Design And Optical Properties Of Polarization Control Devices On The Lithium-niobate-on-insulator

In recent years,with the continuous upgrading of big data technology and its expanding application,people’s requirements for data transmission and processing speed are getting higher and higher,and more and more attention is paid to the miniaturization and low power consumption of devices.Especially in data transmission and operation occasions such as data centers,there is an urgent need for a more effective technical means to break through the performance bottleneck problem of ICs.Currently,silicon waveguides are widely used due to their technical advantages of being compatible with CMOS processes,but they still suffer from high power consumption and poor performance,which will limit their further development.The emergence of integrated optics provides a feasible and effective means to solve the bottleneck problem.Lithium niobate has significant performance advantages on PICs due to their excellent electro-optical and non-linear optical effects.PICs based on the LNOI have become a hot spot and a difficult research area.Compared with the conventional silicon waveguides,the mode hybridization in lithium niobate waveguides becomes a problem affecting the large-scale application of PICs on the LNOI.In this paper,based on the polarization diversity system,the method of controlling polarization that is designing different polarization control devices on the LNOI is proposed to solve the mode hybridization problem of PICs,which is of great significance for large-scale optoelectronic integration on the LNOI.In this paper,the causes of mode hybridization in lithium niobate waveguides are systematically studied and polarization control devices on the LNOI are designed,and the main research contents are as follows:（1）The mode hybridization in lithium niobate waveguides was simulated to reveal the relationship between the structural parameters of waveguide and the degree of mode hybridization.Based on Lumerical FDTD and Mode software,the mode hybridization in straight and bent waveguides on the LNOI was analyzed,respectively.It is found that the film thickness is the essential parameter affecting the mode hybridization.In straight waveguides,the mode hybridization will occur between TM₀and TE₁modes.The smaller the film thickness and refractive index of cladding are,the smaller the widths and width range of waveguides corresponding to the mode hybridization are,which leads to stronger mode hybridization.On the X-cut LNOI platform,mode hybridization occurs between the fundamental modes in bent waveguides.As for the mode hybridization caused by polarizations coupling,increasing the bending radius has almost no effect.Therefore,the smaller film thickness should be selected to design devices considering the mode hybridization.（2）A PBS based on an ADC consisting of three waveguides is designed and the optical properties such as polarization extinction ratio,insertion loss and bandwidth are investigated.The phase mismatch for TM polarization is increased by the silicon nitride layer covered with the top of the lithium niobate waveguide,which can achieve separating TE and TM polarizations.In addition,the three-waveguides structure improves device compatibility.The device length is 112.2μm finally,and the PER is over 35 d B.The IL is less than 0.1 d B.And the PBS has a broad bandwidth of>80 nm with a large fabrication tolerance in addition to the variation of the waveguide width.（3）A PSR consisting of an adiabatic taper and an ADC is designed,which can be fabricated by a single-step etching process.The length of the PSR is 271μm.The PERs are 40.9 and 26.9 d B for TE and TM polarizations,respectively,and IL is less than 0.03 d B.The PSR has a relatively broad bandwidth of>50 nm.And the problem that the fabrication tolerance of the waveguide width is small is discussed,which can be improved by adding a filter.