光纤通信简介翻译级英文原文.doc

单位代码 01 学号 070110096 分 类 号 密 级 文献翻译光纤通信简介 院（系）名称信息工程学院 专业名称通信工程 学生姓名李琳琳 指导教师李利平2011年 4 月 2 日 黄河科技学院毕业论文（文献翻译） 第 10 页 英文译文光纤通信简介维基百科（美国）一、基本概念光纤通信是一种通过发射光脉冲在光纤上传递信息的一种方式。光经过调制成为电磁载波后便能携带信息。成熟于上世纪80年代，光纤通信系统对于电信工业产生了革命性的改变，并且也在信息时代扮演了非常重要的角色。因为光纤通信对比电子通信有绝对的优势，在未来主要核心网络光纤将大量的取代铜线通信。在利用光纤传输的过程中包含以下几个基本步骤：利用发射机产生光信号，在长的光纤中进行中继转发，确保光信号在光纤中不会衰减或是严重变形，接收光信号，并且将其变换成电信号。二、应用光纤被很多通讯公司用于传输电话信号，因特网通讯，或是有线电视信号，与传统的铜线相比，光纤的信号衰减与遭受干扰的情形都有很大的改善，特别是长距离以及大量传输的使用场合中。然而，城市的基础建设发展是相当困难且十分耗时的，并且光纤系统复杂并且安装和操作十分昂贵。由于这些困难，早期光纤通信系统主要应用于长距离通信中，这样使得光纤的优势彻底发挥，且抑制住不断增加的成本。自从2000年起，光纤通信的成本大幅下降，目前光纤的价格已经比铜缆为骨干的通信系统划算。自从1990年光放大器系统进入商业市场，通信产业开始大量在城市间和海底中铺设。到了2002年时，越洋海底电缆的总长已经超过25万公里，每秒能携带的数据量超过2.56Tb/s，而且根据电信业者的统计，这些数据从2004年后仍然不断的大幅成长中。 三、技术现代的光纤通信系统一般包括一个发射器，将电信号变换成光信号，再通过光纤将光信号传递。光纤束埋在地下或建筑下，多种光放大器，以及一个光接收器将光信号变换回电信号。在光纤通信系统中传递的多半是数字信号，来源包括电脑、电话系统，或是有线电视系统。(1)发射器通常在光纤通信系统中光源为半导体组件，例如发光二极管（ LEDs）或是激光二极管。LEDs与激光二极管的主要区别是LEDs所发出的光为非同调性，而激光二极管产生的是同调性的光。在光纤系统中，半导体作为光源的设计目的是体积小、效率高、可靠度高，以及可以操作最佳的波长范围，可以在高频操作下立刻调制。在简单的方式下，LED是有偏向的p-n结，通过自发发射物发出光，这种现象被称为电激发光，发射光是不连贯的有相当宽的频谱，分散在30纳米至60纳米间。LED光传送也是低效率的，通常只有输入功率的1%可以变换成光功率，约是100微瓦特左右。但是由于LED十分简单的设计，LEDs在低价的应用中非常的有用。常用于光通信的LED主要材料是是砷化镓磷（GaAsP）或是砷化镓（GaAs）。由于GaAsP LEDs比GaAs LEDs在较长的波长内操作（1300纳米vs.810纳米至870纳米）,他们的输出光谱很宽大约170。由于LED的频谱范围较广，导致色散较为严重，也限制了其传输速率与传输距离的乘积。LED通常用在传输速率10Mb/s至100Mb/s的局域网，并且传输距离也只有几公里。目前也有LED内包含了数个量子井的结构，使得LED可以发出不同波长的光，涵盖较宽的频谱，并且被广泛应用在区域性的波分复用网络中。现在，LEDs已经被垂直腔面发射激光器(VCSEL)所取代，VCSEL可以在低成本的情况下改善速度，功率和谱性质。一般的VCSEL设备对多模光纤很好。半导体激光器发射光通过受激发射好过于自发发射，自发发射导致高输出功率（100mW）与其他利益相关的自然相干光一样，方向性较强，通常和单模光纤的耦合效率可达到50%。激光的输出频谱较窄，有助于增加传输速率以及降低色散。此外，半导体激光亦可在相当高的操作频率下进行调制，原因是其复合时间非常短。通常在光纤中应用半导体激光器传送信号包括垂直腔面发射激光器（VCSEL）,法布里佩罗和分布反馈（DFB）。半导体激光通常直接进行调制，光输出被一个电流应用调制设备所控制。对于某些传输速率非常高或是传输距离很长的应用，激光光源可能会以连续波的形式控制，例如使用外置的电吸收光调制器或是马赫任德干涉仪对光信号加以调制。外置的调制组件可以大幅减少激光的啁啾脉冲。啁啾脉冲会使得激光的谱线宽度变宽，使得光纤内的色散变得严重。(2)接收器构成光接收器的主要组件是光探测器（photodetector），利用光电效应将入射的光信号转为电信号。光探测器通常是半导体为基础的光二极管，很多类型的二极管包括p-n结二极管、p-i-n二极管，或是雪崩型二极管，金属-半导体-金属（MSM）光探测器也因为与电路集成性佳，而被应用在光再生器或是波分复用器中。光电转换器由转阻放大器和限幅放大器结合，在电领域由入射光信号产生数字信号，光信号通过海底产生衰减和失真。之后通过时钟频率及数据回复电路以及锁相回路将信号做适度处理再输出。(3)光纤 光纤缆线包含一个核心，包层以及套塑。光利用全内反射在核心中传输。核心与折射率较低的包层通常用质量较好的硅石玻璃制成，尽管他们也可以用塑料制成。连接两个光纤是通过融合技术或者机械接头拼接，由于光纤拼接要求在是微观精密的这要求特殊的技术和互联技术。主要有两种光纤被用于光纤通信，分别是多模光纤和单模光纤。多模光纤有较大的核心（50微米），不需要很精准，传输成本低且接收机在接通需要的连接器也较便宜。然而，多模光纤会失真，它限制了频带宽度和传输长度。此外，因为它有较高的掺杂，多模光纤通常很贵并且有很高的衰减。单模光纤的核心较小（10微米）并且需要很贵的组件和互联方法，但是拥有较长传输距离和较高的传输效率。为了包装光纤成为商用产品，光纤的外层有经过紫外线固化后的压克力（acrylate）被覆，终端使用光纤连接器，且组装成电缆。之后，它可以如铜缆一样埋藏于地下传输信息，这些光纤需要的维护费用低于普通的双绞线。（4）光放大器过去光纤的传输距离被光纤内的衰减和信号变形所限制，通过光电感应中继已经解决了这些问题。这种中继器先将光信号转回电信号放大后再变换成较强的光信号传往下一个中继器，然而这样的系统架构较为复杂，不适用于新一代的波分复用技术，同时每隔20公里就需要一个中继器，这些中继器的成本也非常高。一种变换方法是用光放大器，它直接放大光信号不需要转换光信号为电信号。它是通过在一段光纤内掺杂稀土族元素如铒（erbium），再以短波长激光激发之。在新的设备中光放大器大量的取代了中继器。五、与传统通信系统的比较在选择是利用光纤传输还是利用铜缆进行传输需要就几点问题进行权衡，为系统选择光纤通常要求有较高的频带宽度或者相对于电缆可以有较长的传输距离。最主要的优点是光纤有非常小的损耗，它没有地上的电流和其他信号干扰，功率并行传输且本身有较高的携带信息能力。很多电缆都被要求换成光缆，光纤的另一个优点是在长距离传输时即使数条光纤并行也不会产生串讯的干扰，这和传输电信号的传输线正好相反。对于短距离和相对低带宽的通信应用而言，使用电信号的传输有下列好处：(1)材料成本地(2)发射器和接收机的成本低(3)可以利用电力系统传递信息(4)组装容易光纤的拼接比电缆的拼接困难且昂贵。在高能量时，光纤对光纤维的融化极其敏感，会对光纤核心产生灾难性的破坏并且会破坏传输部件。因为这些好处，所以在很短的距离传输信息，例如主机之间、电路板之间，甚至是集成电路芯片之间，通常还是使用电信号传输。然而目前也有些还在实验阶段的系统已经改采光来传递信息。在某些短距离和低带宽的场合，光纤通信仍然有其独特的优势：(1)能抵抗电磁干扰（EMI），包括核子造成的电磁脉冲。