“光纤极客”怎么说|第2篇:色散
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As a quick refresher, article 1 in this series focused on growth in b和width dem和. We also looked at attenuation in optical fibers caused by factors external to the fiber e.g. 弯曲，以及内置的衰减机制.e. 散射和吸收.
In this second article, we will focus on the several types of dispersion that exist in fiber.
色散是什么?
多, 但并非全部, of the traffic traveling through fiber networks takes the form of pulses of laser light. 这种脉冲是通过打开和关闭激光产生的, 创建光脉冲，其中“无光”代表数字“0”，“全光”代表数字“1”。. 因此，数字信息是一系列“无光”和“全光”的编码，光纤另一端的接收器可以理解并将其转换为数字电信号.
Illustrating such a signal would be a series of square pulses as shown in Figure 1.
当这样的信号受到色散影响时, 方形脉冲的边缘将被磨圆, 脉冲会随着时间扩散. 所以色散会使脉冲变宽.
如果弥散较小, the detector at the other end of the fiber will still be able to detect the signal correctly. 一旦色散过大, the broadened pulses will overlap each other 和 the detector will start misreading the signal, creating errors that will effectively hamper the transmission quality. 这种质量的一个度量是误码率(BER)，它表示传输错误的数量相对于传输的总比特数.
Since a faster transmission rate requires pulses to be of a shorter duration, 这也意味着一定的色散水平对传输速率更快的信号更有害. 此外, dispersion is almost always dependent on the fiber length – the longer the fiber, 色散越大.
Hence transmission is limited by: A) The dispersion of the fiber B) The transmission rate, C)纤维的长度. Dispersion can be described as a “speed limiter”- 和 the 3 main types are:
Modal Dispersion, Chromatic Dispersion 和 Polarization Mode Dispersion.
Modal Dispersion is the most serious of the dispersion types, 因此是最严格的“限速器”.
Light “modes” are different types of waves carrying the light through the fiber. 在“多模”光纤中, the core is rather large 和 may typically allow up to 17 different modes to propagate. In a “Single Mode” fiber, the core is so small that it will allow only one mode to propagate.
问题是不同的模式在光纤中遵循不同的路径，而这些路径的长度也不同. Some modes travel close to the center of the core – others bounce against the outer edges of the core, 和 these modes travel a longer way than the ones close to the center. So the different modes travel different distances – 和 hence some tend to travel faster than others. 被注入光纤的部分光将通过一种模式传播——其他部分通过另一种模式传播——以此类推. 如果不采取措施缓解这种情况, 部分输入信号将比其他部分晚到达输出——这将导致输出信号相对于输入信号“分散”，如图1所示.
To try to minimize the dispersion of the signal to the output of the fiber, 多模光纤的纤芯被设计成延迟光模在纤芯附近传播(这是最短距离)，而加速光模在纤芯附近传播(这是最短距离). 在一个完美的世界里，这将导致所有模式同时把光带到光纤的输出端. 唉，这个世界并不完美, 和 as such a bit of Modal Dispersion cannot be avoided in real life.
这意味着, 尽管多模光纤能够使用非常便宜的光源(如led或vcsel)，但它们仍然局限于通常小于2公里的传输距离, 实际上通常不到几百米.
The way to avoid Modal Dispersion is to shrink the size of the fiber core. In a small fiber core there is only room for one light mode to exist, called the Fundamental Mode. 在这种单模光纤中, higher order modes may indeed be generated at splices or connectors, but they will leak out of the fiber after traveling a short distance through the fiber.
现在找到了避开最重要的限速器的方法，我们可以把注意力转向下一个排队的人.
色散意味着不同波长的光沿着光纤以不同的速度传播. 再一次。, 这种差异导致光纤输出端的信号“模糊”，并有效地起到限速器的作用.
有人可能会问，为什么这是一个大问题, since lasers used to inject light into the fiber have very precisely defined 和 stable wavelengths. 然而, 快速打开和关闭激光实际上会产生一些接近原始激光波长的新波长. 幸运的是，这些新波长中的大多数都相当微弱，不会造成问题——但不幸的是，激光的开启和关闭速度越来越快, 产生的波长范围变宽(图5).
