multiplexers based on the directional coupling structure play a crucial role.Based on the mature microelectronics industry technology of complementary metal oxide semiconductor process（CMOS）,silicon photonics technology has the unique advantages such as low cost,small device size,and large transmission capacity,which has emerged as one of the most promising methods for on-chip integrated optics applications.Under this background,this thesis mainly studies the silicon waveguide mode（de）multiplexers based on directional coupler.The main contents include the following parts:First,the development of integrated silicon photonics and the advantages of silicon photonics technology are expounded.Besides,the research background of the mode（de）multiplexers and its application in optical communication and other fields are introduced,and the theory of optical waveguide mode is also analyzed.Secondly,based on the two-dimensional planar optical waveguide structure,the silicon-based waveguide mode converter based on the asymmetric directional coupler structure is studied.The designed mode converters can realize the mode conversion between E₁₁and E₂₁,and E₁₁and E₃₁modes,and the conversion efficiency can reach more than 97.51%.On this basis,a silicon-based waveguide mode filter based on the symmetrical directional coupler structure is further proposed.The effective refractive index,coupling spacing,coupling length and coupling efficiency of the device are simulated and optimized by using the Lumerical-FDTD software.Finally,two kinds of silicon-based waveguide mode filters that can realize the mode filtering function are obtained.The simulation results show that the device can realized the filtering function among E₁₁,E₂₁,and E₃₁modes,and the transmission efficiency of the three modes is above 95%.Moreover,the mode output port can be changed by adjusting the length of the coupling region.Finally,the silicon-based waveguide is extended from a two-dimensional planar structure to a three-dimensional vertical structure.The three-dimensional silicon-based waveguide mode（de）multiplexer based on an asymmetric directional coupler is proposed.This device can realize the mode conversion between the E₁₂and E₂₁ modes in the few-mode silicon waveguide and the E₁₁mode in a single-mode waveguide.This device is based on silicon platform that uses silicon as the waveguide core and silica as the waveguide cladding.By optimizing the coupling spacing and coupling length of the two cascaded directional couplers,the coupling efficiencies between the E₁₂-E₁₁ and E₂₁-E₁₁ modes are 97.6%and 95.1%,respectively,and the crosstalk is less than-30 dB.At the same time,based on the wafer bonding technology,the fabrication process of the three-dimensional silicon-based waveguide mode （de）multiplexers based on the asymmetric directional coupler structure was also designed.