色散磁导率对异向介质中的调制不稳定性的影响

基于Drude模型研究了异向介质的色散磁导率对调制不稳定性的影响. 结果表明,在反常色散情形,赝五阶非线性在异向介质的负折射区中增大了调制频谱的范围及增益值,这与常规正折射介质中出现的现象正好相反;自陡峭效应在异向介质中有可能为负值,但无论正负,也无论在正折射区还是负折射区,它都抑制调制不稳定性的产生;二阶非线性色散效应在正、负折射区中分别促进和抑制调制不稳定性的产生. 在正常色散情形,由于二阶非线性色散效应的作用,使本来在常规正折射介质中不可能出现的调制不稳定性现象也能出现,这一特性为在正常色散区形成孤 关键词： 异向介质 调制不稳定性 色散磁导率 二阶非线性色散     

超常介质中暗孤子的形成和传输特性研究

利用一种扩展的双曲函数级数方法求解超常介质中的传输方程，得到了各种不同情形下的暗孤子解，分析了可控自陡效应和二阶非线性色散效应对孤子形成和传输特性的影响.结果表明，超常介质中负的自陡效应使得暗孤子的中心位置随传输距离向脉冲前沿方向漂移，与常规介质中自陡效应(恒为正)的作用相反；特别是，由于二阶非线性色散的作用，使得在没有线性群速度色散的情形下同样可形成孤子，而且在反常线性色散情形也可形成暗孤子.     

超常介质中超短电磁脉冲的传输特性研究

将超常介质的色散磁导率合并到非线性极化项中,借鉴常规介质中超短脉冲传输方程的推导方法,得到了非线性超常介质中超短脉冲的传输方程。在德鲁德(Drude)色散模型下,根据脉冲中心频率的不同在传输方程中出现了可正、可负、可为零的自陡峭系数,以及高阶非线性色散项。此外,利用矩方法对传输方程进行分析,得到了超常介质中超短脉冲传输方程的能量守恒定律表达式,揭示了色散磁导率导致的超短脉冲传输的新特性,发现二阶非线性色散使超短脉冲的能量、脉冲频移、脉冲宽度、中心位置和啁啾都随传输距离呈现振荡式变化。     

Role of dispersion in multiple-collapse dynamics

The multiple-collapse dynamics of ultrashort pulses along the propagation direction are investigated under conditions of both normal and anomalous group-velocity dispersion (GVD). In the anomalous-GVD regime we find that collapse events can occur at locations in the medium many diffraction lengths beyond the initial collapse point, in contrast with the normal-GVD regime in which multiple collapse occurs within a diffraction length. Numerical simulations of a modified nonlinear envelope equation are found to be in good qualitative agreement with the observed lengths of the filaments.     

高相干度超连续谱产生的理论研究

超连续谱的相干性直接影响到很多光学测量器件的分辨率及准确性, 如何获得高相干度的超连续谱是当前非线性光学领域的研究热点之一. 本文首先分析超连续谱产生的机理, 认为调制不稳现象是降低超连续谱相干性的重要因素. 因此抑制调制不稳定性, 利用其他非线性效应(如自相位调制)是得到高相干性超连续谱的有效方法. 最后设计了一种全波段正常色散光子晶体光纤,仿真结果表明, 在脉宽400 fs无啁啾高斯脉冲抽运, 50 cm长该光子晶体光纤的条件下, 产生的超连续谱具有在相对功率大于-80 dB的全波段高度相干的特性.     

Breathers and Rogue Waves Derived from an Extended Multi-dimensional N-Coupled Higher-Order Nonlinear Schrödinger Equation in Optical Communication Systems

In this paper, an extended multi-dimensional N-coupled higher-order nonlinear Schrödinger equation (NCHNLSE), which can describe the propagation of the ultrashort pulses in wavelength division multiplexing (WDM) systems, is investigated. By the bilinear method, we construct the breather solutions for the extended (1+1), (2+1) and (3+1)-dimensional N-CHNLSE. The rogue waves are derived as a limiting form of breathers with the aid of symbolic computation. The effect of group velocity dispersion (GVD), third-order dispersion (TOD) and nonlinearity on breathers and rogue waves solutions are discussed in the optical communication systems.     

