大容量、高速DWDM光纤传输系统的性能研究

来源于：本站原创论文及翻译添加日期：2006-2-8

大容量、高速DWDM光纤传输系统的性能研究

摘要

本文对波分复用(DWDM)系统中通过提高单信道传输速率、扩展可用传输带宽的方法提高系统容量进行了讨论，比较了分布式FRA，色散管理式FRA和级联放大式FRA，并对高速系统的色度色散(CD) 和偏振模色散(PMD)进行了讨论。进而讨论了FRA的各种噪声问题以及如何消除噪声和码间串扰，同时讨论了拉曼泵浦消耗引起的噪声(RPDIN) ，比较RZ码和NRZ码两种码型受的RPDIN噪声对多信道DWDM系统的影响。最后讨论光孤子通信技术中孤子稳定传输问题，对抑制Gordon-Haus效应进行了论述，并对各种控制方案进行了比较。

关键字：拉曼光纤放大器、色散管理、色度色散、偏振模色散、码间串扰、拉曼泵浦消耗噪声、光孤子通信、Gordon-Haus效应

引言

随着社会的发展，信息传输的需求急剧增长和对通信带宽的需求急剧扩大，对密集波分复用(DWDM)系统的设计也提出了新的挑战，大容量、高速DWDM成为系统设计的趋势。

在DWDM系统中，拉曼光纤放大器技术成为研究和应用的热点，本文主要针对在分布式拉曼光纤放大器和色散补偿条件下的有关技术问题进行研究分析，主要包括：1、如何提高光纤系统的传输能力（CAPACITY）；2、各种噪声对系统的影响和消除以及如何消除码间串扰；3、各种码型在传输中的优劣和选择； 4、光孤子通信技术及高速孤子脉冲稳定传输。

提高光纤系统的传输能力（CAPACITY）

提高DWDM系统容量可通过提高单信道传输速率、减小信道间隔、扩展可用传输带宽三种方法实现。最后一种方法是在现有技术水平和已有传输干线基础上解决传输容量问题最直接、经济的方法。对于光纤的整个低损耗区1270nm-1670nm ，光纤作为光信号传输介质具有极大的可用带宽，而目前光通信的DWDM系统主要在C波段(1530nm-1565nm)和与之相邻的L(1570nm-1610nm)，^[1]~[3]可用带宽主要受放大器宽带的限制，原则上，FRA只要能得到所需的泵浦波长，可在1292-1660nm光谱范围进行光放大。当采用多个抽运源时，光纤喇曼放大器可实现较宽的增益带宽，同时我们可利用滤波器对光纤喇曼放大器的增益曲线进行平坦化，因此光纤喇曼放大器受到了人们的瞩目。

国外对C波段和L波段的光纤喇曼放大器进行了大量的理论分折，优化设计工作和实验研究，取得了较大的进展，并开发了光纤喇曼放大器产品，朗讯实验室等一些通讯领域的大公司已经将多级串接分布式喇曼放大器用于光纤DWDM传输系统,在实验室多波长抽运的喇曼光纤放大器光谱带宽已实现100nm^[4][5]。虽然目前光纤放大器的研究工作主要集中在C+L波段，但S波段光纤喇曼放大器更有发展前景^[6][7]，而且是稀土掺杂光纤放大器难以与其竞争的。利用多个泵浦光来实现光纤拉曼放大器增益谱的宽带和平坦放大，根据拉曼增益谱线的特点和泵浦与泵浦、泵浦与信号、信号与信号之间的SRS作用，多泵浦波的个数、输入功率和频率间隔的适当搭配可以实现DWDM 系统多路信号功率非常好的增益平坦度^[8]。

通常，分布式光纤喇曼放大器与分立式光纤喇曼效大器均存在色散问题，例如：G652 分布式光纤喇曼放大器为正色散，DCF 分立式光纤喇曼效大器为负色散，色散导致光纤喇曼放大器传输特性的变坏，会引起光谱变宽，产生伴线，造成串扰、噪声和脉冲展宽。国内外也开展了色散补偿型分布式光纤喇曼放大器的研究工作，主要集中在C 波段，在S 波段有一些初步的理论分析和实验工作^[9][10]。

对于DWDM信号来说，通过选择合适的抽运波长，拉曼放大器能使在光纤低损耗窗口的任何波长获得增益。在DWDM的抽运方案中，要将多个抽运光与不同的中心波长合并，以便同时进入光纤。它能使拉曼放大器有超过100nm的谱线宽度，超长带宽和超平坦的增益光谱。

DWDM的抽运光在通过拉曼散射时会相互干扰，将能量从短波长向长波长转化。正是这个原因，每个波长的每路平均抽运功率，或有效抽运功率与发射功率不相同，差值取决于干扰的强度。然而，由于被不同抽运波长引起的每一个受激拉曼散射都独立发生，产生的总增益只不过是在每个波长下相对于有效抽运功率的每个指数增益比例的和。波长的数量和分配取决于增益波段和系统要求的平坦程度。抽运波长范围决定了增益波段。

反向DWDM抽运的拉曼放大器在短波长区有很大的噪声，这主要是由于在抽运通道中利用拉曼散射进行了抽运能量的转化。换句话说，在短波长区，高抽运功率是必要的，因为有相当大一部分能量转化为长波的抽运能量。结果，短波长信号获得高噪声，因为它们比长波信号经历更大的损耗和更大的增益。可以通过引入短波长的组合抽运方式，减小和平滑噪声谱线。组合抽运与反向抽运的共同使用，有助于减小来自反向抽运光源的总发射抽运功率，与反向抽运相比，在光信噪比方面，组合抽运用较低的抽运功率获得较大的增益。另一方面，太多组合抽运也会增加非线性效应。