在这种传输系统中，色散引起的问题随着传输速度的增加和光纤长度的延长而恶化(与光纤长度成线性比例)。.
Trying to minimize problems with Chromatic Dispersion the “Dispersion Shifted” (ITU-T G.纤维型是最初发展起来的. 在经典标准单模(ITU-T G.652) fibers the Chromatic Dispersion is zero around 1310 nm. The Dispersion Shifted fibers were targeted for the Chromatic Dispersion to be zero around 1550 nm, because the attenuation of the fiber is lower at 1550 nm 和 so this combination seemed ideal.
Basically, this worked fine right up until DWDM arrived. In DWDM systems a number of individual channels are transmitted over the same fiber. 每个信道被分配一个唯一的波长, 但不幸的是，在DWDM系统中，如果光纤中的色散很低，则光纤的非线性(称为四波混频(FWM))往往会引起不必要的噪声问题.
因此，为了限制DWDM系统中的光纤非线性问题，实现一定程度的色散是可取的, 非零色散位移光纤(ITU-T G).655)被开发. 这种类型的光纤在1550 nm左右有少量的色散(明显小于标准G.因此“速度限制”较小，但色散仍然足够高，可以非常显著地减少非线性问题. 后来的G.非零色散位移光纤的发展是对DWDM系统中信道数量不断增加的需求的响应. 当通道数量增加时, 单个通道需要更紧密地挤在一起，这反过来又需要光纤中更多的色散来减少四波混频的影响.
In parallel with the development of new fiber types with different Chromatic Dispersion characteristics, special devices with negative Chromatic Dispersion were developed. Since transmission fibers normally have positive Chromatic Dispersion, 这两者的结合可以用来减少全光纤链路的总色散几乎为零.
With the ability to reduce the total chromatic dispersion of a transmission link, G的色散越高.因此，656根纤维是一种可接受的技术折衷办法，只剩下费用问题有待考虑.
In many of the recent high-capacity transmission systems, the Chromatic Dispersion of the transmission fiber is compensated electronically with high efficiency, 对于这样的系统，高色散的光纤实际上可能是有利的，因为它有助于限制光纤的非线性.
只是为了使混乱完全, a single-mode fiber will actually be able to carry TWO versions of the fundamental light mode. The reason for this is that light may exist in two different polarizations, 它们的模态彼此垂直. The phenomenon is known from some sunglasses which cut away one of those polarization modes. 从海面或潮湿的道路反射的阳光将主要由这两种偏振模式之一的光组成，而其他物体反射的光将由两种偏振模式的混合组成. Cutting away the polarization mode of the reflected light will “kill” the reflections, but let the other polarization mode pass through the glasses, 让其他对象可见.
在光纤中, 两种极化模式都将存在, 但可能在纤维中以不同的速度传播. 如果光纤芯不是完美的圆形，并且光纤中存在应力，则会出现这种速度差异. 如果纤维的几何形状不是绝对完美的，在制造过程中应力会被“冻结”到纤维中, 例如, 如果包层或涂层不是圆形的, or if the center of the core is different from the center of the cladding or coating.
甚至使用最先进的技术, 高品质制造工艺, 纤维不会在几何上100%完美, hence there will be a speed-difference between the two polarization modes, 会导致分散, 和 it may limit high speed transmission through the fiber. 即使纤维是100%完美的, the slight bending of the fiber in a cable would introduce stress in the fiber – creating PMD. 这是第三个限速器.
Looking at a fiber from a “PMD-perspective” it may be thought of as having a “fast” 和 a “slow” lane. 减少PMD的一种有效方法是在制造过程中来回扭转光纤，以便通过光纤传播的光有效地看到“快”和“慢”通道之间的大量移位.
Because stress is an important cause of PMD, externally applied stress will also affect fiber PMD. In reality just holding a fiber between two fingers may change PMD. 结果是, the PMD of a fiber may be affected both by the cabling of the fiber 和 by external stresses, 比如附近铁路的振动.
与其他色散类型一样，PMD的效果随着传输距离的增加(PMD随距离的平方根缩放)和传输速度的增加而增加. 传输速率为2.5 Gbps或更小，PMD通常不是问题. 对于非常高的传输速率系统, the compensation of PMD is today made electronically 和 built into the transmission system.