Lax pair and vector semi-rational nonautonomous rogue waves for a coupled time-dependent coefficient fourth-order nonlinear Schr?dinger system in an inhomogeneous optical fiber

Optical fibers are seen in the optical sensing and optical fiber communication. Simultaneous propagation of optical pulses in an inhomogeneous optical fiber is described by a coupled time-dependent coefficient fourth-order nonlinear Schr?dinger system, which is discussed in this paper. For such a system, we work out the Lax pair, Darboux transformation, and corresponding vector semi-rational nonautonomous rogue wave solutions. When the group velocity dispersion(GVD) and fourth-order dispersion(FOD) coefficients are the constants, we exhibit the first-and second-order vector semirational rogue waves which are composed of the four-petalled rogue waves and eye-shaped breathers. Both the width of the rogue wave along the time axis and temporal separation between the adjacent peaks of the breather decrease with the GVD coefficient or FOD coefficient. With the GVD and FOD coefficients as the linear, cosine, and exponential functions, we respectively present the first-and second-order periodic vector semi-rational rogue waves, first-and second-order asymmetry vector semi-rational rogue waves, and interactions between the eye-shaped breathers and the composite rogue waves.     

Lax pair and vector semi-rational nonautonomous rogue waves for a coupled time-dependent coefficient fourth-order nonlinear Schrodinger system in an inhomogeneous optical fiber

Optical fibers are seen in the optical sensing and optical fiber communication. Simultaneous propagation of optical pulses in an inhomogeneous optical fiber is described by a coupled time-dependent coefficient fourth-order nonlinear Schr?dinger system, which is discussed in this paper. For such a system, we work out the Lax pair, Darboux transformation, and corresponding vector semi-rational nonautonomous rogue wave solutions. When the group velocity dispersion(GVD) and fourth-order dispersion(FOD) coefficients are the constants, we exhibit the first-and second-order vector semirational rogue waves which are composed of the four-petalled rogue waves and eye-shaped breathers. Both the width of the rogue wave along the time axis and temporal separation between the adjacent peaks of the breather decrease with the GVD coefficient or FOD coefficient. With the GVD and FOD coefficients as the linear, cosine, and exponential functions, we respectively present the first-and second-order periodic vector semi-rational rogue waves, first-and second-order asymmetry vector semi-rational rogue waves, and interactions between the eye-shaped breathers and the composite rogue waves.     

Generation of Bragg solitons through modulation instability in a Bragg grating structure

In this article, we consider the continuous wave (cw) propagation through the nonlinear periodic structure that consists of alternating layers of both positive and negative Kerr coefficients along the propagation direction. We investigate the modulational instability (MI) conditions required for the generation of ultrashort pulses for the nonlinearity management system. We study the occurrence of MI at the top and bottom edges of the photonic band gap (PBG) where the forward and backward propagating waves are strongly coupled because of the presence of the grating structure. We also study the MI when cw is detuned from the edges of the PBG into the anomalous and normal dispersion regimes. In addition, we discuss the existence of gap solitons for the nonlinearity management system in the upper and lower branches of the dispersion curve through the MI gain spectra. We observe the generation of higher order solitons in the nonlinear periodic structure when the input power is increased beyond a certain critical level. Finally, we discuss the generation of higher order Bragg grating solitons through the intensity evolution of the forward and backward propagating fields.     

Kuznetsov–Ma soliton and Akhmediev breather of higher-order nonlinear Schrdinger equation

In terms of Darboux transformation, we have exactly solved the higher-order nonlinear Schrdinger equation that describes the propagation of ultrashort optical pulses in optical fibers. We discuss the modulation instability(MI) process in detail and find that the higher-order term has no effect on the MI condition. Under different conditions, we obtain Kuznetsov–Ma soliton and Akhmediev breather solutions of higher-order nonlinear Schrdinger equation. The former describes the propagation of a bright pulse on a continuous wave background in the presence of higher-order effects and the soliton's peak position is shifted owing to the presence of a nonvanishing background, while the latter implies the modulation instability process that can be used in practice to produce a train of ultrashort optical soliton pulses.     

饱和非线性正折射异向介质中的调制不稳定性

直接从异向介质中包含饱和非线性、自陡峭和二阶非线性色散效应的非线性扩展传输方程出发,采用线性稳定性分析法,导出了调制不稳定性的色散关系、不稳定条件、无量纲的临界扰动频率和增益谱。计算和讨论了异向介质正折射区无量纲的增益谱随归一化角频率和入射功率密度的变化关系。结果表明,在异向介质正折射区,随归一化角频率和入射功率密度的不同,增益谱将会出现扰动频率大于零、扰动频率大于某个非零临界值、扰动频率大于零而小于某个非零临界值3种形式。在归一化角频率较小时,调制不稳定性可能出现阈值入射功率密度;且在较小的入射功率密度时,调制不稳定性只能出现在大于某个临界频率时。     

Measurement of ultrashort laser pulses using single-crystal films of 4-aminobenzophenone

Single-crystal thin-film of an organic second-order nonlinear optical material, 4-aminobenzophenone (ABP), is used to measure the pulsewidth of a Ti-Sapphire laser producing 45 fs pulses at 1 kHz repetition rate, by the non-collinear second-harmonic generation (SHG) intensity autocorrelation technique. These films are suitable for measurements over a broad wavelength range, down to 780 nm, due to their wide optical transparency. The single-crystal film with thickness (3 μm) less than the coherence length requires no phase-matching for efficient broadband SHG. Pulse walk-off due to group-velocity mismatch (GVM) and temporal broadening of the pulses due to group-velocity dispersion (GVD) are found to be negligible. These effects have been estimated for pulse width down to few-cycle pulses (10 fs), and the analyses show that these films can be used to characterize such ultrashort optical pulses.     