人们提出许多优化的方法，如龙格-库塔法(Rung-Kutta algorithm)，模拟退火算法(simulated annealing algorithm)，遗传算法(genetic algorithm)，组合算法等，以上各法大都比较繁琐，实际使用中难以达到动态优化。笔者就动态优化方面做了一些尝试，采用多点反馈法通过计算机进行实时动态优化控制，针对每1个bump都有相应的控制通道依据系统中不同检测点的增益情况实时反馈以调整相应Raman bump光强度，反馈计算采取了半经验的均衡算法。要实现多泵浦的平坦增益放大，泵浦波长的个数和波长分布十分重要，研究发现恰当安排多泵浦波长和功率分布后，泵浦波个数越多，DWDM信号的拉曼增益曲线的波动越小，但泵浦数目到一定程度，再增加泵浦个数对系统性能也没什么太大意义，应选择合适的泵浦便可。泵浦波长等间隔分布时虽然也实现了很好的平坦度，但平坦带宽却比较小，相比之下，当采用边上波长分布较密而中间分布较疏时，系统的整体平坦带宽明显增加。选取好泵浦波长分布后，就可以通过伺服系统通过反馈进行Raman bump光强度调整，在增益为10--20 dB时，平坦度(波动范围)均能保持在0.8 dB 之内。

分布式拉曼放大器利用分布放大低噪声的特点,可实现大容量、长距离的透明传输,其增益介质为常规的G. 622 、G. 655光纤。在现今的全拉曼传输系统中，一种是采用色散管理式的拉曼放大，另一种链路结构是采用单光纤分布式和分立式级联放大。色散管理式的拉曼放大器利用精细的色散管理技术,一方面通过平衡自相位调制和色散的作用,可加大入纤功率,提高信噪比；另一方面,通过控制脉冲的动态特性，可抑制高速光纤传输系统中信道内的非线性效应。当经过级联标准单模光纤和色散补偿光纤放大器放大后，净增益均为0 dB时，此种色散管理式拉曼放大器构成的级联放大结构具有很高的光信噪比。由于组成色散管理放大器的正、负色散光纤的色散斜率可以很好地匹配，同时它有具有相对较高的光信噪比，在大容量、长距离传输系统中具有很大的应用前景。

当光纤通信系统单信道速率升级到40 Gbit/ s 及以上时, 色度色散(CD) 和偏振模色散(PMD) 已经成为严重影响系统性能的主要因素。CD导致的脉冲展宽将产生严重的码间干扰, 特别是对于大量铺设的标准单模光纤(SMF ,G. 652) ,CD 问题尤为严重。PMD在模拟系统中产生高阶畸变效应和与偏振有关的损耗，导致非线性效应：在数字通信系统中，造成脉冲失真变形，增加脉冲之间的相互作用，使误码率增加，降低系统的传输距离，限制系统的传输带宽。与CD不同的是, PMD 是随着时间变化且与波长相关的统计量, 这样其对系统的影响很难消除。当系统的单信道速率上升到40 Gbit/ s 时, 高阶PMD 对于系统的传输限制作用已经明显, 因此, 在考虑PMD 的补偿时, 同时要考虑高阶PMD 的补偿。

PMD是一个服从Maxwllian分布的随机变量，其瞬时的PMD值随波长、时间、温度、移动和安装条件的变化而变化。要想尽可能地减轻PMD对系统性能的影响，就必须采用动态补偿的方法，来减小系统PMD。目前在偏振模色散抑制补偿技术上主要有电域偏振模色散补偿、光域偏振模色散补偿、采用前向纠错技术和选择适当的调制码型抑制偏振模色散、生产低偏振模色散光纤。前三种抑制补偿技术对现已铺设的光纤来说有很大的现实意义。

从目前实用的角度出发, 考虑到现有技术水平, 性能优化的基模色散补偿光纤(FM-DCF) 是CD 及色散斜率分布式补偿和管理的最佳选择，而两段偏振控制器(PC) 加固定差分群时延值(DGD) 组合，并以偏振度(DOP)作为反馈信号的自适应反馈系统则为高阶PMD 补偿的切实可行的方案。^[11]

在入纤功率较小时，色散对系统的性能优劣起主要作用，可采用色散完全补偿，随着WDM系统中复用的波道数越来越多，入纤功率会越来越大，非线性作用随之增强。在不能选取最佳进行色散完全补偿的情况卜，相对来说色散的欠补偿比过补偿好，剩余反常色散可对非线性作用产生一定的抑制作用，改善系统性能。

各种噪声对系统的影响和消除以及如何消除码间串扰

FRA的噪声来源有以下几种:其中主要有自发辐射噪声放大（ASE）、通过泵浦的信道串话，瑞利散射造成的泵浦损耗和噪声、泵浦布里渊散射带来的泵浦损耗和噪声，拉曼泵浦消耗引起的噪声(Raman pump depletion induced noise, RPDIN)。这些都会造成增益饱和、信噪比下降，在FRA中必须设法对其改善。

ASE噪声是由于自发喇曼散射经泵浦光的喇曼放大而产生的覆盖整个喇曼增益谱的背景噪声。显然，在FRA中泵浦光越大，ASE噪声越大。FRA的带宽越窄，FRA的噪声功率越低，从而信噪比越高。而在实际应用中，我们希望放大器的带宽越宽越好，但负面影响就是噪声功率比较大。而带宽窄的一个好处就是它带来的噪声功率较低，即信噪比较高，但同时它放大的路数减少了。

在FRA中，虽然当初入射光只有泵浦光和信号光，但信号光经泵浦光放大后，还有了放大的ASE噪声。自发辐射会引起功率消耗，即会引起一部分功率损失。且功率的损失还跟放大器的带宽有关。所以，在放大器的设计中，要综合考虑带宽、功率损失、放大器长度等之间的关系，使放大器的性能达到最好。

拉曼放大器中的串话噪声分为两种．一种是由于泵浦光源的波动而造成的泵浦信号串话，另一种是由于泵浦同时对多信道放大而导致的泵浦引入信号间串话．对DWDM 系统主要考虑第二种串话．