（不过光纤可能会毁于或射线）(2)对电信号的阻抗极高，所以能在高电压或是地面电位不同的状况下安全工作。(3)重量较轻，这在飞机中特别重要。(4)不会产生火花，在某些易燃的环境中显得重要。(5)没有电磁辐射、不易被窃听，对于需要高度安全的系统而言十分重要。(6)线径小，当绕线的路径被限制时，变得重要。 摘自：维基百科（美国），2009年12月。附：英文原文 Introduction to Optical Fiber Communication冈萨雷斯Wikipedia（USA）I. The basic concept Fiber-optic communicationis a method of transmitting information from one place to another by sending pulses oflightthrough anoptical fiber. The light forms anelectromagneticcarrier wavethat ismodulatedto carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunications industry and have played a major role in the advent of the Information Age. Because of itsadvantages over electrical transmission, optical fibers have largely replaced copper wire communications incore networksin thedeveloped world.The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal.II. ApplicationsOptical fiberis used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lowerattenuationandinterference, optical fiber has large advantages over existing copper wire in long-distance and high-demand applications. However, infrastructure development within cities was relatively difficult and time-consuming, and fiber-optic systems were complex and expensive to install and operate. Due to these difficulties, fiber-optic communication systems have primarily been installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost. Since 2000, the prices for fiber-optic communications have dropped considerably. The price for rolling out fiber to the home has currently become more cost-effective than that of rolling out a copper based network. Since 1990, whenoptical-amplificationsystems became commercially available, the telecommunications industry has laid a vast network of intercity and transoceanic fiber communication lines. By 2002, an intercontinental network of 250,000km ofsubmarine communications cablewith a capacity of 2.56Tb/s was completed, telecommunications investment reports indicate that network capacity has increased dramatically since 2004.III. TechnologyModern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, acablecontaining bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typicallydigital informationgenerated by computers,telephone systems, andcable televisioncompanies.（1）TransmittersThe most commonly-used optical transmitters are semiconductor devices such aslight-emitting diodes(LEDs) andlaser diodes. The difference between LEDs and laser diodes is that LEDs produceincoherent light, while laser diodes producecoherent light. For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient, and reliable, while operating in an optimal wavelength range, and directly modulated at high frequencies.In its simplest form, an LED is a forward-biasedp-n junction, emitting light throughspontaneous emission, a phenomenon referred to aselectroluminescence. The emitted light is incoherent with a relatively wide spectral width of 30-60nm. LED light transmission is also inefficient, with only about 1% of input power, or about 100 microwatts, eventually converted intolaunched powerwhich has been coupled into the optical fiber. However, due to their relatively simple design, LEDs are very useful for low-cost applications.Communications