Researches On Formation And Evolution Dynamics Of Ultrashort Pulse And Its Application In Ultrafast Process Measurement

Ultrashort pulse,as the shortest time event that human has created,has pulse width of the order of femtosecond(10^-15s),providing the most precise measurement in many fields.These high-precision measurements are strict with the ultrashort pulse in duration and stability,and bely the precise tuning of ultrashort pulses,which make it necessary to understand the evolution characteristics of ultrashort pulses in depth.Therefore,the introcavity evolution dynamics of ultrashort pulse is the research focus in ultrafast optics.The emerging dispersive Fourier transform（DFT）technique can single-shot probe the spectrum of ultrashort pulse,but it cannot retrieve pulse information in time domain.In some high-precision measurement like frequency comb,the jitters of pulse phase and roundtrip time can severely affect the measurement precision.In this thesis,we develop the dispersive temporal interferometer（DTI）,and study the intracavity pulse evolution in-depth from the perspective of roundtrip time and phase.which is very important to improve the stability and optimize the performance of ultrashort pulses.Meanwhile,we propose that the dispersive temporal interferometer can be used to fabricate time-modulated sensors,intended to extend the application of ultrashort pulse.The main work and chief achievements of this thesis are as follows:1、We develop a new technique to single-shot probe time events from attosecond to picosecond,the dispersive temporal interferometer（DTI）,which is a temporal analogy of double-slit interference:the ultrashort pulse is assembled into a dual-pulse structure with time separation after it passes different paths,and after large dispersion,interferogram arises in time domain,from which the time separation can be decoded and the time signal exerted into one path is retrieved.This technique can realize precise single-shot measurement for ultrafast time signal with resolution of sub-20as（0.004optical cycle）,range of more than 2ps（400 optical cycle）,and range-to-resolution ratio of 10⁵.Additionally,its sampling rate can reach to 42.8MHz.The experiment shows:this technique has broken the 2plimitation for traditional interferometer in single-shot measurement,and improves interferometer’s single-shot time measurement range to400 optical cycles.Compared with the time lens technique,its resolution is not limited by the pulse width,but can reach sub-optical cycle with single-shot measurement.DTI is the only single-shot time measurement technique that can achieve attosecond resolution and picosecond range.It can be used not only for novel optical sensors based on time measurement,but also to study pulse characteristics in ultrafast optics.It is expected to play a role in life activity monitoring,astronomical observation,basic physics research and other aspects.2、Employing dispersive temporal interferometer,we propose the method to probe pulse-to-pulse interaction and pulse-to-cavity-parameter interaction,and analyze the variation of the time interferogram when the pulse time shifts due to refractive index change and gain depletion.It is proved that the gain depletion changes the pulse roundtrip time,but does not affect its relative phase.As a typical example,we study the relaxation process of the laser after mode-locking,and capture three stages after pulse formation:the soliton breath,relaxation oscillation,and long-term relaxation.In the soliton stage,the roundtrip time of pulse jitters violently,which is greatly affected by gain depletion effect,and meanwhile,the Kerr effect makes the pulse phase change.During the relaxation oscillation stage,affected by Kerr effect,the pulse phase jitters synchronously with intensity periodically.In the long-term relaxation stage,the relaxation of gain medium makes the refractive index change,making the increase of roundtrip time.The experimental result shows that the change of pulse shape can make the parameters like gain change,producing the pulse-cavity parameter interaction.The gain depletion will strongly affect pulse’s roundtrip time,which is the important source of laser’s timing jitter.Moreover,Kerr effect brings the obvious change of pulse phase.These studies have an instructive significance for understanding the dynamics of mode-locked lasers,and will have profound impact in precise timing and frequency comb generation.3、For the first time,we observe the periodic oscillation of breathing soliton with dispersive temporal interferometer.Stationary breathing soliton is obtained experimentally and numerically.The breathing period increase linearly as the pump power decreases from 470m W to 350m W.Not only the pulse energy and spectrum breathe,but its roundtrip time and phase fluctuate periodically,synchronously.Furthermore,we capture the period-doubling breathing soliton for the first time as the pump power decrease to 340m W.The experiment shows:the breathing soliton is metastable pulse at the transition state of stable mode-locking and unlocking,and the breathing frequency can be controlled by tunning the pump power.Breathing soliton source can improve the srability of dual-comb,and will have great attraction and development potential in the fields of soliton dual-frequency comb,optical communication,supercontinuum generation.4、Using dispersive temporal interferometer,we capture the subharmonic entrainment breathing soliton for the first time,and obtain breathing soliton with breathing period being 20 folds to roundtrip time.It is observed that the time-phase diagram of subharmonic entrainment breathing soliton is localized at some discrete points,rather than a closed curve of common breathing soliton.Breathing frequency increases with pump power,but will deviates from the expected value near the subharmonic soliton frequency and rigidly snaps to the subharmonic of soliton frequency.Experiment result indicates that the stability of subharmonic entrainment breathing soliton is obviously better than common breathing soliton,which provides experimental evidence to capture subharmonic entrainment breathing soliton in Kerr microresonator.This phenomenon is helpful for the measurement of ultra-high pulse repetition frequency which cannot be achieved by electrical methods.Theoretically,we propose the non-Hermitian model of breathing soliton lasers,analyze the coupling dynamics of breathing frequency and cavity frequency,and explaine the arisement of subharmonic entrainment breathing soliton.Meanwhile,it is proved theoretically and experimentally that the subharmonic entrainment can effectively improve the stability of breathing soliton,which is helpful to broaden the application of breathing soliton in many fields.Furthermore,we develop a numerical simulation method for breathing soliton lasers,and numerically obtain the subharmonic entrainment breathing soliton in mode-locked lasers for the first time.The simulation result agrees well with the experiment and the theory.Moreover,the exceptional point（EP）physics is introduced into the mode-locked laser for the first time,which will make the mode-locked laser an effective platform for the study of EP physics,attract more scientists to devote themselves to the study of related novel phenomena,and spur a new course in ultrafast optics.5、We present the concrete method to fabricate optical sensor with dispersive temporal interferometer,and introduce several optical fiber sensors,including angular velocity sensor,displacement sensor,vibration sensor.Firstly,we fabricate a fiber optic gyroscope,whose sensitivity is 348 as/（deg/s）.Additionally,the gyroscope can also measure the transient angular velocity with sampling rate of 42MHz.Then,we real-time measure the displacement of the mirror in delay-line,and calculate out that the speed of the mirror is 6mm/s.In addition,the vibration of the optical fiber is measured,and the vibration frequency is about 500Hz.These experiments qualitatively demonstrate that the dispersive temporal interferometer can be used to fabricate time-modulated sensors.These sensors have sampling rate preceding tranditional modulation method,like intendity,phase,wavelength,polarization,and has advantage in resolution and stability.This is a new type of optical fiber sensing method,will be applied in many fields...