光纤通信简介专业英语

Optical Fiber Communication

-introduction Foreword

The use of light to send messages is not new .Fires were used for signaling in biblical times, smoke signals have been used for thousands of years and flashing lights have been used to communicate between warships at sea since the days of Lord Nelson.

The idea of using glass fiber to carry an optical communication signal originated with Alexander Graham Bell. However this idea had to wait some 80 years for better glasses and low-cost electronics for it to become useful in practical situations.

The predominant use of optical technology is for transmission of data at high speed. Optical fibers replace electric wire in communications systems and nothing much else changes. Perhaps this is not quite fair. The very speed and quality of optical communications systems has itself predicated the development of a new type of electronic communications itself designed to be run on optical connections. ATM (Asynchronous Transfer Mode) and SDH (Synchronous Digital Hierarchy) technologies are good examples of the new type of systems.

It is important to realize that optical communications is not like

electronic communications. While it seems that light travels in a fiber much like electricity does in a wire this is very misleading. Light is an electromagnetic wave and optical fiber is a waveguide. Everything to do with transport of the signal even to simple things like coupling (joining) two fibers into one is very different from what happens in the electronic world. The two fields (electronics and optics) while closely related employ different principles in different ways.

Some people look ahead to “true”optical networks. These will be networks where routing is done optically from one end-user to another without the signal ever becoming electronic. Indeed some experimental local area (LAN) and metropolitan area (MAN) networks like this have been built. In 1998 optically routed nodal wide area networks are imminently feasible and the necessary components to build them are available. However, no such networks have been deployed operationally yet.

In 1998 the “happening”area in optical communications was Wavelength Division Multiplexing (WDM). This is the ability to send many (perhaps up to 1000) independent optical channels on a single fiber. The first fully commercial WDM products appeared on the market in 1996. WDM is a major step toward fully optical networking.

1. Transmitting Light on a Fiber

An optical fiber is a very thin strand of silica glass in geometry quite like a human hair. In reality it is a very narrow, very long glass cylinder with special characteristics. When light enters one end of the fiber, it travels (confined within the fiber) until it leaves the fiber at the other end. Two critical factors stand out:

Very little light is lost in its journey along the fiber.

Fiber can bend around corners and the light will stay within it and be guided around the corners.

An optical fiber consists of two parts: the core and the cladding. The core is a narrow cylindrical strand of glass and the cladding is a tubular jacket surrounding it. The core has a (slightly) higher refractive index than the cladding. This means that the boundary (interface) between the core and the cladding acts as a perfect mirror. Light traveling along the core is confined by the mirror to stay within it-even when the fiber bends around a corner.

When light is transmitted on a fiber, the most important consideration is “what kind of light?”The electromagnetic radiation that we call light exists at many wavelengths. These wavelengths go from invisible infrared through all the colours of the visible spectrum to invisible ultraviolet. Because of the attenuation characteristics of fiber, we are only interested in infrared “light”for communication applications. This light is usually

invisible, since the wavelengths used are usually longer than the visible limit of around 750 nanometers ( nm ) .

If a short pulse of light from a source such as a laser or an LED is sent down a narrow fiber, it will be changed (degraded) by its passage down the fiber. It will emerge (depending on the distance) much weaker, lengthened in time (“smeared out”), and distorted in other ways.

2. Optical Transmission System Concepts

The basic components of an optical communication system are optical transmitter and receiver,Fiber jumpers,Optical,fiber splice tray Optical fiber.

A serial bit stream in electrical from is presented to a modulator, which encodes the data appropriately for fiber transmission.

A light source (laser or Light Emitting Diode—LED) is driven by the modulator and the light focused into the fiber. The light travels down the fiber (during which time it may experience dispersion and loss of strength).

At the receiver end the light is fed to a detector and converted to electrical form. The signal is then amplified and fed to another detector, which isolates the individual state changes and their timing. It then decodes the sequence of state changes and reconstructs the original bit stream.

The timed bit stream so received may then be fed to a using device. Optical communication has many well-known advantages.

