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1、Fiber-Optic Communication SystemsIntroductionA communication system transmits information from one place to another, whether separated by a few kilometers or by transoceanic distances.一个通信系统是用来从一个地方向另一个地方传递信息的，无论相隔几公里或者是跨越海洋。Information is often carried by an electromagnetic carrier wave whose frequ

2、ency can vary from a few megahertz to several hundred terahertz. 信息经常以频率为几兆赫兹到几百T赫兹的电磁波为载波传递。Optical communication systems use high carrier frequencies (100 THz) in the visible or near-infrared region of the electromagnetic spectrum. 光通信系统用处于电磁波谱中可见光或者近红外附近的高频载波（大于100THz）。They are sometimes called l

3、ightwave systems to distinguish them from microwave systems, whose carrier frequency is typically smaller by ve orders of magnitude (1 GHz). 光通信系统有时也被称为光波系统，用以区别于载波频率远远小了5个数量级的微波系统（小于1GHz）。Fiber-optic communication systems are lightwave systems that employ optical bers for information transmission.光

4、纤通信系统是一种用光纤来传递信息的光波系统。Such systems have been deployed worldwide since 1980 and have indeed revolutionized the technology behind telecommunications.这种系统从1980开始被全球使用并且确实已经从根本上改变了通信技术。Indeed, the lightwave technology, together with microelectronics, is believed to be a major factor in the advent of the

5、 'information age'. 确实，光波技术和微电子技术被称为“信息时代”出现的首要原因。The objective of this book is to describe ber-optic communication systems in a comprehensive manner.这本书的主要目的是为了全面描述光纤通信系统。The emphasis is on the fundamental aspects, but the engineering issues are also discussed. 重点是讲述基础原理方面，但是也会涉及到一些工程方面。The

6、 purpose of this introductory chapter is to present the basic concepts and to provide the background material.这篇绪论的作用是展现基本概念和提供背景材料。Section 1.1 gives a historical perspective on the development of optical communication systems.第一章第一节讲述了光通信系统的发展的历史背景。In Section 1.2 we cover concepts such as analog an

7、d digital signals, channel multiplexing, and modulation formats.第一章第二节涉及到了一些概念比如模拟和数字信号，信道多路复用技术和调制格式。Relative merits of guided and unguided optical communication systems are discussed in Section 1.3. 光导通信系统和非光导通信系统的优缺点在1.3章节介绍。The last section focuses on the building blocks of a ber-optic communica

8、tion system.最后一个章节主要介绍光纤通信系统的中心模块。1.1 Historical PerspectiveThe use of light for communication purposes dates back to antiquity if we interpret optical communications in a broad sense 1.如果我们把光通信放在一个广义的角度来说，早在古时候人们就用光来进行通信。Most civilizations have used mirrors, fire beacons, or smoke signals to convey

9、 a single piece of information (such as victory in a war). 很多居民使用镜子，火把，或者烟信号来传递一部分信息。Essentially the same idea was used up to the end of the eighteenth century through signaling lamps, flags, and other semaphore devices. 本质上说，18世纪末，人们利用相同的想法用了信号灯，旗子还有其他设备。The idea was extended further, following a s

10、uggestion of Claude Chappe in 1792, to transmit mechanically coded messages over long distances (100 km) by the use of intermediate relay stations 2, acting as regenerators or repeaters in the modern-day language.这种想法在克劳德查铺的建议下被推广，利用中继器能够传输很长距离（超过100km）的机械编码信息，也就是今日人们所说的中继器。Figure 1.1 shows the basi

11、c idea schematically. 图1.1体现了开始想法的产生。The rst such “optical telegraph”was put in service between Paris and Lille (two French cities about 200 km apart) in July 1794.第一个光电报1794年7月被使用在两个相距200km的法国城市巴黎和里尔。By 1830, the network had expanded throughout Europe 1. 到了1830年，这种网络被扩展到全欧洲。The role of light in suc

12、h systems was simply to make the coded signals visible so that they could be intercepted by the relay stations. 光在那种系统中的角色是简单的使编码信号可见以至于可以被中继器拦截。The opto-mechanical communication systems of the nineteenth century were inherently slow.19世纪的光机通信系统传输速度非常慢。In modern-day terminology,the effective bit rat

