Research On The Transient Dynamics Of The Ultrafast Lasers Using The Dispersive Fourier Transformation Technique

Serving as a high-quality light source for ultra-short pulses,the ultrafast fiber lasers play inestimable roles in both practical applications and basic scientific research.On the one hand,the output pulses with the advantages of narrow pulse-width,high peak power,high repetition rate,wide bandwidth and other technical features have brought a breakthrough in optical fiber communication,sensing,and precision measurement.On the other hand,it can provide an excellent platform for studying the ultrafast and ultra-strong nonlinear optical physics phenomena of ultrashort pulses due to its compact structure,simple operation,low cost,etc.However,the research on the transient dynamics of ultrashort pulses requires real-time measurement technology.As a pioneering tool,the time stretch-dispersive Fourier transform technology overcomes the limitations of traditional spectroscopy instruments and realizes the functions of continuous single-shot spectral measurement,imaging and so on.Not only it is used in the frontier fields such as communications,sensing,and biological diagnosis,but also opened up new horizons for revealing transient phenomena in nonlinear dynamics.In this thesis,based on the exploration of laser physics,oriented to the application demand of ultrafast fiber laser technology,and considering the dispersive Fourier transform technology as a tool,we systematically studied the soliton complexes including vector pulses and soliton molecules in ultrafast fiber lasers,unveiling the transient polarization dynamics of the vector pulses and the internal motions and transition dynamics of the soliton molecules.The main results of this thesis are as follows:(1)Transient polarization dynamics of vector pulses.The passive mode-locking mechanism based on nonlinear multimode interference is studied.An experimental platform for vector passively mode-locked fiber lasers is built using graded index multimode fiber,realizing the generation of group-velocity-locking vector solitons and vector noise-like pulses.Combining the real-time spectral observation method based on the dispersive Fourier transform technology and the polarization resolved measurement,the transient polarization dynamics of the vector pulse is further explored,and the pulsation of the group-velocity-locked vector noise-like pulses is observed.The result is beneficial to expand the research on the potential applications of ultrashort pulses in the field of precise processing.(2)Transient dynamics of soliton pairs.A dispersion-managed passively mode-locked fiber laser is built and combined with the real-time spectral observation method to serve as a research platform for the transient dynamics of soliton molecules.Based on the first-order autocorrelation based inversion method,the transient dynamics of steady-state soliton pairs,conventional evolutionary soliton pairs,and shaking soliton pairs are analyzed.For the first time,the internal motions of the shaking soliton pair are experimentally studied.And the spectrum of oscillatory frequency is proposed to analyze the evolution of shaking soliton pair with superposition of multiple oscillatory frequencies.The experimental results enrich the evolution of soliton molecules in the dissipative system and reveal the dynamic of complex internal motions of soliton molecules.(3)Transient dynamics of multi-pulse soliton molecules.Based on the study of soliton pairs,the transient dynamics of multi-pulse soliton molecules are explored.On account of the first-order autocorrelation inversion based pulse distribution analysis method,the internal motions of the equally-spaced soliton triplets and unequally spaced soliton triplets are detailly explored.The transient dynamics of stable soliton triplets,oscillating soliton triplets,shaking soliton triplets,and flipping soliton triplets are analyzed.Then,the fitting method is used to restore the real-time spectral evolution of the flipping unequally-spaced soliton triplets.Finally,the transient dynamics of unequally-spaced multi-pulse soliton molecules is analyzed.The results provide a basis for explaining the dynamics of the multi-pulse soliton molecules,and are conducive to expand the research on the stability of ultrafast fiber lasers and the manipulation of soliton molecules.(4)Transition dynamics of soliton molecules.Under the unstable mode-locking state of a free-running fiber laser,the transient dynamics of the generation,disappearance of different types of pulses are studied,as well as the transition between soliton molecules.Using dispersive Fourier transform technique to randomly record the real-time spectrum of ultrashort pulses,the generation and disappearance of soliton and soliton molecules are observed,and four transient processes are summarized as relaxation oscillation,transition,mode-locking establishment and mode-locking degradation.Furthermore,the transition dynamics between soliton molecules are observed.More importantly,the evolution and degeneration of soliton molecules are analyzed.The results will deepen the understanding of dynamics of the ultrashort pulses in the nonlinear dissipative systems,and lay the foundation for the realization of precise control of ultrafast fiber laser,and further expand the research on intelligent control of passively mode-locked fiber lasers.