Science English 期末复习(1)

A 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 frequency 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 lightwave systems to distinguish them from microwave systems, whose carrier frequency is typically smaller by five 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 fibers for information transmission.

光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 '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 fiber-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 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 and 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 fiber-optic communication 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 Perspective

The 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 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 suggestion 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 basic idea schematically. 图1.1体现了开始想法的产生。

The first 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 such 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 rate 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 Communications

The 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 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 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 electric 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 networks 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 1940, 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缆功耗所，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 1–10 GHz is used to transmit the signal by using suitable modulation techniques. 这种使（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 advanced 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。 A commonly used figure of merit for communication systems is the bit

rate–distance product, BL, where 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 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 ways 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光的传输。 It was suggested in 1966 that optical fibers might 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 fibers—fibers available during the 1960s had losses in excess of 1000 dB/km.

最主要的问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 μm [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 temperature, 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 of 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 an 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 rate–distance product of modern lightwave systems 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 Systems

The research phase of fiber-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-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 quantified through various laboratory experiments[12].

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 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.

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 first 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 field trials during the period1977–79,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 maintenance 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 μm, where fiber loss is below 1 dB/km.

在20世纪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 1.3 μm.

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在

100Mbps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。

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 1987, 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 operating wavelength of 1.3 μm (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在波长1.3um。 Losses of silica fibers become minimum near 1.55 μm. 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-generation lightwave systems operating at 1.55 μm was considerably delayed by a large fiber dispersion near 1.55 μm. 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 longitudinal 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 μm 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 approaches 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 available 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-generation 1.55-um systems is that the signal is regenerated periodically by using electronic repeaters spaced apart typically by 60–70 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 improves 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 19.

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-division multiplexing (WDM)

for increasing the bit rate.

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 lightwave 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，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 60–80 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，用一个循环回路配臵

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 submarine cables [21], but commercial transatlantic and transpacific cable systems also became available.

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 international 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 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，而且长达35000km的传输距离。

Several WDM systems were deployed across the Atlantic and Pacific oceans during 1998–2001 in response to the Internet-induced increase in 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 ( WDM channels at 10 Gb/s over 4 fiber pairs) is scheduled to be operational in 2002 [25]. Clearly, the fourth-generation systems have revolutionized the whole field of fiber-optic communications.

显然，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 possible to amplify all channels using a single amplifier.

随着WDM信号的带宽的提升，它通常不可能使用一个单一的放大器将所有通道放大。 As a result, new kinds of amplification schemes are being explored for covering the spectral region extending from 1.45 to 1.62 μm.

最后，许多新的放大计划被探索，使得光谱区域从1.45提升到1.62um。

This approach led in 2000 to a 3.28-Tb/s experiment in which 82 channels, each operating at 40 Gb/s, were transmitted over 3000 km, resulting in a BL product of almost 10,000 (Tb/s)-km.

这种方法使得在2000年，一个在83个信道中的3.25tbps的实验，每个

Within a year, the system capacity could be increased to nearly 11 Tb/s(273 WDM channels, each operating at 40 Gb/s) but the transmission distance was limited to 117 km [26].

在一年内，系统容量提升到11Tbps。

In another record experiment, 300 channels, each operating

at 11.6 Gb/s, were transmitted over 7380 km, resulting in a BL product of more than 25,000 (Tb/s)-km [27].

Commercial terrestrial systems with the capacity of 1.6 Tb/s were available by the end of 2000, and the plans were underway to extend the capacity toward 6.4 Tb/s.

商用的地面系统 提升容量的计划在处理之中。

Given that the first-generation systems had a capacity of 45 Mb/s in 1980, it is remarkable that the capacity has jumped by a factor of more than 10,000 over a period of 20 years.

考虑到，20年内容量提升了10^4倍是有纪念意义的。

The fifth generation of fiber-optic communication systems is concerned with extending the wavelength range over which a WDM system can operate simultaneously.

第五代光波系统考虑到扩大波长范围，波分复用系统可以同时运行。

The conventional wavelength window, known as the C band, covers the wavelength range 1.53–1.57μm.

传统的波长范围，被叫做C波段，覆盖的波长范围在1.53-1.57um。

It is being extended on both the long- and short-wavelength sides, resulting in the L and S bands, respectively. 这也被扩展到长波和短波，分别对应于

The Raman amplification technique can be used for signals in all three wavelength bands. Moreover, a new kind of fiber, known as the dry fiber has been developed with the property that fiber losses are small over the entire wavelength region extending from 1.30 to 1.65 μm [28].

拉曼放大技术可以用在这三个波长范围内的信号上。另外，一种新型的光纤，被叫做，被发明出来，它的性能是光纤损耗低于1.30-1.65波长范围内的所有光纤。

Availability of such fibers and new amplification schemes may lead to lightwave systems with thousands of WDM channels.