LEDs are most commonly made fromgallium arsenide phosphide(GaAsP) orgallium arsenide(GaAs). Because GaAsP LEDs operate at a longer wavelength than GaAs LEDs (1.3 micrometers vs. 0.81-0.87 micrometers), their output spectrum is wider by a factor of about 1.7. The large spectrum width of LEDs causes higher fiber dispersion, considerably limiting their bit rate-distance product (a common measure of usefulness). LEDs are suitable primarily forlocal-area-networkapplications with bit rates of 10-100 Mbit/s and transmission distances of a few kilometers. LEDs have also been developed that use severalquantum wellsto emit light at different wavelengths over a broad spectrum, and are currently in use for local-areaWDMnetworks.Today, LEDs have been largely superseded byVCSEL(Vertical Cavity Surface Emitting Laser) devices, which offer improved speed, power and spectral properties, at a similar cost. Common VCSEL devices couple well to multi mode fiber.A semiconductor laser emits light throughstimulated emissionrather than spontaneous emission, which results in high output power (100mW) as well as other benefits related to the nature of coherent light. The output of a laser is relatively directional, allowing high coupling efficiency (50%) into single-mode fiber. The narrow spectral width also allows for high bit rates since it reduces the effect ofchromatic dispersion. Furthermore, semiconductor lasers can be modulated directly at high frequencies because of shortrecombination time.Commonly used classes of semiconductor laser transmitters used in fiber optics includeVCSEL(Vertical Cavity Surface Emitting Laser), Fabry Perot andDFB(Distributed Feed Back).Laser diodes are often directlymodulated, that is the light output is controlled by a current applied directly to the device. For very high data rates or very long distancelinks, a laser source may be operatedcontinuous wave, and the light modulated by an external device such as anelectro-absorption modulatororMach-Zehnder interferometer. External modulation increases the achievable link distance by eliminating laserchirp, which broadens thelinewidthof directly-modulated lasers, increasing the chromatic dispersion in the fiber.（2）ReceiversThe main component of an optical receiver is aphotodetector, which converts light into electricity using thephotoelectric effect. The photodetector is typically a semiconductor-basedphotodiode. Several types of photodiodes include p-n photodiodes, a p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability forcircuit integrationinregeneratorsand wavelength-division multiplexers.Optical-electrical converters are typically coupled with atransimpedance amplifierand alimiting amplifierto produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted while passing through the channel. Further signal processing such as clock recovery from data performed by aphase-locked loopmay also be applied before the data is passed on.