泵浦引入信号间串话主要是由于泵浦光对放大单一信道与放大多个信道的增益不同而造成的。具体表现为当两个相邻的信道同时传号时，信号的增益小于一个信道传号另一个信道空号时的增益．从总体上来看就表现为两信道间传、空号的相互影响，且信道数越多，串话影响越大。

研究表明，采用后向泵浦（backward bump）比前向泵浦(forward bump)能获得更好的信号干扰比(SIR)，因此在实际应用中，应尽量采取后向泵浦，即使采用前向泵浦也不能用太高的泵浦功率，一般要采用高阶泵浦来降低串话影响，串话噪声本质上与泵浦光到信号光的转换效率有关，转换效率越高，串话越严重，在DWDM 系统中，拉曼放大器的能量转换效率都很低。

光纤中的瑞利散射对分布喇曼放大器的性能产生不利的影响。瑞利散射是由于光纤制造过程中，局部浓度微观起伏导致折射率在比波长小的尺度上发生随机变化而引起的。瑞利散射噪声是由瑞利后向散射引起的。

它在光纤后端反射输出形成噪声，导致信噪比的恶化。瑞利噪声包括单瑞利噪声和双瑞利噪声，单瑞利散射经过一次后向散射再反射到输出端，表现为信号自发拍频噪声，双瑞利散射经过两次后向散射返回到输出端，表现为多径串扰。^[12]

当泵浦功率较大时，瑞利散射噪声相应较大，而信号的增益则被限制，因此信噪比也会下降．因此为了抑制瑞利散射噪声的影响，可以采用多级放大的方式，避免泵浦功率过高和传输距离过长，另外还可以采用双向泵浦的方法降低瑞利散射。

布里渊散射带来的泵浦损耗和噪声，可描述为抽运波、斯托克斯波通过声波进行的非线性相互作用。这个散射过程可看成是抽运光子的湮灭，同时产生斯托克斯光子和声频声子，在散射过程中必须满足能量和动量守恒。

在前向抽运和背向抽运方式下，当抽运功率超过SBS的阈值时，均观察到前向SBS现象。随着SBS的抽运功率的提高，将抽运的功率转移到布里渊散射中去，甚至限制了FRA 的增益。当FRA的抽运功率过高时，就会在斯托克斯区FRA中出现的级联的多级SBS线，信号的功率转换为SBS功率，当进一步增加FRA的抽运功率，其增益下，噪声变大，SBS的串扰破坏了FRA特性，使其无法在密集波分复用(DWDM)光纤传输系统中使用，因此需要严格地控制入纤的信号功率和FRA抽运功率。^[13]

RPDIN噪声是拉曼光纤放大器中，暂态泵浦消耗引起的光噪声和串扰。由于光纤的传输损耗和受激拉曼散射引起的损耗，泵浦功率沿着光纤长度方向逐渐变小。在光纤当中的某个点上，不同信号值(即比特“0”和“1”)所需要的泵浦功率不同，因此会导致不同的瞬态泵浦消耗。这样，就造成了光增益的瞬态变化，形成一定的噪声和串扰。RPDIN噪声与传统的拉曼噪声有所区别，它主要是由于其他n-1个信道引起的泵浦瞬态消耗而引起的。通过研究，发现这种噪声对比特“1”的影响较大。

各种码型在传输中的优劣和选择

比较RZ码和NRZ码两种码型，同样的信息序列，RZ码需要的能量较少，所以FRA泵浦消耗也较少，在DWDM系统，这种泵浦消耗不均匀性更为明显。在所受到FRA噪声的情况下，RZ码要比NRZ码健壮(robust)的多。经过FRA放大后，NRZ波的幅度波动比RZ的剧烈。从而，泵浦就能向所有的信道和比特提供几乎相等的能量，所以每个信道的所有比特也就被放大到几乎一样的水平。

所以，RZ信号相对于NRZ信号受到的RPDIN噪声要小。在DWDM 传输系统中，使用分布式拉曼放大器的长距离光纤链路(尤其是具有64个信道以上)中，RZ码型比NRZ码型好，这在进行DWDM系统的设计中应加以考虑。

光孤子通信技术及高速孤子脉冲稳定传输

光孤子通信技术是一种利用强脉冲在光纤中产生的非线性压缩来补偿脉冲的色散展宽，实现高速孤子脉冲稳定地传输。光纤孤子传输系统既可以实现单信道多路长距离光孤子通信，又可以利用WDM技术构成多信道光孤子通信系统，从而更加彻底地解决WDM系统中的色散问题。

光纤中的非线性与群速色散(GVD)平衡时，就会产生孤子，光纤没有损耗时，孤子传输任意距离，其形状和能量将保持不变。但事实上，由于光纤中总是存在损耗，光纤的非线性效应不足以抵消群速色散的影响，要实现光孤子超远距离传输，必须对光孤子的能量进行放大。但在放大的同时，自发辐射噪声(ASE)将会随之而产生，引起孤子脉冲到达时间的抖动，即Gordon-Haus效应^[14]。

要抑制Gordon-Haus效应，使光孤子稳定传输，目前应用较多的是色散管理孤子传输方案(DMS^)[15]，也称色散补偿控制技术, 即周期性地采用色散系数符号相反的光纤——在一段反常群速色散(色散系数D>O)单模光纤后面，紧接着是一段正常群速色散(D<O)的色散补偿单模光纤。

对色散控制孤子DMS方案研究表明^[16]：用色散补偿校正由色散与非线性引起的波形畸变，可利用已铺设光纤系统稍加改动，接入某种色散补偿光纤或元件，即可用普通单模光纤实现孤子通信；色散管理孤子解决了Gordon-Haus定时抖动与信噪比的矛盾，通过合理配置色散补偿光纤，使路径平均色散接近于零，降低了ASE噪声引起的定时抖动，同时DMS本身则具有增强的脉冲能量机制使系统的信噪比要高于普通孤子，增大了系统的信噪比； DMS不仅可以运行在反常色散区，还可以运行在正常色散区甚至零平均色散区，这一特点有利于DMS密集波分复用。