这种光纤的使用和新放大方案的实施可以使得光波系统有上千个波分复用信道。

The fifth-generation systems also attempt to increase the bit rate of each channel within the WDM signal.

第五代系统也尝试着提高每个信道波分复用信号的比特率。

Starting in 2000, many experiments used channels operating at 40 Gb/s; migration toward 160 Gb/s is also likely in the future.

从2000年开始，许多实验；以后改为160Gbps也是可能的。

Such systems require an extremely careful management of fiber dispersion. 这种系统需要一个很谨慎的管理光纤色散。

An interesting approach is based on the concept of optical solitons—pulses that preserve their shape during propagation in a lossless fiber by counteracting the effect of dispersion through the fiber nonlinearity.

一个有趣的方法是基于光孤子脉冲的概念，通过非线性光纤来抵消色散效应，从而使其在无损光纤中传播时保持波形。

Although the basic idea was proposed [29] as early as 1973, it was only in 1988 that a laboratory experiment demonstrated the feasibility of data transmission over 4000km by compensating the fiber loss through Raman amplification [30]. 尽管这种想法的基础在1973年就被提出，但是在1988年才有一个实验室实验证明了使用拉曼放大器补偿光纤损耗 使得数据传输超过4kkm的可行性。

Erbium-doped fiber amplifiers were used for soliton amplification starting in 19. 掺饵光纤从19年开始用于孤子放大器。

Since then, many system experiments have demonstrated the eventual potential

of soliton communication systems.

自那以后，许多系统实验都证明了孤子通信系统的最终潜力。

By 1994, solitons were transmitted over 35,000 km at 10 Gb/s and over 24,000 km at 15 Gb/s [31]. 到1994年，孤子被

Starting in 1996, the WDM technique was also used for solitons in combination with dispersion management.

从1996年开始，波分复用技术也被用于孤子与色散管理的结合。

In a 2000 experiment, up to 27 WDM channels, each operating at 20 Gb/s, were transmitted over 9000 km using a hybrid amplification scheme [32]. 在一个2000年的实验中，用了一个混合放大器方案。

Even though the fiber-optic communication technology is barely 25 years old, it has progressed rapidly and has reached a certain stage of maturity. 虽然光纤通信技术还不到25年，但它的发展很快，已经达到了一定的成熟阶段。 This is also apparent from the publication of a large number of books on optical communications and WDM networks since 1995 [33]–[55]. 在一些有关光通信系统和波分复用网络的书上也可以很明显的看到。

This third edition of a book, first published in 1992, is intended to present an up-to-date account of fiber-optic communications systems with emphasis on recent developments.

一本书的第三版，首次出版于1992，旨在提出光纤通信系统，重点写其最近的发展。 1.2 Basic Concepts

This section introduces a few basic concepts common to all communication systems.

这一节介绍了所有通信系统的一些基础概念。

We begin with a description of analog and digital signals and describe how an analog signal can be converted into digital form.

我们从模拟和数字信号的描述和如何将模拟信号转化成数字信号形式开始。

We then consider time- and frequency division multiplexing of input signals, and conclude with a discussion of various modulation formats.

然后我们介绍输入信号的时分和频分复用，然后以一个不同调制格式的讨论结尾。 1.2.1 Analog and Digital Signals

In any communication system, information to be transmitted is generally available as an electrical signal that may take analog or digital form [56]. 在任何通信系统里，信息都被用数字或者模拟信号的电子信号传输。

In the analog case, the signal (e. g., electric current) varies continuously with time, as shown schematically in Fig. 1.6(a).

在模拟形式下，信号（电流）随时间变化，像图1.6（a）所示。

Familiar examples include audio and video signals resulting when a microphone converts voice or a video camera converts an image into an electrical signal. 类似的例子比如音频信号和视频信号，由麦克风将声音或者由录像机将图像转化成为电子信号。

By contrast, the digital signal takes only a few discrete values. 相对来说，数字信号只有一些离散值。

In the binary representation of a digital signal only two values are possible. 一个数字信号的二进制表达法只有可能是2个值。

The simplest case of a binary digital signal is one in which the electric current is either on or off, as shown in Fig.1.6(b).

最简单的情况就是一个二进制数字信号是一个电流电平或者高或者低如图1.6所示。 These two possibilities are called ‚bit 1‛ and ‚bit 0‛ (bit is a contracted form of binary digit).