（3）FiberAn optical fiber consists of a core,cladding, and a buffer (a protective outer coating), in which the cladding guides the light along the core by using the method oftotal internal reflection. The core and the cladding (which has a lower-refractive-index) are usually made of high-qualitysilicaglass, although they can both be made of plastic as well. Connecting two optical fibers is done by fusion splicing or mechanical splicing and requires special skills and interconnection technology due to the microscopic precision required to align the fiber cores. Two main types of optical fiber used in optic communications includemulti-mode optical fibersandsingle-mode optical fibers. A multi-mode optical fiber has a larger core ( 50micrometres), allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. However, a multi-mode fiber introducesmultimode distortion, which often limits the bandwidth and length of the link. Furthermore, because of its higherdopantcontent, multi-mode fibers are usually expensive and exhibit higher attenuation. The core of a single-mode fiber is smaller (10 micrometres) and requires more expensive components and interconnection methods, but allows much longer, higher-performance links.In order to package fiber into a commercially-viable product, it is typically protectively-coated by using ultraviolet (UV), light-curedacrylate polymers, then terminated withoptical fiber connectors, and finally assembled into a cable. After that, it can be laid in the ground and then run through the walls of a building and deployed aerially in a manner similar to copper cables. These fibers require less maintenance than common twisted pair wires, once they are deployed. （4）Amplifiers The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than it was before. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that they had to be installed about once every 20km), the cost of these repeaters is very high.An alternative approach is to use anoptical amplifier, which amplifies the optical signal directly without having to convert the signal into the electrical domain. It is made bydopinga length of fiber with the rare-earth mineralerbium, andpumpingit with light from alaserwith a shorter wavelength than the communications signal (typically 980nm). Amplifiers have largely replaced repeaters in new installations.IVComparison with electrical transmissionThe choice between optical fiber and electrical (orcopper) transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring higherbandwidthor spanning longer distances than electrical cabling can accommodate.The main benefits of fiber are its exceptionally low loss (allowing long distances between amplifiers/repeaters), its absence of ground currents and other parasite signal and power issues common to long parallel electric conductor runs (due to its reliance on light rather than electricity for transmission, and the dielectric nature of fiber optic), and its inherently high data-carrying capacity. Thousands of electrical links would be required to replace a single high bandwidth fiber cable. Another benefit of fibers is that even when run alongside each other for long distances, fiber cables experience effectively nocrosstalk, in contrast to some types of electricaltransmission lines. In short distance and relatively low bandwidth applications, electrical transmission is often preferred because of its（1） Lower material cost, where large quantities are not required（2