Thin-Film Lithium Niobate Based On-Chip Acousto-Optic Modulator

In recent years,with the rapid development of communication technology and the continuous improvement of the speed,capacity and integration of communication system,the performance index that can be achieved by traditional electrical interconnection technology is close to the bottleneck.Optical interconnection technology has attracted more and more attention.Silicon on insulator（SOI）has the most common platform as photonics integrated circuits（PICs）,but it lacks optical modulation characteristics and has limitations in the application of optical modulation.At the same time,as a new technology compatible with CMOS manufacturing process and excellent optical modulation characteristics of materials,photonic integrated circuit based on lithium niobate（LN）has gradually come into people’s vision.This paper mainly introduces the acousto-optic modulator based on LNOI platform,which can modulate light through elastic wave.The inefficient electromechanical coupling caused by the mode mismatch between different physical fields is the main challenge to be overcome.The key problem studied in this paper is to improve the conversion efficiency from microwave input to sound cavity mode and improve the acousto-optic coupling.The main contents of this paper are as follows:1.This paper briefly summarizes the development history of PICs,introduces the research status of low loss waveguide,electro-optic modulator and acousto-optic modulator at home and abroad,and compares and analyzes the development advantages of optical devices based on LNOI.At the same time,the main performance indexes of electro-optic modulator and acousto-optic modulator are defined.2.The basic concept of acousto-optic crystal and the analysis and calculation methods of its band gap are analyzed,including frequency domain finite difference method,time domain finite difference method and finite element method.The basic equations of electromagnetic wave and elastic wave are described.Based on these equations,the related effects of the interaction between acousto-optic in micro nanostructures are discussed,which provides a theoretical basis for the design of acousto-optic modulation devices.3.An independent LN thin film acoustic resonator and low loss optical resonator are realized on the LNOI platform.A TE₀ acousto-optic modulator with a central wavelength of1550 nm is designed.The device uses Mach-Zehnder interferometer structure to efficiently modulate the single arm optical region through surface acoustic wave.Compared with the traditional electro-optic modulator,the metal electrode can be placed far away from the optical waveguide,and the metal absorption loss of the device is also quite low.By introducing the double slot scheme into the waveguide,the optimized device performance has been further improved,and a lower driving voltage can be realized.Therefore,the device can promote the on-chip low-power modulation scheme to achieve better efficiency and integration.4.For the analysis of high-order mode TE₁,an acousto-optic modulator working in high-order TE₁ mode is proposed.The modulation region of MZI structure is optimized,and a mode converter is designed to realize TE₀ to TE₁ conversion.Thanks to the phase matching of parallel straight waveguides,the device has larger working bandwidth and wider application.At the same time,the manufacturing tolerance of the key parameters of the device is analyzed.On the basis of optimizing its performance,the smaller size of the device can be realized,which makes the device have strong application potential in the realization of on-chip compact photonic integrated circuit.Compared with the modulator usually working in fundamental optical mode,the device is of great significance to the development of on-chip high-performance multimode acousto-optic modulation.5.The work of this paper is summarized and the future application prospect is prospected.