Study On Graphene-based Optical Modulator Based On Racetrack Resonator Structure

With the fast development of optical communication,the research about optical modulators with high-speed,high-integration and large modulation-bandwidth has been immediately increased.Due to the weak electro-optic properties of silicon crystal,the modulation-speed of silicon-based optical modulator is limited,and it is difficult to be further integrated.Among graphene's unique electro-optic properties,perhaps its optical properties are the most promising in leading to practical applications in the near future.In this paper,the basic theory of graphene-based optical modulator is discussed.On the basis of the electro-absorption optical modulator with a dual-graphene layer,combined with the advantages of the racetrack resonator,this paper presents an optical modulator with graphene embedded in the racetrack resonator.The main work of this paper is proposed:Firstly,after understanding the mechanism of electro-absorption of graphene optical modulator,combining with the MATLAB calculation,this paper obtains the relationship between the refractive index of graphene and the applied voltage,providing theoretical support for the subsequent design of the graphene optical modulator.Secondly,based on the beam propagation method(BPM),the mode characteristics of the racetrack resonator are analyzed.By the analysis of single-mode and polarization independent,this paper determines the approximate size of the waveguide.When the waveguide core thickness fixed 0.36?m,ridge height valued 0.28?m,ridge width is in the range of 0.31?m to 0.35?m,especially 0.33?m.Or when ridge height valued 0.30?m,ridge width is in the range of 0.35?m to 0.37?m.Based on finite difference time method(FDTD),this paper analyzes the resonator's steady-state response,optimizing the cavity structure parameters,when the structure of the waveguide resonant cavity length valued3?m,spacing between the straight waveguide valued 0.1?m,bending radius valued 3?m,the steady-state response of the device is good.Thirdly,based on the COMSOL software,the structure of graphene-straight-waveguide is designed.By analyzing the relationship between the imaginary part of the effective refractive index and the Fermi level,it is found that the absorption ability of graphene at0.4eV or 0eV or 1eV is weak,and the ability to absorb light is strong when the Fermi level is 0.51eV.The different buffer materials and their thickness are compared and analyzed,and finally the 5nm thick hBN material is selected.By calculating,when the active modulation length is about 2?m,the modulator extinction ratio is 15dB,modulation bandwidth is 86.9GHz,the insertion loss is 1.074dB,the unit energy loss is 2.23fJ/bit.Based on the FEM method,the 2D structure of the racetrack resonator is simulated,and the corresponding Q value distribution under different resonance frequencies is obtained,which is 10~4.On this basis,established the 3D racetrack resonator model and analyzed the influence of structure parameters of R and L on the quality factor of the resonator,when R=3?m,L=2?m,the resonant wavelength valued 1.546?m,the Q value is 9394.Finally,the model of graphene-racetrack cavity structure optical modulator is established.When the ridge width is 0.33?m,core thickness valued 0.36?m,ridge height valued 0.28?m,the coupling distance between straight waveguides valued 0.1?m,bending radius valued 3?m,resonator straight waveguide length valued 2?m,dielectric material selected 5nm thick hBN,graphene thickness valued 0.7nm,the length of graphene on the racetrack resonator valued 0.24?m,the extinction ratio of the device is 37dB,the modulation depth is 0.83,the modulation bandwidth is 86.9GHz,the insertion loss is 0.537dB,and the energy loss is1.125fJ/bit.