光纖通信畢業(yè)論文外文翻譯

1、東華理工大學(xué)長江學(xué)院畢業(yè)設(shè)計外 文 翻 譯 學(xué)生姓名： 學(xué) 號： 09323119 專 業(yè)： 信息工程 系 別： 信息工程 指導(dǎo)教師： 職稱： 講師 二0一三年六月 五 日originaloptical fiber communicationsthe general system communication may be broadly dened as the transfer of information from one point to another. when the information is to be conveyed over any distance a communi

2、cation system is usually required. within a communication system the information transfer is frequently achieved by superimposing or modulating the information onto an electromagnetic wave which acts as a carrier for the information signal. this modulated carrier is then transmitted to the required

3、destination where it is received and the original information signal is obtained by demodulation. sophisticated techniques have been developed for this process by using electromagnetic carrier waves operating at radio frequencies as well as microwave and millimeter wave frequencies. however, communi

4、cation may also be achieved by using an electromagnetic carrier which is selected from the optical range of frequencies.an optical fiber communication system is similar in basic concept to any type of communication system.the communication system therefore consists of a transmitter or modulator link

5、ed to the information source, the transmission medium,and a receiver or demodulator at the destination point. in electrical communications the information source provides an electrical signal, usually derived from a message signal which is not electrical (e.g. sound), to a transmitter comprising ele

6、ctrical and electronic components which converts the signal into a suitable form for propagation over the trans-mission medium. this is often achieved by modulating a carrier, which, as mentioned previously, may be an electromagnetic wave. the transmission medium can consist of a pair of wires, a co

7、axial cable or a radio link through free space down which the signal is transmitted to the receiver, where it is transformed into the original electrical information signal (demodulated) before being passed to the destination. however, it must be noted that in any transmission medium the signal is a

8、ttenuated, or suffers loss, and is subject to degradations due to contamination by random signals and noise, as well as possible distortions imposed by mechanisms within the medium itself. therefore, in any communication system there is a maximum permitted distance between the transmitter and the re

9、ceiver beyond which the system effectively ceases to give intelligible communication. for long-haul applications these factors necessitate the installation of repeaters or line amplifiers at intervals,both to remove signal distortion and to increase signal level before transmission is continued down

10、 the link. for optical fiber communications system shown in figure (a) may be considered in slightly greater detail, as given in figure (b). fig(a) the general communication system (b)the optical fiber communication systemin this case the information source provides an electrical signal to a transmi

11、tter comprising an electrical stage which drives an optical source to give modulation of the light wave carrier. the optical source which provides the electricaloptical conversion may be either a semiconductor laser or light-emitting diode (led). the transmission medium consists of an optical fiber

12、cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier. photodiodes (pn, pin or avalanche) and, in some instances, phototransistors and photoconductors are utilized for the detection of the optical signal

13、and the opticalelectrical conversion. thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually performed electrically.the optical carrier may be modulated using either an analog or digital information signal. in the syst

14、em shown in figure (b) analog modulation involves the variation of the light emitted from the optical source in a continuous manner. with digital modulation,however, discrete changes in the light intensity are obtained (i.e. onoff pulses). although often simpler to implement, analog modulation with

15、an optical fiber communication system is less efficient, requiring a far higher signal-to-noise ratio at the receiver than digital modulation. also, the linearity needed for analog modulation is not always provided by semiconductor optical sources, especially at high modulation frequencies. for thes

16、e reasons,analog optical fiber communication links are generally limited to shorter distances and lower bandwidth operation than digital links.figure (c) shows a block schematic of a typical digital optical fiber link. initially, the input digital signal from the information source is suitably encod

17、ed for optical transmission. the laser drive circuit directly modulates the intensity of the semiconductor laser with the encoded digital signal. hence a digital optical signal is launched into the optical fiber cable. the avalanche photodiode (apd) detector is followed by a front-end amplifier equa

18、lizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction. fig(c)a digital optical fiber link using a semiconductor laser source and an avalanche photodiode(apd) detector待添加的隱藏文字內(nèi)容2finally, the signal obtained is decoded to give the original digital informatio

19、n. however, at this stage it is instructive to consider the advantages provided by light wave communication via optical fibers in com-parison with other forms of line and radio communication which have brought about the extensive use of such systems in many areas throughout the world.譯文光纖通信一般來說把信息從一

20、點(diǎn)傳送到另一點(diǎn)就稱為通信。當(dāng)信息跨越一段距離進(jìn)行傳送時，就需要一個通信系統(tǒng)。在通信系統(tǒng)中，信息傳輸是通過把信息疊加在電磁波上或?qū)﹄姶挪ㄟM(jìn)行調(diào)制來實(shí)現(xiàn)的，電磁波起著載送信號的作用。這一經(jīng)過調(diào)制的載頻隨后被傳送到要求（到達(dá)）的目的地，在那里被接收，并且通過解調(diào)還原成原始信息。在運(yùn)用電磁載波的領(lǐng)域，高新技術(shù)得到進(jìn)一步的發(fā)展，比如射頻、微波以及毫米波的頻率都被用來作為載頻使用。亦可選用光波范圍內(nèi)的頻率作為載波來實(shí)現(xiàn)通信。從概念上講，一個光纖通信系統(tǒng)與任何類型的通信系統(tǒng)是一樣的。通信系統(tǒng)通常由這樣幾部分組成，即一個發(fā)射機(jī)或與信源相關(guān)聯(lián)的調(diào)制器、傳送媒介以及一個接收機(jī)或是放在目的地的解調(diào)器。在電通信系統(tǒng)中

21、，信源的作用是將原始的非電信號（如語音信號等）轉(zhuǎn)變成電信號，然后通過電或電子器件將該信號轉(zhuǎn)變成適合在傳輸媒介上傳輸?shù)男盘?，這一過程通常是通過調(diào)制載頻來完成的。如前所述，這個載頻可以是電磁波，傳輸介質(zhì)可以是一對導(dǎo)線、一條同軸電纜或通過自由空間的無線電，鏈路通過介質(zhì)將調(diào)制好的信號送到接收端，信號在傳送給終端之前，先復(fù)原成原是信號（即解調(diào)）。必須引起注意的是：經(jīng)過任何傳輸介質(zhì)，信號都會衰減或損耗，并且經(jīng)常容易為隨機(jī)信號和噪聲所損害，以及可能有傳輸媒介自身引起的失真等，這些都會導(dǎo)致信號惡化。因此，在任何通信系統(tǒng)中，接收機(jī)和發(fā)射機(jī)之間都存在一個最大允許距離，否則，就不可能有可理解的通信。在長途干線中，上述因素要求按一定間隔設(shè)置再生中繼站或線路放大器，而再生中繼站和線路放大器的設(shè)備都是為了消除失真，提高信號電平，以便傳輸?shù)靡岳^續(xù)