本科毕业生外文文献翻译.doc

本科毕业生外文文献翻译 学生姓名：指导教师：所在学院：信息技术学院专 业：通信工程Chapter 1IntroductionA communication system transmits information from one place to another, whether separated by a few kilometers or by transoceanic distances. Information is often carried by an electromagnetic carrier wave whose frequency can vary from a few megahertz to several hundred terahertz. Optical communication systems use high carrier frequencies (. 100 THz) in the visible or near-infrared region of the electromagnetic spectrum. 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). Fiber-optic communication systems are lightwave systems that employ optical fibers for information transmission. Such systems have been deployed worldwide since 1980 and have indeed revolutionized the technology behind telecommunications. Indeed, the lightwave technology, together with microelectronics, is believed to be a major factor in the advent of the "information age." The objective of this book is to describe fiber-optic communication systems in a comprehensive manner. The emphasis is on the fundamental aspects, but the engineering issues are also discussed. The purpose of this introductory chapter is to present the basic concepts and to provide the background material. Section 1.1 gives a historical perspective on the development of optical communication systems. In Section 1.2 we cover concepts such as analog and digital signals, channel multiplexing, and modulation formats. Relative merits of guided and unguided optical communication systems are discussed in Sec-tion 1.3. The last section focuses on the building blocks of a fiber-optic communication system.1.1 Historical PerspectiveThe use of light for communication purposes dates back to antiquity if we interpret optical communications in a broad sense 1. Most civilizations have used mirrors, fire beacons, or smoke signals to convey a single piece of information (such as victory in a war). Essentially the same idea was used up to the end of the eighteenth century through signaling lamps, flags, and other semaphore devices. 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 sta-tions 2, acting as regenerators or repeaters in the modern-day language. Figure 1.1 shows the basic idea schematically. The first such "optical telegraph" was put in service between Paris and Lille (two French cities about 200 km apart) in July 1794. By 1830, the network had expanded throughout Europe 1. 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. The opto-mechanical communication systems of the nineteenth century were inherently第一章介绍通信系统把信息从一个地方传播到另一个地方，是否要分开几千米或更远的距离，信息通常被电磁波运送，它的频率能够从几MHZ变化到几百THZ，光通信系统使用高频率载波（大约100THZ）在可见的范围内或电磁光谱接近红外线的区域内。光波系统与微波系统之间的差别有时候被称为光通信系统，基本上比它们小5倍（大约1GHZ），光纤通信系统是采用光纤的光波系统来传送信息，从1980年起，这样的系统已经被全世界所使用，而且在电信之后，已经宣传了这个技术，实际上，光波技术和微电子学被认为是信息时代到来的主要因素，本书的目的就是以可以理解的形式描述了光纤通信系统，重点是最基本的方面，但工程问题已经被解决了，本章的主要目的就是最基本的概念和提供了这个背景材料，1.1节给出了这个光纤通信系统发展的历史观点，1.2节介绍了这个概念。例如：模拟和数字信号，多元信道和调制方式，1.3节讨论了引导和非引导光纤通信系统的相关优点，最后一节的焦点是光纤通信系统的结构板块。1.1 历史观点如果我们以宽广眼界解释光通信，通信使用光起源于太古时代.大部分文明社会就已经使用反光镜，火把，或是烟信号来传达信息（例如在战争中成功），事实上，这同一个方法在18世纪末期就被使用了，通过使用信号灯，旗帜和其它的信号装置。在1972年，这个方法被Claude Chappe提出，使用中继站扩大更长的距离来传送编码信息.中继站用现代语言叫做再生器或中继器.如图1.1所示这个基本方法的示意图。1974年7月，第一个光电报机在法国的巴黎和里尔（法国大约相距200KM的两个城市）之间运转.到1830年为止，网络已经扩展到了欧洲，在那样的系统中光的主要作用是制作可见的编码信号以至于它们能够被中继站拦截，19世纪的光机械通信系统是固有的缓慢。在现代的术语中，那样系统的有效比特率小于1比特每秒（B1b/s）1.1.1 光纤通信的要求18世纪30年代，电信技术的到来用电代替了光的使用，开始了电通信的新纪元。使用新的编码技术，比特率大约能够被增加10b/s，例如，摩尔斯式电码。使用中继站允许的通信距离大约是1000千米远。实际上，在1866年，第一个成功的长距离的电报机开始工作，电信技术通过不同持续期间的两个电脉冲使用了根本上的数字式计划（摩尔斯式电码的小点和破折号）。1976年，电话的发明带