Modulation instability in lossy silicon optical waveguide

Taking into account the small linear loss of silicon-on-insulator(SOI) waveguide, conditions and gain spectra of modulation instability(MI) induced by combined effects of self-phase modulation and waveguide dispersions are investigated. The impacts of various parameters to gain spectra of MI are analyzed theoretically. Results show that strong MI takes place even in the existence of low light power. The MI peak gain is 2 to 3 orders of magnitude larger than that achieved in optical fibers with the same light power. The linear loss of waveguide impacts gain spectra of MI, even within ultra-short propagation distance. The peak gain, peak gain frequency and bandwidth of gain spectra decrease to 66.828%, 41.683% and 41.6879% of their maxima at propagation distance z = 5mm, respectively.     

线性偏振激光在相对论等离子体中的调制不稳定性

从相对论等离体中电磁波的非线性色散方程出发,利用Karpman方法获得了线性偏振波模所满足的非线性控制方程,在非线性色散方程和非线性控制方程的基础上对线性偏振激光在相对论等离体中传播的调制不稳定性进行分析,给出了调制不稳定的时间增长率与扰动态波数之间的函数关系。     

高阶色散和指数饱和非线性光纤的调制不稳定性

采用线性稳定性分析法,解析和计算了指数饱和非线性和高阶色散光纤中的调制不稳定条件和增益谱。结果表明:当二阶和四阶色散同为负时,随着参数的不同,增益谱可能只有一个谱区,也可能出现两个分离的谱区;当二阶和四阶色散分别为正和负时,无调制不稳定性;在其它色散区,则只有一个谱区。指数饱和非线性可能使各谱区的谱宽、峰值增益随入纤功率的增大呈现出先增大后减小的特点,即出现两个不同的输入功率对应同一个不稳定增益峰值和谱宽的情形。在其它参数相同时,指数饱和非线性下增益谱的谱宽和峰值增益随入纤功率的变化速度将比传统饱和非线性更快。     

Compensation for depolarisation by a fibre coil in the presence of self-phase modulation

We report on the dependence of the degree of polarisation of an 80 fs pulsed laser beam propagating through a length of a single-mode birefringent fibre on the illumination power. Due to the birefringence and the nonlinear effect of self-phase modulation, the measured depolarisation dependence is oscillatory. It is, however, demonstrated that the oscillatory depolarisation can be compensated for by introducing a loop into the fibre geometry, which maintains the degree of polarisation of the pulsed beam as high as 90%.     

Modulation instability of quasi-plane-wave optical beams in biased photorefractive-photovoltaic crystals

We investigate the modulation instability of quasi-plane-wave optical beams in biased photorefractive－photovoltaic crystals by globally treating the space-charge field. The modulation instability growth rate is obtained, which depends on the external bias field, on the bulk photovoltaic effect, and on the ratio of the optical beam's intensity to that of the dark irradiance. Our analysis indicates that this modulation instability growth rate is identical to the modulation instability growth rate studied previously in biased photorefractive－nonphotovoltaic crystals when the bulk photovoltaic effect is negligible for shorted circuits, and predicts the modulation instability growth rate in open- and closed-circuit photorefractive－photovoltaic crystals when the external bias field is absent.     

具有分布损耗波导结构的回旋行波放大器绝对不稳定性

 详细推导了具有分布损耗波导结构的回旋行波放大器的色散特性。通过绝对不稳定性振荡出现的条件,给出求解具有损耗波导结构回旋行波管放大器的绝对不稳定性起振电流的数值计算方法。研究结果表明：绝对不稳定性起振电流与损耗波导的集肤深度有关,选择有较大的集肤深度的损耗波导可以提高绝对不稳定性起振电流;绝对不稳定性起振电流同时也与工作磁场偏离饱和磁场的程度以及电子束的纵横速度比有关;通过设计具有分布损耗波导结构的注-波互作用电路,以及工作磁场、电子束的纵横速度比,可以在兼顾带宽、效率的条件下,保证回旋行波管放大器稳定工作。     

梯形截面介质光波导的模吸收损耗

本文运用微分法由梯形截面介质光波导的近似模方程导出了模吸收损耗系数的公式.并结合计算实例进行了误差分析.     

