Numerical Study Of The Pulse Compression And Broadening Based On The Optical Fibers With Large Group-velocity Dispersions

The characteristics of ultrashort pulses with high peak powers and narrow durations make them have a wide range of applications in the fields of optical communication,biomedicine and laser processing.The pulse compression and broadening are very important to obtain ultrashort pulses.There are mainly two types of optical devices to realize optical pulse compression and broadening:bulk optical components and fiber optic components.Bulky optical components,such as grating pairs and prism pairs,can exhibit large and tunable dispersion.Fiber devices with large group-velocity dispersion?GVD?characteristics include dispersion compensation fibers,chirped fiber Bragg gratings and microstructure fibers.We propose two fiber structures,which are the asymmetric step-index dual-core fiber and three-core photonic crystal fiber with large GVD characteristics accompanied with low nonlinearity and confinement loss.Consequently,they can be used for pulse broadening and compression.The main work in this paper is to analyze the dispersion,nonlinearity and loss characteristics for the two optical fibers.The generalized pulse propagation equation is numerically solved to analyze the evolution of pulses along the optical fibers.Further,the influences of input pulse and fiber parameters on pulse stretching and compression are discussed.The main contents are as follows:First,the basic principles of pulse propagation along the optical fiber are presented,including the Maxwell equations and the Helmholtz equation,and the numerical method for solving the equation to obtain the mode fields in the fibers,i.e.,the finite element method.Then,the concepts and formulas of the GVD,nonlinear parameter,and confinement loss of the fibers are presented.Additionally,the generalized nonlinear Schr?dinger equation and its numerical solving method are described.Second,an asymmetric step-index dual-core fiber is proposed.The supermode of the fiber can provide anomalous-and normal-GVD profiles with large peak values by properly designing the fiber parameters.The peak values of the normal and anomalous dispersions are-3942ps/?nm·km?and 3040ps/?nm·km?at 1550nm,respectively.The corresponding nonlinear parameters of the two supermodes are 0.0006W^-1/m and0.0007W^-1/m,and the confinement losses are 9.69×10^-8d B/m and 1.1×10^-5d B/m,respectively.The curves of the GVD,nonlinear parameter and confinement loss can be shifted in the wavelength window of 1550 nm by properly tuning the fiber parameters.The asymmetric dual-core fiber with the normal-GVD supermode 1 can be employed in the pulse broadening,and the effects of parameters such as initial pulse width and fiber length on the output pulse are also analyzed.Last,a three-core photonic crystal fiber?TC-PCF?structure with large dispersion is proposed.The TC-PCF can exhibit large dispersion,low nonlinearity and loss characteristics at three wavelength windows of 1030nm,1550nm and 1900nm by tuning fiber parameters such as air-hole diameter and spacing,the refractive index of the two cores.The curves of the dispersions and nonlinear parameters can be shifted within a certain wavelength range by adjusting fiber parameters.When the supermodes 1 and 2of the fiber are excited respectively,the optical fiber can exhibit high normal-and anomalous-GVD profiles.Consequently,the designed optical fibers can be employed for pulse broadening and compression.For the pulse broadening with high peak powers,the short fiber length and low nonlinear parameter are helpful to avoid pulse distortions induced by the nonlinearity.For the pulse compression with high peak powers,a comparison is made between the one-fiber-segment and two-fiber-segment pulse-compression schemes.In the latter,both the group-velocity dispersion and third-order dispersion?TOD?of the pulses can be compensated.As a result,the pulse compression with the high quality can be achieved.