Heterogeneous Integrated Electro-optical Modulator Based On Lithium Niobate Thin Film

Traditional electrical interconnection technology has gradually reached the bottleneck in data transmission rate and bandwidth,while electro-optical interconnection has become the development direction of future data transmission due to its advantages of low power consumption and high bandwidth.Electro-optical interconnection relies on the conversion of electrical signals to optical signals,electro-optic modulator is a key device that converts electrical signals into optical signals,and is crucial for the development of optical switching,communication,ranging and sampling,and radio frequency photonics.Lithium Niobate(LN)crystal has become one of the ideal materials for manufacturing EOMs due to its wide transparent window(from 400 nm to 5μm),high electro-optic coefficient,and stable physicochemical properties.In this paper,a silicon nitride-lithium niobate thin film hetero-integrated electro-optic modulator is proposed.The modulator is implemented on the lithium niobate on insulator(LNOI)platform,and the modulation waveguide adopts silicon nitride waveguide.The lithium niobate thin film is bonded to the top of the silicon nitride waveguide by bonding with the BCB adhesive.A silicon dioxide layer is added between the modulating electrode and the lithium niobate thin film.Modeling and calculations are performed using COMSOL Multiphysics and FDTD Solutions to verify that the structure can effectively reduce the half-wave voltage-length product(V_πL)of the EOM,increase the3 dB modulation bandwidth,and still have the ability to integrate with CMOS electronic devices on a single chip.Subsequently,a silicon-based 1×2 multimode interference splitter structure and a stepped lithium niobate thin film coupler structure are designed to further improve the performance of this electro-optic modulator.1.The main research contents of this paper are as follows:This paper proposes a silicon nitride-lithium niobate thin film hetero-integrated electro-optic modulator.By summarizing and analyzing various types of electro-optic modulators at present,it is found that thin-film lithium niobate not only perfectly inherits the excellent performance of bulk lithium niobate in terms of its own material properties,but also has better optical field confinement ability than bulk niobium lithium,so the modulation volume of the electro-optic modulator based on lithium niobate thin film is small,and the modulation bandwidth is large.Although the light confinement ability of lithium niobate thin film is better than that of traditional bulk lithium niobate,its ability to confine light is still weaker than that of silicon,so electro-optic modulators based on lithium niobate thin film have a higher driving voltage.In conclusion,the electro-optic modulator designed in this paper is able to maintain high modulation bandwidth while minimizing the driving voltage.:by filling a certain thickness of BCB layer between the lithium niobate thin film and the modulating waveguide,the half-wave voltage-length product(V_πL)and optical loss of the electro-optic modulator can be effectively reduced;by filling a certain thickness of silica layer between the lithium niobate thin film and the modulating electrode,the microwave loss and effective refractive index of the RF can be effectively reduced.After optimizing the modulation area structure,the proposed electro-optic modulator in this paper has good performance(at a wavelength of 1550 nm and an electrode spacing of 3.7μm,the device can achieve V_πL=1.76 V·cm and a 3 dB bandwidth of 140 GHz).2.In this device,we choose the multimode interference beam splitter as the light field input and output structure of the electro-optic modulator,because the multimode interference beam splitter has a large bandwidth and is not sensitive to manufacturing tolerances.The polarized TE mode light enters the two modulator arms through a multimode interference beam splitter.After the modulation is completed,the light fields on the two modulation arms are combined and output through a multimode interference coupler again.Based on the self-imaging principle of multimode waveguides,we designed a silicon nitride-based 1×2 multimode interference beam splitter,and the input and output waveguides of the multimode interference beam splitter are compatible with the modulation area waveguide.After optimization,the performance parameters obtained by the multimode interference beam splitter are:the transmission loss is 0.29 dB,and the total length of the device is35μm.3.Achieving low coupling loss in heterogeneously integrated electro-optic modulators based on lithium niobate thin films has been a great challenge.The traditional electro-optic modulator based on lithium niobate heterogeneous integration uses a taper-type waveguide structure to reduce the coupling loss.This structure can achieve adiabatic transmission of the light field in two different regions,but the structure has bonding difficulties,length long and other disadvantages.Therefore,a ladder-type thin-film lithium niobate coupling structure is designed in this paper,which is short in length and does not require precise alignment during the bonding process.After optimization,the single-ended coupling loss of the structure is 0.73 dB,and the total length of the coupling region is15μm.In conclusion,the proposed electro-optic modulator structure in this paper can effectively reduce V_πL,increase the modulation bandwidth,and it is hoped that this device structure will contribute to the improvement of the performance of hetero-integrated electro-optic modulators,and promote the development of photonic integrated devices and circuits.