具有分布损耗波导结构的回旋行波放大器绝对不稳定性

详细推导了具有分布损耗波导结构的回旋行波放大器的色散特性。通过绝对不稳定性振荡出现的条件,给出求解具有损耗波导结构回旋行波管放大器的绝对不稳定性起振电流的数值计算方法。研究结果表明：绝对不稳定性起振电流与损耗波导的集肤深度有关,选择有较大的集肤深度的损耗波导可以提高绝对不稳定性起振电流;绝对不稳定性起振电流同时也与工作磁场偏离饱和磁场的程度以及电子束的纵横速度比有关;通过设计具有分布损耗波导结构的注-波互作用电路,以及工作磁场、电子束的纵横速度比,可以在兼顾带宽、效率的条件下,保证回旋行波管放大器稳定工作。     

硅基纳米波导交叉相位调制不稳定性分析

在考虑线性损耗的情况下,对硅基纳米波导中皮秒脉冲传输交叉相位调制不稳定性进行了仿真研究。采用微扰和线性稳定性分析方法,理论推导了调制不稳定性产生所满足的色散关系和调制不稳定性增益谱表达式。分析了脉冲光功率、线性损耗和群速度色散系数等参量对交叉相位调制不稳定性的影响,并进行了仿真分析。结果表明,即使在微弱的入射光功率下,仍然存在强烈的交叉相位调制不稳定性现象;交叉相位调制不稳定性不仅可以在反常色散区出现,而且在正常色散区也会发生。波导的线性损耗对交叉相位调制不稳定性增益谱有明显影响。     

饱和非线性下零色散附近的交叉相位调制非稳

从光纤中扩展的耦合非线性薛定谔方程组出发,在饱和非线性光纤零色散附近,研究了不同二、四阶色散参量下交叉相位调制不稳定性增益谱及其临界扰动频率、谱宽和谱峰随两光波入射功率的演化特点.研究表明,随色散参量不同,增益谱随两光波入射功率的增大可能出现三种演化形式：一是始终是两个分离的谱区;二是由开始时的两个合成一个,最后再分离成两个;三是始终是一个谱区.饱和非线性的存在则使每个谱区的谱宽、谱峰及远离零点的临界扰动频率,随两光波入射功率的增大可能呈现出先增大后减小的特点,使第二谱区靠近零点的临界扰动频率呈现先减小后增大的特点,从而可能出现两个不同的输入功率对应同一个增益峰值和谱宽的情形.色散参量对增益谱的谱峰影响小,对谱宽影响大.越靠近零色散区,每个谱区谱宽越大,越易连成一个谱区.     

四阶色散和五阶非线性对零色散附近交叉相位调制不稳定性的综合影响

从包含高阶色散和高阶非线性的广义非线性薛定谔方程出发,研究了零色散附近交叉相位调制不稳定性增益谱,分析了四阶色散和五阶非线性系数对增益谱的影响.结果表明：当光脉冲工作在零色散附近时,四阶色散对交叉相位调制不稳定性起决定作用,它使增益谱变宽.正（负）五阶非线性使增益谱的谱宽和峰值增大（减小）.随着二阶色散的增加,四阶色散对增益谱的影响越来越小.     

Cross-phase modulation instability in optical fibres with exponential saturable nonlinearity and high-order dispersion

Utilizing the linear-stability analysis, this paper analytically investigates and calculates the condition and gain spectra of cross-phase modulation instability in optical fibres in the case of exponential saturable nonlinearity and high-order dispersion. The results show that, the modulation instability characteristics here are similar to those of conventional saturable nonlinearity and Kerr nonlinearity. That is to say, when the fourth-order dispersion has the same sign as that of the second-order one, a new gain spectral region called the second one which is far away from the zero point may appear. The existence of the exponential saturable nonlinearity will make the spectral width as well as the peak gain of every spectral region increase with the input powers before decrease. Namely, for every spectral regime, this may lead to a unique value of peak gain and spectral width for two different input powers. In comparison with the case of conventional saturable nonlinearity, however, when the other parameters are the same, the variations of the spectral width and the peak gain with the input powers will be faster in case of exponential saturable nonlinearity.     

ICF研究中的Rayleigh-Taylor不稳定性实验用靶

 概述了当前研究Rayleigh-Taylor不稳定性增长实验所用的靶型情况。从实验用靶来看，调制靶的设计正从平面向立体发展，出现了球形和柱形等新靶形，制备手段包括精密机械微加工、激光束、电子束和离子束加工、半导体工艺和微米纳米技术加工等。根据对国外新靶型的研究，结合国内工艺条件和实验需求，对国内可能采用的靶型进行了研究探索。     

光子晶体光纤中调制不稳定现象与超连续光谱的产生

研究了光子晶体光纤中调制不稳定性效应.从非线性薛定谔方程出发，计算和分析了光子晶体光纤中反常色散区以及正常色散区内的调制不稳定性现象，详细讨论了超短脉冲的脉宽、峰值功率、高阶色散和高阶非线性效应（如脉冲内喇曼散射、自陡峭效应）对调制不稳定性产生的影响.结果表明：二阶色散对调制不稳定性的影响要远大于三阶色散，同时也发现随着初始脉冲宽度的减小，调制不稳定性旁瓣增大但是强度有所降低.另外还发现高阶非线效应如自陡峭和喇曼效应会在不同程度上抑制调制不稳定性.     

