Efficient dispersion tailoring by designing alternately arranged dispersion compensating fibers and fiber amplifiers to create self-similar parabolic pulses

The parabolic similariton pulse formation by alternate arrangements of passive and active dispersion compensating fibers (DCFs) is presented here. These combinations of passive and active DCFs with constant core radii and constant nonlinearities are suggested as equivalent profiles of a dispersion tailored fiber amplifier in normal dispersion regime. The dispersion tailored fibers, usually known as dispersion decreasing fibers (DDFs) in normal dispersion regime, are capable of producing linearly chirped parabolic self-similar pulses. The DDF is designed and optimized with proper choice of fiber parameters so that considerable variation of nonlinearity can be achieved, which in turn enhances the effective gain coefficient of the fiber. Inclusion of this nonlinear variation along the DDF amplifier length leads to obtain the simulated output pulses with very small misfit parameters with respect to perfect parabolic pulse at sufficiently reduced optimum length. At the same time to avoid the fabrication difficulties of the DDF, the alternately arranged passive and active DCFs are suggested as suitable alternatives of the DDF. The performances of the cascaded systems for generation of self-similar parabolic pulses are compared with that of the DDF amplifier as well as combined systems consisting of DCFs with equal gain. The results show that the proposed alternately arranged cascaded system with less pumping requirements, are efficient enough to produce similar parabolic pulses as compared to the previously designed DDF, even when considerable amount of splice loss at each joint is included.     

压缩平稳重调脉冲的色散渐减光纤的设计

文章根据平稳重调脉冲(SRP)在梳状光纤(CPF)结构中的压缩原理,对色散渐减光纤(DDF)的色散特性进行设计,结果发现该色散渐减光纤的色散特性呈线性递减.对于平稳重调脉冲其压缩比与功率比等于光纤始末两端二阶色散系数的比值.当色散渐减光纤的斜率足够小时,无啁啾基阶孤子可以近似为平稳重调脉冲,当色散渐减光纤的色散斜率较大时,无啁啾基阶孤子不能近似为平稳重调脉冲.当基阶孤子带有与光纤色散斜率成正比的线性啁啾时,脉冲的压缩比与功率比更接近于光纤始末两端二阶色散系数的比值.说明带有线性啁啾的基阶孤子比不带啁啾的基阶孤子更接近于平稳重调脉冲. 关键词： 平稳重调脉冲(SRP) 梳状光纤(CPF) 色散渐减光纤(DDF) 色散递减表达式     

凹形色散分布光纤中超连续谱特性分析

比较了在不同色散分布的色散渐减光纤中超连续（SC）谱的产生。结果表明,当光纤的色散值关于中心波长对称、且随着传输距离增加,经由反常色散区过渡到正常色散区的、凹形色散分布的色散渐减光纤更有利于产生平坦、带宽的超连续谱。从频域的全场方程出发,模拟了脉冲在光纤中的传输情形,发现脉冲在凹形色散分布的光纤中传输时,不仅在反常色散区能更大程度被压缩,在正常色散区由于存在零色散波长点,还能更进一步展宽,从而得到更宽、更平坦的超连续谱。在谱强为-27dB时,谱宽可达到298nm,比相同情况下的凸形色散的光纤中超连续谱增宽97nm。结果显示,凹形色散分布的光纤比凸形色散分布的光纤在超连续谱的产生上有更明显的优势,可以得到更宽的超连续谱。     

色散渐减光纤的脉冲压缩研究

介绍了色散渐减光纤的绝热孤子压缩(ASC)过程，分析结果表明利用线性渐减的色散渐减光 纤(DDF)对飞秒基态孤子进行绝热压缩可以得到高质量的压缩脉冲，但用于压缩的DDF长度很 长，因此又提出DDF的高阶孤子(1＜N≤21)脉冲压缩技术，以在较短的DDF长度下获得 较大压缩比和高质量的压缩脉冲. 关键词： 色散渐减光纤 绝热孤子压缩 脉冲压缩 高阶孤子     

Stationary rescaled pulse in dispersion-decreasing fiber for pedestal-free pulse compression

By using the concept of a stationary rescaled pulse (SRP), we analyze an adiabatic soliton compression system based on dispersion-decreasing fiber (DDF). We show that a SRP can exist in a DDF with a linearly decreasing dispersion profile and that the SRP resembles a linearly chirped sech2 pulse. According to the analysis, we show numerically that pedestal-free pulse compression is possible by using the SRP.     

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.     

Propagation of short soliton pulses through a parabolic index fiber with dispersion decreasing along length

A parabolic index dispersion decreasing fiber (DDF) has been designed and optimized to produce high capacity soliton communication system. Variation of different fiber parameters such as core radius, effective core area and GVD factor along the 25 km of DDF length has been carried out to optimize a best possible DDF which can sustain the propagation of fundamental soliton. The variation of non-linearity with length along with the conventional power and GVD factor variation has been included in the generalized non-linear Schrodinger equation (NLSE). This NLSE has been solved numerically by split step Fourier method for shorter pulse propagation, incorporating the term for third order dispersion and intrapulse Raman scattering. Stable soliton pulses in transmission system have been achieved by our simulation, when a correction factor due to Raman induced soliton mean frequency shift is incorporated to the GVD profile predicted by the fundamental soliton condition. The interaction of neighboring soliton pulse pair through the proposed fiber has also been studied.     