色散缓变光纤中飞秒孤子压缩与稳定传输

运用数值方法研究了色散缓变光纤中飞秒基本孤子的传输特性，发现基本孤子脉冲不仅能被压缩，而且压缩光脉冲能保持脉宽不变转输即稳定传。光脉冲压缩过程中，发现三阶色散效应有较大的影响。适当地选取色散缓变光纤结构参数可能获得高质量的超短压缩光脉冲。对压缩光脉冲稳定伟机理探索表明，光纤二阶色散纵向变化率，三阶色散与喇曼自散射效应共同作用导致压缩光脉冲稳定传输。     

部分相干光束经过湍流大气传输研究进展

相较于相干光束,部分相干光束经过湍流大气传输能够有效地抑制湍流引起的光束展宽、光斑漂移及光强闪烁等扰动效应,在自由空间光通信、激光雷达和激光遥感等方面有重要的应用前景.近年来,部分相干光束湍流大气传输研究受到越来越多学者的关注.本文回顾了部分相干光束在湍流大气中传输特性研究的发展历程、理论基础及常用的理论方法,介绍了处理光束经过湍流大气传输的相位屏数值模拟方法,以及如何把该方法运用到处理部分相干光束传输.     

负折射介质中超短脉冲的反常分裂现象

对超短脉冲在负折射介质中的传输特性进行了研究,着重分析其不同于常规介质的反常脉冲分裂现象。结果表明:由于负的折射率影响,介质将通过反常群速度色散与自散焦非线性的相互作用机理来诱导脉冲的对称分裂。同时,分析由色散磁导率所导致的可控自陡效应对脉冲分裂的影响,发现负折射介质中正负的自陡效应都会引脉冲非对称分裂。其中负的自陡效导致脉冲裂变后其后沿比前沿高,这与常规介质自陡效应（恒为正）情形相同;而负折射介质正的自陡效应对脉冲分裂影响刚好与前者相反,即裂变后脉冲后沿比前沿低。     

Temporal solitons in magnetooptic and metamaterial waveguides

The important topic of temporal soliton propagation in double-negative metamaterials is discussed with an emphasis upon short pulses that exhibit self-steepening controlled by the frequency dependence of the relative permittivity and permeability. In addition, magnetooptic control is included, leading to some fascinating outcomes that should have practical application. The role of self-steepening, Raman scattering, third-order dispersion and magnetooptics is thoroughly investigated, and it is shown that pulses can acquire signatures in the form of additional velocities with respect to the moving frame. The metamaterial influence upon self-steepening has such a strong frequency dependence that it can be used to combat Raman scattering. The self-steepening can change sign, and it is shown that it is possible to arrange pulses in special switching formats to organise the output times. The metamaterial influence upon bit-patterns admits an important degree of control over multi-pulse interactions, and this is combined with magnetooptics to restore patterns. The role of third-order dispersion is also presented. Again, a control of the pulse behaviour in the neighbourhood where the frequency dependence causes the group-velocity dispersion parameter to approach zero is a direct consequence of using this kind of metamaterial. Finally, a Lagrangian analysis is used to support simulations of the positions of the pulse maxima.     

Singularity analysis and integrability for a HNLS equation governing pulse propagation in a generic fiber optics

Taking into account many developments in fiber optics communications, we propose a higher nonlinear Schrödinger equation (HNLS) with variable coefficients, more general than that in [R. Essiambre, G.P. Agrawal, Opt. Commun. 131 (1996) 274], which governs the propagation of ultrashort pulses in a fiber optics with generic variable dispersion. The study of this equation is performed using the Painlevé test and the zero-curvature method. Also, we prove the equivalence between this equation and its anomalous integrable counterpart (the so-called Sasa-Satsuma equation). Finally, in view of its physical relevance, we present a soliton solution which represents the propagation of ultrashort pulses in a dispersion decreasing fiber.     

All-photonic-crystal-fiber coherent black-light source

Photonic-crystal fibers (PCFs) are widely recognized as efficient sources of white-light supercontinuum. Here, we show that a combination of an ultrashort-pulse ytterbium PCF laser with a highly nonlinear PCF enables all-fiber generation of ultrashort pulses of coherent UV radiation in the black-light spectral region with all the key processes, such as lasing, amplification, and conversion of the amplified PCF-laser output to the UV range, taking place in specifically designed fiber components.