色散缓变掺铒光纤放大器的调制不稳定性

由于掺铒光纤放大器(EDFA)存在增益,相比于传输光纤它有较小的调制不稳定性阈值,使其很容易受到调制不稳定性的影响.本文将用微扰法分析基本的非线性薛定谔方程,研究色散缓变掺铒光纤放大器的调制不稳定性,分析其调制不稳定性积分增益谱与输入信号功率、放大器增益、放大器的长度、光纤纵向色散变化参量的关系.结果显示增大光纤纵向色散变化参量值是减小调制不稳定性对放大器影响的有效途径.通过分析调制不稳定增益产生长度,表明合理的选择放大器的长度可以消除调制不稳定性增益的产生.     

Theoretical analysis of the all-fiberized, dispersion-managed regenerator for simultaneous processing of WDM channels

The concept of the all-fiberized multi-wavelength regenerator is analyzed, and the design methodology for operation at 40 Gb/s is presented. The specific methodology has been applied in the past for the experimental proof-of-principle of the technique, but it has never been reported in detail. The regenerator is based on a strong dispersion map that is implemented using alternating dispersion compensating fibers (DCF) and single-mode fibers (SMF), and minimizes the nonlinear interaction between the wavelength-division multiplexing (WDM) channels. The optimized regenerator design with + 0.86 ps/nm/km average dispersion of the nonlinear fiber section is further investigated. The specific design is capable of simultaneously processing five WDM channels with 800 GHz channel spacing and providing Q-factor improvement higher than 1 dB for each channel. The cascadeability of the regenerator is also indicated using a 6-node metropolitan network simulation model.     

Spatiotemporal instability in dispersive nonlinear Kerr medium with a finite response time

We investigated the spatiotemporal modulation instability (MI) in a medium with a non-instantaneous Kerr response in which the nonlinear contribution to the refractive index is governed by a relaxing Debye type equation. The expression for the MI gain spectrum in non-instantaneous Kerr medium is obtained, which clearly reveals the influence of the finite time of the nonlinear response (relaxation effect) on the spatiotemporal MI. It is shown that, due to the relaxation effect, the spatiotemporal MI can appear for the case of anomalous dispersion and defocusing nonlinearity, while it cannot appear in instantaneous Kerr medium for the same case, and a new MI gain spectrum appears, adding to the conventional MI gain spectrum similar to that in an instantaneous Ker medium. In addition, the bandwidth of MI gain spectrum is extended and the maximum MI gain is reduced with increasing of the relaxation time. Interestingly, spatiotemporal MI can appear for any spatial frequencies in non-instantaneous Kerr medium for any combination of dispersion and nonlinearity. We have performed an experiment of MI in carbon disulfide (CS₂), a typical non-instantaneous Kerr medium, which shows quantitative agreement with the theoretical analyses.     

Modulation of spatial spectrum in tunable two crystal optical parametric generator

We report on theoretical and experimental investigations of two crystal traveling-wave optical parametric generator (OPG). It is shown that the efficiency and angular structure of traveling-wave tunable OPG output depends on the distance between the two crystals. Air dispersion driven efficiency modulation at different OPG output wavelengths is observed experimentally and confirmed by theoretical calculations.     

白光和激光产生的调制不稳定性的比较

在非线性自散焦LiNbO_3∶Fe晶体中,实验观察了用激光作光源和用白光作光源两种情况下所产生的调制不稳定性,并从三个方面对这两种情况进行了比较。实验结果表明:在两种情况下,达到稳定状态前,MI现象都会随辐照时间的增加而变得越来越清晰,并且MI图样都会随着入射光束形状的变化而变化。不同的是,用白光作光源时,灯丝长度方向与晶体c轴方向的夹角大于等于45°时出现MI,而用激光作光源时,入射光束方向与c轴方向的夹角不大于45°才出现MI。     

Modulation instability in metamaterials with fourth-order linear dispersion,second-order nonlinear dispersion,and three kinds of saturable nonlinearites

The linearized nonlinear propagation equation and its coefficients, gain spectrum of modulation instability (MI) in metamaterials (MMs) with fourth-order linear dispersion, second-order nonlinear dispersion, and three kinds of saturable nonlinearites, are analytically deduced by utilizing the linear stability analysis and Drude electromagnetic model. Then variations of gain spectra of MI with the normalized angular frequencies and the optical power densities are calculated in real units. In the negative refractive region, two kinds of gain spectra are discovered. The first (second) one is close to (far from) the zero perturbation frequencies and it corresponds to the lower (higher) normalized angular frequencies. Moreover, the second one has higher cutoff frequency, which is obviously beneficial to generation of high-repetition-rate pulse trains. While in the positive refractive region, only the first kind of gain spectra is found. With increase of the optical power densities, the peak gains and the spectral widths of MI increase before decrease, but they vary the most rapidly (slowly) for the exponential (conventional) saturable nonlinearities. The MI characteristics and their corresponding applications can be adjusted by several methods.