Research On Mode Control For Space Division Multiplexing Based On Silicon Integrated Devices

With the rapid development of the Internet,the surge in demand for related services such as instant messaging,search engines,high-definition video,and live webcasts has driven the expansion of fiber-optic communication systems.However,the transmission capacity of traditional single-mode fiber-optic communication systems is limited by nonlinear effects such that it cannot meet the ever-increasing demand for communication capacity in the future.Space division multiplexing can further enhance the optical fiber transmission capacity based on technologies such as wavelength division multiplexing,polarization division multiplexing,and advanced modulation formats.In order to make full use of the diversity of spatial modes,it is of great practical significance to study how to excite and multiplex as many modes as possible on the transmitter side.Silicon photonic integrated devices are favored by their small size,low loss,low cost and stability and thus considered as one of the most promising options for interfacing with fiber-optic transmission links.On the other hand,on-chip all-optical signal processing technology also needs to be upgraded for space division multiplexing for applications such as chip interconnects in data centers.This thesis focuses on the research of mode control for space division multiplexing based on silicon photonic integrated devices.The research content includes device design for the expansion of on-chip modal dimensions,device design and experiments for the excitation of orbital angular momentum modes in ring-core fibers,along with device design and experiments for all-optical logic operations in mode divison multiplexing system.The main research results of the thesis are as follows:(1)A high-order mode converter based on asymmetric waveguide is designed.The mode hybridization and polarization dependence of the device are analyzed.The effects of device etching parameters on mode conversion efficiency and device length are studied for TM and TE polarization states,respectively,and the parameter selection guide of the device is given in combination with the fabrication tolerance analysis.The simulation results show that the mode conversion efficiency can reach 98%.The mode conversion in the adiabatic taper structure is studied.Based on the above devices,a two-mode multiplexing system is designed with mode extinction ratio exceeding 15 dB in a wide wavelength range,which realizes the vertical expansion of the on-chip modal dimensions.(2)A novel orbital angular momentum emitter is designed and fabricated.A micro-ring resonator embedded with angular gratings is combined with a multimode interference coupler to generate four orbital angular momentum modes with different orders and polarizations at a specific wavelength,making full advantage of the diversity of spatial modes.A micro-ring resonator design scheme based on semi-analytic method is proposed.The fabricated device is tested and proved to be in line with the design expectations.The device successfully excites the corresponding orbital angular momentum modes in the ringcore fiber.The modal field of the fiber output remains stable even if the fiber is substantially perturbed.The optimal coupling condition for the device as a photon source is theoretically analyzed,and the system loss is tested based on the existing setup,indicating that the device is expected to be applied to quantum optics under the condition of improving device emission efficiency and reducing system loss.(3)All-optical logic operation compatible with mode division multiplexing system is proposed.The multimode nonlinear Schr?dinger equation is numerically solved and a simulation model is established,which can be used to study the transmission evolution process of multimode signals in nonlinear media.An on-chip two-mode multiplexing system is designed and fabricated.By changing the size of the multimode waveguide,the dispersion characteristics of the modes are controlled and the phase matching condition is satisfied.The feasibility of mode-selective four-wave mixing is verified by the above model,and the dual-channel logic operations of the mode division multiplexing system are verified by experiments.Bit error rates of the system are measured,and the results show that the additional power penalty for the simultaneous operation of the two modes compared to the single mode operation is 1.3 dB and 1.1 dB,respectively.