Dynamic Characterization And Spectral Measurement Application Of Microresonator-based Optical Frequency Comb

Optical frequency combs,a new type of light source whose spectra consist of a series of equally spaced coherent spectral lines,have revolutionized the precision measurements of time and frequency.Compared with optical frequency combs produced by traditional mode-locked lasers,microresonator-based optical frequency combs(microcombs)have the advantages of small size,low power consumption,high repetition rate,and on-chip integration.Under the double balance of loss and parametric gain as well as dispersion and nonlinearity,dissipative Kerr solitons are generated through the cascaded four-wave mixing in a high-quality-factor microresonator pumped by a continuous-wave laser,which can provide stable ultra-short pulse source or coherent microcomb source.As a unique platform for studying nonlinear physics,soliton microcombs exhibit rich soliton dynamics,and realtime observation of them is helpful to deeply understand the generation mechanism of soliton microcombs.In recent years,the miniaturized integration of soliton microcombs has provided new solutions for many pioneering applications,such as coherent communications,ultrafast ranging,dual-comb spectroscopy,low-noise microwave generation and many other fields.This thesis focuses on the experimental generation,dynamic characterization and spectral measurement application of soliton microcombs.The representative research results of the thesis are summarized as follows:(1)The characteristic parameters of the microresonator and the basic theory of the microcomb are analyzed and summarized in detail.Combining the pulse propagation equation and the microcavity boundary conditions,the Lugiato-Lefever equation model describing the generation of the microcomb is deduced,and the characteristic parameters of the microcomb are discussed,which lays a theoretical foundation for the experiment.(2)A complete experimental platform for the generation of soliton microcombs is built,and the characteristic parameters of high-index doped silica glass microring are characterized.A dual-pump thermal compensation scheme is used to realize stable excitation of the soliton microcombs.In both cases of slow tuning and fast scanning of pump laser frequency,a single soliton microcomb is stably generated,and a variety of different soliton dynamics are realized,such as soliton self-starting and soliton number bidirectional switching.Finally,the feedback control system is established by monitoring the changes of the comb power and beat frequency,and the single soliton comb can work stably for up to22 hours.(3)The nonlinear dynamical phenomena of solitons are observed based on two ultrafast time-domain measurement techniques,showing the rich evolution process of the solitons in microresonators.Firstly,the radio frequency spectral measurement system is used to characterize different steady-state solitons,and the autocorrelation functions are obtained by Fourier transform of radio frequency spectrum,so as to analyze the temporal characteristics of solitons.The dynamics of soliton switching from four-soliton state to single soliton state is observed by measuring the radio frequency spectra in real time.Secondly,the temporal waveforms of the microcavity solitons are characterized by the temporal magnification imaging system.The soliton dynamics are presented more intuitively,which is beneficial to observe soliton states with a larger number.Rich dynamical phenomena such as relative soliton motion,soliton collision and separation are observed by changing the frequency sweep condition of pump laser.(4)Taking advantage of the large bandwidth of soliton microcombs,a scheme of continuously scanning a soliton microcomb based on four-wave mixing effect is proposed to achieve broadband and high-resolution spectral measurements in the C and L bands.The swept-frequency laser and the soliton microcomb are used as the pump and the signal light,respectively,to generate the idler soliton microcomb.The fast and high-precision frequency scanning of the pump light can realize the continuous,fast and high-precision scanning of the idler soliton microcomb.Based on the continuously scanned idler soliton microcomb,spectral measurements with a bandwidth of 20 nm,a resolution of 2 MHz,and a frame rate of 200 Hz are achieved in the C and L bands,respectively.(5)A hybrid asynchronous optical sampling technique is proposed to realize a fast and high-resolution spectroscopy for emission spectrum based on the high repetition rate characteristic of the soliton microcomb.Based on the focusing mechanism of time lens,the Fourier transform of signal spectrum is realized using a fiber comb with a relatively low repetition rate.On the premise of eliminating the third-order dispersion of the system,the spectrum of the signal is accurately mapped into the temporal waveform.The soliton microcomb with a high repetition rate performs hybrid asynchronous optical sampling on the frequency-mapped waveform with a low repetition rate,which can increase the acquisition frame rate of the signal by 3 orders of magnitude while ensuring high temporal resolution,and achieve a resolution below 1 pm and a measurement frame rate of 1 MHz.