在色散管理孤子传输方案中，根据沿传输系统光纤色散的分布方式, 可以采用终端正色散补偿、终端正色散补偿在线滤波控制混合补偿、周期性集总式色散补偿、周期性分布式补偿等几种方案。

为克服ASE 噪声和Gordon Haus 效应对通信容量的限制,Nakazawa 等人提出了另一种传输控制方案, 称为同步幅度调制控制, 此方案是在孤子传输线上, 周期性地提取时钟脉冲, 控制接入线路的电光幅度调制器, 对通过调制器的孤子脉冲进行整形和定时, 实现抑制孤子到达时间抖动的目的。这是一种时域控制技术, 不仅能克服Gordon Haus效应影响, 对抑制相邻孤子的相互作用亦十分有效。同步相位调制控制方案是利用从孤子传输系统中提取的时钟脉冲, 控制经过相位调制器的光孤子脉冲, 对光孤子中心频率进行调整, 达到抑制孤子到达时间抖动的目的。

采用滤波器构成的带宽限制频域控制系统^[17],能扼制Gordon Haus效应, 即在光孤子传输系统中，插入窄带滤波器，孤子的幅度变得稳定，孤子的相互作用得到减弱，拉曼自频移也得到抑制。但由于补偿滤波器插入损耗的附加增益, 会引起滤波器中心频率附近色散波累积, 导致系统稳定性下降, 通信容量得不到很大的提高。为抵消滤波器的损耗，必须给系统提供额外的线性增益，但是这种线性增益，同样会放大与孤子链同时存在的线性波，导致背景的不稳定。

非线性增益控制方案是利用系统增益特性随光强非线性变化的控制机制, 使强光透射率高, 弱光透射率低, 可以对受扰或畸变的孤子脉冲整形和消除线性色散波, 实现孤子稳定传输。

WDM光纤孤子系统的一个新的重要特征就是由于群速度不同引起的不同信道孤子间的碰撞。信道间碰撞对WDM光纤孤子系统性能的影响，可以通过考虑最简单的、频率间隔为的两个WDM信道的例子来理解。在归一化单位中，孤子间的角频率间隔是，为脉冲宽度。在WDM情况下的两个孤子是以不同的群速度传播的，由于孤子碰撞受到孤子能量变化(不同频率的集总放大增益不同)的不利影响，信道间的碰撞情况更严重。物理上讲，发生在一个碰撞长度上的大能量变化破坏了碰撞的天然对称性。信道间碰撞严重地限制了WDM技术的有效性和WDM系统的信道数量。

结论

本文主要对DWDM系统中通过提高单信道传输速率、扩展可用传输带宽的方法提高DWDM系统容量进行了讨论，比较了分布式FRA，色散管理式FRA和级联放大式FRA，并对高速系统升级的, 色度色散(CD) 和偏振模色散(PMD)进行了讨论。进而讨论了FRA的各种噪声问题以及如何消除噪声和码间串扰，同时讨论了拉曼泵浦消耗引起的噪声(RPDIN) ，比较RZ码和NRZ码两种码型受的RPDIN噪声对多信道DWDM系统的影响。最后讨论光孤子通信技术及高速孤子脉冲稳定传输问题，对抑制Gordon-Haus效应进行了论述，色散管理孤子传输方案(DMS⁾ ，时间抖动传输控制方案，插入窄带滤波器，非线性增益控制方案进行了比较。同时也对WDM光纤孤子系统不同信道孤子间的碰撞问题进行了讨论。

参考文献

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Property Studies of Large-capacity and High-speed DWDM Optical Fiber Transmission System

Abstract:

This article discussed the method to improve the system’s capacity by improving the transmission rate of the single signal channel and by expanding available transmission bandwidth; compared distributed Fiber Raman Amplifier, optical dispersion managed Fiber Raman Amplifier and cascade amplification-type Fiber Raman Amplifier; and discussed the chromatic dispersion and polarization mode dispersion of the high speed system. This paper further discussed all kinds of noise problems of Fiber Raman Amplifier, and how to eliminate the noises and the intersymbol interference. In the meantime, the article discussed Raman pump depletion induced noise, compared the influences of Raman pump depletion induced noises undertaken by the two code patterns - RZ code and NRZ code – on the multi signal channel DWDM system. In the last, the paper argued the stable transmission question of optical soliton in the optical soliton communication technology, discoursed upon the inhibition of the Gordon – Haus effect, and compared different kinds of control projects.

Keyword: Raman optical fiber amplifier, optical dispersion management, chromatic dispersion, polarization mode dispersion, intersymbol interference, Raman pump wastage noise, optical soliton communication, Gordon-Haus effect

Introduction

With the development of the society, the rapid increase in the demand for information transmission and the dramatic increase in the demand for communication bandwidth, there are new challenges for the design of the DWDM system. Large-capacity and high-speed DWDM optical fiber transmission system became a trend in system design.

