Soliton States And Dynamics In Passively Mode-Locked Fiber Lasers With Higher-order Effects

As an ultra-fast light source with low-cost and high reliability,passively mode-locked fiber lasers have been widely studied in recent years.The pulse generated by the device has a series of remarkable advantages,including high conversion efficiency,narrow pulse width and high power,which are widely used in micro-processing,medical treatment,communication,national defense and other fields.Since the fact that the transmission characteristics of optical solitons are affected by gain,loss,dispersion,and nonlinear effects during actual transmission,it is of important theoretical value and practical significance to study the transmission characteristics of optical solitons in optical fibers for the design and implementation of high-speed optical soliton communication systems.Based on the extended nonlinear Schr?dinger equation,this thesis mainly analyzes the generation conditions of different types of optical solitons,the interaction behavior,the influence of different media and higher-order effects on the propagation of optical solitons.The main content of this paper is as follows:(1)Based on the nonlinear Schr?dinger equation,we study the beam propagation in the focused Kerr type medium with non-uniform gain and loss distribution.The propagation characteristics of optical solitons passing through PT symmetric barrier are studied and the phenomena are numerically simulated.It is found that with the increase of power,symmetric solitons will occur symmetry breaking phenomenon in the transmission process,and asymmetric solitons will appear.When the power exceeds a certain threshold,symmetric solitons and asymmetric solitons can exist simultaneously.The effective bandwidth of solitons increases with the enhancement of fractional diffraction effect.This achievement not only deepens another development direction of traditional soliton theory research,but also has a potential technical impact on optical systems.(2)By adding higher-order effect to the complex Swift-Hohenberg equation,the characteristics of the pulse with input pulse width less than 100 femtoseconds are investigated.The results show that solitons with high energy will be limited by the soliton area theorem,and the soliton splitting phenomenon will occur.With the increase of the small signal gain coefficient or self-steepening coefficient,the number of soliton molecules will gradually increase.However,the M-type soliton can still maintain stable self-shape transfer when the system parameters remain unchanged.Finally,due to the addition of the higher-order filtering effect,the fourth order filtering realizes the asymmetry of the spectral sideband,thus changing the output center wavelength of the laser.(3)Based on the basic theory of polarization mode dispersion and its expression,the coupled Ginzburg-Landau equation including gain and higher-order effect is obtained to describe the propagation of optical pulses in birefringent fibers.Synchronous and asynchronous transmission of vector solitons can be realized by controlling the angle of the polarizer and the small signal gain coefficient.In addition,it can also be observed that the peak amplitude of the pulse sequence changes from period doubling to chaos and then to period doubling,and the soliton splitting phenomenon can also be generated by reducing the output ratio of the coupler.It is also found that the interaction between soliton molecules can occur by adjusting the beat length of the fiber.The research results provide a theoretical basis for the realization of ultrashort pulse all-optical switch and information transfer.