Impact of higher-order dispersion in the modulational instability spectrum of a relaxing coupled saturable media

A theoretical analysis of modulational instability (MI) of optical pulses propagating near the zero dispersion wavelength of a lossless fibre with the effect of delayed saturable nonlinear response is presented. We calculate the exact dispersion relation with the effect of higher dispersion for the harmonic perturbation. We analysed the impact of fourth-order dispersion effects in the MI spectrum. We examine the possibility of MI in both dispersion regimes, regardless of the sign of the group velocity dispersion.     

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.     

Supercontinuum spectra generation in the single-mode optical fibre with concave dispersion profile

In this paper, a new method is proposed to generate broad supercontinuum (SC) spectra in the single-mode optical fibre with concave dispersion profile. We numerically simulate pulse evolutions and discuss physics mechanism in detail for SC spectrum generation in the optical fibre with concave dispersion profile. Furthermore, general criteria are presented for specifying the shape of SC spectrum by introducing normalized parameters, which are related to the fibres and the initial pump pulses. The results show that the flat and broad SC spectra are indeed generated in our proposed optical fibre.     

基于周期极化晶体MgO:PPLN的纠缠光子对制备过程的研究

基于二阶非线性效应的自发参量转换技术制备纠缠光子对过程中,以掺5 mol%MgO:PPLN周期极化晶体为研究对象,将光参量变换过程中的动量守恒和能量守恒条件与该晶体的色散方程,以及晶体极化周期随温度变化的热膨胀方程相联系,得到了355 nm、405 nm、532 nm、780 nm和1 064 nm这5个实验室常用波长点在制备纠缠光子对时的周期调谐特性和温度调谐特性。研究过程中发现,出现了晶体极化周期过小和产生两对纠缠光子对问题,总结并归纳了各波长点在一定极化周期和温度下与非线性晶体作用所产生的纠缠光波段范围。当选用其他非线性周期极化晶体进行实验时,改变QPM动量守恒条件中的极化周期项,同时根据具体使用的晶体改变色散方程。该研究方案可直接推广到使用不同非线性晶体产生通信光波段或红外光波段的纠缠光子对研究中,在制备量子光源等领域具有重要研究价值。     

单个波长到多个波长的变换实验

报道一种波长变换的新方法.采用此方法不需要任何泵浦光便能够直接将信号光变换成在特定谱宽内的任意多个任意波长的变换光,这对于将来的波分复用光网络提供了较大的方便.此波长变换的原理是利用色散位移光纤的非线性效应产生超连续光谱(SC)信号,由于这种SC信号携带有变换光信号,对这种连续谱进行滤波便能够得到所需要的任意频率的变换信号.此变换方式具有变换频带宽,偏振不敏感的优点。     

光纤拉曼放大器中增益的偏振相关特性研究

提出了一种改进的、可用于计算偏振相关拉曼增益的光纤拉曼放大器的非线性耦合波方程。建立了单模双折射光纤拉曼增益的数学模型,分析了线偏振抽运光以与光纤快轴成45°激励的单模双折射光纤拉曼放大器模型与实际具有随机双折射的光纤拉曼放大器的拉曼增益偏振相关特性的等价性。基于上述模型,提出了一个可定量表征单模光纤偏振模色散统计特性的拉曼增益偏振相关因子,用以替代常规的光纤拉曼放大器非线性耦合波方程中的偏振相关因子。计算结果与已报道文献的实验数据非常吻合。同时对抽运增益在同向和反向抽运方式下截然不同的增益偏振相关特性给出了合理的解释。     

拉曼效应和参量放大共同作用下增益谱特性

根据激光脉冲在双折射光纤中传输时, 拉曼效应和参量放大共同作用下所所遵循的耦合模方程, 基于平行拉曼增益的洛伦兹模型, 给出了输入抽运波偏振方向沿相互正交的双折射轴时, 拉曼效应和参量放大共同作用所导致的增益. 讨论并分析了在不同色散区相关参量对增益谱特性的影响. 结果表明, 拉曼效应改变了非线性和色散的相互平衡, 使得参量放大斯托克斯波与反斯托克斯波增益谱彼此不对称; 当输入功率一定时, 其增益谱结构确定, 非线性系数和色散系数两者之间相对变化时, 增益谱的强度和展宽有所改变.     

Enhanced pulse compression induced by the interaction between the third—order dispersion and the cross—phase modulation in birefringent fibres

In this paper,we report on the enhanced pulse compression due to the interaction between the positive third-order dispersion (TOD) and the nonlinear effect (cross-phase modulation effect) in birefringent fibres.Polarization soliton compression along the slow axis can be enhanced in a birefringent fibre with positive third-order dispersion,while the polarization soliton compression along the fast axis can be enhanced in the fibre with negative third-order dispersion.Moreover,there is an optimal third-order dispersion parameter for obtaining the optimal pulse compression.Redshifted initial chirp is helpful to the pulse compression,while blueshifted chirp is detrimental to the pulse compression.There is also an optimal chirp parameter to reach maximum pulse compression.The optimal pulse compression for TOD parameters under different N-order solitons is also found.     

