Research On Single-crystal Lithium Niobate Thin Film Integrated Modulator Based On Strip-loaded Waveguide

Microwave photonics technology has the advantages of large bandwidth,low loss,and strong anti-electromagnetic interference ability.It has important application prospects in the fields of wireless communication,phased array radar,and deep space exploration.However,in a traditional microwave photonic link,a cascade of multiple devices is usually required to implement signal processing functions such as phase modulation,frequency conversion,and optical frequency comb.The cascade of these separate devices causes large transmission loss and performance degradation.The traditional lithium niobate electro-optic modulator is relatively large,and its substrate is not easy to integrate with other materials.The newly developed single-crystal lithium niobate thin film makes full use of the modulation characteristics of lithium niobate,and at the same time,because its substrate is silicon,it is easy to mix heterogeneous integration with a variety of materials.This paper intends to design and process a new type of cascade modulator.The modulator consists of a phase and intensity modulator cascade structure formed on a lithium niobate film through etching and other processes.The modulation unit is separately controlled by voltage to achieve simultaneous modulation of the phase and intensity of the light wave.The main work of this paper is as follows:1.Strip-loaded waveguide,Y branch and coupling structure is optimized based on the finite element software COMSOL.The influence of the loading strip on the optical mode field distribution of the waveguide was studied,and the single-mode geometric size of the loading strip was determined.The energy flow ratio between the waveguide region and the non-waveguide region was calculated to characterize the confinement ability of the loaded strip waveguide,the energy flow ratio between lithium niobate layer and the waveguide region was calculated to characterize the confinement ability of lithium niobate layer,eventually,the refractive index range of the loaded strip waveguide with high modulation efficiency and strong confinement ability was determinded.Three simulation models of Y-branch structures,Cos,Sin,Arc,was established and analyased,the results show that the Cos-type Y-branch has smaller transmission loss under the same conditions.On this basis,the influence of length and high of the COS-type Y-branch are respectively explored for the propagation loss,a Cos-type Y branch with a low loss(0.5d B)and a small size was purposed.In view of the serious mode field mismatch between the waveguide and the optical fiber,the wedge coupling structure of the waveguide was optimized to reduce the coupling loss from 8.2d B to 6.1d B.2.Sample processing process was design,Si N-LN loaded strip straight waveguide samples were prepared,coupling platform which was used to test the Si N-LN straight waveguide samples was built.L-edit software was used to draw the sample layout of the straight waveguide and Y branch waveguide,a 500 nm thick silicon nitride layer on the LNOI substrate was deposited by enhanced chemical vapor deposition(PECVD)technology,and electron beam lithography(EBL)and Reactive ion beam etching(RIE)technology were used to form Si N-LN loaded strip waveguides,and the end surface of the sample was polished and profiled.The coupling platform was built with components such as six-dimensional precision adjustment frames and fiber clamps.The platform can realize the nano-level alignment of the Si N-LN loaded strip waveguide and the fiber,and feedback the relative position of the sample and the fiber and the coupled output optical power in real time.Thex coupling experiment of the Si N-LN loaded strip waveguide and the optical fiber was completed,the optical transparency of the waveguide was verified,and the insertion loss was tested.3.Coplanar waveguide electrode model was established,the parameters of coplanar waveguide electrode were optimized by the model,the simulation experiment of the intensity-phase modulator was carried out to test its optical and electrical performance.The influence of electrode parameters on electrode performance was explored,the results show that reducing the electrode spacing can significantly improve the electro-optic overlap integral,and reducing the electrode thickness is conducive to achieving impedance matching,but it is difficult to achieve optimal electrode performance indicators at the same time.In response to this phenomenon,the electrode structure was optimized by a optimization function.The optimized electrode has the advantages of high electro-optical overlap integration,impedance matching and phase velocity matching.Based on the optimized waveguide and electrode parameters,simulation models of intensity modulators,phase modulators and cascade modulators are established.The output light field of each modulator under different voltages was calculated to verify its modulation function,the intensity(phase)-voltage relationship curve was given to calculate the half-wave voltage.