A Study On High-Order Mode Conversion Of Thin Film Lithium Niobite Optical Waveguide

Lithium niobate-on-insulator（LNOI）has attracted attention because of its high electro-optical coefficient,excellent piezoelectric,elastic,nonlinear properties and stable physical and chemical properties,promoting integrated photonics into a new period of development.Wideband,high-capacity,high-rate on-chip optical interconnect is the goal of integrated photonics,and multiplexing techniques are the key to achieving these goals.For on-chip optical interconnects,mode-division multiplexing（MDM）is an effective way to improve transmission capacity.The generation and multiplexing of high-order optical waveguide modes usually require the use of mode generators or converters,and then the information to be transmitted can be loaded into the high-order mode,and the capacity of the transmission system can be improved by mode division multiplexing.Thus,a variety of device schemes have been developed,focusing on the design of mode conversion functional areas,including deep etching,shallow etching grooves,optimization algorithms,etc.Almost all of these schemes are changes to the surface structure of the waveguide,which can be collectively referred to as the metasurface method.Based on this,seven mode converters based on LNOI with different functions are designed in this paper.By etching the designed metasurface structure on the surface of waveguide,mode conversion of TE polarization is realized.The main research contents of this paper are summarized as follows:First,based on LNOI platform,a mode converter using nanowire loaded structure is proposed.This device introduces a silicon nanowire array and Sb₂Se₃ rectangular nanowires on top of the thin film lithium niobate ridge waveguide,and the nanowires can achieve the accumulation of phase difference.After parameter optimization,the proposed TE₀-TE₁ mode converter has good performance,with a conversion efficiency of 97.6%,crosstalk of-19.7 d B,and insertion loss of 0.31 d B at 1550 nm.Through horizontal expansion,TE₀-TE₂ mode conversion is achieved with a conversion efficiency of 90.9%,crosstalk of-15.3 d B,and insertion loss of 0.62 d B.If we further extend the device structure,we can also obtain the mode conversion from input TE₀ mode to output TE₃ and TE₄ mode.Second,a mode converter based on"V"shaped silicon on LNOI waveguide is proposed.Compared to the previous structure,the metasurface in this design is made of only silicon,greatly simplifying the processing process.In this design,the"V"shaped silicon structure as metasurface,introduce perturbation in the waveguide,change the phase of light transmission,so as to achieve the conversion of low-order mode to high-order mode.After parameter optimization,the design can achieve TE₀-TE₁ mode conversion with a conversion efficiency of96.8%,crosstalk of-28.6 d B,and insertion loss of 0.78 d B.At the same time,the bandwidth and manufacturing tolerances of the device are analyzed.Third,due to the increased demand for high-order modes of devices,the scalability of devices has become a concern issue.In order to verify the scalability of the second structure,a TE₀-TE₂ mode converter based on"V"shaped silicon is proposed.The"V"shaped silicon metasurface in the design changes the material uniformity distribution,causing a change in the local refractive index of the material,resulting in a phase difference when the light is transmitted in the waveguide.The design can achieve TE₀-TE₂（TE₀-TE₃）conversion without increasing the length of the conversion area,using the same structure and optimizing only the parameters,with a conversion efficiency of 98.1%（82%）,crosstalk of-25.6 d B（-15 d B）,and a loss of 0.6 d B（1.4 d B）.This proves that the design is highly scalable.So far,the design scheme proposed by this institute can realize the mode conversion from TE₀ to TE₁,TE₂ and TE₃,and the device has high performance,compact structure,easy manufacturing and high scalability.