1、Fiber-Optic Communication SystemsIntroductionA communication system transmits information from one place to another, whether separated by a few kilometers or by transoceanic distances.一個通信系統(tǒng)是用來從一個地方向另一個地方傳遞信息的，無論相隔幾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. 信息經(jīng)常以頻率為幾兆赫茲到幾百T赫茲的電磁波為載波傳遞。Optical communication systems use high carrier frequencies (100 THz) in the visible or near-infrared region of the electromagnetic spectrum. 光通信系統(tǒng)用處于電磁波譜中可見光或者近紅外附近的高頻載波（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). 光通信系統(tǒng)有時也被稱為光波系統(tǒng)，用以區(qū)別于載波頻率遠遠小了5個數(shù)量級的微波系統(tǒng)（小于1GHz）。Fiber-optic communication systems are lightwave systems that employ optical bers for information transmission.光

4、纖通信系統(tǒng)是一種用光纖來傳遞信息的光波系統(tǒng)。Such systems have been deployed worldwide since 1980 and have indeed revolutionized the technology behind telecommunications.這種系統(tǒng)從1980開始被全球使用并且確實已經(jīng)從根本上改變了通信技術。Indeed, the lightwave technology, together with microelectronics, is believed to be a major factor in the advent of the

5、 information age. 確實，光波技術和微電子技術被稱為“信息時代”出現(xiàn)的首要原因。The objective of this book is to describe ber-optic communication systems in a comprehensive manner.這本書的主要目的是為了全面描述光纖通信系統(tǒng)。The emphasis is on the fundamental aspects, but the engineering issues are also discussed. 重點是講述基礎原理方面，但是也會涉及到一些工程方面。The purpose o

6、f this introductory chapter is to present the basic concepts and to provide the background material.這篇緒論的作用是展現(xiàn)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.第一章第一節(jié)講述了光通信系統(tǒng)的發(fā)展的歷史背景。In Section 1.2 we cover concepts such as analog and digital

7、signals, channel multiplexing, and modulation formats.第一章第二節(jié)涉及到了一些概念比如模擬和數(shù)字信號，信道多路復用技術和調制格式。Relative merits of guided and unguided optical communication systems are discussed in Section 1.3. 光導通信系統(tǒng)和非光導通信系統(tǒng)的優(yōu)缺點在1.3章節(jié)介紹。The last section focuses on the building blocks of a ber-optic communication syste

8、m.最后一個章節(jié)主要介紹光纖通信系統(tǒng)的中心模塊。1.1 Historical PerspectiveThe use of light for communication purposes dates back to antiquity if we interpret optical communications in a broad sense 1.如果我們把光通信放在一個廣義的角度來說，早在古時候人們就用光來進行通信。Most civilizations have used mirrors, fire beacons, or smoke signals to convey a single

9、piece of information (such as victory in a war). 很多居民使用鏡子，火把，或者煙信號來傳遞一部分信息。Essentially the same idea was used up to the end of the eighteenth century through signaling lamps, flags, and other semaphore devices. 本質上說，18世紀末，人們利用相同的想法用了信號燈，旗子還有其他設備。The idea was extended further, following a suggestion

10、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.這種想法在克勞德查鋪的建議下被推廣，利用中繼器能夠傳輸很長距離（超過100km）的機械編碼信息，也就是今日人們所說的中繼器。Figure 1.1 shows the basic idea sch

11、ematically. 圖1.1體現(xiàn)了開始想法的產(chǎn)生。The rst such “optical telegraph”was put in service between Paris and Lille (two French cities about 200 km apart) in July 1794.第一個光電報1794年7月被使用在兩個相距200km的法國城市巴黎和里爾。By 1830, the network had expanded throughout Europe 1. 到了1830年，這種網(wǎng)絡被擴展到全歐洲。The role of light in such systems