Weight and Size

Fiber cable is significantly smaller and lighter than electrical cables to do the same job. In the wide area environment a large coaxial cable system can easily involve a cable of several inches in diameter and weighing many pounds per foot. A fiber cable to do the same job could be less than one half an inch in diameter and weigh a few ounces per foot. This means that the cost of laying the cable is dramatically reduced. Material Cost

Fiber cable costs significantly less than copper cable for the same transmission capacity.

Information Capacity

The idea rate of system in 1998 was generally 150 or 620Mbps on a single (unidirectional) fiber. This is because these systems were installed in past years. The usual rate for new systems is 2.4Gbps or even 10Gbps. This is very high in digital transmission terms.

In telephone transmission terms the very best coaxial cable systems give about 2,000 analog voice circuits. A 150Mbps fiber connection gives just over 2,000 digital telephone (64kbps) connections. But the 150Mbps

fiber is at a very early stage in the development of fiber optical systems. The coaxial cable system with which it is being compared is much more costly and has been developed to its fullest extent.

Fiber technology is still in its infancy. Using just a single channel per fiber, researchers have trial systems in operation that communicate at speeds of 100Gbps.By sending many (“wavelength division multiplexed ”) channels on a single fiber, we can increase this capacity a hundred and perhaps a thousand times. Recently researchers at NEC reported a successful experiment where 132 optical channels of 20Gbps each were carried over 120km. This is 2.64 terabits per second! This is enough capacity to carry about 30 million uncompressed telephone calls (at 64kbps per channel). Thirty million calls is about the maximum number of calls in progress in the world at any particular moment in time. That is to say, we could carry the world’s peak telephone traffic over one pair of fibers. Most practical fiber systems don’t attempt to do this because it costs less to put multiple fibers in a cable than to use sophisticated multiplexing technology.

No Electrical Connection

This is an obvious point but nevertheless a very important one . Electrical connections have problems. In electrical systems there is always the possibility of “ground loops” causing a serious problem,especially in the

LAN or computer channel environment . When you communicate electrically you often have to connect the grounds to one another or at least go to a lot of trouble to avoid making this connection. One little known problem is that there is often a voltage potential difference between “ground”at different locations. The author has observed as much as 3 volts difference in ground potential between adjacent buildings (this was a freak situation). It is normal to observe 1or 2 volt differences over distance of a kilometer or so.With shielded cable there can be a problem if you earth the shields at both ends of the connection. Optical connection is very safe. Electrical connections always have to be protected from high voltages because of the danger to people touching the wire . In some tropical regions of the world, lightning poses a severe hazard even to buried telephone cables! Of cause, optical fiber isn’t subject to lightning problems but it must be remembered that sometimes optical cables carry wires within them for strengthening or to power repeaters . These wires can be a target for lightning.

No Electromagnetic Interference

Because the connection is not electrical, you can neither pick up nor create electrical interference (the major source of noise). This is one reason that optical communication has so few errors. There are very few source of things that can distort or interfere with the signal. In a building

this means that fiber cables can be placed almost anywhere electrical cables would have problems, (foe example near a lift motor or in a cable duct with heavy power cables). In an industrial plant such as a steel mill, this gives much greater flexibility in cabling than previously available.

In the wide area networking environment there is much greater flexibility in route selection. Cables may be located near water or power lines without risk to people or equipment.

Distances between Regenerators

As a signal travels along a communication line it loses strength (is attenuated) and picks up noise. The traditional way to regenerate the signal, restoring its power and removing the noise, is to use either a repeater or an amplifier. Indeed it is the use of repeaters to remove noise that gives digital transmission its high quality.

In long-line optical transmission cables now in use by the telephone companies, the repeater spacing is typically 40 kilometers. This compares with 12 km for the previous coaxial cable electrical technology. The number of required repeaters and their spacing is a major factor in system cost.

Open Ended Capacity

The maximum theoretical capacity of installed fiber is very great (almost

infinite). This means that additional capacity can be had on existing fibers as new technology becomes available. All that must be done is change the equipment at either end and change or upgrade the regenerators.

Better Security

It is possible to tap fiber optical cable. But it is very difficult to do and the additional loss caused by the tap is relatively easy to detect.There is an interruption to service while the tap is interested and this can alert operational staff to the situation. In addition, there are fewer access points where an intruder can gain the kind of access to a fiber cable necessary to insert a tap.