In DWDM system, Raman optical fiber amplifier technique became a hotspot in research and application. This article mainly studied and analyzed relevant technical issues under the conditions of distributed Raman optical fiber amplifier and chromatic dispersion compensation. The main contents are as follows:

1. How to improve the transmission capacity of an optical fiber system?

2. The influence of different kinds of noises on the system; the elimination of noises; and how to eliminate intersymbol interference;

3. The advantages of, disadvantages of different kinds of code patterns, and how to choose different kinds of code patterns;

4. The optical soliton communication technology and the stable transmission of high-speed optical soliton pulse.

1. How to improve the transmission capacity of an optical fiber system?

The improvement of the capacity of the DWDM system can be realized by three methods including improving the transmission velocity of the single channel, decreasing the channel interval, and expanding available transmission bandwidth. The last method is the most direct and the most economic method to solve issues on transmission capacity based on present technical level and available transmission trunk line. To the overall low wastage range of optical fibers from 1270nm to 1670nm, optical fibers have extremely huge available bandwidth as optical signal transmission media. But the present DWDM system in optical communication is mainly in the C waveband (1530nm-1565nm) and its adjacent L waveband (1570nm-1610nm).^[1]-[3] Its available bandwidth is limited mainly by the amplifier bandwidth. In principle, whenever FRA obtains the pump wavelength that it needs, light amplification can be achieved in the spectrum range 1292-1660nm. When multiple pumping sources are adopted, optical fiber Raman amplifier can realize a relative broad gain bandwidth. In the meantime, we can utilize electric filter to make the gain curve of the optical fiber Raman amplifier flat. Therefore, optical fiber Raman amplifier attracted attention of people.

Foreign researchers have performed a great amount of theoretical analysis on the optical fiber Raman Amplifier in the C waveband and the L waveband. Optimization design work and lab research have achieved relatively big progress. And optical fiber Raman amplifier products have been developed. Some giant companies in the field of communication such as Lucent laboratories have applied multilevel concatenated distribution-type Raman amplifier to optical fiber DWDM transmission system. In laboratories, the spectrum bandwidth of multi-wavelength pumping Raman optical fiber amplifiers has reached 100nm. ^[4][5] Although presently the research work of optical fiber amplifier is concentrated mainly in the C + L waveband, the optical fiber Raman amplifier in the S waveband has more developmental prospect,^[6][7] and rare earth doped fiber amplifier can’t compete with it. Utilizing multi pump light we can realize the broad band and flat amplification of the gain spectra of optical fiber Raman amplifier; utilizing the characteristics of Raman gain spectra, the SRS interactions between pump and pump, pump and signal, signal and signal, and suitable arrangement of the number of multi pump waves, the input power and the frequency interval we can realize a very good gain flatness of multi-path signal power of the DWDM system.^[8]

Generally, both the distributed optical fiber Raman amplifier and the discrete optical fiber Raman amplifier have optical dispersion problems. For example, the G652 distributed optical fiber Raman amplifier has positive optical dispersion, and the DCF discrete optical fiber Ramen amplifier has negative optical dispersion. Optical dispersion can cause the deterioration of the transmission property of the optical fiber Raman amplifier, can broaden spectra, produce satellite lines, and cause crosstalk, noise and widening of pulses. At home and abroad, researches on the optical dispersion compensated distributed optical fiber Raman amplifier are carried out also, and are mainly concentrated in the C waveband. There are also some basic theoretically analytical and experimental works in the S waveband.^[9][10]

To the DWDM signal, by choosing suitable pump wavelength, Raman amplifier can enable any wavelength to obtain a gain in the optical fiber low wastage window. In the DWDM pump project, we should combine multi pump light with different central wavelength so that they can enter the optical fiber simultaneously. It can enable Raman amplifier to have more than 100nm spectrum width, overlong bandwidth and over flat gain spectra.

The pump lights of DWDM can interfere with each other when going through the Raman scattering so that the energy can be converted from short wavelength to long wavelength. Just because of this reason, each average pump power of each wavelength, or effective pump power and emission power are different. The difference value is determined by the intensity of the interference. However, because each excited Raman optical dispersion which is caused by different pump wavelength can come into being independently, the overall gain thus produced is merely the sum of each index gain relative to the effective pump power under each wavelength according to its proportion. The number of wavelengths and their distribution are determined by the gain wave range and the degree of flatness required by the system. The range of the pump wavelength determined the gain wave range.

The Raman amplifier of backward DWDM pump has very large noise in the short wavelength region, which is mainly caused by that in the Raman scattering the pump energy is transformed from the short wavelength region to the long wavelength region. In another word, in the short wavelength region, high pump power is necessary because a great proportion of energy is converted to the pump energy of the long wavelength. The result is that the short wavelength signals obtain high noise because they have bigger wastage and larger gain than the long wavelength signals. We can introduce the composite pump manner of the short wavelength to reduce and smooth the noise spectral lines. The simultaneous usage of composite pump and backward pump is helpful to reduce the overall emission pump power which comes from the backward pump light source. Compared to the backward pump, in the aspect of light signal to noise ratio, the composite pump can obtain a bigger gain by utilizing a lower pump power. On the other hand, a composite pump which has too many combinations can also increase the non-linear effect.

Scholars have proposed many optimization methods such as Rung-Kutta Algorithm, Simulated Annealing Algorithm, Genetic Algorithm, Combinatorial Algorithm, etc. Most of the methods mentioned above are relatively rather cumbersome, and it is hard to realize dynamic optimization in practical applications by these methods. I have made several tests in the field of dynamic optimization. I adopted multi-point feedback method, and performed real-time dynamic optimization control by computers. To each bump, there is a corresponding control channel which according to the gain conditions of different checkpoints in the system performs real-time feedback so as to adjust corresponding Raman bump light intensity. The feedback calculation adopts semi-empirical equalization algorithm. In order to realize the flat gain amplification of multi bumps, the number of the bump wave and the wavelength distribution are very important. Research showed that after suitable arrangement of the multi bump wavelength and the power distribution, the bigger the number of the bump wave, the smaller the fluctuation of the Raman gain curve of the DWDM signal. But when the number of bumps reaches to a certain extent, to increase further the number of bumps hasn’t much meaning to the performance of the system so that we should choose suitable number of bumps. Although very good flatness is achieved when the bump wavelength is distributed at equal intervals, the flat wavelength is relatively small. On the contrary, when the distribution of the peripheral wavelength is relatively dense and the distribution of the central wavelength is relatively sparse, the overall flat wavelength of the system is increased obviously. After choosing well the distribution of bump wavelength, we can perform the adjustment of the Raman bump light intensity by feedback. When the gain is between 10 dB and 20 dB, the flatness (the range of fluctuation) can maintain within 0.8 dB.

