Research On The Properties Of Optical Solitons In Fiber Lasers

Since it was firstly predicted in 1973,soliton formation in single mode fibers(SMFs)has attracted great attention of research and been extensively investigated both theoretically and experimentally.The light propagation in SMFs is governed by the nonlinear Schr?dinger equation(NLSE),which admits various analytical solutions of the envelope solitons.The same equation also governs the nonlinear wave dynamics in a wide range of physical systems,such as the spin waves in magnetic films,water waves in a tunnel,and matter wave in a Bose-Einstein condensate.Therefore,study on the solitons in SMFs and their dynamics is not only of fundamental importance,but also providing an insight into the nonlinear wave dynamics of various physical systems.Particularly,as compared with the soliton formation in other physical systems,optical soliton formation in SMFs is experimentally easy to achieve and precisely controllable,which make the system an ideal test bed for the study of nonlinear waves.Physically,the transition of optical pulses to optical soltons requires balance between fiber dispersion and the self-phase modulation.Depending on the sign of fiber dispersion,either optical bright or dark solitons can be formed.Specifically,while the bright solitons,which are localized bell shaped pulses,are formed in fibers with anomalous group velocity dispersion(GVD),dark solitons,which are intensity dips on a continuous wave(CW)background,are formed in fibers with normal GVD.Another important difference between the bright and dark NLSE solitons is that the formation of the bright solitons always requires a certain initial pulse intensity threshold,but that of the dark solitons is thresholdless.An arbitrary initial intensity dip could evolve into one or multiple dark solitons.All the features of the NLSE solitons in SMFs have been experimentally confirmed.Fiber laser,mainly formed by various optical fibers,is a perfect platform to generate and study the scalar and vector optical solitons.Different from the case of optical single mode fibers,the optical solitons formation in fiber lasers is a result of the mutual interaction among the cavity dispersion,fiber nonlinearity,laser gain saturation,gain bandwidth filtering and saturate absorption.Strictly speaking,the solitons in fiber lasers are dissipative solitons which are governed by the Ginzburg-Landau Equation(GLE).However,NLSE solitons could also be obtained in fiber lasers under certain conditions.In fiber lasers,bright solitons have been obtained by passive mode locking of the lasers that could be achieved by inserting a saturable absorber into the laser cavity.Similarly,dark solitons can be selected and stabilized by inserting a reversed saturable absorber into the laser canvity.In the current dissertation,we studied various solitons formation and dynamics in fiber lasers which are dissipative systems.Our studies are not only of fundamental importance,but also help us with a deeper understanding on fiber lasers and may have potential applications in the future optical communication systems.The main research results are listed below:Firstly,we derived the averaged equation mode describing fiber laser operations,this equation simplified the complex fiber laser system to a single extended NLSE,making our theoretical studies easier and more convenient.Meanwhile,we also designed a full numerical simulation method which will reveal all the details in fiber lasers,our program can reproduce most experimental results and provide theoretical evidence for those experimental observations.Furthermore,our numerical method is faster and cheaper than experimental studies.Secondly,combing theoretical and numerical method,we studied modulation instability in fiber lasers and also the related P-soliton formation.We further studied the cavity modulation instability only exists in fiber lasers.We also studied the effect of saturate absorption on bright soliton formation in anomalous dispersion cavity fiber lasers,and the formation of cavity solions under effect of gain bandwidth filtering.Thirdly,we studied the formation of high repetition rate dark soliton trains in fiber lasers under effect of gain bandwidth,and dark soliton formation and dynamics in fiber lasers with and without reversed saturate absorption.We also show results of vector dark soliton formation in fiber lasers with reversed saturate absorption.Finally,we studied the controlled generation of bright or dark solitons in fiber lasers using nonlinear polarization rotation method,and also their mutual transformation.For other types of solitons,such as domain wall solitons and anti-dark solitons,we have also studied in detail.