Research On Dispersive Wave And Soliton Trapping In Cascaded Photonic Crystal Fibers

Since the fiber has dispersion and nonlinear effects,when the pump pulse transmits in it,both the frequency domain and the time domain are broadened,resulting in new spectral components.The photonic crystal fiber has the characteristics of controllable dispersion and rich nonlinearity,so the double-zero dispersion fiber can be obtained by adjusting the microstructure of the photonic crystal fiber.When the high-order soliton transmits in the fiber,it will be splitted into fundamental solitons by the effects of nonlinear and dispersion effects,and radiate blue-shifted dispersive waves and red-shifted dispersive waves to generate supercontinuum.However,studies have shown that when the width of the two zero-dispersion wavelength is wide,the supercontinuum with extremely wide spectrum can be obtained,but we can find that there is an obvious wave structure in the long wavelength range of the spectrum.In this paper,we analyze the causes of wave structure and propose that we can improve the flatness of the supercontinuum through cascading the fibers.In the meantime,we explore the feasibility of this method from the theoretical and simulation perspective.The main contents of the study are as follows:First,we introduce the transmission equation of the pulse in the fiber,that is,the nonlinear Schr?dinger equation.The equation is solved by the split-step Fourier method,and the cross-correlation frequency-resolved optical switching technique is introduced.Starting from the equations,we analyze the dispersion and various nonlinear effects in the broaden process of the pulse.Secondly,according to the experimental requirements,four fibers with different zero-dispersion wavelength spacing are designed.The time-frequency diagram of the output spectrum is compared and analyzed.When the width of the two zero-dispersion wavelengths is wide,the blue-shifted dispersive wave in the time domain will move from the trailing edge to the cutting edge of the soliton,the new spectral component produced by soliton capture can increase the spectral width slightly at the short wavelength range;however,the red-shifted dispersive wave shifts to the soliton trailing edge,and the capture wave and the red-shifted dispersive wave coincide in the frequency domain;the frequency shift of the second fundamental soliton which is splitted from the high-order soliton is very slow,and there is a significant energy gap between two fundamental soliton.Thirdly,the soliton order jump theory is introduced to analyze the reasons of the deterioration of the supercontinuum flatness caused by the increase of two zero-dispersion wavelengths' width.Through selecting appropriate fibers to cascade,we can shape the supercontinuum spectrum,compensate the spectral gap and generate a wide and flat supercontinuum.