| Passively mode-locked fiber lasers are increasingly widely used in scientific research and industrial production due to their compact structure,low cost,high stability,and short pulse duration.The output soliton pulse of passively mode-locked fiber lasers is the result of dispersion effect and nonlinear effect of optical fibers.Dispersion effect refers to the different frequency components of optical pulse propagating at different speeds when transmitted in optical fibers.Among them,group velocity dispersion is an important factor leading to pulse width broadening;thirdorder dispersion can lead to pulse time domain profile and spectral asymmetry,compress or broaden pulse,and lead to pulse chaos;fourth-order dispersion can make soliton have a flatter spectral center and generate high-energy pure-quartic soliton.Researchers have been studying the influence of dispersion on the characteristics of soliton pulse.Firstly,this paper introduces research background and basic theory of modelocked fiber lasers.In the research background section,mode-locked fiber lasers are divided into active mode-locked fiber lasers and passively mode-locked fiber lasers according to mode locking method,and their implementation technologies and research status are introduced respectively.In the theoretical part,dispersion effect and nonlinear effect of fibers,optical pulse propagation equation,and nonlinear Schrodinger equation are studied.The main work of this paper is to numerically simulate the output soliton pulse characteristics of a passively mode-locked fiber laser and study the influence of dispersion on the soliton characteristics.Firstly,group velocity dispersion on the soliton pulse characteristics of passively mode-locked fiber laser is studied.By changing the length of the dispersion compensation fiber,the intracavity group velocity dispersion is adjusted to positive group velocity dispersion region and negative group velocity dispersion region,respectively.The influence of group velocity dispersion on pulse width,single pulse energy,and pulse peak power are studied in two different dispersion regions.In the positive and negative group velocity regions,group velocity dispersion can broaden pulse width.Then the influence of small signal gain and gain saturation energy on pulse characteristics under two dispersion conditions are studied.Subsequently,the influence of third-order dispersion on the output soliton pulse characteristics of passively mode-locked fiber laser is explored.By adjusting thirdorder dispersion values of optical fibers,the soliton characteristics under different third-order dispersion values are analyzed.In the time domain,third-order dispersion results in distortion of pulse shape,asymmetric pulse profile and temporal shift;in the spectrum,third-order dispersion leads to changes in the position and intensity of sidebands.Finally,the influence of fourth-order dispersion on the output soliton pulse characteristics of passively mode-locked fiber laser is studied.Analyzing the pulse time domain evolution diagram and spectrum diagram,it is found that the pulse width changes with fourth-order dispersion,and also have an impact on spectral sidebands intensity.In the presence of fourth-order dispersion,the influence of saturation energy on pulse peak power and single pulse energy are studied.It is proved that increasing saturation energy can lead to asymmetric pulse temporal profile,because phase distribution caused by self-phase modulation and fourth-order dispersion does not match.Finally,the influence of net second-order dispersion and third-order dispersion on the quartic soliton are studied.In addition,it is found that spectrum and time domain evolution of soliton are not completely the same when changing the fiber at different positions.The peak power and single pulse energy of the quartic soliton are different.In this paper,a theoretical model of passively mode-locked fiber laser is constructed.The influence of group velocity dispersion and high-order dispersion on the output soliton properties are studied by numerical simulation in both time domain and frequency domain.In this paper,the research on the influence of intracavity dispersion on the output soliton characteristics of passively mode-locked fiber lasers has certain guiding significance for the generation of high-quality soliton pulse and the stable transmission of soliton pulse in passively mode-locked fiber lasers. |
