Design Of Multimode Photonic Passive Devices On Silicon Nitride/Lithium Niobate Hybrid Platform

Lithium niobate has attracted widespread attention in recent years due to its strong electro-optic effect,tight optical constraint,and wide optical transparent window from visible to mid-infrared.A series of high-speed active and nonlinear devices have been verified on this platform successfully,such as photoelectric modulators,mode switches,and frequency converters.The large-scale integration of photonic chips requires not only active and nonlinear devices,but also multimode passive devices to expand communication capacity.However,there are still challenges in the processing and manufacturing of high-precision microstructures on the LNOI platform,However,there are still challenges in manufacturing high-precision microstructures on the LNOI platform,resulting in a significant gap in multimode passive devices based on this platform.Since the platform has realized hybrid integration with other materials,such as silicon nitride,which can also utilize mature manufacturing processes while taking into account the advantages of various materials.Based on the silicon nitride lithium niobate hybrid platform,some passive components have been simulated and verified in this thesis.aiming at providing mature and high-performance solutions for the passive device design,based on the silicon nitride-lithium niobate hybrid platform.The main work is as follows:（1）As an essential connecting device in multimode photonic integrated circuits,multimode waveguide bending requires low insertion loss and low mode crosstalk in multimode transmission.This thesis designs a multimode waveguide bending that supports low loss transmission of different modes on the lithium niobate silicon nitride hybrid platform.By etching air grooves at the bend of the waveguide,the refractive index distribution of the material at the bend can be adjusted by introducing different numbers of air grooves,while avoiding the problem of immature processing technology faced by grayscale etching（etching out gradient waveguide thickness）.By utilizing the overlap integral of the mode distribution between the straight waveguide and the waveguide bend as the criterion,the low crosstalk transmission can be achieved by selecting the appropriate groove position.Then,the fabricated S-shaped waveguide bending with double air slots and supporting three modes is measured.The experimental results show that the insertion loss of TE₀-TE₂propagating in a pair of90°multimode bent waveguides is 0.72～1.42 d B,0.69～1.12 d B and 1.5～2.5 d B respectively in the range of 50 nm with the center wavelength of 1550 nm,and the mode crosstalk is less than-12.2 d B.（2）A compact and functional multimode multiplexer/demultiplexer is an essential component in modern high-speed optical communication and optical connectivity.In this thesis,An asymmetric Y-branch structure with multimode waveguide width along the Z propagation direction is designed on the lithium niobate silicon nitride hybrid platform,which can achieve multiplexing/demultiplexing of four TE modes（i.e.TE₀～TE₃）.The structure follows the approximate matching principle of refractive index,and the refractive index of multiple modes supported in the branch arm and corresponding modes in the bus waveguide are approximate,thus realizing the function of multimode demultiplexing.The experimental results show that the insertion loss of the four modes is above-1.68d B and the mode crosstalk is-13.0d B below in the wavelength range of 1500-1600nm.The device is expected to provide a new idea for the design of broadband mode demultiplexer devices on the LNOI platform.（3）A Y-branch structure that propagates along the Z direction of the beam has been designed on the same hybrid platform.This structure can realize the polarization and mode demultiplexing function by processing TE and TM modes simultaneously.When designing this structure,the polarization-independent mode separation effect is achieved by finding an appropriate waveguide width from the function curve of the mode refractive index changing with the waveguide width.The simulation results show that the insertion loss of different polarization states is greater than-0.06d B and the mode crosstalk is less than-15d B in the range of 100nm bandwidth.This work blazed a new trial for the design of polarization-independent demultiplexers.