3. Wavelength Division Multiplexing

Wavelength Division Multiplexing (WDM) is the basic technology of optical networking. It is a technique for using a fiber (or optical device) to carry many separate and independent optical channels. The principle is identical to that used when we tune our television receiver to one of many TV channels. Each channel is transmitted at a different radio frequency and we select between them using a “tuner” which is just a resonant circuit within the TV set. Of course wavelength in the optical world is just the way we choose to refer to frequency and optical WDM is

quite identical to radio FDM.

There are many varieties of WDM. A simple form can be constructed using 1310nm as one wavelength and 1550 as the other or 850 and 1310. This type of WDM can be built using relatively simple and inexpensive components and some applications have been in operation for a number of years using this principle.

Wavelength selective couplers are used both to mix (multiplex) and to separate (de-multiplex) the signals. The distinguishing characteristic here is the very wide separation of wavelengths used (different bands rather than different wavelengths in the same band).

Th ere are many variations around on this very simple theme. Some systems use a signal fiber bidirectionally while others use separate fibers for each direction . Other systems use different wavelength bands from those illustrated in the figure (1310and 1550 for example). The most common systems run at very low data rates. Common application areas are in video transport for security monitoring and in plant process control.

Dense WDM however is another thing.Dense WDM refers to the close spacing of channels.Sadly，＂dense＂is a qualitative measure and just what dense means is largely in the mind of the description.Others use the term to distinguish systems where the wavelength spacing is 1nm per channel or less.

Each optical channel is allocated its own wavelength —or rather range of wavelengths.A typical optical channel might be 1nm wide. This channel is really a wavelength range within which the signal must stay. It is normally much wider than the signal itself. The width of a channel depends on many things such as the modulated line width of the transmitter,its stability and the tolerances of the other components in the system. In practical terms the transmitter is always a laser.It must have a line width which (after modulation) fits easily within its allocated band. It must not go outside the allocated band so it should have chirp and drift characteristics that ensure this. Depending on the width of the allocated band,these characteristics don't need to be the most perfect obtainable.However they do have to be such that the signal stays where it is supposed to be. The receiver is relatively straightforward and is generally the same as a non-WDM receiver .This is because the signal has been de-multiplexed before it arrives at the detector.

光纤通信简介

前言

使用光来传送信息并不新鲜。旧约时代就开始用火来传递信息，烟雾信号已使用千年。从纳尔逊勋爵时代开始，海上舰船间的通信就采用闪烁的灯光。

Alexander Graham Bell 最先提出用玻璃纤维来传送光通信信号。但直到80多年后，有了更好的玻璃纤维及低成本电子设备，该想法才真正切实可行。

光技术主要用于高速数据传输。除了用光纤代替电缆外，与其他通信系统没有什么区别。不过，这样说也许并不太公平。光通信系统的高速及高质量的传输预示了以光纤作为传输媒介的电通信系统的新的发展方向。异步传输模式（Asynchronous Transfer Mode,ATM ）和同步数字体系（Synchronous Digital Hierarchy, SDH）技术就是这种新系统的很好的例子。

认识到光通信不同于电通信很重要。人们很容易误解成光在光纤上传输就像电在电缆上传输一样。光是一种电磁波而光纤是一种波导。在光的世界里，任何的和信号的传输相关的方面（即使是诸如将两条纤维连在一起这样简单的事情）和电的世界中都完全不同。虽然电和光这两个领域紧密相关，但却在不同的方面采用不同的原理。

有人预测未来将会出现“纯”光网络。这些网络中，完全通过光路由将信号从发射端传输到接收端，无须转换成电信号。实际上，已经建立了一些这样的实验局域网和广域网。1998年，光路由节点广域网已经完全可行，且建造该网络的必要部件都是现成的。不过，这样的网络至今都没有部署运营。

1998年，光通信中的热门领域就是波分复用。该技术能够在一条单独的光纤上传输很多（也许超过1000个）独立的光信道。1996年，市场上出现了第一个完全商业化的波分复用设备。波分复用是迈向全光网络互联的重要的一步。

1. 光在光纤上传输

光纤是非常细的硅玻璃线，很像人的头发。实际上，光线是具有特殊性质的又窄又长的玻璃柱体。光从光纤的一端进入后，将一直在光纤内传输，直到从另一端离开。光纤有两个关键因素：

