The Effect Of Positive Chirp On Dispersion And Soliton Trapping In Photonic Crystal Fibers

Photonic crystal fiber(PCF)is an excellent medium for the study of supercontinuum(SC),and the rise of PCF has opened new horizons for effectively SC generation.Compared with the traditional optical fiber,it has some very special properties,such as "endlessly single-mode" behavior,controllable group velocity dispersion characteristics and high nonlinear properties,these characteristics have brought great convenience to our research.The generation of SC in PCFs is a very complex nonlinear process,many factors such as the structure of the optical fiber,group-velocity dispersion(GVD),self-phase modulation(SPM),the self-steepening,the cross-phase modulation and so on,can influence the generation of SC.When a femtosecond optical pulse pumped in the anomalous dispersion regime of PCF,the nonlinear effect and high-order dispersion effects are two prominent factors that greatly affect the ideal periodic high-order soliton evolution.Consequently,the higher-order soliton can break up into red-shifted fundamental solitons through soliton fission.During the soliton fission process,dispersive wave(DW)is emitted due to the energy transfer from soliton to narrow-band resonance in the normal dispersion regime.On the basis of the generalized nonlinear Schrodinger equation,we use numerical methods to detailedly simulate the influence of positive chirp on the SC generation and the stability of optical solitons in PCF with two zero dispersion wavelengths(ZDWs).The main research contents are listed below:Firstly,based on the Maxwell equation,we derive the nonlinear Schrodinger equation for the propagation of optical pulses in fibers.Then we use the standard split-step Fourier method solve the equation and analyzes the calculating precision.We study the characteristics of the soliton and DW by cross-correlation frequency-resolved optical gating technique(XFROG),and get the process of soliton trapping DW.Secondly,we analyzed the influence of GVD and nonlinear effect on optical pulse propagation in the PCF.And find GVD broadens the pulse.The extent of broadening is governed by the dispersion length.SPM induced spectral changes,new frequency components are generated continuously as the pulse propagates down the fiber,these SPM-generated frequency components broaden the spectrum.Intrapulse Raman Scattering cause the energy from blue components is continuously transferred to red components.Such an energy transfer appears as a red shift of the soliton spectrum,with shift increasing with distance.Self-steepening leads to an asymmetry in the SPM-broadened spectra of ultrashort pulses.As the pulse propagates inside the fiber,it becomes asymmetric,with its peak shifting toward the trailing edge.Thirdly,we numerically studied the generation of DW and its interaction with higher-order solitons.In an ideal lossless fiber,the pulse will propagate undistorted without change in shape for arbitrarily long distances.But if we consider the perturbation of solitons,high-order soliton breaks up into fundamental solitons.During the soliton fission,DW is emitted due to the energy transferred from soliton to narrow-band resonance in the normal dispersion regime.Then we use numerical methods to detailedly simulate the influence of positive chirp on the SC generation and stability of optical solitons.The results show that the positive chirp can significantly improve the probability of energy transfer from the soliton to dispersive wave.At the same time,the intensity of dispersive wave is proportional to the chirp value.Especially,solitons will not appear when the chirp value exceeds a certain threshold.Therefore,choosing an appropriate positive chirp,we can regulate the generation of dispersive wave and solitons in photonic crystal fibers.