Utilizing its property of amplifying low noise by distribution, distributed Raman amplifier can realize a high-capacity and long-distance transparent transmission. Its gain medium is regular G. 622 and G. 655 optical fibers. In modern whole Raman transmission system, one is to adopt optical dispersion management Raman amplification, the other link architecture is to adopt single optical fiber distributed and discrete cascade amplification. Optical dispersion management Raman amplifier utilizes fine optical dispersion management technique. On the one hand, by equilibrium self phase adjustment and the optical dispersion effect, it can enlarge the power of entering fiber, and increase the signal to noise ratio; on the other hand, by controlling the dynamic property of pulses, it can inhibit non-linear effects in the signal channel in the high-speed optical fiber transmission system. After the amplification by the cascade standard single-mode optical fiber and optical dispersion compensation optical fiber amplifiers, when the net gains are all 0 dB, this kind of cascade amplification structure constituted by optical dispersion management-type Raman amplifier has very high light signal to noise ratio. Because the optical dispersion slopes of the positive and negative optical dispersion optical fibers which constitute the optical dispersion management amplifier can match each other very well, and at the same time the optical dispersion management amplifier has relatively high light signal to noise ratio, it has very large application prospect in the high capacity and long distance transmission system.

When the rate of the single signal channel of the optical fiber transmission system is upgraded to 40 Gbit/s or above, the chromatic dispersion (CD) and the polarization mode dispersion (PMD) become the main factors that severely influence the performance of the system. The pulse widening which is caused by the chromatic dispersion can produce severe intersymbol interference. Especially to the standard single mode fiber (SMF, G. 652) which is laid down to a great number the chromatic dispersion problem is more severe. In a simulation system, the polarization mode dispersion can produce high order distortion effect and wastage which is related to polarization so that non-linear effects can be caused. In the digital communication system, it can cause the pulse to distort and deform, increase the interactions between pulses, increase the error code ratio, decrease the transmission distance of the system, and limit the transmission bandwidth of the system. Different from the chromatic dispersion, the polarization mode dispersion is a statistical quantity which changes with time and is related to the wavelength. Thus the influence of the polarization mode dispersion on the system is very hard to eliminate. When the single signal channel of the system is increased to 40 Gbit/s, the transmission limitation effect of the high order polarization mode dispersion on the system has been clear already. Therefore, when considering the compensation of the polarization mode dispersion, we should also consider the compensation of the high order polarization mode dispersion.

Polarization mode dispersion is a stochastic variable that abides to the Maxwllian distribution. Its transient polarization mode dispersion value changes with the change in the wavelength, time, temperature, movement, and installation condition. In order to the largest extent to reduce the influence of polarization mode dispersion to the performance of the system, we must adopt the method of dynamic compensation to reduce the system’s PMD. Currently, on the technology of polarization mode dispersion inhibition compensation, there are mainly electric domain polarization mode dispersion compensation, light domain polarization mode dispersion compensation, inhibition polarization mode dispersion that adopts forward error correction and chooses suitable modulation code pattern, and the production of optical fibers with low polarization mode dispersion. The first three inhibition compensation techniques have very big practical meaning to the optical fibers which have been laid down.

From the angle of current practicability, considering the present technical level, the fundamental mode – dispersion compensation fiber (FM-DCF) whose performance is optimized is the best choice for the chromatic dispersion and the dispersion slope distributed compensation and management. In the meantime, the self adaptation feedback system with two segments polarization controller (PC) and fixed differential group delay (DGD) combination together with degree of polarization (DOP) to be the feedback signal is a feasible plan for high order polarization mode dispersion compensation. ^[11]

When the power of entering fiber is relatively small, the optical dispersion plays a leading role in determining the performance of the system. We can adopt complete compensation of optical dispersion. When there is more and more multiplex wave channels in the WDM system, the power of entering fiber will be bigger and bigger, and the non-linear effect will increase correspondingly. In a situation that we can not optimally completely compensate the optical dispersion, generally speaking under compensation of optical dispersion is better than over compensation. The residue abnormal optical dispersion can have certain inhibition effect on the non-linear effect and thus can improve the performance of the system.

The influence of different kinds of noises on the system; the elimination of noises; and how to eliminate intersymbol interference

The source of noises of Fiber Raman Amplifier (FRA) has the following five main types: spontaneous radiation noise amplification (ASE), signal channel cross talk through pumping, pump wastage and noise caused by Rayleigh scattering, pump wastage and noise brought by pump Brillouin scattering, Raman pump depletion induced noise (RPDIN). All of these can cause gain saturation, decrease in signal to noise ratio. And thus we must try to improve them in the Fiber Raman Amplifier.

The spontaneous radiation noise amplification is a kind of background noise which covers the whole Ramen gain spectrum and is produced by Ramen amplification of the spontaneous Raman scattering by the pump light. It’s very obvious that in the Fiber Raman Amplifier the bigger the pump light, the bigger the spontaneous radiation noise. The narrower the bandwidth of the Fiber Raman Amplifier, the lower the noise power of the Fiber Raman Amplifier, and thus higher signal to noise ratio. But in practical application, we hope the bandwidth of the amplifier is wider, and we think the wider the bandwidth the better the situation is. But the adverse effect is there is relatively large noise power. On the contrary, an advantage of a narrow bandwidth is the noise power it brings is relatively low, i.e. relatively high signal to noise ration. But at the same time the path number it amplifies is decreased.

In FRA, although the original incident light only has pump light and signal light, after the amplification of the signal light by the pump light, there is amplified spontaneous radiation noise. Self radiation can cause the consumption of power, i.e. a certain amount of power will be wasted. In addition, the wastage of power is related to the bandwidth of the amplifier. Therefore, when designing an amplifier, we should consider comprehensively the correlations among the bandwidth, the wastage of power and the length of the amplifier, etc, and let the performance of the amplifier to achieve it’s best condition.