12、was simply to make the coded signals visible so that they could be intercepted by the relay stations. 光在那種系統(tǒng)中的角色是簡單的使編碼信號可見以至于可以被中繼器攔截。The opto-mechanical communication systems of the nineteenth century were inherently slow.19世紀的光機通信系統(tǒng)傳輸速度非常慢。In modern-day terminology,the effective bit rate of such

13、systems was less than 1 bit per second (B 1 b/s).在現(xiàn)代術語中，這種系統(tǒng)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世紀30年代電報的發(fā)明使電代替了光從而開始了電子通信的時代。The bit rate B could be in

14、creased to 10 b/s by the use of new coding techniques, such as the Morse code. 比特率B通過新的編碼技術可以被提高到10b/s，比如莫爾斯電碼。The use of intermediate relay stations allowed communication over long distances ( 1000 km).中繼站的使用使得通信可以跨越更長的距離。Indeed, the first successful transatlantic telegraph cable went into operatio

15、n in 1866.確實，第一次成功的跨洋電報電纜在1866年開始投入使用。Telegraphy used essentially a digital scheme through two electrical pulses of different durations (dots and dashes of the Morse code).電報本質上使用的是數(shù)字格式的兩個不同長度的電子脈沖。（莫爾斯電碼的點和橫）The invention of the telephone in 1876 brought a major change inasmuch as electric signals

16、were transmitted in analog form through a continuously varying electric current 4.1876年電話的發(fā)明帶來了一個巨大的改變，因為電子信號通過不斷變化的電流以模擬形式傳輸。Analog electrical techniques were to dominate communication systems for a century or so.模擬電子技術開始主導通信系統(tǒng)并持續(xù)了將近一個世紀甚至更多。The development of worldwide telephone networks during th

17、e twentieth century led to many advances in the design of electrical communication systems.20世紀全球電話網(wǎng)絡的發(fā)展使電子通信系統(tǒng)設計得到很大進展。The use of coaxial cables in place of wire pairs increased system capacity considerably.同軸電纜取代了對稱電纜，大大改進了系統(tǒng)的容量。The first coaxial-cable system, put into service in 1940, was a 3-MHz

18、 system capable of transmitting 300 voice channels or a single television channel.第一個同軸電纜系統(tǒng)在1940年被投入使用，它是一個2MHz容量的系統(tǒng)，包含300多個音頻信道和一個視頻信道。The bandwidth of such systems is limited by the frequency-dependent cable losses, which increase rapidly for frequencies beyond 10 MHz.這種系統(tǒng)的帶寬被與頻率有關的電纜功耗所限制，可以快速提升至

19、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.這種限制使（頻率以1-10GHz頻率的以合適的調制技術來傳遞信號的電磁波為載波的）微波通信系統(tǒng)得到了發(fā)展。The first mi

20、crowave system operating at the carrier frequency of 4 GHz was put into service in 1948.第一個微波系統(tǒng)在1948年投入使用，它的載波頻率是4GHz。Since then, both coaxial and microwave systems have evolved considerably and are able to operate at bit rates 100 Mb/s.從那以后，同軸電纜和微波系統(tǒng)逐步改進并且都能夠在100Mbps的比特率上運行。The most advanced coaxia

21、l system was put into service in 1975 and operated at a bit rate of 274 Mb/s.最前沿的同軸電纜系統(tǒng)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.高速同軸系統(tǒng)的一個小缺點就是他們的中繼站間距較小，使得系統(tǒng)運行起來較昂貴

22、。Microwave communication systems generally allow for a larger repeater spacing, but their bit rate is also limited by the carrier frequency of such waves.微波通信系統(tǒng)允許較大的中繼站間距，但是也被那種波的載波頻率限制。A commonly used figure of merit for communication systems is the bit ratedistance product, BL, where B is the bit