这两种可能性被叫做比特1和比特0.bit是二进制数字的简单形式。

Each bit lasts for a certain period of time TB, known as the bit period or bit slot. 每个比特持续一个确定的时间TB

Since one bit of information is conveyed in a time interval TB, the bit rate B, defined as the number of bits per second, is simply B=T −1B . 因为一比特的信息需要传输TB个时间间隔，比特率是B，被叫做每秒的比特数。被简写成 A well-known example of digital signals is provided by computer data. 一个有名的例子是数字信号被计算机数据所提供。

Each letter of the alphabet together with other common symbols (decimal

numerals, punctuation marks, etc.) is assigned a code number (ASCII code) in the range 0–127 whose binary representation corresponds to a 7-bit digital signal. 字母表的每个字母和其他普通符号（十进制数字，标点符号等等）一样被分配一个编码（ASCII编码）在0-127的范围内，也就是二进制对应于7比特的数字信号。

The original ASCII code has been extended to represent 256 characters transmitted through 8-bit bytes.

最初的ASCII编码被扩展到256个字符，可以传送8比特（1byte）的信号。

Both analog and digital signals are characterized by their bandwidth, which is a measure of the spectral contents of the signal.

模拟和数字信号都由他们的带宽决定，带宽是衡量一个信号的光谱含量。

The signal bandwidth represents the range of frequencies contained within the signal and is determined mathematically through its Fourier transform. 信号带宽代表信号内的频率范围，并通过其傅立叶变换来确定其数学范围。 An analog signal can be converted into digital form by sampling it at regular intervals of time [56].

一个模拟信号可以由在相同时间间隔里抽样转化成数字形式。 Figure 1.7 shows the conversion method schematically. 图1.7体现了转换的思想。

The sampling rate is determined by the bandwidth Δf of the analog signal. 抽样频率由模拟信号的带宽决定。

According to the sampling theorem [57]–[59], a bandwidth-limited signal can be fully represented by discrete samples, without any loss of information, provided that the sampling frequency fs satisfies the Nyquist criterion [60], fs ≥ 2Δf . 根据采样定理–[ 57 ] [ 59 ]，一个有限带宽信号可以完全由离散的样本表示出来，而不会丢失任何信息，只要采样频率fs满足Nyquist准则[ 60 ]，FS≥2ΔF

The first step consists of sampling the analog signal at the right frequency. 第一步就是在正确的频率采样。

The sampled values can take any value in the range 0 ≤ A ≤ Amax, where

Amax is the maximum amplitude of the given analog signal.

Let us assume that Amax is divided into M discrete (not necessarily equally spaced)

intervals.

采样值可以在0到Amax的范围内，Amax是该模拟信号的幅度最大值。

Each sampled value is quantized to correspond to one of these discrete values. 每个采样值都量化为对应于这些离散值中的一个。

Clearly, this procedure leads to additional noise, known as quantization noise, which adds to the noise already present in the analog signal.

很明显，这些过程会导致额外的噪声，被叫做量化噪声，它会将噪声加在已经显示的模拟信号中。

The effect of quantization noise can be minimized by choosing the number of discrete levels such thatM >Amax/AN, where AN is the root-mean-square noise amplitude of the analog signal.

量化噪声的影响可以通过选择离散水平：M > AMAX /AN，其中AN是模拟信号的噪声振幅的平方 来减小。

The ratio Amax/AN is called the dynamic range and is related to the

signal-to-noise ratio (SNR) by the relation SNR = 20log10(Amax/AN), (1.2.1) where

SNR is expressed in decibel (dB) units.

这个比率被叫做动态范围，它由信噪比决定。信噪比=20log10（），信噪比的单位是分贝。 Any ratio R can be converted into decibels by using the general definition 10log10

R (see Appendix A).

所有比率都可以转化成分贝，用一个公式

Equation (1.2.1) contains a factor of 20 in place of 10 simply because the SNR for electrical signals is defined with respect to the electrical power, whereas A is related to the electric current (or voltage).

方程（1.2.1）包含10的地方因为电气信号的信噪比是相对于电功率定义20的一个因素，而A是电流（或电压）相关。

The quantized sampled values can be converted into digital format by using a suitable conversion technique.

利用合适的转化技术可以将量化采样值转化成数字形式。

In one scheme, known as pulse-position modulation, pulse position within the bit slot is a measure of the sampled value.

在一个被叫做脉冲位臵调制方案中，一个位槽中的脉冲位臵是一个测量的采样值。 In another, known as pulse-duration modulation, the pulse width is varied from bit to bit in accordance with the sampled value.

在另一个方案中，被叫做脉宽调制，由于采样值，脉宽在每个比特中都是不同的。 These techniques are rarely used in practical optical communication systems, since it is difficult to maintain the pulse position or pulse width to high accuracy during propagation inside the fiber.

这些技术很少被用在现实的光通信系统中，因为对于脉位或脉宽来说，在光纤中传播保持一个高的准确性是很难的。

The technique used almost universally, known as pulse-code modulation (PCM), is based on a binary scheme in which information is conveyed by the absence or the presence of pulses that are otherwise identical.