The cross talk noise of the Ramen amplifier has two types. The first type is the pump signal cross talk which is caused by the fluctuation of the pump light. The second type is the cross talk among signals induced by pump which is caused by the simultaneous amplification of multi signal channels by pump. To the DWDM system we mainly consider the second type of cross talk.

The main reason for the cross talk among signals induced by pump is that the gain for the pump light to amplify a single signal channel is different from the gain for the pump light to amplify multi signal channels. In detail the situation can be described as when two adjacent signal channels transmit signals at the same time, the gain of the signal if smaller than the gain when one signal channel transmit signal while the other signal channel has no signal. In the mass, we can see the interactions of alternated signal transmission by two signal channels and the influence of no signal, and that the more the signal channels, the bigger the influence of cross talk.

Research shows that adopting backward pump instead of forward pump can enable us to achieve better signal interference ratio (SIR). Therefore, in practical application, we should try our best to adopt backward pump, and if we adopt forward pump in any case we should not use high pump power. Generally we should adopt high order pump to lower the influence of cross talk. The nature of the cross talk noise is correlated to the translation efficiency from the pump light to the signal light. The higher the translation efficiency, the severer the cross talk. In the DWDM system, the energy translation efficiency of the Raman amplifier is always very low.

The Rayleigh scattering in the optical fiber can cause adverse effect on the performance of the distributed fiber Raman amplifier. In the production process of optical fibers, the local concentration fluctuates microscopically so that the reflective index changes randomly on a scale which is smaller than the wavelength. Thus this process causes the appearance of Rayleigh scattering. The Rayleigh scattering noise is caused by Rayleigh backward scattering.

It reflects and outputs at the back end of the optical fiber, and thus noise comes into being and this process causes the deterioration of the signal to noise ratio. The Rayleigh noise includes single Rayleigh noise and double Rayleigh noise. The single Rayleigh scattering goes through a backward scattering and then reflects back to the input end. This process behaves as signal spontaneous beat frequency noise. The double Rayleigh scattering undergoes two backward scattering and then returns back to the input end, which behaviors as multi path crosstalk.^[12]

When the pump power is relatively large, the Rayleigh scattering noise is correspondingly large, and the signal’s gain is in turn inhibited so that the signal to noise ratio will decrease. Therefore, in order to inhibit the influence of Rayleigh scattering noise, we can adopt the method of multistage amplification so as to prevent the pump power from too high and transmission distance from too long. Besides, we can also adopt bi-directional pump method to reduce Rayleigh scattering.

The pump wastage and noise brought by pump Brillouin scattering can be described by the nonlinear interaction of the Stocks wave which is carried by sound wave. This scattering process can be regarded as the annihilation of the pump photo. At the same time Stocks photos and audio frequency phonons are produced. In the scattering process, the conservation of energy and the conservation of momentum must be satisfied.

In the mode of forward pump and backward pump, when the pump power exceeds the threshold of SBS, we can observe the forward SBS phenomenon. With the increase of the pump power of SBS, the power of pump is transferred into Brillouin scattering, and the gain of FRA is limited. When the pump power of FRA is too high, in the Stocks region FRA cascade multi-level SBS lines will appear, and the power of the signal will be transformed to SBS power. When the pump power of FRA is increased further, under this gain, the noise is increased. And the crosstalk of SBS destroyed the property of FRA so that it can not be used in the DWDM optical fiber transmission system. Therefore, it is required to control rigorously the signal power of the entering fiber and the FRA pump power. ^[13]

RPDIN noise is light noise and crosstalk caused by transient pump consumption in the Raman fiber amplifier. Because of the transmission wastage of the optical fiber and the wastage caused by stimulated Raman scattering, the pump power decreases gradually along the optical fiber. At one point on the optical fiber, different signal value (i.e. bit “0” and “1”) requires different pump power so that different transient pump wastage will happen. Therefore, it causes transient changes of the light gain, and causes certain noise and crosstalk. RPDIN noise is different from the traditional Raman noise. It is mainly caused by the transient wastage of pump which is caused by the other n – 1 signal channels. By research, it is found that this kind of noise has relatively big influence on bit “1”.

The advantages of, disadvantages of different kinds of code patterns, and how to choose different kinds of code patterns

Comparing the two code patterns, RZ code and NRZ code, for the same information sequence, the RZ code requires less energy so that its FRA pump wastage is relative less. In the DWDM system, this kind of inhomogeneity of pump wastage is more severe. When experiencing FRA noise, the RZ code is more robust than the NRZ code. After FRA amplification, the amplitude fluctuation of the NRZ wave is more severe than that of RZ wave. Thus, the pump can provide almost equal energy to all signal channels and bits. Therefore, all of the bits of each signal channel will be amplified to almost equal level.

Thus, compared to the NRZ signal, the RZ signal experiences less RPDIN noise. In the DWDM transmission system, when using the long distance optical fiber chain circuit of the distributed Raman amplifier (especially having more than 64 signal channels), the RZ code pattern is better than the NRZ code pattern, which should be considered in the design of the DWDM system.

The optical soliton communication technology and the stable transmission of high-speed optical soliton pulse

The optical soliton communication technology is a kind of transmission which utilizes the nonlinear compression which is produced by strong pulse in the optical fiber to compensate the optical dispersion expansion of the pulse and to realize the stable transmission of the high speed soliton pulse. The optical fiber soliton communication technology can not only realize single channel multi-path long distance optical soliton communication, but also can utilize WDM technique to constitute multi signal channel optical soliton communication system so as to solve more thoroughly the optical dispersion problem in the WDM system.