23、rate and L is the repeater spacing.一個衡量通信系統(tǒng)優(yōu)點的數(shù)據(jù)是比特率，距離乘積為BL，其中B是比特率，L是中繼站間距。Figure 1.2 shows how the BL product has increased through technological advances during the last century and a half.圖1.2展現(xiàn)了B*L的乘積通過技術提升而提高了在過去的一個半世紀里。Communication systems with BL 100 (Mb/s)-km were available by 1970 and we

24、re 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世紀下半葉，人們發(fā)現(xiàn)使BL乘積提升一個數(shù)量級是可能的，如果光波可以當做載波的話。However

25、, neither a coherent optical source nor a suitable transmission medium was available during the 1950s.然而，在1950s，既沒有一個同步光源，也沒有一個合適的傳輸媒介。The invention of the laser and its demonstration in 1960 solved the first problem 5.激光的發(fā)明和1960年它的第一次試行解決了第一個問題。Attention was then focused on finding ways for using l

26、aser light for optical communications.人們開始聚焦在尋找方法來利用激光作為光通信的媒介。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很多想法都很先進。最值得注意的一個想法是利用一系列的氣體透鏡限制光的傳輸。It was suggested in 1966 that optical fibers might be the bes

27、t 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，有人說，光纖可能是最好的選擇，當他們有能力使光像電子在銅線中傳播那樣。The main problem was the high losses of optical fibersfibers availableduring the 1960s had losses in excess of 1000 dB/km.最主要的問題是光纖有的巨大的能量損耗1

28、960s的時候可使用的光纖的功耗在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取得了一個較大的突破，當波長范圍在1微米附近時，光纖的損耗可以降低到20dB/km以下。At about the same time, GaAs semiconductor lasers, operating continuously at room temperature, were d

29、emonstrated 10.同時，砷化鎵半導體激光在室溫下能夠持續(xù)運行。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.緊湊型光源和低損耗光纖的同時發(fā)放使得光纖通信系統(tǒng)有了一個世界范圍的大發(fā)展。Figure 1.3 shows the increase in the capacity of lightwave sy

30、stems realized after 1980 through several generations of development.圖1.3體現(xiàn)了1980年后光波系統(tǒng)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.可見，研究與開發(fā)緊緊跟隨著光波系統(tǒng)的商業(yè)調度。The progress has indeed been rapid as evident from an increase in

31、the bit rate by a factor of 100,000 over a period of less than 25 years.這個進步確實是飛快的，可以由比特率在25年內(nèi)上升了100000倍看出。Transmission distances have also increased from 10 to 10,000 km over the same time period.傳輸距離在同一時期也從10km提升到10000km。As a result, the bit ratedistance product of modern lightwave systems can exc

32、eed by a factor of 107 compared with the first-generation lightwave systems.最后，如今光波系統(tǒng)的比特率-距離乘積也超出了第一代光波系統(tǒng)的107倍。1.1.2 Evolution of Lightwave SystemsThe research phase of ber-optic communication systems started around 1975.對于光纖通信系統(tǒng)的研究階段從1975年開始。The enormous progress realized over the 25-year period ex

33、tending from 1975 to 2000 can be grouped into several distinct generations.從1975到2000年這25年的時間里，這些巨大的進展可以被分為幾個不同的時代。Figure 1.4 shows the increase in the BL product over this time period as quantied through various laboratory experiments12.圖1.4展現(xiàn)了通過不同的室內(nèi)實驗BL乘積在這些年里的增長情況。The straight line corresponds t

34、o a doubling of the BL product every year. 直線表示了BL乘積每年成倍增長。In every generation, BL increases initially but then begins to saturate as the technology matures.每一代里，BL都是開始增長但是后來當技術成熟時會逐漸飽和。Each new generation brings a fundamental change that helps to improve the system performance further.每新的一代里都會帶來一個能

35、提高系統(tǒng)性能的基礎的改變。The rst generation of lightwave systems operated near 0.8 m and used GaAs semi conductor lasers.第一代光波系統(tǒng)在0.8um的波長工作并且使用了砷化鎵半導體激光器。After several eld trials during the period197779,such systems became available commercially in 1980 13.在1977-1979年中進行了幾次實地測驗之后，這種系統(tǒng)在1980年投入了商業(yè)化使用。They operate