一种被广泛运用的技术，被叫做脉冲编码调制，是基于一种 通过脉冲的有无来传播信息 的二进制方案。

A binary code is used to convert each sampled value into a string of 1 and 0 bits. 二进制编码被用于转化采样值为一系列1或者0比特。

The number of bits m needed to code each sample is related to the number of quantized signal levels M by the relation M = 2m or m = log2M. (1.2.2) 比特数M需要代码的每个样本的量化信号水平的关系M＝2m m或m = log2M数相关（1.2.2）。

The bit rate associated with the PCM digital signal is thus given by B = mfs ≥ (2Δf )log2M, (1.2.3) where the Nyquist criterion, fs ≥ 2Δf , was used. By noting that

M > Amax/AN and using Eq. (1.2.1) together with log2 10 ≈ 3.33, B > (Δf /3)SNR, (1.2.4) where the SNR is expressed in decibel (dB) units.

Equation (1.2.4) provides the minimum bit rate required for digital representation of an analog signal of bandwidth Δf and a specific SNR.

等式1.2.4证明了最小比特率需要一个带宽为德尔塔F和一个特定的信噪比的模拟信号的数字形式。

When SNR > 30 dB, the required bit rate exceeds 10(Δf ), indicating a considerable increase in the bandwidth requirements of digital signals.

当信噪比大于30dB， 他需要比特率超过10德尔塔Ｆ，说明需要数字信号提升一个很大的带宽。

Despite this increase, the digital format is almost always used for optical communication systems.

除去这个提升，数字形式也经常被用到光通信系统。

This choice is made because of the superior performance of digital transmission systems.

之所以被用到是因为数字形式在数字传输系统中有着卓越的性能。

Lightwave systems offer such an enormous increase in the system capacity (by a

factor ∼ 10 5) compared with microwave systems that some bandwidth can be traded for improved performance.

相对于微波系统，光波系统使得系统熔炼更有了很大的提升。有些带宽可以被交易用来提升性能。

As an illustration of Eq. (1.2.4), consider the digital conversion of an audio signal generated in a telephone.

The analog audio signal contains frequencies in the range 0.3–3.4 kHz with a bandwidth Δf = 3.1 kHz and has a SNR of about 30 dB. Equation (1.2.4) indicates that B > 31 kb/s.

In practice, a digital audio channel operates at kb/s.

The analog signal is sampled at intervals of 125 μs (sampling rate f s =8 kHz), and each sample is represented by 8 bits.

The required bit rate for a digital video signal is higher by more than a factor of 1000.

The analog television signal has a bandwidth∼4 MHz with a SNR of about 50 dB. The minimum bit rate from Eq. (1.2.4) is 66 Mb/s.

In practice, a digital video signal requires a bit rate of 100 Mb/s or more unless it is compressed by using a standard format (such as MPEG-2).

在实践中，一个数字视频信号需要100Mbps的比特率，甚至于被压缩到一个标准的形式。 1.2.2 Channel Multiplexing

As seen in the preceding discussion, a digital voice channel operates at kb/s. 我们在之前的讨论中，一个数字音频信道工作在。。。

Most fiber-optic communication systems are capable of transmitting at a rate of more than 1 Gb/s.

To utilize the system capacity fully, it is necessary to transmit many channels simultaneously through multiplexing.

为了充分使用系统容量，利用信道复用来传递信息是必要的。

This can be accomplished through time-division multiplexing (TDM) or

frequency-division multiplexing (FDM).

这个可以由 时分复用和频分复用 来实现。

In the case of TDM, bits associated with different channels are interleaved in the time domain to form a composite bit stream.

在时分复用中，将提供给不同信道传输信息的时间划分成若干时间片，并将这些时隙分配给形成一个复合比特流。

For example, the bit slot is about 15 μs for a single voice channel operating at kb/s. 比如，

Five such channels can be multiplexed through TDM if the bit streams of successive channels are delayed by 3 μs.

5条这样的信道可以通过时分复用，如果比特流在连续的信道中被延误超过3us。 Figure 1.8(a) shows the resulting bit stream schematically at a composite bit rate of 320 kb/s.

图1.8（a）展现了比特流最终达到了一个复合比特率

In the case of FDM, the channels are spaced apart in the frequency domain. 信道被分割成不同的频率空间。

Each channel is carried by its own carrier wave. 每个信道有着自己的载波。

The carrier frequencies are spaced more than the channel bandwidth so that the channel spectra do not overlap, as seen Fig. 1.8(b). 载波频率比信道带宽大，由此信道光谱不会过载。

FDM is suitable for both analog and digital signals and is used in broadcasting of radio and television channels.

频分复用对模拟和数字信号都很使用。而且被用于录音机和电视机信道的广播节目。 TDM is readily implemented for digital signals and is commonly used for telecommunication networks.

时分复用对于数字信号更容易实施，被广泛用于电信网络。

It is important to realize that TDM and FDM can be implemented in both the electrical and optical domains; optical FDM is often referred to as WDM.

