Theoretical And Experimental Investigation On The Novel Nonlinear Dynamics In Mode-locked Fiber Lasers

Ultrafast fiber lasers have important application values in optical communication systems,material processing,biomedicine and optical precision metrology.As a typical nonlinear and dissipative system,ultrafast fiber lasers are a good platform to research the nonlinear dynamics of optical waves.Although a servo feedback system can be used to overcome external perturbations on the stable operation of a mode-locked laser,the ultrashort pulse is still intrinsically unstable under the combination of many inner effects of the laser when the parameters of the laser deviate a lot from the steady-state mode-locked operation conditions.The complexity and detailed physical mechanisms of the nonlinear dynamics of optical solitons in lasers have yet to be thoroughly explored.This is of great significance for us to better design and optimize ultrafast fiber lasers and to understand and excavate the nonlinear phenomena of lasers.In this dissertation,the nonlinear behaviors of solitons in ultrafast fiber lasers are studied in detail and in depth from two aspects:theoretical simulation and experimental exploration.The main contents of the thesis include:(1)The interactions between the dispersive waves and solitons in the ultrafast fiber laser were researched.A novel kind of the resonant dispersive wave sidebands induced by the continuous-wave was observed experimentally,and the phase-matching condition that fits well with the experiments was proposed.Based on the theoretical simulation,the mechanism of the dip-type Kelly sidebands in the conventional soliton fiber laser was analyzed.A kind of the broadened Kelly sidebands was experimentally observed,whose mechanism was theoretically revealed to be induced by the fast oscillation of the wavelength of the Kelly sidebands with frequency magnitude of 10⁵Hz.Based on the simulation,the resonant dispersive wave emitted by the dissipative soliton observed,whose phase-matching condition was also proposed.The results enrich the theories and phenomena of the interactios between the dispersive wave and solitons,which can be used for the theoretical guidance of the optimization of the laser.(2)The polarization evolution of the vector soliton in ultrafast fiber lasers was researched based on the simulation of the coupled Ginzburg-Landau equations.The fast-axis instability of the vector soliton in the fiber laser was researched.The polarization dynamics and the molecular vibration of the orthogonal modes of the soliton molecule formed by the bounding of two single solitons were researched,and the effects of linear birefringence and pumping strength on the polarization evolution of the two sub-pulses of the soliton molecules were explored.The results offer a further understanding of the polarization evolution of the ultrafast laser,which is important to useful and control the polarization of the ultrafast pulse.(3)The evolution and properties of the high-order soliton in the ultrafast fiber laser were explored based on numerical simulations.A novel configuration of the high-order soliton dispersion-managed fiber laser was proposed,achieving the direct output of the transform-limited pulse with width shorter than 40 femtoseconds.The pulse splitting due to the high-order soliton in the fiber laser was researched when the soliton-order of the pulse was larger than 2.The work motivates the researching in high-order soliton of the laser,in addition,it gives a practical method to design the ultrafast fiber laser that outputs ultrashort and ultra-broaband pulse.(4)Based on numerical simulations,the pulse-shaping mechanism in a fiber laser at normal-dispersion were studied.The effects of the pump strength and spectral filtering on different pulse mechanisms were analyzed.For examples,the optical pulse changed from a coherent single pulse to an incoherent noise-like pulse;the pulse changed from the dissipative soliton to the dissipative-similar pulse and to the amplifier similariton.The results give a further understanding of the pulse-shaping mechanism in lasers,which can be used for the optimization of the high-energy dissipative soliton laser.(5)Based on theoretical simulations,the classification and mechanisms of soliton explosions in dissipative soliton fiber lasers were researched.It was pointed out that the linear loss and spectral filtering can be used to controll the soliton explosion.The existence and evolution characteristics of dark solitons in the soliton explosion in fiber laser were also proved.The unique erupting structures of the exploding soliton were induced by the clustering of the dark solitons embedded in the bright soliton.The results have significances in understanding the chaotic evolution of the dissipative soliton and the nonlinear dynamics of the dark soliton.(6)A real-time spectroscopy system based on the dispersion Fourier transform was built,whose wavelength-resolution was 0.138 nanometer.The pulsating dissipation solitons with pulsating frequency of 183 k Hz in a normal-dispersion erbium-doped fiber laser was observed.By further adjusting the laser parameters,we also found that the longitudinal modes of the cavity are randomly switched between the synchronization and unsynchronization with a time scale of 10^-2second,and the mechanism of the spontaneous switching of the lasing states is analyzed by revealing its real-time spectral evolution.The results enrich the experimental nonlinear dynamic behaviors in ultrafast fiber lasers,which are meaningful to understand the chaotic evolution in ultrafast lasers.