36、d at a bit rate of 45 Mb/s and allowed repeater spacings of up to 10 km.他們工作在45Mbps的比特率，并且允許中繼器間距達到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 maintenance costs associated w

37、ith each repeater.和同軸系統(tǒng)的1km的中繼站距離相比，大型中繼站的距離對于系統(tǒng)設計者來說是一個很大的激勵，因為它減少了每個中繼站的安裝和維修費用。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世紀70年代內(nèi)，可以很清楚的看到，如果使光波

38、系統(tǒng)工作在波長范圍在1.3um附近的話，中繼站距離可以被很大的提升。而且光纖損耗也會低于1dB/km。Furthermore, optical fibers exhibit minimum dispersion in this wavelength region.此外，在這個波長區(qū)間內(nèi)，光纖的散射達到最小。This realization led to a worldwide effort for the development of InGaAsP semiconductor lasers and detectors operating near 1.3 m.這個想法使得全世界的人都在為使砷

39、化鎵半導體激光器和探測器能夠工作在1.3um附近而努力著。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.第二代光纖通信系統(tǒng)在20世紀八十年代早期投入使用，但是早期系統(tǒng)的比特率被限制在100Mbps以下，多模光纖的散射。This limi

40、tation was overcome by the use of single-mode fibers.單模光纖的使用克服了這個限制。A laboratory experiment in 1981 demonstrated transmission at 2 Gb/s over 44 km of single-mode fiber 15.一個1981年的室內(nèi)實驗演示了能夠工作在 2Gbps的傳輸超過44km的單模光纖。The introduction of commercial systems soon followed.接下來引入了商業(yè)系統(tǒng)。By 1987, second-generati

41、on 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，第二代光波系統(tǒng)，比特率超過1.7Gbps，中繼器間距大約50km，被投入商用。The repeater spacing of the second-generation lightwave systems was limited by the fiber losses at the operating wavelengt

42、h of 1.3 m (typically 0.5 dB/km).第二代光波系統(tǒng)的中繼站間距因為光纖損耗被限制在波長1.3um。Losses of silica fibers become minimum near 1.55 m.石英光纖的波長損耗最小在1.55um附近。Indeed, a 0.2-dB/km loss was realized in 1979 in this spectral region 16.確實，在1979年人們發(fā)現(xiàn)了光譜范圍內(nèi)的0.2dB/km的損耗。However, the introduction of third-generation lightwave sys

43、tems operating at 1.55 m was considerably delayed by a large fiber dispersion near 1.55 m.但是，第三代光波系統(tǒng)只能工作在1.55um主要是因為光纖色散導致的。Conventional InGaAsP semiconductor lasers could not be used because of pulse spreading occurring as a result of simultaneous oscillation of several longitudinal modes.傳統(tǒng)的磷砷化鎵因半

44、導體激光器不能被使用因為縱模的同時震蕩導致了脈沖展寬的發(fā)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.色散問題可以被解決，無論是1.55um附近色散最小的用色散位移光纖還是將激光光譜先知道一個單縱模式。Both approaches were follow

45、ed during the 1980s.20世紀80年代各種想法都層出不窮。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，室內(nèi)實驗表明了的可能性。Third-generation lightwave systems operating at 2.5 Gb/s became available commercially in

46、 1990.第三代光波系統(tǒng)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.最好的辦法是使用色散位移光纖和激光器震動在同一個單縱模結構。A drawback of third-generation 1.55-um systems

47、 is that the signal is regenerated periodically by using electronic repeaters spaced apart typically by 6070 km.第三代1.55um系統(tǒng)的一個缺點是信號在用電子中繼器（間距在60-70km左右）時，會出現(xiàn)周期性再生。The repeater spacing can be increased by making use of a homodyne or heterodyne detection scheme because its use improves receiver sensit