偏振模色散及非线性效应对40 Gbit/s密集波分复用系统的影响

高速率、大容量的密集波分复用系统是光纤通信系统的最终发展方向 ,单信道速率达到 4 0Gbit/s时 ,光纤的非线性效应、偏振模色散现象对系统的影响更加突出。在综合考虑群速度色散、自相位调制、交叉相位调制、四波混合、偏振模色散等因素的基础上 ,推导了密集波分复用系统中任意信道的耦合非线性薛定谔方程组。利用扩展的分步傅里叶方法对该方程进行了数值计算 ,通过对 8× 4 0Gbit/s密集波分复用系统的仿真 ,分别研究了非线性效应和偏振模色散对密集波分复用系统的影响。发现由于交叉相位调制和四波混合作用 ,多波长的密集波分复用系统比单波系统受非线性效应影响严重 ;系统受偏振模色散与非线性效应的影响程度与输入信号功率有关 ,在入射光单信道平均功率较低 0 .1mW时 ,偏振模色散是影响系统性能的主要因素 ;当入射光单信道平均功率较高1mW时 ,系统受非线性效应影响严重。而偏振模色散在使信号脉冲展宽的同时 ,类似于非零色散位移光纤中的微小色散 ,对非线性效应又有一定的抑制作用。     

基于复Ginzburg-Landau方程的双核光纤中调制不稳定性的仿真研究

在双核光纤光学系统中,应用复Ginzburg-Landau方程,研究了连续波的不稳定性问题.双核光纤光学系统是由一个非线性离散主核和一个线性附核构成的.研究发现,在线性微扰下存在调制不稳定性.系统仿真结果表明：如果充分考虑调制不稳定性,则该系统将产生规则或者不规则的脉冲序列.反之,如果不考虑调制不稳定性它将产生一连串具有连续增长振幅的离散峰.这表明在反常群速度色散情况下,一串归零脉冲的峰值或者单一归零脉冲峰值仍然是增强的.在光纤中产生归零序列脉冲源,这一研究结果对全光纤通信有一定的价值,对光纤光学及物理学 关键词： 光孤子 复Ginzburg-Landau方程 双核光纤 调制不稳定性     

Fibre Optical Parametric Amplification in Defect Bragg Fibres with Zero Dispersion Slow Light Effect

Nonlinearity enhancement by slow light effect and strong light confinement in defect Bragg fibres is demonstrated and anMysed in applications of fibre optical parametric amplifiers. Broadband low group velocity and zero dispersion as well as the strong light confinement by band gap enhances the nonlinear coefficient up to more than one order than the conventional high nonlinear fibres. Moreover, the zero dispersion wavelength of coupled core mode can be designed arbitrarily, under which the phase-matching bandwidth of the nonlinear process can be extended.     

Continuous-wave pumping in the anomalous- and normal-dispersion regimes of nonlinear fibers for supercontinuum generation

Supercontinuum (SC) growth in highly nonlinear fibers is compared for cw pumping in the anomalous- and normal-dispersion regimes. For anomalous-dispersion pumping, the combined effects of modulation instability (MI) and stimulated Raman scattering (SRS) contribute to spectral broadening. Furthermore, breakup of the cw light into ultrashort pulses by MI leads to the formation of a Raman pulse that evolves into a soliton, as evidenced by the observation of soliton self-frequency shift. Blueshifted, nonsolitonic radiation associated with the fission of higher-order solitons is also present in the SC spectra. For normal-dispersion pumping, SRS seeds the SC growth by generating several cascaded Stokes orders. When the Stokes orders are shifted into the anomalous-dispersion regime at higher launch powers, MI again causes soliton formation. Broadband continua are generated when the laser is positioned as far away as 191 nm from the zero-dispersion wavelength in normal dispersion.     

双折射光纤中拉曼效应对参量放大增益谱的影响

根据激光脉冲在光纤中传输时, 所满足的波动方程, 导出了拉曼效应和参量放大共同作用下, 在双折射光纤中所遵循的耦合模方程, 并引入平行拉曼增益的洛伦兹模型, 给出了输入抽运波偏振方向同双折射轴成45^o 时, 拉曼效应和参量放大共同作用所导致的增益. 讨论并分析了拉曼效应在不同色散区对参量放大增益谱的影响. 结果表明, 在考虑拉曼效应后, 使得参量放大斯托克斯波与反斯托克斯波增益谱彼此不对称; 在反常色散区, 产生的增益以反斯托克斯波为主, 正常色散区则以斯托克斯波为主.     

磁光非线性光纤中光参量增益的研究

将光纤中磁光效应和非线性效应作为微扰,推导了磁光四波混频的耦合模方程.通过解析解研究了各向同性磁光非线性光纤的参量过程,并指出利用磁光耦合系数的色散特性可以实现四波混频的相位匹配.同时,采用龙格-库塔法分析了在线双折射磁光非线性光纤中,忽略费尔德常量的波长依赖性时,左旋圆偏振光参量增益的磁控特性,指出了实验中采用较高费尔德常量的非线性光纤的必要性.研究表明:1)对于低线双折射磁光非线性光纤,优化双折射大小可以获得最大的参量增益;2)根据参量增益对磁光耦合系数的单调依赖特性,适当选择光纤长度、泵浦功率以及输入导波光的偏振态,可使参量增益的磁可调范围大大提高.     