Analysis of loss influence on the generation of soliton trains in a dispersion decreasing fibre

The influence of fibre loss on the generation of high bit rate and high quality soliton pulse trains using dual-frequency beat-signal and dispersion decreasing fibre (DDF) is analysed theoretically and numerically. The results show that when fibre loss is taken into account, the propagation behaviour of the pulse train is quite different from that when loss is neglected even though the condition Γ (normalized loss) ≪ σ0 (equivalent gain of DDF) is satisfied. As a result, when designing experiment work, one must be cautious in neglecting loss. This revised version was published online in November 2006 with corrections to the Cover Date.     

色散缓变光纤中的调制不稳定性分析

研究了色散缓变光纤中调制不稳定效应,得到了增益谱与光纤纵向色散参量的一般关系式.结果表明,色散缓变光纤较常规光纤具有较宽的增益谱.选取色散级变光纤的色散参量可以获得较大的增益带宽.数值模拟显示,利用色散缓变光纤中的调制不稳定性可以产生高重复率的基本孤子脉冲串.     

色散渐减光纤中超连续谱产生的数值模拟及优化

从脉冲在光纤中传输所遵循的广义非线性薛定谔方程出发,通过数值模拟对色散渐减光纤产生超连续谱(SC)进行了研究.结果表明:色散渐减光纤的色散斜率k、色散参量D0、参量L0和R及脉冲的峰值功率、脉冲宽度对SC谱的产生有着极其重要影响|在色散渐减光纤产生超连续谱的过程中,三、四阶群速度色散甚至更高阶群速度色散对超连续谱的影响完全可以忽略,但高阶非线性效应对SC谱的产生影响很小|计算表明对光纤和脉冲的各参量进行优化选择后,可以获得宽而平坦的超连续谱.     

色散渐减光纤中自相似脉冲传输区域的研究

从非线性薛定谔(NLS)方程出发,用分步傅里叶方法结合对数值解的波形分析,确定了色散渐减光纤(DDF)中能实现自相似脉冲传输的区域,并研究了初始脉冲和光纤参数对自相似区域和演化速度的影响。结果表明,初始脉冲能量的减小有利于扩宽自相似区域,但会使自相似演化进程略为减慢;初始脉宽有一个最佳值,在最佳值上自相似区域最宽,演化较快且输出脉冲和啁啾较为稳定;高斯脉冲比双曲正割脉冲更快转化为自相似脉冲,传输区域也更广。选择具有较小非线性参量的DDF可以获得较广的自相似区域,同时非线性参量的增大可以加快自相似演化,而群速度色散参量和增益系数必须选择在最佳值附近,才能获得最大自相似区域和最快演化速度。     

Enhanced Compression of Fundamental Solitons in Dispersion Decreasing Fibers with Negative Third-order Dispersion Ⅱ. Effects of Fiber Characteristics and Initial Soliton Width

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Enhanced Compression of Fundamental Solitons in Dispersion Decreasing Fibers with Negative Third-order Dispersion Ⅱ. Effects of Fiber Characteristics and Initial Soliton Width

Previous work have shown that the combined effects of negative third-order dispersion (TOD) and Raman self-scattering (RSS) can significantly enhance soliton compression in dispersion decreasing fibers (DDFs). In this paper, the effects of the negative TOD coefficient and the effective amplification of DDF′s on the performance of soliton compression are investigated. It is shown that for a given initial soliton width and a given effective amplification, there exists an optimum value of the negative TOD coefficient of the DDF at which the enhancement in soliton compression is maximum. It is also shown that the compression enhancement saturates when the effective amplification exceeds a certain value, which has been explained as a compromise between the higher-order effects induced increase of the ratio of input to output group-velocity dispersion coefficients of the DDF and the TOD induced non-adiabatic compression characteristics. The dependence of the compression enhancement on the initial soliton width have also be studied and the scheme is found works well for solitons with initial widths less than 3 ps.     

Nonautonomous spatiotemporal localized structures in the inhomogeneous optical fibers: Interaction and control

We develop a systematic way to find the similarity transformation and investigate nonautonomous optical similariton dynamics for (n + 1)-dimensional nonlinear Schrödinger equation in the inhomogeneous optical fibers. A condition between the parameters of the mediums, which hints a exact balance between the dispersion/diffraction, nonlinearity and the gain/loss, has been obtained. Under this condition the optical similariton transmission in the dispersion-decreasing fibers (DDF) can be exactly controlled by proper dispersion management. Moreover, novel propagation dynamics of bright and dark similaritons on the background waves and optical rogue waves (rogons) in DDF are investigated too.     