13、e of such systems was less than 1 bit per second (B <1 b/s).在现代术语中，这种系统的有效比特率低于1bps。1.1.1 Need for Fiber-Optic CommunicationsThe advent of telegraphy in the 1830s replaced the use of light by electricity and began the era of electrical communications 3.19世纪30年代电报的发明使电代替了光从而开始了电子通信的时代。The bit rate

14、 B could be increased to 10 b/s by the use of new coding techniques, such as the Morse code. 比特率B通过新的编码技术可以被提高到10b/s，比如莫尔斯电码。The use of intermediate relay stations allowed communication over long distances ( 1000 km).中继站的使用使得通信可以跨越更长的距离。Indeed, the first successful transatlantic telegraph cable went

15、 into operation in 1866.确实，第一次成功的跨洋电报电缆在1866年开始投入使用。Telegraphy used essentially a digital scheme through two electrical pulses of different durations (dots and dashes of the Morse code).电报本质上使用的是数字格式的两个不同长度的电子脉冲。（莫尔斯电码的点和横）The invention of the telephone in 1876 brought a major change inasmuch as ele

16、ctric signals were transmitted in analog form through a continuously varying electric current 4.1876年电话的发明带来了一个巨大的改变，因为电子信号通过不断变化的电流以模拟形式传输。Analog electrical techniques were to dominate communication systems for a century or so.模拟电子技术开始主导通信系统并持续了将近一个世纪甚至更多。The development of worldwide telephone netw

17、orks during the twentieth century led to many advances in the design of electrical communication systems.20世纪全球电话网络的发展使电子通信系统设计得到很大进展。The use of coaxial cables in place of wire pairs increased system capacity considerably.同轴电缆取代了对称电缆，大大改进了系统的容量。The first coaxial-cable system, put into service in 194

18、0, was a 3-MHz system capable of transmitting 300 voice channels or a single television channel.第一个同轴电缆系统在1940年被投入使用，它是一个2MHz容量的系统，包含300多个音频信道和一个视频信道。The bandwidth of such systems is limited by the frequency-dependent cable losses, which increase rapidly for frequencies beyond 10 MHz.这种系统的带宽被与频率有关的电

19、缆功耗所限制，可以快速提升至10MHz以上。This limitation led to the development of microwave communication systems in which an electromagnetic carrier wave with frequencies in the range of 110 GHz is used to transmit the signal by using suitable modulation techniques.这种限制使（频率以1-10GHz频率的以合适的调制技术来传递信号的电磁波为载波的）微波通信系统得到了发

20、展。The first microwave system operating at the carrier frequency of 4 GHz was put into service in 1948.第一个微波系统在1948年投入使用，它的载波频率是4GHz。Since then, both coaxial and microwave systems have evolved considerably and are able to operate at bit rates 100 Mb/s.从那以后，同轴电缆和微波系统逐步改进并且都能够在100Mbps的比特率上运行。The most a

21、dvanced coaxial system was put into service in 1975 and operated at a bit rate of 274 Mb/s.最前沿的同轴电缆系统在1975年被投入使用，并且工作在274Mbps的比特率。A severe drawback of such high-speed coaxial systems is their small repeater spacing (1 km), which makes the system relatively expensive to operate.高速同轴系统的一个小缺点就是他们的中继站间距

22、较小，使得系统运行起来较昂贵。Microwave communication systems generally allow for a larger repeater spacing, but their bit rate is also limited by the carrier frequency of such waves.微波通信系统允许较大的中继站间距，但是也被那种波的载波频率限制。A commonly used figure of merit for communication systems is the bit ratedistance product, BL, where

23、 B is the bit rate and L is the repeater spacing.一个衡量通信系统优点的数据是比特率，距离乘积为BL，其中B是比特率，L是中继站间距。Figure 1.2 shows how the BL product has increased through technological advances during the last century and a half.图1.2展现了B*L的乘积通过技术提升而提高了在过去的一个半世纪里。Communication systems with BL 100 (Mb/s)-km were available

24、by 1970 and were limited to such values because of fundamental limitations.It was realized during the second half of the twentieth century that an increase of several orders of magnitude in the BL product would be possible if optical waves were used as the carrier.20世纪下半叶，人们发现使BL乘积提升一个数量级是可能的，如果光波可以

