Effect Of Soliton Collisions On Tunable Ultra-short Pulse Sequences

Ultrashort pulse plays an important role in the optical field such as supercontinuum generation,soliton splitting and the generation of dark soliton.In recent years,many scholars focus on the generation of ultrashort pulse sequences and have achieved many excellent results.This paper focuses on the generation of tunable ultrashort pulse sequences in photonic crystal fibers.Two beams of Gaussian light pulses with different time delays are pumped into the input of the fiber.When the two beams of pulses transmit some distance in the fiber,they will collide,then causing severe frequency changes and energy conversion.This is due to the fact that the two beams of pulses are continuously broadening under the effect of group velocity dispersion to produce time-domain overlap,leading to the interaction between the pulses in the overlapping region,which results in the generation of ultrashort pulse sequences and a train of dark solitons.In the frequency domain,new frequency components are continuously generated due to self-phase modulation,which allows the two pulse spectra to produce a mixing effect,forming a very obvious comb-shaped supercontinuum.Besides,a series of dark solitons are generated in the normal dispersion region and bright solitons are produced in the anomalous dispersion region.In addition,the seed pulse also has a non-negligible impact on the generation of ultrashort pulse sequences.In this paper,based on the generalized nonlinear Schr(?)dinger equation,numerical simulation and analysis of the effect of the seed pulse on the ultrashort pulse sequences and dark soliton generated in the normal dispersion region in the photonic crystal fiber are carried out,the main research contents are as follows:(1)Based on Maxwell’s equations,an equation that can be generally applied to describe the mathematical and physical processes that light waves follow when propagating in PCF is derived,that is,the generalized nonlinear Schrodinger equation.The fractional-step Fourier algorithm for solving the transmission equation is expounded.The common factors that affect the evolution of optical pulses,ie,dispersion and nonlinear effects,are analyzed in detail.(2)The process of pumping two Gaussian light pulses into photonic crystal fibers to generate ultrashort pulse sequences and dark solitons is studied,and the effect of seed pulses on ultrashort pulse sequences and dark solitons generated in the normal dispersion region is also emphatically analyzed.The simulation results show that selecting appropriate seed pulse parameters can effectively promote the energy enhancement of ultrashort pulse sequences.(3)The regulation and influence of seed pulses on ultrashort pulse sequences under different modulation frequencies and different modulation depths are studied.We find out that when the modulation frequency corresponds to the frequency value at the highest point of the modulation instability gain spectrum,the increase in the peak value of the ultrashort pulse sequence compared to the unseeded case is more severe than that of the other modulation frequencies.In addition,simulation results show that appropriate modulation depth helps to improve the peak energy of the ultrashort pulse sequence.If the modulation depth is too small,it has basically no effect,and if the modulation depth is too large,the energy enhancement of the ultrashort pulse sequence is hindered.