Theoretical And Experimental Study On Ultrafast Transient Dynamics In Passively Mode-locking Fiber Lasers

Passive mode-locking ultrafast fiber lasers,which take the advantages of small size,high energy and wide wavelength range,have been widely used in bioimaging,optical communication,spectroscopy and other fields in recent decades.As a dissipative system,it also provides a perfect experimental platform for the study of soliton dynamics.Due to this dissipative property,the soliton pulses in fiber lasers can evolve into new soliton states or exhibit complex nonlinear evolution dynamics under certain conditions.Limited by detector bandwidth or response speed,it is still a difficult task to experimentally observe the evolution of ultrafast transient dynamics in a passively mode-locking fiber laser in real time.The research in this thesis focuses on the ultrafast transient evolution dynamics,to reveal and analyze the generation and evolution dynamics of passive mode-locking fiber laser in theoretical simulations and experimental studies,to achieve the real-time observation of its transient dynamics process.The main results achieved in this thesis are as following.(1)The vector asymmetric solitons and dual-states vector solitons induced by the cross-phase modulation(XPM)effect are found in a mode-locked fiber laser under weak birefringence.The key conditions for the formation of these two kinds of vector solitons,including proper birefringence,pulse energy and dispersion,are theoretically simulated by coupling the nonlinear Schrodinger equation,and the evolution dynamics is verified experimentally by the dispersive Fourier transform(DFT)technique.Under certain conditions of birefringence and pumping energy,the spectra of the vector asymmetric solitons exhibit periodic redshifts and blueshifts with a peak wavelength offset of 4.3nm and a period of two cavity roundtrips.By further controlling the system parameters,we obtained dual-states vector solitons with alternating evolution of double-peak and Lorentz-type spectra.The two orthogonal polarization components of the double-peak spectrum exhibit an XPM-induced asymmetric distribution,while the frequency shift of the Lorentz-type spectrum is much weaker.These two vector solitons refresh the concept of stable mode-locking and reveal the physical mechanism by which XPM causes spectral instability,which will be useful for laser design and understanding of ultrafast lasers to improve laser performance.(2)To achieve high performance laser generation,firstly,glass-graphene saturable absorbers with excellent performance were fabricated by sol-gel method and applied to ultrafast laser mode-locking.The fabricated glasses are valuable for improving the damage threshold of saturable absorbers and the average laser power.Then,a broadband tunable all-fiber laser based on the single-wall carbon nanotube mode-locker was constructed by inserting a tunable filter into the cavity,with operation bandwidth covering the C+L band,with the output center wavelength tuning range of>70 nm and continuous tuning,providing a new light source for various applications requiring variable spectral wavelength or bandwidth.(3)In order to realize a high-energy broadband laser source,firstly,we built a mode-locked laser source based on self-similarity amplification evolution,with a 3dB spectral bandwidth that increases with the increase of pump power to 31nm and an average output power of about 150mW.Then,we built a Mamyshev oscillator for communication band based on self-similarity evolution and spectral offset filtering effect.A stable pulse output with a pulse width of 1.6ps and a spectral 10dB bandwidth of more than 33nm was obtained experimentally,and the intracavity evolution dynamics was also analyzed by theoretically simulations,which provides new research ideas and technical ways for the future study of extreme ultra-short pulse fiber lasers.(4)An ultrafast temporal magnification system based on asynchronous four-wave mixing was built,and real-time observation of artificial soliton molecules and creeping soliton dynamics in passively mode-locking ultrafast fiber laser were realized.By optimizing the system and introducing Raman amplification,the recording length can be greatly increased to more than 5000 cavity roundtrips.For the external motion dynamics of the artificial soliton molecule,experiment results show that vibration-like dynamics can be seen at narrower spacing of soliton pairs,while the two solitons with wider separation remain relatively unchanged.In addition,by combing with DFT technique the full-filed information in the temporal-spectral domain of stable single solitons and creeping solitons can be measured.The tiny motion distances of the solitons are magnified by time lens so that they can be resolved beyond the resolution limits of conventional oscilloscopes.In addition,the spectra of the creeping solitons exhibit breathing dynamics,suggesting that the pulse width and peak power are periodically pulsating.By adjusting the pump power in the experiment,steady mode-locking can be transformed into a pulsating state,indicating that soliton pulsation is a Hopf bifurcation behavior in a dissipative system.These results also highlight the importance of the simultaneous use of time-lens and DFT techniques for full-field characterization of ultrafast phenomena,and show how single-shot measurements can provide new insights into ultrafast transient dynamics in nonlinear optics.