25、当做载波的话。However, neither a coherent optical source nor a suitable transmission medium was available during the 1950s.然而，在1950s，既没有一个同步光源，也没有一个合适的传输媒介。The invention of the laser and its demonstration in 1960 solved the first problem 5.激光的发明和1960年它的第一次试行解决了第一个问题。Attention was then focused on finding wa

26、ys for using laser light for optical communications.人们开始聚焦在寻找方法来利用激光作为光通信的媒介。Many ideas were advanced during the 1960s 6, the most noteworthy being the idea of light confinement using a sequence of gas lenses 7.1960s很多想法都很先进。最值得注意的一个想法是利用一系列的气体透镜限制光的传输。It was suggested in 1966 that optical fibers mi

27、ght be the best choice 8, as they are capable of guiding the light in a manner similar to the guiding of electrons in copper wires.在1966年，有人说，光纤可能是最好的选择，当他们有能力使光像电子在铜线中传播那样。The main problem was the high losses of optical fibersfibers availableduring the 1960s had losses in excess of 1000 dB/km.最主要的问

28、题是光纤有的巨大的能量损耗1960s的时候可使用的光纤的功耗在1000dB/km以上。A breakthrough occurred in 1970 when fiber losses could be reduced to below 20 dB/km in the wavelength region near 1 m 9.在1970取得了一个较大的突破，当波长范围在1微米附近时，光纤的损耗可以降低到20dB/km以下。At about the same time, GaAs semiconductor lasers, operating continuously at room tempe

29、rature, were demonstrated 10.同时，砷化镓半导体激光在室温下能够持续运行。The simultaneous availability of compact optical sources and a low-loss optical fibers led to a worldwide effort for developing fiber-optic communication systems 11.紧凑型光源和低损耗光纤的同时发放使得光纤通信系统有了一个世界范围的大发展。Figure 1.3 shows the increase in the capacity o

30、f lightwave systems realized after 1980 through several generations of development.图1.3体现了1980年后光波系统容量一代又一代的提升。As seen there, the commercial deployment of lightwave systems followed the research and development phase closely.可见，研究与开发紧紧跟随着光波系统的商业调度。The progress has indeed been rapid as evident from a

31、n increase in the bit rate by a factor of 100,000 over a period of less than 25 years.这个进步确实是飞快的，可以由比特率在25年内上升了100000倍看出。Transmission distances have also increased from 10 to 10,000 km over the same time period.传输距离在同一时期也从10km提升到10000km。As a result, the bit ratedistance product of modern lightwave s

32、ystems can exceed by a factor of 107 compared with the first-generation lightwave systems.最后，如今光波系统的比特率-距离乘积也超出了第一代光波系统的107倍。1.1.2 Evolution of Lightwave SystemsThe research phase of ber-optic communication systems started around 1975.对于光纤通信系统的研究阶段从1975年开始。The enormous progress realized over the 25-

33、year period extending from 1975 to 2000 can be grouped into several distinct generations.从1975到2000年这25年的时间里，这些巨大的进展可以被分为几个不同的时代。Figure 1.4 shows the increase in the BL product over this time period as quantied through various laboratory experiments12.图1.4展现了通过不同的室内实验BL乘积在这些年里的增长情况。The straight line

34、 corresponds to a doubling of the BL product every year. 直线表示了BL乘积每年成倍增长。In every generation, BL increases initially but then begins to saturate as the technology matures.每一代里，BL都是开始增长但是后来当技术成熟时会逐渐饱和。Each new generation brings a fundamental change that helps to improve the system performance further

35、.每新的一代里都会带来一个能提高系统性能的基础的改变。The rst generation of lightwave systems operated near 0.8 µm and used GaAs semi conductor lasers.第一代光波系统在0.8um的波长工作并且使用了砷化镓半导体激光器。After several eld trials during the period197779,such systems became available commercially in 1980 13.在1977-1979年中进行了几次实地测验之后，这种系统在1980年投

36、入了商业化使用。They operated at a bit rate of 45 Mb/s and allowed repeater spacings of up to 10 km.他们工作在45Mbps的比特率，并且允许中继器间距达到10km以上。The larger repeater spacing compared with 1km spacing of coaxial systems was an important motivation for system designers because it decreased the installation and maintenanc