When the nonlinear and group-velocity dispersion (GVD) in the optical fiber reach equilibrium, solitons will be produced. When there isn’t any wastage in the optical fiber, solitons transmit any distance, and their shape and energy remain constant. But in fact, because there is always wastage in the optical fiber, the nonlinear effect in the optical fiber is not sufficient enough to counteract the influence of the group-velocity dispersion. In order to realize the super long distance transmission of the optical soliton, we must amplify the energy of the soliton. However when we amplify the energy of the soliton, spontaneous radiation noise will happen correspondingly, which in turn causes the fluctuation of the arrival time of the soliton pulse, i.e., Gordon-Haus effect.^[14]

In order to inhibit the Gordon-Haus effect, and to enable the optical soliton to transmit stably, presently people use most often the Dispersion Managed Soliton (DMS) System Schemes ^[15] which is also called the optical dispersion compensation control technique, i.e. adopting periodically optical fibers whose optical dispersion coefficient sign is reversed – after a segment of abnormal group-velocity dispersion (the dispersion coefficient D > 0) single-mode optical fiber, there is a segment of dispersion compensation single-mode optical fiber of normal group-velocity dispersion (D < 0).

Research on the dispersion control soliton DMS system schemes shows that^[16] : when utilizing optical dispersion compensation to proofread the waveform aberration caused by optical dispersion and nonlinear effect, we can utilize the optical system which has already been laid down, change it to some extent, and connect a certain type of optical dispersion compensation optical fiber or element, i.e., we can use normal single-mode optical fiber to realize soliton communication. Optical dispersion managed soliton solved the contradiction between the Gordon-Haus timed jitter and the signal to noise ratio. By rationally arranging the optical dispersion compensation optical fiber, the path average optical dispersion is close to zero, and the timed fluctuation caused by ASE noise is reduced. In the meantime, because DMS itself has enhanced pulse energy mechanism, which causes the signal to noise ratio of the system is higher than a normal soliton, the signal to noise ration of the system is thus increased. DMS can not only operate in abnormal optical dispersion region, but also can operate in normal optical dispersion region and even zero average optical dispersion region. This property favors the DMS Dense Wave Division Multiplexer.

In the optical dispersion managed soliton transmission schemes, according to the distribution mode of the optical fiber dispersion which is along the transmission system, we can adopt several schemes including terminal positive optical dispersion compensation, terminal positive optical dispersion compensation online filtering control mixed compensation, periodically centralized optical dispersion compensation, periodical distributed compensation, and so on.

In order to overcome the inhibition of the ASE noise and the Gordon-Haus effect on the communication capacity, Nakazawa, et al proposed another kind of transmission control scheme which is called synchronized magnitude modulation control. On the soliton transmission line, in this scheme the clock pulse is extracted periodically, the electro-optic magnitude modulator of the input circuit is controlled, and the shaping and timing are performed on the soliton pulse which goes through the modulator. Thus this scheme achieves its goal of inhibiting the arrival time of the soliton from fluctuation. This is a kind of time domain control technique. It not only can overcome the influence of Gordon-Haus effect, but also is very effective in inhibiting the interactions between adjacent solitons. The synchronized phase modulation control scheme utilizes the clock pulse extracted from the soliton transmission system, controls the light soliton pulse which goes through the phase modulator, and adjusts the central frequency of the light soliton as so to inhibit the fluctuation of the soliton arrival time.

The bandwidth limitation frequency domain control system constituted by wave filters can inhibited the Gordon-Haus effect.^[17] In the light soliton transmission system, narrow band wave filters are inserted, the amplitude of solitons tends to be stable, the interaction among solitons is reduced, and the Raman self frequency shift is also inhibited. However because of the extra gain of the compensation wave filter plug wastage, it can cause the accumulation of the optical dispersion wave near the central frequency of the wave filter, and cause the decrease in the system’s stability, and the communication capacity can’t obtain very large improvement. In order to counteract the wastage of the wave filter, we must provide the system with extra linear gain. But this kind of linear gain can also amplify the linear wave which coexists with the soliton chain, and can cause instability of the background.

The nonlinear gain control scheme utilizes a control mechanism that the system’s gain property changes nonlinearly with the light intensity to realize high transmission rate of the strong light and low transmission rate of the weak light. It can shape soliton pulses which are interfered or distorted, can eliminate linear optical dispersion wave, and can realize the stable transmission of solitons.

One new and important characteristic of the WDM optical fiber soliton system is that because of different group velocity collisions among solitons in different signal channels happen. The influence of collisions between signal channels on the WDM optical fiber soliton system’s property can be understood by a most simple example in which the frequency interval is two WDM signal channel. In normalized unit, the angular frequency interval between solitons is pulse width. Under the WDM circumstance, two solitons transmit with different group velocity. Because the soliton collision obtains adverse influence from the soliton’s energy change (the centralized amplification gain of different frequency is different), the collision between signal channel is more severe. In Physics, it says that a big energy change which happens on a collision length destroyes the natural symmetry of collision. The collision between signal channel limits severely the effectiveness of the WDM technique and the number of signal channels of the WDM system.

Conclusion

This article mainly discussed method to improve the DWDM system’s capacity by improving the transmission rate of single signal channels and expanding available transmission bandwidth, compared distributed FRA, optical dispersion managed FRA and cascade amplification-type FRA, and discussed chromatic dispersion and polarization mode dispersion of high speed system upgrading. This paper further discussed all kinds of noise problems of FRA, and how to eliminate noise and intersymbol interference. In the meantime, this article discussed Raman pump depletion induced noise, compared the influence of RPDIN noise experienced by RZ code and NRZ code on the multi channel DWDM system. At last, this paper discussed optical soliton communication technique and issues on the stable transmission of high speed soliton pulse, described how to inhibit Gordon-Haus effect, and compared Dispersion Managed Soliton System Schemes, dispersion managed soliton transmission scheme, time jitter transmission control scheme, narrow band insertion filters, and nonlinear gain control scheme. Meanwhile, this paper discussed the collision question among solitons in different signal channel in the WDM optical fiber soliton system.

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