Study On The Transmission Characteristics Of Soliton Molecules In Optical Fiber Communication Systems

Optical solitons can play an increasingly important role as excellent carriers of information in the future communication field because of their unique advantages.And with the growth of communication capacity demand and capacity crisis,using soliton molecules as carriers of information will become a new information transmission means and solution for future communication systems.In this thesis,the propagation states of different nonlinear waves in molecular form in optical fiber communication systems are the main research objectives,and corresponding theoretical models are proposed for different optical fiber systems,and the analytical expressions of nonlinear waves such as bright solitons,dark solitons,and breathers are obtained by solving these models analytically.Further,the propagation states of these different types of soliton molecules under different parameters are discussed in conjunction with three-dimensional plots,density plots and contour plots to achieve precise parameter control of the soliton molecules.The research can provide theoretical references for the experimental observation of bound states of different types of optical pulses in various types of optical fiber communication systems in the future,reduce the experimental sunk cost,and explore the potential practical applications of optical solitons and their molecular forms.The specific main research of this thesis is as follows:(1)The transmission characteristics of bright solitons and bright soliton molecules in inhomogeneous optical fibers:For the higher-order nonlinear Schrodinger model describing the dynamic characteristics of femtosecond optical solitons in inhomogeneous fibers,the one-,two-,and three-soliton solutions of this model are obtained by using the Hirota method.The propagation states and interactions of solitons in inhomogeneous fibers are analyzed.By adjusting the parameters,the periodic oscillation of two solitons and the splitting of three solitons into multiple solitons under the effect of distribution function are analyzed,and the constraints for soliton molecules composed of different numbers of solitons are obtained,respectively.The interaction between soliton molecules and solitons,as well as the stable propagation and internal interaction of soliton molecules composed of three solitons are analyzed.The results can provide a theoretical reference for the coding method of quaternary data transmission using solitons.(2)The transmission characteristics of dark solitons and dark soliton molecules in inhomogeneous optical fibers:Aiming at the problem of dark soliton pulse propagation in inhomogeneous optical fibers,the corresponding analytical model is proposed,and the analytical solutions of dark one-and two-solitons are solved.The expressions for the variation of the dark soliton propagation velocity and amplitude are derived from the results,and the effects of different parameters on the dark soliton propagation state and the interactions between dark solitons are discussed.Depending on the values of different functions of the group velocity dispersion coefficient and the third-order dispersion coefficient,dark solitons can show a wealth of propagation states,including the abrupt changes of amplitude and plane background wave in the propagation process.In addition,the formation conditions of dark soliton molecules are obtained by velocity resonance conditions.It is found that dark soliton molecules are more stable than bright soliton molecules,and will not deform in a short interval.This result verifies the idea of applying dark soliton molecules to optical fiber communication and provides a theoretical reference for future experimental observation and practical application of dark soliton molecules.(3)The transmission characteristics of optical solitons and soliton molecules in birefringence optical fibers:The variable coefficient coupled(3+1)-dimensional nonlinear Schrodinger equation describing the optical soliton propagation in birefringence fibers is the main object of investigation.The model is transformed into a bilinear form using the Hirota method,and the bilinear equation is used to solve the one-and two-soliton solutions.After the analytical derivation,the expressions of the soliton propagation velocity and amplitude of the model are obtained.The effects of different parameters on the soliton propagation process are analyzed.The interaction phenomena of fusion,splitting and deformation of two solitons are revealed by parameter control and analysis.Moreover,interestingly,we find that the energy between the two solitons is redistributed under certain parameter conditions.Different values of the parameter can adjust the vibrational synchronization of the two solitons during the propagation of the two solitons oscillating in a trigonometric function type.Finally,we obtain the formation conditions of the stable propagation of the soliton molecules in different planes by the velocity resonance conditions and analyzed the propagation states of the soliton molecules under the influence of different parameters.(4)The transmission characteristics of pure quartic solitons and soliton molecules:The corresponding propagation models are developed based on the characteristics of pure quartic solitons.The one-and two-quartic soliton solutions are obtained by the Hirota method.The dynamics of the one-and two-pure quartic solitons are discussed based on the obtained analytical soliton solutions.The static and non-static pure quartic soliton molecules are obtained by velocity analysis.We find that under certain conditions,static and non-static pure quartic soliton molecules can stably propagate in a new form:the higher middle main peak accompanies two smaller energy sub-peaks on both sides.Moreover,the pure quartic soliton-asymmetric soliton molecules are obtained by parameter control.The related study helps to further explore the practical applications of pure quartic solitons.(5)The transmission characteristics of femtosecond breathers and breathers molecules:The one-and two-breather solutions of the higher-order nonlinear Schrodinger equation describing the propagation state of femtosecond breathers are successfully solved by the Hirota method.Based on the obtained results,the propagation velocity of the breather is analyzed,and different mode conversions between the breather and other nonlinear waves are realized by parameter control.At the same time,the interactions between the breathers and different nonlinear waves are also investigated in depth.Notably,we find that the interaction between two breathers is elastic regardless of whether they collide head-on or chase collisions.Moreover,when the breathers interact with bright solitons and periodic waves on a non-zero plane,the amplitudes of the bright solitons and periodic waves will be amplified or compressed by the breathers.This phenomenon can be used to overcome the problem of weakening of the optical pulse energy due to losses and has practical applications.Finally,the formation conditions of general breather molecules,Kuznetsov-Ma breather molecules and Akhmediev breather molecules are obtained separately,and the dynamics properties of different types of breather molecules are discussed.The related research results can provide a more intuitive and in-depth understanding of the modulation instability of optical nonlinear systems,and also facilitate the experimental observation of breathers and breather molecules.