37、e costs associated with each repeater.和同轴系统的1km的中继站距离相比，大型中继站的距离对于系统设计者来说是一个很大的激励，因为它减少了每个中继站的安装和维修费用。It was clear during the 1970s that the repeater spacing could be increased considerably by operating the lightwave system in the wavelength region near 1.3 m, where fiber loss is below 1 dB/km.在20世纪

38、70年代内，可以很清楚的看到，如果使光波系统工作在波长范围在1.3um附近的话，中继站距离可以被很大的提升。而且光纤损耗也会低于1dB/km。Furthermore, optical fibers exhibit minimum dispersion in this wavelength region.此外，在这个波长区间内，光纤的散射达到最小。This realization led to a worldwide effort for the development of InGaAsP semiconductor lasers and detectors operating near 1.

39、3 m.这个想法使得全世界的人都在为使砷化镓半导体激光器和探测器能够工作在1.3um附近而努力着。The second generation of fiber-optic communication systems became available in the early 1980s, but the bit rate of early systems was limited to below 100 Mb/s because of dispersion in multimode fibers 14.第二代光纤通信系统在20世纪八十年代早期投入使用，但是早期系统的比特率被限制在100Mbps

40、以下，多模光纤的散射。This limitation was overcome by the use of single-mode fibers.单模光纤的使用克服了这个限制。A laboratory experiment in 1981 demonstrated transmission at 2 Gb/s over 44 km of single-mode fiber 15.一个1981年的室内实验演示了能够工作在 2Gbps的传输超过44km的单模光纤。The introduction of commercial systems soon followed.接下来引入了商业系统。By 1

41、987, second-generation lightwave systems, operating at bit rates of up to 1.7 Gb/s with a repeater spacing of about 50 km, were commercially available.到了1987年，第二代光波系统，比特率超过1.7Gbps，中继器间距大约50km，被投入商用。The repeater spacing of the second-generation lightwave systems was limited by the fiber losses at the

42、 operating wavelength of 1.3 m (typically 0.5 dB/km).第二代光波系统的中继站间距因为光纤损耗被限制在波长1.3um。Losses of silica fibers become minimum near 1.55 m.石英光纤的波长损耗最小在1.55um附近。Indeed, a 0.2-dB/km loss was realized in 1979 in this spectral region 16.确实，在1979年人们发现了光谱范围内的0.2dB/km的损耗。However, the introduction of third-gene

43、ration lightwave systems operating at 1.55 m was considerably delayed by a large fiber dispersion near 1.55 m.但是，第三代光波系统只能工作在1.55um主要是因为光纤色散导致的。Conventional InGaAsP semiconductor lasers could not be used because of pulse spreading occurring as a result of simultaneous oscillation of several longitud

44、inal modes.传统的磷砷化镓因半导体激光器不能被使用因为纵模的同时震荡导致了脉冲展宽的发生。The dispersion problem can be overcome either by using dispersion-shifted fibers designed to have minimum dispersion near 1.55 m or by limiting the laser spectrum to a single longitudinal mode.色散问题可以被解决，无论是1.55um附近色散最小的用色散位移光纤还是将激光光谱先知道一个单纵模式。Both ap

45、proaches were followed during the 1980s.20世纪80年代各种想法都层出不穷。By 1985, laboratory experiments indicated the possibility of transmitting information at bit rates of up to 4 Gb/s over distances in excess of 100 km 17.到了1985年，室内实验表明了的可能性。Third-generation lightwave systems operating at 2.5 Gb/s became avail

46、able commercially in 1990.第三代光波系统投入商用。Such systems are capable of operating at a bit rate of up to 10 Gb/s 18.The best performance is achieved using dispersion-shifted fibers in combination with lasers oscillating in a single longitudinal mode.最好的办法是使用色散位移光纤和激光器震动在同一个单纵模结构。A drawback of third-genera

47、tion 1.55-um systems is that the signal is regenerated periodically by using electronic repeaters spaced apart typically by 6070 km.第三代1.55um系统的一个缺点是信号在用电子中继器（间距在60-70km左右）时，会出现周期性再生。The repeater spacing can be increased by making use of a homodyne or heterodyne detection scheme because its use impr

