Theoretical And Experimental Research Of Optical Frequency Comb Generation Based On CMOS-compatible Microresonators

The high-Q microresonators have strong light field confinement capability and high nonlinear coefficient,which provide a new method for the development of optical frequency comb(OFC)technology to the higher integration and lower threshold.The OFC based on microresonators naturally has the advantages of small size and ultra-high repetition frequency,making it promising in the fields of high-speed coherent communication,precise molecular spectroscopy detection,optical frequency synthesis,arbitrary waveform generation,etc.At present,microcavity based OFC generation has been implemented on a variety of different materials.Among them,CMOS-compatible material microcavities are suitable for semiconductor processing technology and easy to integrate with other photonic devices,revealing special significance for the development and application of technology in this field.This dissertation systematically investigates the basic devices structure and their physical characteristics,as well as the nonlinear theory and dynamic process involved in the microcavity OFC.The evolution rules of bright/dark solitons under the influence of high-order dispersion,Raman effect and self-steepening effect are analyzed.Moreover,the broadband mid-infrared OFC can be generated in normal-dispersion silicon microcavity by adjusting the multiphoton absorption and carrier effect.Based on high-index doped silica glass microcavity,a variety of soliton states are experimentally observed.The main contents and achievements are as follows:Firstly,the numerical solutions of the nonlinear coupled mode equation and the Lugiato-Lefever equation are realized by using the split-step Fourier and Runge-Kutta algorithm.The influences of high-order dispersion,Raman and self-steepening effects,radiation loss in mode coupling on the evolution process and distribution characteristics of microcavity solitons in the time and frequency domain are analyzed.The results show that the Raman effect causes a red shift of the overall spectrum profile,which appears as a slight change in the pulse repetition rate in the time domain.The self-steepening effect has little effect on the time-frequency domain of the soliton microcomb,while the larger mode coupling can help to realize the single soliton state with the local jump on the spectrum.Secondly,the moment method and dispersive wave theory are used to analyze the time and frequency domain characteristics of bright and dark solitons and breathers.It is found that the high odd-order dispersion causes the time-domain drift of microcavity solitons and the drift speed mainly depends on the absolute value of third-order dispersion.The periodic oscillation characteristics of bright and dark breathers can be stabilized by designing high odd-order dispersion.In the frequency domain,the high-order odd-even dispersion jointly determines the dispersion curve envelope and ultimately affects the position and number of dispersion waves,providing a theoretical basis for optimizing the required dispersion of broadband OFC generation.Thirdly,we theoretically analyzed the influence of multi-photon absorption and carrier effects on the formation of OFC in normal-dispersion silicon microcavities.In the communication band,the two-photon absorption becomes the dominant loss factor and suppresses the parametric oscillation process.In the 2200 nm-3300 nm band,the three-photon absorption and accompanying carrier effects mainly inhibit OFC formation,while OFC is achieved by adding the PIN diode to sweep out the carrier.The four-photon absorption and accompanying carriers beyond 3300 nm are smaller than the first two,thus OFC can be generated at an appropriate pump power.Furthermore,small positive dispersion is obtained by designing the waveguide cross-section,and the appropriate microcavity radius(with FSR of 129 GHz)is used to suppress the strong Raman effect of silicon material.The broadband mid-infrared OFC spanning an octave(2-4μm)with adjustable FSR can be achieved.Different from the time-domain Gaussian pulse in anomalous dispersion,the obtained flat-top pulse has a higher conversion efficiency(up to 67%).Finally,the pump-assisted laser scheme is adopted on the high-index doped silica glass platform for soliton microcomb generation.By optimizing the waveguide structure and processing technology to avoid mode crossing,a single soliton with ultra-smooth spectrum is realized(with bandwidth of 160 nm).Raman self-frequency shift of the standard sech~2 spectrum is 4.4 nm,and the Raman shock time of the material is accurately calculated to be 2.7 fs.Meanwhile,the auxiliary light can be controlled to stabilize the thermal effect to obtain different“perfect”multi-soliton states(1-,2-,4-solitons).The spacing of the dual-solitons in the cavity is adjustable and soliton tail oscillation vanishes simultaneously.The effects of thermal instability on the formation of solitons is analyzed via the LLE model containing thermal effects.It is found that the thermal change during the pump laser tuning process can cause the existence or annihilation of solitons.