Influence of nonlinear dispersion on modulation instability of coherent and partially coherent ultrashort pulses in metamaterials

The combination of dispersive magnetic permeability with nonlinear polarization leads to a series of nonlinear dispersion terms in the propagation equations for ultrashort pulses in metamaterials. Here we present an investigation of modulation instability (MI) of both coherent and partially coherent ultrashort pulses in metamaterials to identify the role of nonlinear dispersion in pulse propagation. The Wigner–Moyal equation for partially coherent ultrashort pulses and the nonlinear dispersion relation for MI in metamaterials are derived. Combining the standard MI theory with the unique properties of the metamaterial, the influence of the controllable first-order nonlinear dispersion, namely self-steepening, and the second-order nonlinear dispersion on both coherent and partially coherent MI, in both negative-index and positive-index regions of the metamaterial for all physically possible cases is analyzed in detail. For the first time to our knowledge, we demonstrate that the role of the second-order nonlinear dispersion in MI is equivalent to that of group-velocity dispersion (GVD) to some extent, and thus due to the role of the second-order nonlinear dispersion, MI may appear in the otherwise impossible cases, such as in the normal GVD regime. PACS 42.25.Kb; 42.65.Sf; 78.20.Ci     

Effects of pulse chirp on supercontinuum produced in dispersion decreasing fibre

The supercontinuum (SC) source plays a key role in wavelength division multiplexing over the optical time division multiplexing (WDM/OTDM) transmission system. The SC produced in dispersion decreasing fibre (DDF) is wider and flatter than that produced in dispersion shifted fibre (DSF). In this paper, a SC producing process in DDF is numerically simulated, the effects of pump pulse chirp on the SC width and flatness are initially studied and the corresponding theoretical explanations are given. Particularly, we have analysed and compared the evolutions of the SC along DDF pumped by different chirp pulses. Several valuable conclusions are obtained.     

Quantum Statistical Theory of Polarization Mode Dispersion

Polarization mode dispersion is modelled as decoherence of polarization under the disturbance of environment and the coupling with frequency. This model is described by the quantum master equation and the Langevin equation. It can be predicted that the optical beam is depolarized exponentially in a fibre and the differential group delay （DGD） is proportional to the square root of the propagation distance. The distribution of the DGD can also be calculated.     

Supercontinuum generation at 1.55μm using highly nonlinear photonic crystal fiber for telecommunication and medical applications

In this paper, we present a theoretical calculation of a highly nonlinear germanium (Ge) doped photonic crystal fiber with all-normal group velocity dispersion to design a supercontinuum (SC) light source at 1.55 μm. By doping 3% higher refractive index Ge inside the host silica, the nonlinear coefficient is increased to a value as large as 60.5 W^?1 km^?1 at 1.55 μm. A 10 dB bandwidth of a 120 nm SC spectrum for a 2.5 ps input optical pulse and a 10 dB bandwidth of a 190 nm SC spectrum for a 1.0 ps input optical pulse have been found using the same fiber length of 200m and input optical power of 18 W. The coherent lengths of the generated SC light sources are found to be 8.8 μm for a 2.5 ps input optical pulse and 5.6 μm for a 1.0 ps input optical pulse. Therefore, the highest longitudinal resolution at 1.55 μm is found to be about 4.0 μm for biological tissues.     

单模光纤中受激喇曼散射对调制不稳定性的影响

基于修正的非线性薛定谔方程，利用线性扰动理论和数值方法研究了单模光纤中的调制不稳定性.由于受激喇曼散射的作用，使得喇曼增益谱叠加到光纤中的调制不稳定性增益谱上.这样，原本调制稳定的光纤正常色散区也出现了调制不稳定性；而在反常色散区，随着初始功率的增加，常规的调制不稳定性增益谱的增益和频谱范围均增大，而喇曼增益谱的增益增大但其频谱范围基本不变，这样导致常规的不稳定区域逐渐侵入并最终掩盖喇曼增益区.数值模拟验证了解析结果的正确性，并证明了利用反常色散情形下的调制不稳定性可以产生超短脉冲序列，但这种脉冲序列的进一步传输将会出现喇曼孤子自频移现象.     

受激拉曼散射对非线性介质波导的时空不稳定性影响

基于非线性包络方程我们研究了平面非线性介质波导中的时空不稳定性,得到了有拉曼散射效应情形下不稳定性调制的增益谱的表达式.结果表明在正常和反常两种群速度色散情形下,拉曼散射效应都会导致出现新的不稳定性区域,并且使原有的每一个谱分量的增益上升从而使谱的范围扩大.并通过分析反常色散情况下四种不同函数分布的非线性色散介质系数对不稳定性增益谱的影响,得到呈双曲函数分布的非线性色散介质最为理想.