Theoretical And Experimental Study Of Ultrashort Pulses Based On Passively Mode-Locked Fiber Lasers

Compared with other types of lasers,Owing to their compactness,low-cost and no cooling system,fiber lasers will be the main development of lasers compared with other type of lasers.It may have chance to substitute solid state lasers as the lead of the lasers in the world.Now with the development of the all normal dispersion fiber lasers,fiber lasers can compete with solid state lasers in obtain pulse energy.However,how to generate ultrashort pulse is still a technique problem for fiber lasers and now lots of researchers work on it.Passively mode-locking is the main method to solve the pulse duration problem and it can generate pulse in femtosecond regime easily.This thesis focus on how to generate ultrashort in passively mode-locked fiber lasers.Analysis several type of solitons in fiber lasers by theoretical modeling and experiment,numerical simulations on influence of the saturable absorber in passively mode-locked fiber lasers,achieved high performance tunable self-similar fiber laser and high pulse energy Mamyshev fiber laser,by putting high nonlinear photonic crystal fiber and dechirped stage in passively mode-locked fiber laser we obtain the the broadest spectrum and shortest pulse in the fiber laser.The theoretical and experimental research of this thesis has important scientific significance and practical application for fiber lasers in generating high energy and ultrashort pulse.The main research of this thesis are as follows:1.Numerical simulations are conducted on dynamic evolution of the soliton molecules in an all-normal dispersion fiber laser.Soliton molecule is generated by the excessive nonlinear effect in the cavity,which causes the pulse to split into a soliton molecule from the single soliton.With the increase pump power,soliton molecules do not split into three bound states directly while it shows to avoidgenerating three bound states through a series of adaptive adjustments,thus we conclude that the soliton molecules have high nonlinearity tolerance.After adaptive adjustments each soliton has the same physical properties and a fixed peak to peak separation to keep propagation2.Numerical simulations on black phosphorus as a new saturable absorber in passively mode-locked(PML)fiber laser to generate dissipative solitons.It shows that under the same conditions,the dissipative soliton generated by PML fiber laser based on BP as a saturable absorber has better physical properties(such as need less time to generate pulse,has narrow pulse duration and high pulse energy)than other2 D materials(Graphene,TMDs,TIs)as a saturable absorber.It prove that black phosphorus is an ideal saturable absorber and can be used for ultrashort optical pulse generation.After that we further investigated the impact of saturable absorber parameters in passively mode-locked fiber laser.Numerical simulations show that modulation depth,nonsaturable absorption and saturation energy can influence the dissipative solitons' physical properties.We need deep modulation depth and less nonsaturable absorption at the same time if we want to generate ultrashort pulse.We can deeply understand the importance of saturable absorber in passively mode-locked fiber lasers and develop a more suitable saturable absorber in experiments based on the analysis of saturable absorber parameters.3.Theoretical and experimental research on self-similar fiber lasers.Dynamic evolution of the pulse in self-similar fiber laser by numerical simulation,and then built Yb-doped self-similar fiber laser in our lab and 5.8 nJ and 83 fs output.In experiment we find that by adjusting a narrow intracavity 4 nm spectral filter we can tune the pulse central wavelength.Hence,we focus on how to achieve tunable selfsimilar fiber laser,we successful achieved highly performing pulses with pulse durations around 118 fs and pulse energies of 3.9 nJ are demonstrated over a 70 nm tuning range.By appropriately adjusting the nonlinear polarization rotation,another pulse duration less than 100 fs is obtained.As far as we know,this is the first tunable fiber laser with pulse duration less than 100 fs in Yb-doped fiber laser.The results reveal that self-simialr fiber lasers have a strong potential for applications requiring wavelength tunable pulses.4.New type of ultrashort pulse fiber laser——Mamyshev fiber laser has been studied.Mamyshev fiber laser has lead of the fiber lasers in generate high puleenergy.We achieved 60 fs with 30 nJ pulse energy from Mamyshev fiber laser in our lab.After that,we present a method to achieve 17 fs with ultra-broad spectrum(394nm@-20dB)in Mamyshev fiber laser by inserting dispersion delay line and high nonlinear fiber inside the cavity.To our best knowledge,this is the broadest spectrum and shortest pulse(17 fs)directly generate from a mode-lock fiber.It will give us new insights into generate few cycle pulse or even one cycle pulse in a passively mode-locked fiber laser.