《电子信息类专业英语(第二版)第一课32061.pptx
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1、Unit One Optical Fiber Communication Unit One Optical Fiber Communication Passage A The General System Passage B Advantages of Optical Fiber Communication Passage C Fiber-optics CommunicationUnit One Optical Fiber Communication Passage A The General System Communication may be broadly defined as the
2、 transfer of information from one point to another.When the information is to be conveyed over any distance,a communication system is usually required.Within a communication system the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic
3、 wave which acts as a carrier for the information signal.This modulated carrier is then transmitted to the required 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
4、carrier waves operating at radio frequencies as well as microwave and millimeter wave frequencies.However,“communication”may also be achieved by using an electromagnetic carrier which is selected from the optical range of frequencies.Unit One Optical Fiber Communication An optical fiber communicatio
5、n system is similar in basic concept to any type of communication system.A block schematic of a general communication system is shown in Figure 1.1(a),the function of which is to convey the signal from the information source over the transmission medium to the destination.The communication system th
6、erefore consists of a transmitter or modulator linked 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 electric
7、al(e.g.sound),to a transmitter comprising electrical and electronic components which converts the signal into a suitable form for propagation over the transmission medium.This is often achieved by modulating a carrier,which,as mentioned previously,may be an electromagnetic wave.The transmission medi
8、um can consist of a pair of wires,a coaxial 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.Unit One Optical Fiber Communication Ho
9、wever,it must be noted that in any transmission medium the signal is attenuated,or suffers loss,and is subject to degradations due to communication by random signals and noise,as well as possible distortions imposed by mechanisms within the medium itself.Therefore,in any communication system there i
10、s a maximum permitted distance between the transmitter and the receiver 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 an
11、d to increase signal level before transmission is continued down the link.Unit One Optical Fiber Communication For optical fiber communications system shown in Figure1.1(a)may be considered in slightly greater detail,as given in Figure1.1(b).In this case the information source provides an electrical
12、 signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of the lightwave carrier.The optical source which provides the electrical-optical conversion may be either a semiconductor laser or light emitting diode(LED).The transmission medium consists of
13、an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier.Photodiodes(p-n,p-i-n or avalanche)and,in some instances,phototransistors and photoconductors are utilized for the detection of the o
14、ptical signal and the optical-electrical 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.Unit One Optical Fiber Communication Unit One Optical Fiber Communication Figure 1.1(a)T
15、he general communication system;(b)The optical fiber communication systemUnit One Optical Fiber Communication The optical carrier may be modulated using either an analog or digital information signal.In the system shown in Figure1.1(b)analog modulation involves the variation of the light emitted fro
16、m the optical source in a continuous manner.With digital modulation,however,discrete changes in the light intensity are obtained(i.e.on-off pulses).Although often simpler to implement,analog modulation with an optical fiber communication system is less efficient,requiring a far higher signal to nois
17、e ratio at the receiver than digital modulation.1 Also,the linearity needed for analog modulation is not always provided by semiconductor optical sources,especially at high modulation frequencies.For these reasons,analog optical fiber communication links are generally limited to shorter distances an
18、d lower bandwidths than digital links.Unit One Optical Fiber Communication Figure 1.2 shows a block schematic of a typical digital optical fiber link.Initially,the input digital signal from the information source is suitably encoded for optical transmission.The laser drive circuit directly modulates
19、 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 and equalizer or filter to provide gain as well as linear signal processing and
20、 noise bandwidth reduction.2 Finally,the signal obtained is decoded to give the original digital information.However,at this stage it is instructive to consider the advantages provided by lightwave communication via optical fibers in comparison with other forms of line and radio communication which
21、have brought about the introduction of such systems in many areas throughout the world.Unit One Optical Fiber Communication Figure 1.2Unit One Optical Fiber Communication KEY WORDS superimposing 叠加,重叠sophisticated 复杂的,高级的,现代化的coaxial 同轴的degradation 恶化,降级distortion 失真,畸变 LED(light-emitting-diode)发光二极
22、管 photoconductor 光敏电阻discrete 离散,不连续linearity 线性度intensity 强度avalanche 雪崩front-end 前置,前级Unit One Optical Fiber Communication NOTES1 Although often simpler to implement,analog modulation with an optical fiber communication system is less efficient,requiring a far higher signal to noise ratio at the r
23、eceiver than digital modulation.虽然具有光纤系统的模拟调制易于实现,但其效率较低,且要求在接收端有比数字调制高得多的信噪比。signal to noise ratio为信噪比。2 The avalanche photodiode(APD)detector is followed by a front-end amplifier and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction.前置放大器与均衡器或滤波器
24、跟在雪崩二极管检测器的后面以获得增益、线性信号处理及噪声带宽的抑制。Unit One Optical Fiber Communication EXERCISES1.Fill in the blanks.(1)Within a communication system the information transfer is frequently achieved by or the information on to an electromagnetic wave.(2)The general communication system consists of a or modulator,the
25、 transmission medium,and a receiver or at the destination point.(3)The optical source which provides the conversion may be either a semiconductor or LED.(4)The laser drive circuit directly modulates the of the semiconductor laser with the encoded digital signal.Unit One Optical Fiber Communication 2
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