对人们来说，时分复用和频分复用可以被用在电子和光域中，光的时分复用也被叫做WDM。

Chapter 8 is devoted to optical-domain multiplexing techniques. 第八章着重于讲光域复用技术。

This section covers electrical TDM, which is employed universally to multiplex a large number of voice channels into a single electrical bit stream.

这节涉及到电子时分复用，被广泛用于大量的声音信道复用，到一个小的电子比特流。 The concept of TDM has been used to form digital hierarchies. TDM的概念用于数字分级体系。

In North America and Japan, the first level corresponds to multiplexing of 24 voice channels with a composite bit rate of 1.4 Mb/s (hierarchy DS-1), whereas in Europe 30 voice channels are multiplexed, resulting in a composite bit rate of 2.048 Mb/s. 在北美和日本

The bit rate of the multiplexed signal is slightly larger than the simple product of kb/s with the number of channels because of extra control bits that are added for separating (demultiplexing) the channels at the receiver end.

The second-level hierarchy is obtained by multiplexing 4 DS-1 TDM channels. This results in a bit rate of 6.312 Mb/s (hierarchy DS-2) for North America or Japan and 8.448 Mb/s for Europe.

This procedure is continued to obtain higher-level hierarchies.

For example, at the fifth level of hierarchy, the bit rate becomes 565 Mb/s for Europe and 396 Mb/s for Japan.

The lack of an international standard in the telecommunication industry during the 1980s led to the advent of a new standard, first called the synchronous optical network (SONET) and later termed the synchronous digital hierarchy or SDH

[61]–[63]. 同步光纤网

It defines a synchronous frame structure for transmitting TDM digital signals. The basic building block of the SONET has a bit rate of 51.84 Mb/s.

The corresponding optical signal is referred to as OC-1, where OC stands for optical carrier.光学载波。

SONET的基本构建是比特率相应地光信号被叫做。

The basic building block of the SDH has a bit rate of 155.52 Mb/s and is referred to as STM-1, where STM stands for a synchronous transport module.

A useful feature of the SONET and SDH is that higher levels have a bit rate that is an exact multiple of the basic bit rate.

Table 1.1 lists the correspondence between SONET and SDH bit rates for several levels.

The SDH provides an international standard that appears to be well adopted. Indeed, lightwave systems operating at the STM- level (B ≈ 10 Gb/s) are available since1996 [18].

Commercial STM-256 (OC-768) systems operating near 40 Gb/s became available by 2002.

1.2.3 Modulation Formats

The first step in the design of an optical communication system is to decide how the electrical signal would be converted into an optical bit stream. Normally, the output of an optical source such as a semiconductor laser is modulated by applying the electrical signal either directly to the optical source or to an external modulator.

There are two choices for the modulation format of the resulting optical bit stream.

These are shown in Fig. 1.9 and are known as the return-to-zero (RZ) and

nonreturn-to-zero (NRZ) formats.

In the RZ format, each optical pulse representing bit 1 is shorter than the bit slot, and its amplitude returns to zero before the bit duration is over.

In the NRZ format, the optical pulse remains on throughout the bit slot and its amplitude does not drop to zero between two or more successive 1 bits.

As a result, pulse width varies depending on the bit pattern, whereas it remains the same in the case of RZ format.位模式

An advantage of the NRZ format is that the bandwidth associated with the bit

stream is smaller than that of the RZ format by about a factor of 2 simply because on–off transitions occur fewer times.

However, its use requires tighter control of the pulse width and may lead to bit-pattern-dependent effects if the optical pulse spreads during transmission.位模式相关效应

The NRZ format is often used in practice because of a smaller signal bandwidth associated with it.

The use of the RZ format in the optical domain began to attract attention around 1999 after it was found that its use may help the design of high-capacity lightwave systems[]–[66].

An example of the RZ format is provided by the dispersion-managed soliton systems where a chirped pulse propagates inside the fiber link in a periodic fashion, and the average dispersion is used to counteract the buildup of the nonlinear effects [67].

In an interesting variant of the RZ format, known as the chirped RZ(or CRZ) format, optical pulses in each bit slot are chirped before they are launched into the fiber link but the system is operated in a quasi-linear regime [68].

In other schemes, modulation formats well known in the field of microwave communications are applied to the optical domain.

Such formats are known as carrier-suppressed RZ(CSRZ), 载波抑制归零码single-sideband, or vestigial-sideband formats [59].残留边带

Such RZ formats benefit from a reduced bandwidth compared to the standard RZ format.

不归零模式相对于标准归零模式，受益于带宽的减少。

An important issue is related to the choice of the physical variable that is modulated to encode the data on the optical carrier. 一个重要的问题是有关 在光载波上解调来解码的物理变量的选择。

The optical carrier wave before modulation is of the form E(t) = ˆeAcos(ω0t +φ ), (1.2.5) where E is the electric field vector, ˆe is the polarization unit vector, A is the amplitude, ω0 is the carrier frequency, and φ is the phase.

