Research On Soliton Dynamic Behaviors In Ultrafast Fiber Lasers

Ultrafast fiber lasers,as excellent light sources generating ultrashort pulses,have important applications in various fields,such as laser processing,biomedical science,optical communications,and optical frequency combs.In addition to their practical applications,they are also typical nonlinear dissipative systems,which have been demonstrated as good platforms for exploring diverse soliton dynamics.In recent years,with the development of high-precision real-time measurement techniques,the soliton dynamic behaviors in ultrafast fiber lasers have become one of the research hotspots.The research on soliton dynamics can deepen the understanding of soliton characteristics,and further improve the laser performance.This could promote the development of ultrafast laser technology.In this dissertation,experimental and theoretical investigations on transient characteristics of soliton dynamic behaviors in ultrafast fiber lasers,such as soliton pulsations,soliton collisions,and soliton explosions,are carried out by utilizing high-precision real-time measurement techniques.The main research contents are as follows:1.By virtue of the dispersive Fourier transform(DFT)technique,a novel type of pulsating solitons is observed in an all-anomalous-dispersion fiber laser based on singlewall carbon nanotubes,which features periodic central-wavelength redshift and blueshift.These solitons are referred to as pulsating solitons with central wavelength oscillation.By adjusting the pump power,the intermittent pulsation as well as synchronous and asynchronous dual-soliton pulsations are also obtained.The formation mechanisms for the pulsating solitons with central wavelength oscillation are discussed.Secondly,various types of soliton-molecule pulsations are measured in real-time,including symmetric and asymmetric soliton-molecule pulsations.The results show that the transient interferencespectrum evolution of symmetric soliton-molecule pulsations exhibits periodic oscillations,while the transient interference-spectrum evolution of asymmetric solitonmolecule pulsations shifts towards one direction.Both symmetric and asymmetric soliton molecules experience internal energy exchange during the pulsating process.Finally,the dissipative soliton pulsation is investigated in a bidirectional mode-locked fiber laser.A novel double-periodic pulsation is obtained,whose evolution is a combination of two pulsation periods.It is found that the dissipative solitons in different directions exhibit synchronization during the pulsating process.2.The transient dynamics of collisions between a two-soliton molecule and a soliton singlet are experimentally investigated in a single-wavelength mode-locked fiber laser.By means of the DFT technique,the evolving spectral interferograms of different collision scenarios are measured in real time,including the “quasi-elastic” collision,the annihilation of a soliton in the soliton molecule,and the formation of a stable unequally spaced soliton triplet.The collision mechanisms are analyzed.Secondly,the collision between a stable soliton molecule and a single soliton as well as the collision between an oscillatory soliton molecule and a single soliton in a dual-wavelength mode-locked fiber laser are observed,which reveals the self-stabilization process of soliton molecules under the perturbance of soliton collisions.It is found that the relative phase and soliton separation of the soliton molecules undergo sudden changes due to cross-phase modulation(XPM)during collisions.After collisions,soliton molecules undergo transient time-varying processes until return to their original states.Finally,the elastic collision between single solitons is obtained in an ultrafast fiber laser.It is shown that,in the elastic soliton collision,the soliton with higher group velocity do not overlap the soliton with lower group velocity,but rather exchange their states through the gain-absorption depletion and recovery effect of the gain fiber.Our findings can deepen the understanding for particle-like properties of solitons.3.A normal-dispersion fiber laser mode-locked by the nonlinear-polarizationrotation technique is constructed.By adjusting the pump power,the stationary dissipative soliton and noise-like pulses are obtained.The dissipative soliton explosions are observed in the transition regime between the stationary dissipative soliton and noise-like pulses by using the DFT technique.The transient dynamic process of soliton explosions is revealed.By further increasing the pump power,soliton explosions in the double-soliton state are also captured,and the corresponding physical mechanisms are discussed.Secondly,soliton explosions triggered by vector dissipative soliton collisions are observed for the first time in a polarization-multiplexed ultrafast fiber laser.Numerical simulations validate the experimental observations and give more insights into the physical mechanism for soliton explosions.It is demonstrated that the XPM-induced pulse interaction is the origin of collision-triggered soliton explosions.Finally,the explosions of pulsating solitons are observed in a bidirectional mode-locked fiber laser,and the evolution characteristics are analyzed.Our work would contribute to the design of ultrafast fiber lasers.4.A temporal magnifier based on electro-optic phase modulation is designed and constructed,and its accuracy is verified through the simulation and experiment.This temporal magnifier provides a powerful tool for investigating the transient temporal characteristics of soliton dynamics.By utilizing the temporal magnifier and DFT technique,the temporal-spectral transient dynamics of pulsating solitons are simultaneously measured in an ultrafast fiber laser.It is found that both the pulse peak power and temporal width vary periodically.The evolution of pulse peak power is synchronous with that of spectral width but asynchronous with that of pulse width,which could be attributed to the mutual interaction between the self-phase modulation and group-velocity dispersion effects.Finally,the temporal-spectral transient characteristics of the internal motion within soliton molecules are investigated.It is shown that the two solitons in the oscillating asymmetric soliton molecule periodically approach and move away from each other in the temporal domain.Meanwhile,the energy and peak power of the two solitons oscillate synchronously,but the oscillation amplitude is different.The corresponding evolution mechanisms are analyzed.