48、ivity.中繼器間距可以被提升，用同步檢波或者外差檢波方案，因為他的使用改善了接收器的靈敏度。Such systems are referred to as coherent lightwave systems.這種系統(tǒng)被稱作相干光波系統(tǒng)。Coherent systems were under development worldwide during the 1980s, and their potential benefits were demonstrated in many system experiments 19.相干系統(tǒng)在20世紀80年代被世界所研究，而且他們很多系統(tǒng)實驗證明了他

49、的潛在利益。However, commercial introduction of such systems was postponed with the advent of fiber amplifiers in 1989.但是，這種系統(tǒng)的商業(yè)引進因為光纖放大器的發(fā)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-division multiplexing (WDM)

50、 for increasing the bit rate.第四代光波系統(tǒng)使用了光學放大器用來提高中繼站間距，還使用了波分復用技術來提高比特率。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 lightwave systems operating

51、 at a bit rate of 10 Tb/s by 2001.圖1.3中可以看到不同的數(shù)據(jù)，在1992年之前和之后的，波分復用技術的發(fā)明開始了一場革命，使得系統(tǒng)容量每六個月或者更少就提升一倍，而且使得光波系統(tǒng)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.在大部分波分復用系統(tǒng)中，光纖損耗用間距60-80km的摻鉺光纖放大器周期性的補償。Such amplifiers

52、were developed after 1985 and became available commercially by 1990.這些放大器在1985年之后被發(fā)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 20.一個1991年的實驗展現(xiàn)了的可能性，用一個循環(huán)回路配置This perform

53、ance indicated that an amplifier-based, all-optical, submarine transmission system was feasible for intercontinental communication.這種性能表明一個對各大陸之間通信的基于放大器，全光的，海底的傳輸系統(tǒng)是切實可行的。By 1996, not only transmission over 11,300 km at a bit rate of 5 Gb/s had been demonstrated by using actual submarine cables 21,

54、 but commercial transatlantic and transpacific cable systems also becameavailable.到了1996年，不光是傳輸距離達到11300km，用了一個真正的海底電纜，而且商業(yè)化的橫渡大西洋和橫渡太平洋的電纜系統(tǒng)也投入使用。Since then, a large number of submarine lightwave systems have been deployed worldwide.從那以后，大量的海底光波系統(tǒng)被全世界的使用。Figure 1.5 shows the international network o

55、f submarine systems around 2000 22.圖1.5體現(xiàn)了海底系統(tǒng)的國際網(wǎng)絡。The 27,000-km fiber-optic link around the globe (known as FLAG) became operational in 1998, linking many Asian and European countries 23.連接全世界的長達27000km的光纖被投入使用在1998，連接了許多亞歐國家。Another major lightwave system, known as Africa One was operating by 200

56、0; it circles the African continent and covers a total transmission distance of about 35,000 km 24.另一個主要的光波系統(tǒng)，被稱作非洲1號，他包圍了非洲大陸，而且長達35000km的傳輸距離。Several WDM systems were deployed across the Atlantic and Pacific oceans during 19982001 in response to the Internet-induced increase in the data traffic; t

57、hey have increased the total capacity by orders of magnitudes.許多波分復用系統(tǒng)被分布在穿越大西洋和穿越太平洋，為了應對互聯(lián)網(wǎng)引起的數(shù)據(jù)流量增加。他們也增加了總容量的數(shù)量級。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.Clearly, the fourth-g

58、eneration systems have revolutionized the whole field of fiber-optic communications.顯然，第四代系統(tǒng)已經(jīng)徹底改變了整個光纖通信領域。The current emphasis of WDM lightwave systems is on increasing the system capacity by transmitting more and more channels through the WDM technique.目前波分復用光波系統(tǒng)的重點是 通過波分復用技術傳輸更多信道 來提升系統(tǒng)容量。With increasing WDM signal bandwidth, it is often not possible