The spatial dependence of E is suppressed for simplicity of notation.

One may choose to modulate the amplitude A, the frequency ω0, ω0, or the phase φ .

In the case of analog modulation, the three modulation choices are known as amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).

在模拟调制中，三种调制方式有幅度调制，频率调制和相位调制。

The same modulation techniques can be applied in the digital case and are called amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK), depending on whether the amplitude, frequency, or phase of the carrier wave is shifted between the two levels of a binary digital signal.

相同的调制技术在数字调制中叫振幅移位键控，频率……和相位……。决定于两个二进制数字信号的两级中载波的振幅，频率和相位哪个是在变化。

The simplest technique consists of simply changing the signal power between two levels, one of which is set to zero, and is often called on–off keying (OOK) to reflect the on–off nature of the resulting optical signal.

最简单的方法是通过简单地改变两级之间的信号功率，其中一个被设臵为零，并且经常被叫做开关键控（OOK）反映光学信号的亮-灭的性质。

Most digital lightwave systems employ OOK in combination with PCM. 1.3 Optical Communication Systems

As mentioned earlier, optical communication systems differ in principle from microwave systems only in the frequency range of the carrier wave used to carry the information.

像之前说的那样，光通信系统在规则上有不同：微波系统只能在一定载波频率范围内用来传递信息。

The optical carrier frequencies are typically ∼200 THz, in contrast with the microwave carrier frequencies (∼1 GHz).

An increase in the information capacity of optical communication systems by a factor of up to 10,000 is expected simply because of such high carrier frequencies

used for lightwave systems.

光通信系统的信息容量的以10000的因数的提升被期待，因为这种高载波频率在光波系统中被使用。

This increase can be understood by noting that the bandwidth of the modulated carrier can be up to a few percent of the carrier frequency.

这种提升可以被理解成为调制载波的带宽可以到达载波频率的部分百分比。

Taking, for illustration, 1% as the limiting value, optical communication systems have the potential of carrying information at bit rates ∼1 Tb/s. 就以1%来说明吧，光通信系统能够传递信息的最大速率是1Tbps。

It is this enormous potential bandwidth of optical communication systems that is the driving force behind the worldwide development and deployment of lightwave systems.

光通信系统的这种潜在的巨大的带宽是使得世界光波系统的发展和实施的主要驱动力。 Current state-of-the-art systems operate at bit rates ∼10 Gb/s, indicating that there is considerable room for improvement.

Figure 1.10 shows a generic block diagram of an optical communication system.框图

现行的国家的最先进的系统，暗示着有很大的提升空间。

It consists of a transmitter, a communication channel, and a receiver, the three elements common to all communication systems.

它包含了发射器，通信信道，接收器，这是所有通信系统的三种基本元素。 Optical communication systems can be classified into two broad categories:

guided and unguided.

As the name implies, in the case of guided lightwave systems, the optical beam emitted by the transmitter remains spatially confined. 空间

This is realized in practice by using optical fibers, as discussed in Chapter 2.这被人们所察觉在现实中，

Since all guided optical communication systems currently use optical fibers, the

commonly used term for them is fiber-optic communication systems.

The term lightwave system is also sometimes used for fiber-optic communication systems, although it should generally include both guided and unguided systems. In the case of unguided optical communication systems, the optical beam emitted by the transmitter spreads in space, similar to the spreading of microwaves.

However, unguided optical systems are less suitable for broadcasting applications than microwave systems because optical beams spread mainly in the forward direction (as a result of their short wavelength).

Their use generally requires accurate pointing between the transmitter and the receiver.精确定向

In the case of terrestrial propagation, the signal in un-guided systems can deteriorate considerably by scattering within the atmosphere. 陆地传播

This problem, of course, disappears in free-space communications above the earth atmosphere(e.g. intersatellite communications).

Although free-space optical communications systems are needed for certain applications and have been studied extensively [69], most terrestrial applications make use of fiber-optic communication systems. This book does not consider unguided optical communication systems.

The application of optical fiber communications is in general possible in any area that requires transfer of information from one place to another.

However, fiber-optic communication systems have been developed mostly for telecommunications applications.

This is understandable in view of the existing worldwide telephone networks which are used to transmit not only voice signals but also computer data and fax messages.

The telecommunication applications can be broadly classified into two

categories, long-haul and short-haul, depending on whether the optical signal is transmitted over relatively long or short distances compared with typical intercity

distances(∼100 km).长途和短途

Long-haul telecommunication systems require high-capacity trunk lines and benefit most by the use of fiber-optic lightwave systems.干线

Indeed, the technology behind optical fiber communication is often driven by long-haul applications.

Each successive generation of lightwave systems is capable of operating at higher bit rates and over longer distances.

Periodic regeneration of the optical signal by using repeaters is still required for most long-haul systems.

