Studies Of Soliton Transient Dynamics In Passively Mode-locked Fiber Laser

Fiber lasers have many unique merits,such as compact configuration,heat dissipation and high reliability,which have been exploited applications in optical communications,optical sensors and nonlinear optics.Recently,the dispersive Fourier transform(DFT)technique opens a new way for the study of soliton transient dynamics in the passively mode-locked fiber lasers,which can map the spectral information of light wave into its temporal domain envelope.In this paper,the real-time evolution dynamics of conventional solitons,dissipative solitons,double-soliton and bound solitons are investigated experimentally in a carbon nanotube based mode-locked fiber laser.Moreover,the transition dynamics between the single soliton state and the bound state are studied in detail.Based on the experimental results,the complex nonlinear optical phenomena in these ultrafast processes,such as beating dynamics,modulation instability induced pulse splitting and soliton-soliton interaction,are investigated to further improve people's understanding of the solitons.The main contents of this dissertation are shown as follows:1.The build-up evolution dynamics of solitons in different dispersion regions are studied in a carbon nanotube based mode-locked fiber laser.The formation of conventional soliton with spectral width of 8.7 nm,pulse width of 0.75 ps and dissipative soliton with spectral width of 16.3 nm,pulse width of 8.5 ps have been investigated by DFT technique.The experimental results revealed that relaxation oscillation,Q-switched mode-locking,spectral broadening and stable mode-locking appear in the evolution processes of both types of solitons.Due to the different formation mechanism of conventional soliton and dissipative soliton,beating dynamics exist in the evolution process of conventional soliton mode-locking,sometimes there are also transitional bound state,while a short unstable mode-locking stage,and evident fringes at both edges of spectra of Q-switched mode-locked pulse which could be caused by the large pulse energy appear in the evolution processes of dissipative soliton mode-locking.2.Based on the DFT technique,it is revealed that there are two different evolution paths for the build-up and annihilation of double-soliton states in fiber lasers.It is found that the double-soliton state can be formed not only by pulse splitting but also by mode-locking of two pulses simultaneously in a fiber laser which can output stable double-soliton pulses.In addition,the annihilation dynamics of the double-soliton mode-locked state when the gain of the laser suddenly decreases to zero is studied by suddenly closing the pump light.It is revealed that the two pulses in the cavity can disappear one by one or annihilate simultaneously.The difference between that two type evolutionary processes mainly depends on whether there are effective perturbations when the pulse energy drops rapidly to a critical value.3.The evolution dynamics of different bound states in fiber lasers are studied experimentally.It is found that the interaction between solitons in the formation of different bound states is discrepant.By adjusting the pump power and polarization controller,the bound states with constant temporal interval of ~1.7 ps and ~9.2 ps,and the robust bound state with an oscillating interval at near ~8.2 ps are obtained in the fiber laser.The real-time spectral evolution processes of those three kinds of bound states are captured by DFT technique,and complex soliton-soliton interaction and nonlinear phenomena exist in the formation processes of that bound states.In addition,it is demonstrated that the annihilation processes of bound states at different drop rates of pump power are quite different.4.The transition from bound states to single soliton states when the pump power decreases and the evolution from single soliton states to the bound states when the pump power increases are studied in detail.It is found that,due to the fierce competition among solitons,the evolution processes from bound states to single pulse states are usually accompanied by the annihilation of one soliton pulse.However,pulse splitting caused by modulation instability exists in the evolution processes from single pulse states to bound states.The temporal interval and relative phase between two solitons changed complicatedly during both transition processes due to the complex interaction between solitons.