Theoretical Research On The Highly Nonlinear Two-core Optical Fiber As A Saturable Absorber

Recently,passively mode-locked fiber laser has been widely employed in the fields of industry,medicine,biology and nonlinear optics.As one of the important components in passively mode-locked fiber lasers,saturable absorber(SA)can provide the function of amplitude modulation for mode-locking technique by absorbing low-power optical wave and transmitting high-power optical wave.The saturable absorbers based on optical fibers mainly include Nonlinear Optical Loop Mirror(NOLM),Nonlinear Amplifying Loop Mirror(NALM)and so on.Recently,the two-core fiber is also demonstrated to be a saturable absorber due to its special structure.However,most researches on the two-core-fiber saturable absorber are based on the quasi-continuous coupling equations,which can be employed to analyze the case of input pulses with long temporal width,but not suitable for the case of ultra-short pulses.Consequently,we mainly focuses on the saturable absorption based on highly nonlinear two-core fiber based on numerically solving ultra-short pulse propagation coupling equations.First,the two-core fibers are analyzed by numerically solving the quasi-continuous coupling equations.The results show that standardized distance can be reduced to weaken the linear coupling,or the power of input can be increased to enhance the nonlinear effects for obtaining lower saturation power of SA.According to the above discussions,the parameters selection required for the following investigations on the ultra-short pulse evolutions along the two-core fibers can be obtained.Second,the dynamic characteristics of ultra-short pulses propagating along the two-core fiber as the SA are analyzed based on solving ultra-short pulse propagation coupling equations.Unlike quasi-continuous coupling equations,the ultra-short pulse propagation coupling equations include the terms of stimulated Raman scattering(SRS),dispersion and shock in optical fibers besides the effects of SPM and linear coupling.At the same time,the influences of the parameters of fibers and input pulses on the output pulse shape,spectra and transmission functions are analyzed.Moreover,in order to investigate the influences of SRS,dispersion and shock on the output characters,the ultra-short pulse propagation coupling equations are numerically solving by neglecting the terms of SRS,dispersion or shock,under the condition of material in cores and structure of fiber chosen in this research.The simulation results compare with the results of complete coupling equations.The results show that saturation power will be reduced due to the effect of SRS when the width of input pulse is less than 1 ps.The effect of dispersion can result in a pulse broadening in temporal domain,thus reducing the amplitude of the pulse and weakening the nonlinearity-induced saturation.Moreover,the effect of shock on the transmission can be ignored.Finally,the two-core optical fiber as a saturable absorber is investigated numerically by numerically simulating the model of all-normal-dispersion passively mode-locked fiber laser.The results show that the fiber lasers based on the high-nonlinearity two-core-fiber SA can achieve self-similar parabolic pulses.