However, more than an order-of-magnitude increase in both the repeater spacing and the bit rate compared with those of coaxial systems has made the use of lightwave systems very attractive for long-haul applications.

Furthermore, transmission distances of thousands of kilometers can be realized by using optical amplifiers.

As shown in Fig. 1.5, a large number of transoceanic lightwave systems have already been installed to create an international fiber-optic network.

Short-haul telecommunication applications cover intracity and local-loop traffic.市内的，本地环路流量

Such systems typically operate at low bit rates over distances of less than 10 km. The use of single-channel lightwave systems for such applications is not very cost-effective, and multichannel networks with multiple services should be considered.

The concept of a broadband integrated-services digital network requires a high-capacity communication system capable of carrying multiple services.宽带综合业务数字网的概念需要一个高容量通信系统能够携带多个服务。 The asynchronous transfer mode(ATM) technology also demands high bandwidths.异步传输模式

Only fiber-optic communication systems are likely to meet such wideband distribution requirements.

Multichannel lightwave systems and their applications in local-area networks are discussed in Chapter 8.

1.4 Lightwave System Components

The generic block diagram of Fig. 1.10 applies to a fiber-optic communication system, the only difference being that the communication channel is an optical fiber cable.

图1.10的框图适用于光纤通信系统，唯一的区别就是通信信道是光纤电缆。

The other two components, the optical transmitter and the optical receiver, are designed to meet the needs of such a specific communication channel. 其他两个组成部分，光发射器和光接收器根据各种不同的通信系统进行设计。

In this section we discuss the general issues related to the role of optical fiber as a communication channel and to the design of transmitters and receivers. 在这节我们讨论了光纤作为通信信道和发射器和接收器的设计的一些问题。

The objective is to provide an introductory overview, as the three components are discussed in detail in Chapters 2–4.

目的是为了提供一个总体的介绍，三个部分的具体细节在第二章到第四章进行讨论。 1.4.1 Optical Fibers as a Communication Channel

The role of a communication channel is to transport the optical signal from transmitter to receiver without distorting it.使其失真

Most lightwave systems use optical fibers as the communication channel because silica fibers can transmit light with losses as small as 0.2 dB/km. 许多光波系统用了光纤作为通信信道，因为石英光纤的传输光的损耗低达。 Even then, optical power reduces to only 1% after 100 km. 即使这样，光能量在传播100km以后只剩下了1%

For this reason, fiber losses remain an important design issue and determine the repeater or amplifier spacing of a long-haul lightwave system.

因为这个原因，光纤损耗一直是设计中应该考虑的重要的因素，并且确定长途光波系统中继器或放大器间距。

Another important design issue is fiber dispersion, which leads to broadening of individual optical pulses with propagation.

另一个重要的设计因素是光纤色散，它导致了扩大单个光脉冲的传播。

If optical pulses spread significantly outside their allocated bit slot, the transmitted signal is severely degraded.

如果光脉冲在他们指定的位槽之外大量传播，传输信号就会大量衰减。 Eventually, it becomes impossible to recover the original signal with high accuracy.

最终，将不可能以高准确率恢复原始信号。

The problem is most severe in the case of multimode fibers, since pulses spread rapidly (typically at a rate of ∼10 ns/km) because of different speeds associated with different fiber modes.

在多模光纤的情况问题最严重，因为脉冲传播迅速（通常的速度在∼10 NS /公里）由于在不同纤维模式速度不同。

It is for this reason that most optical communication systems use single-mode fibers.

也是这个原因所以许多光通信系统用单模光纤。

Material dispersion (related to the frequency dependence of the refractive index) still leads to pulse broadening (typically<0.1 ns/km), but it is small enough to be acceptable for most applications and can be reduced further by controlling the spectral width of the optical source.

材料的色散（与折射率频率有关）也导致脉冲展宽。但是对于大部分应用已经足够小到能够接受了，而且可以用控制光源的光谱宽度来减小。

Nevertheless, as discussed in Chapter 2, material dispersion sets the ultimate limit on the bit rate and the transmission distance of fiber-optic communication systems.

虽然如此，材料色散奠定了比特率和光纤通信系统的传输距离的最低。 1.4.2 Optical Transmitters

The role of an optical transmitter is to convert the electrical signal into optical form and to launch the resulting optical signal into the optical fiber. 光发射器是用来转换电子信号到光的形式，并将得到的光信号放入光纤中。

Figure 1.11 shows the block diagram of an optical transmitter. 图1.11体现了光发射器的框图。

It consists of an optical source, a modulator, and a channel coupler. 耦合器

Semiconductor lasers or light-emitting diodes are used as optical sources because of their compatibility with the optical-fiber communication channel; both are discussed in detail in Chapter 3.