1、光在光纤中传输时损失非常小。

2、光在拐弯处能够弯折，折弯后光仍旧在光纤中传输。

光纤由两部分组成：纤芯和纤包。纤芯是一个狭窄的圆柱形玻璃纤，纤包是围绕在其上的管状套。纤芯的折射系数逼纤包大一些，这说明纤芯和纤包间的边界（接口）就像一个平面镜。在纤芯中传输的光因镜子的反射而始终限制在纤芯内，即使当光纤折弯时也如此。光在光线中传输时，最重要的是考虑使用哪种类型的光。那些我们称为光的电磁辐射存在于不同波长处，从不可见的红外光，到可以看见的各种颜色的频谱，一直到不可见的紫外光。由于光纤的衰减特性，我们只对通信应用中的红外“光”感兴趣。由于使用的波长通常比可见波长的极限（大概是75nm）长，因此这种光通常不可见。

如果将光源（如激光器或者发光二极管）产生的短光脉冲送入光纤，它将在其传输路径上衰减。随着距离的不断增加，信号强度将逐渐减弱，传输时间变长，以及产生其他方面的失真。

2. 光传输系统概念

光通信系统由光端机、光纤跳线、光纤熔接盒等组成。

-调制器的输入是一串电比特流，编码后的数据适于在光纤中传输。

-光源（激光器或发光二极管）由调制器驱动，将光送入光纤。光沿光纤传播（这时的光可能会发生散射和能量损失）。

-在接收端，将光送入检测器并转换成电的形式。然后信号经放大并送入另一个检测器，其作用是将各自的状态变化及时序分开，然后对状态变化序列解码并还原成原始比特流。

-由此所得的具有时间标记的比特流可供后续设备使用。

光通信有很多众所周知的优点：

重量和体积

对于完成相同的功能而言，光缆比电缆要小得多且轻得多。在广域环境中，大的同轴电缆系统很容易电缆直径达到数英尺而重量高达每英尺好几磅。对于完成同样功能的光纤系统，光缆直径一般少于半英尺、重量仅为每英尺几盎司。这说明铺设光缆的成本远远低于电缆。

材料成本

对于同样的传输容量，光缆的成本大大低于铜制电缆。

信息容量

1998年，光纤系统的数据传输率一般为每根光纤（单向）150Mbps或620Mbps。这是由于这些系统是当时所建。现在新系统的数据传输速率是为2.4Gbps甚至是10Gbps。就数字传输而言这个数据传输率已经非常高。

在电话传输中最好的同轴电缆系统能够支持大约2000路模拟语音电路。一条150Mbps 的光缆仅能支持超过2000路的数字电话（64kbps）连接。不过，150Mbps光缆仅是光纤系统发展最早阶段的数据传输率。相比之下，同轴电缆系统要昂贵得多，且很难再有新的突破。光纤技术仍处于发展初期。仅在每根光纤上使用一个信道，研究人员就能建立通信速率为100Gbps的实验系统。如果在单根光纤上采用波分复用发送多路信号，可以将通信容量增加100甚至1000倍。最近NEC的研究人员披露了这样一个成功的实验，该实验中每根光纤上承载132个光信道，每个信道速率为20Gbps，传输距离可达120km，也就是每秒2.64万亿比特！这个容量足以传送三千万路未经压缩的电话呼叫（每路64kbps）。任何时候在世界上的任何地方，三千万路呼叫都可以认为是系统运行的最大峰值。也就是说，一对光缆就能承载世界上所有电话的峰值流量。但由于把多条光纤汇成光缆的成本比使用复杂多分复用技术低得多，因此大部分实用的光纤系统并没有这样做。

无电气连接

这一点显而易见但非常重要。电连接形式存在一些问题。在电气系统中总有出现“接地环路”的可能，这会导致严重的问题，尤其是在局域网或者是计算机系统中。当使用电通信的时候，通常要将地线连接起来，或者至少是尽可能避免出现这样的连接。一个不太为人所知的问题就是不同的地方的“地”之间通常存在电势差。作者曾在相邻建筑的地之间发现了最多有3V的电势差（奇怪的情形）。通常相距1公里左右存在1V或2V的电势差是正常的。当使用屏蔽电缆时，如果在连接的两端把屏蔽都接地，会出现问题。