48、oves receiver sensitivity.中继器间距可以被提升，用同步检波或者外差检波方案，因为他的使用改善了接收器的灵敏度。Such systems are referred to as coherent lightwave systems.这种系统被称作相干光波系统。Coherent systems were under development worldwide during the 1980s, and their potential benefits were demonstrated in many system experiments 19.相干系统在20世纪80年代被

49、世界所研究，而且他们很多系统实验证明了他的潜在利益。However, commercial introduction of such systems was postponed with the advent of fiber amplifiers in 1989.但是，这种系统的商业引进因为光纤放大器的发明而被延后了。The fourth generation of lightwave systems makes use of optical amplification for increasing the repeater spacing and of wavelength-divisio

50、n multiplexing (WDM) for increasing the bit rate.第四代光波系统使用了光学放大器用来提高中继站间距，还使用了波分复用技术来提高比特率。As evident from different slopes in Fig. 1.3 before and after 1992, the advent of the WDM technique started a revolution that resulted in doubling of the system capacity every 6 months or so and led to lightwa

51、ve systems operating at a bit rate of 10 Tb/s by 2001.图1.3中可以看到不同的数据，在1992年之前和之后的，波分复用技术的发明开始了一场革命，使得系统容量每六个月或者更少就提升一倍，而且使得光波系统工作在10Tbps在2001年。In most WDM systems, fiber losses are compensated periodically using erbium-doped fiber amplifiers spaced 6080 km apart.在大部分波分复用系统中，光纤损耗用间距60-80km的掺铒光纤放大器周期性

52、的补偿。Such amplifiers were developed after 1985 and became available commercially by 1990.这些放大器在1985年之后被发明，并在1990年投入商用。A 1991 experiment showed the possibility of data transmission over 21,000 km at 2.5 Gb/s, and over 14,300 km at 5 Gb/s, using a recirculating-loop configuration 20.一个1991年的实验展现了的可能性，用

53、一个循环回路配置This performance indicated that an amplifier-based, all-optical, submarine transmission system was feasible for intercontinental communication.这种性能表明一个对各大陆之间通信的基于放大器，全光的，海底的传输系统是切实可行的。By 1996, not only transmission over 11,300 km at a bit rate of 5 Gb/s had been demonstrated by using actual

54、submarine cables 21, but commercial transatlantic and transpacific cable systems also becameavailable.到了1996年，不光是传输距离达到11300km，用了一个真正的海底电缆，而且商业化的横渡大西洋和横渡太平洋的电缆系统也投入使用。Since then, a large number of submarine lightwave systems have been deployed worldwide.从那以后，大量的海底光波系统被全世界的使用。Figure 1.5 shows the int

55、ernational network of submarine systems around 2000 22.图1.5体现了海底系统的国际网络。The 27,000-km fiber-optic link around the globe (known as FLAG) became operational in 1998, linking many Asian and European countries 23.连接全世界的长达27000km的光纤被投入使用在1998，连接了许多亚欧国家。Another major lightwave system, known as Africa One

56、was operating by 2000; it circles the African continent and covers a total transmission distance of about 35,000 km 24.另一个主要的光波系统，被称作非洲1号，他包围了非洲大陆，而且长达35000km的传输距离。Several WDM systems were deployed across the Atlantic and Pacific oceans during 19982001 in response to the Internet-induced increase in

57、 the data traffic; they have increased the total capacity by orders of magnitudes.许多波分复用系统被分布在穿越大西洋和穿越太平洋，为了应对互联网引起的数据流量增加。他们也增加了总容量的数量级。A truly global network covering 250,000 km with a capacity of 2.56 Tb/s (64 WDM channels at 10 Gb/s over 4 fiber pairs) is scheduled to be operational in 2002 25.C

58、learly, the fourth-generation systems have revolutionized the whole field of fiber-optic communications.显然，第四代系统已经彻底改变了整个光纤通信领域。The current emphasis of WDM lightwave systems is on increasing the system capacity by transmitting more and more channels through the WDM technique.目前波分复用光波系统的重点是 通过波分复用技术传输更多信道 来提升系统容量。With increasing WDM signal bandwidth, it is often not possib