半导体激光或发光二极管都被用来当做光源因为他们对光纤通信信道的兼容性较好。 The optical signal is generated by modulating the optical carrier wave. 光信号由调制光载波形成。

Although an external modulator is sometimes used, it can be dispensed with in some cases, since the output of a semiconductor optical source can be modulated directly by varying the injection current.

尽管有时候需要用到外部调制器，它可以在某些情况下被分配，因为半导体光源的输出可以通过改变注入电流直接调制。

Such a scheme simplifies the transmitter design and is generally cost-effective.这种方案简化了发射器设计而且非常的划算。

The coupler is typically a microlens that focuses the optical signal onto the entrance plane of an optical fiber with the maximum possible efficiency.耦合器就像是一个显微镜头，它以最大效率的聚焦光信号到光纤的入口面。 The launched power is an important design parameter. 发射功率是一个重要的设计参数。

One can increase the amplifier(or repeater) spacing by increasing it, but the onset of various nonlinear effects limits how much the input power can be increased.

一方面通过增加它可以增加放大器（或中继器）的间距，但各种非线性效应的发生了多少输入功率可以增加。

The launched power is often expressed in ‚dBm‛ units with 1mW as the reference level.

As light-emitting diodes are also limited in their modulation capabilities, most

lightwave systems use semiconductor lasers as optical sources.

因为发光二极管也了他们的调制能力，许多光波系统用半导体激光作为光源。 The bit rate of optical transmitters is often limited by electronics rather than by the semiconductor laser itself.

With proper design, optical transmitters can be made to operate at a bit rate of up to 40 Gb/s.

Chapter 3 is devoted to a complete description of optical transmitters. 1.4.3 Optical Receivers

An optical receiver converts the optical signal received at the output end of the optical fiber back into the original electrical signal.

一个光接收器将在光纤输出端接收到的光信号转化回原始的电子信号。 Figure 1.12 shows the block diagram of an optical receiver.

It consists of a coupler, a photodetector, and a demodulator.光电探测器 The coupler focuses the received optical signal onto the photodetector. Semiconductor photodiodes are used as photodetectors because of their compatibility with the whole system; they are discussed in Chapter 4.

The design of the demodulator depends on the modulation format used by the lightwave system.

The use of FSK and PSK formats generally requires heterodyne or homodyne demodulation techniques discussed in Chapter 10.

Most lightwave systems employ a scheme referred to as ‚intensity modulation with direct detection‛ (IM/DD). 大多数光波系统采用的方案称为‚强度调制直接探测‛（IM/DD）。

Demodulation in this case is done by a decision circuit that identifies bits as 1 or 0, depending on the amplitude of the electric signal.判定电路

The accuracy of the decision circuit depends on the SNR of the electrical signal generated at the photo detector.

The performance of a digital lightwave system is characterized through the bit error rate (BER).数字光波系统的性能由比特误差率判定。

Although the BER can be defined as the number of errors made per second, such

a definition makes the BER bit-rate dependent.

It is customary to define the BER as the average probability of incorrect bit identification.

Therefore, a BER of 10−6 corresponds to on average one error per million bits. Most lightwave systems specify a BER of 10−9 as the operating requirement; some even require a BER as small as 10−14.

The error-correction codes are sometimes used to improve the raw BER of a lightwave systems. An important parameter for any receiver is the receiver sensitivity.灵敏度 It is usually defined as the minimum average optical power required to realize a BER of 10 −9.

Receiver sensitivity depends on the SNR, which in turn depends on various noise sources that corrupt the signal received.

Even for a perfect receiver, some noise is introduced by the process of photodetection itself.

This is referred to as the quantum noise or the shot noise, as it has its origin in the particle nature of electrons.

这被称为量子噪声或散粒噪声，因为它起源于电子的粒子性质。 Optical receivers operating at the shot-noise limit are called quantum-noise-limited receivers.

No practical receiver operates at the quantum-noise limit because of the presence of several other noise sources.

Some of the noise sources such as thermal noise are internal to the receiver.热噪声 Others originate at the transmitter or during propagation along the fiber link. For instance, any amplification of the optical signal along the transmission line with the help of optical amplifiers introduces the so-called amplifier noise that has its origin in the fundamental process of spontaneous emission.

例如，任何放大的光信号的光放大器的帮助下，介绍了所谓的放大器的噪声，起源于自发辐射的基本过程。

Chromatic dispersion in optical fibers can add additional noise through

phenomena such as intersymbol interference and mode-partition noise. 码间干扰/电流分配干扰

The receiver sensitivity is determined by a cumulative effect of all possible noise mechanisms that degrade the SNR at the decision circuit.

接收器的灵敏度是由所有可能的噪声累积决定，噪声降低了判定电路的信噪比。 In general, it also depends on the bit rate as the contribution of some noise sources (e.g. shot noise) increases in proportion to the signal bandwidth. Chapter 4 is devoted to noise and sensitivity issues of optical receivers by considering the SNR and the BER in digital lightwave systems.