光连接很安全。电气连接由于存在高电压，所以需要保护起来，防止人碰到而发生危险。在某些热带地区，闪电会造成严重的危害，甚至对埋在地下的电话线也不例外。当然，对于光缆来说，不存在闪电的问题，但是别忘了有时在光缆中也会携带金属丝起加固作用或是给放大器供电。这些金属也可能成为闪电攻击的目标。

无电磁干扰

由于光纤系统无电气连接，所以在光缆中既不会收集也不会产生电干扰（噪声的主要来源），这是光通信错误率低的原因之一。对信号发生失真或与之发生干扰的光源很少。这说明在建筑物中，任何铺设电缆存在问题的地方都能用光纤代替（例如电梯发动机旁或高压电缆管道里），在工厂车间里（例如钢厂），光缆铺设比以前更加灵活。

在广域网环境中路径的选择有更大的灵活性。光缆能铺设到近水处或者是电线旁，而不会给人或设备带来任何危害。

中继之间的距离

信号在通信线路上传输时会导致强度衰减并引入噪声。传统的用于再生信号、恢复功率并除去噪声的方法是使用中继器或者放大器。中继器的确可以去除噪声，实现高质量的数字传输。

目前电话公司所使用的长途光传输电缆中，中继器间的间隔通常是40公里。与此相比，以前同轴电缆传输系统的中继器间的距离是12公里。中继器的数量及他们之间的距离是整个系统成本的一个主要因素。

可扩展的容量

光纤容量的理论最大值很大，几乎可以认为是无限大，这说明新的技术一旦出现，可以在已有的光纤上扩展容量。所需的工作只是更换光纤两端的设备及更换或升级再生器。

更高的安全性

光缆可能被窃听。不过这点非常难做到，而且可较容易地检测到由于窃听而造成的额外损失。插入窃听时会导致服务器中断，这会引起操作人员对该情况的警惕。而且，在光纤上只有少数的接入点能够给入侵者插入窃听。

3. 波分复用

波分复用是光互连中的基本技术。这种技术使用单条光纤（或者光设备）承载多个单独的光信道，其原理与调节电视接收机接收某个电视频道一致。每个频道使用不同的频率发送（信号），人们使用“调谐器”来从中选择不同的频道，它是电视中的一个谐振电路。广域中选择波长的方式和这里选择频率的方式一样，光的波分复用等同于频域的频分复用。

波分复用有几种。一种简单的波分复用使用1310nm作为一个波长，1550nm作为另一个波长，或者两个波长分别采用850nm1310nm，使用较简单和便宜的元器件就能实现，有些应用已运行多年。

波长选择性耦合器用于对信号进行混合（复用）和分离（分解复用），其中最明显的特点就是所使用的波长间距非常大（使用不同波段，而不是同一波段中的不同波长）。

基于以上非常简单的思想有多种变形。有些系统使用一根双向光纤，而另一些系统在不同方向各自使用光纤。也有系统使用与一般数字（例如1310和1550）不同的波长段。常见的大多数系统数据传输率都非常低，主要应用于安全监测中的视频传输和工厂进程控制。

密集波分复用是另一个概念，指信道间间隔紧密。不过，“密集”只是一个定性标准，不同人对间隔多近算作密集的看法不同，一些人认为WDM信道间相隔3.6nm就算密集。其他人用该术语区分信道波长区间为1nm或更小的系统。

每个光信道分配的波长就是它自己，或者是某区域范围内的波长。典型的光信道宽度为1nm。信道宽度实际上是其中的光信号所处的波长范围。信道一般比本身宽很多。信道宽度有很多因素决定，例如发送器的调制行宽、稳定性和系统中其他部件的容错性。实际上传送器就是一个激光发射装置，它的线宽经调制后能便于与其分配的波段匹配。信道的线宽不能超出分配的带宽，因此只好利用chirp和漂移特性来实现。根据所分配的波段带宽的不同，并非完全需要这些特征。但是必须具备这样的特性使信号存在于它应有的地方。由于信号在到达检测器之前已经解复用，因此接收器相对简单，通常与非波分复用时的接收器一样。