Self-similar propagation region in passive optical fibers based on low-order approximation

For the first time, we numerically demonstrate that similaritons in passive fibers can only propagate with parabolic profile without oscillations in a limited self-similar propagation region when low-order approximation is applied. Dispersion-decreasing fiber (DDF) and nonlinearity increasing fiber (NIF) are used here to generate self-similar pulse in different passive ways. Moreover, we put forward several self-similar features to judge effective similariton propagation and determine self-similar region by numerical simulations and waveform observation. Finally we compare in DDF and NIF the dependence of these regions on incident pulse and fiber parameters in order to obtain parabolic similariton more effectively and steadily. These results extend the low-order propagation properties in passive fibers and are beneficial for the generation of high-quality similariton pulse by parameter selection in experimental studies and practical applications.     

Stimulated Brillouin scattering in dispersion-decreasing fiber with ultrahigh-speed femtosecond soliton pulse compression

We report that stimulated Brillouin scattering (SBS) in a dispersion-decreasing fiber (DDF) is particularly disadvantageous with ultrahigh-speed femtosecond soliton compression that exceeds 40 GHz. It is important to note that the increase in the longitudinal mode power of a soliton is proportional to the square of the repetition rate. The SBS threshold is determined by the dispersion-decreasing rate of the DDF, rather than its fiber loss. We suppressed the SBS by applying 30-MHz frequency modulation to a mode-locked fiber laser and successfully obtained a stable 40-GHz, 100-fs pulse train.     

Logic Gates Based in Asymmetric Couplers: Numerical Analysis

Abstract

In this work, we present a numerical investigation of the transmission and switching of fundamental solitons in asymmetric nonlinear directional couplers, constructed with dispersion decreasing fibers (DDF). In this configuration, the coupler consists of two separated parallel fibers, one with a DDF profile and the other with a constant profile. We obtained the characteristics of transmission, extinction ratio, and compression factor of the device.

The truth tables for the logic gates AND, OR, and XOR were obtained. We concluded that the device presenting a constant profile provided the best performance of the studied logic gates. Logic gates AND, OR, and XOR operating with extinction coefficient around 16.6 dB were obtained.     

Dark-in-the-Bright solitary wave solution of higher-order nonlinear Schrödinger equation with non-Kerr terms

We present new type of Dark-in-the-Bright solution also called dipole soliton for the higher order nonlinear Schrödinger (HNLS) equation with non-Kerr nonlinearity under some parametric conditions and subject to constraint relation among the parameters in optical context. This equation could be a model equation of pulse propagation beyond ultrashort range in optical communication systems. The solitary wave solution is composed of the product of bright and dark solitary waves. This type of pulse shape to be formed both the group velocity dispersion and third-order dispersion must be compensated. We also investigated the stability of the solitary wave solution under some initial perturbation on the parametric conditions. We have shown that the shape of pulse remains unchanged up to 20 normalized lengths even under some very small violation in parametric conditions.     

包含高阶色散的色散缓变光纤中交叉相位调制不稳定性分析

从包含高阶色散的广义非线性薛定谔方程出发,得到了色散缓变光纤中交叉相位调制不稳定增益谱,研究了增益谱随入射功率及光纤纵向色散参量的变化关系.结果表明：由于四阶色散的影响,在色散缓变光纤的正、反常色散区,交叉相位调制不稳定均发生在两个频谱区.反常色散区两频谱区宽度均比正常色散区宽,且反常色散区第二频谱区更靠近零点,说明色散缓变光纤中交叉相位调制不稳定更容易发生在反常色散区.增益谱宽都随两入射光波功率比值的增加而增大.色散缓变光纤中交叉相位调制不稳定增益谱宽比常规光纤的宽,且随着光纤纵向色散参量μ的增大色散缓变光纤中交叉相位调制不稳定越来越明显.     

Diffraction-like phenomena in a periodic magnetization distribution at 1.5 T using the distant dipolar field (DDF)

In the CRAZED experiment (COSY revamped by asymmetric Z-gradient echo detection, Warren et al.), a spatially anisotropic magnetization distribution is created by application of a magnetic field gradient (strength G, duration tau) which in turn generates a response called the distant dipolar field (DDF). The DDF is a source of intermolecular multiple-quantum coherences (iMQC) which contain information on the distance d=pi/(gammaGtau) between pairs of dipolar-coupled spins. Diffraction-like phenomena may result for periodically structured samples. In this study, we report the observation of diffraction owing to the DDF at 1.5 T using a clinical whole-body tomograph. Based on the semi-classical treatment of the problem by Robyr and Bowtell, diffraction conditions were obtained for a CRAZED-type pulse sequence that selects iMQC of order N. The predicted distinct difference in N=2 and N not equal2 coherences, i.e., a dominant continuous course as a function of tau (N=2) and prominent diffraction peaks otherwise, could be verified in CRAZED experiments in a periodically structured sample selecting coherence orders N=2 and N=3. The diffractive signal component contains information on the geometric structure of the sample. Applications of this technique may permit the detection of changes in composition and geometry of periodic structures.