Research On Polarization Beam Splitting And Mode-order Conversion Based On Thin-film Lithium Niobate

Lithium niobate-on-insulator（LNOI）,as a kind of material with excellent acousto-optic,electro-optic and nonlinear optical properties,has attracted extensive attention.LNOI is expected to replace silicon materials in the photonics field and provide solutions to the issues of large transmission loss and insufficient transmission bandwidth in the communication systems.In the future,photonic integrated chips based on LNOI platform will indicate great application value in optical communication,big data,cloud computing,artificial intelligence and other fields.For the purpose of further improving the data bandwidth,the optical communication systems usually use two multiplexing technologies,that are,mode division multiplexing and partial division multiplexing.The mode division multiplexing system applies the mode converter to generate different modes as the channel of information transmission,while the polarization beam splitter is used to make the two polarization states carry information respectively,which can double the transmission capacity.Currently,polarization beam splitter and mode converter tend to be independent waveguide structures.If the two can be combined reasonably,the overall on-chip integration can be effectively improved.Therefore,on-chip multiplexing transmission especially needs integrated devices with polarization beam splitting and mode conversion functions,which will also contribute to the rapid development of photonic integration technology on LNOI platform.The main research contents and innovations of this paper are summarized as follows:Firstly,based on LNOI platform,a three-waveguide polarization beam splitting and mode converter is proposed in this paper.Such kind of device can realize not only the polarization separation between input TE₀ mode and TM₀ mode,but also the mode conversion from input TE₀ mode to output TE₁ mode with a relatively good performance.Moreover,the simulation results show that when TM₀ mode is input,the extinction ratio of the device is 26.15 d B and the insertion loss is 0.47 d B.When TE₀ mode is input,the mode conversion efficiency from TE₀ mode to TM₁ mode reaches 99.6%,as well as the crosstalk is-22.82 d B and the insertion loss is 0.12 d B.In addition,by increasing the width of the cross-output waveguide,the device scheme can be further extended to the output of other high-order TE modes,prossessing favorable device function scalability.Secondly,at present,the sidewalls of LNOI waveguides are regularly inclined.To study the influence of the inclined sidewall caused by the processing technology on the performance of the device,this paper designs a DC structure polarization beam splitter based on the inclined LNOI waveguide,and optimizes the performance of the device.At the working wavelength of1,550 nm,the extinction ratio of the device reaches 23.15 d B,the working bandwidth exceeds70 nm,and the manufacturing tolerance is 15 nm approximately.It can be seen from the simulation results that the key parameters of the device are able to be effectively reduced.Thirdly,in terms of of fully considering the research status and processing technology of LNOI,a polarization beam splitting and mode converter based on sidewall inclined LNOI waveguide is proposed.The simulation results demonstrate that the extinction ratio of the device is 29.91 d B and the insertion loss is 0.67 d B when TM₀ mode is input.When TE₀ mode is input,the mode conversion efficiency from TE₀ to TE₁ reaches 97.56%,the crosstalk is-16.22 d B and the insertion loss is 0.39 d B.If the strict evaluation criteria is set while keeping the extinction ratio of the device to be greater than 20 d B and the crosstalk to be less than-13d B simultaneously,the bandwidth of the device is about 54 nm and the manufacturing tolerance is 15 nm.