《电子信息类专业英语(第二版)第一课.pptx

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 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 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 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 communication 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 therefore 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 electrical(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 medium 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 However,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 is 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 and 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 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 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 optical 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)The 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 from 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 noise 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 and 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 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 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 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 receiver 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 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.True/False.(1)When the information is to be conveyed over any distance,a communication system is usually required.()(2)In any communication system there is a maximum permitted distance between the transmitter and the receiver beyond which the system effectively ceases to give intelligible communication.()(3)Digital modulation with an optical fiber communication system is less efficient,requiring a far higher signal to noise radio at the receiver than analog modulation.()(4)Digital optical fiber communication links are generally limited to shorter distances and lower bandwidths than analog links.()(5)The information-carrying capacity of optical fiber systems has proved far superior to the best copper cable systems.()Unit One Optical Fiber Communication(6)The WDM(wavelength division multiplexed)operation offers the potential for a fiber information carrying capacity which is may orders of magnitude in excess of that obtained using copper cables or a radio system.()(7)Like the situation with copper cables,a transmitted optical signal can be obtained from a fiber in a noninvasive manner.()(8)Taking the size and weight advantage into account,these optical fiber cables are generally superior in terms of storage,transportation,handling and installation to corresponding copper cables,whilst exhibiting at least comparable strength and durability.()Unit One Optical Fiber Communication(9)Hence with fewer repeaters,system reliability is generally enhanced in comparison with conventional electrical conductor system.()(10)Although the use of the laser for free space optical communication proved somewhat limited,the invention for the laser instigated a tremendous research effort into the study of optical component to achieve reliable information transfer using a lightwave carrier.()(11)In theory,the greater the carrier frequency,the smaller the available transmission bandwidth and thus the information-carrying capacity of the communication system.()Unit One Optical Fiber Communication 3.Fill in the blanks with the best choice.(1)Sophisticated techniques have been developed for the process of modulation,demodulation and transmission using electromagnetic carrier waves operating at radio frequencies as well as .a.microwave frequency b.millimeter wave frequencyc.a and b d.a or b(2)The electrical transmission medium can consist of .a.a pair of wires b.a coaxial cablec.a radio link through free space down d.all of aboveUnit One Optical Fiber Communication(3)are not utilized for the detection of the signal and the optical-electrical conversion.a.Light emitting diodes b.Phototransistorsc.Photoconductors d.Photodiodes(4)The property of electrical isolation makes optical fiber transmission ideally suited for communication in electrically hazardous environments as the fibers create no .a.arcing b.spark hazard at abrasionsc.short circuits d.any of above(5)Optical fibers form a dielectric wave guide but not free from .a.electromagnetic interference b.radio frequency interferencec.mechanical damage d.electromagnetic pulseUnit One Optical Fiber Communication(6)Fibers have been fabricated with losses as low as and this feature has become a major advantage of optical fiber communication.a.5 dB/km b.0.2 dB/kmc.1 dB/kmd.0.5dB/km(7)To obtain both the low loss and low dispersion at the same operating wavelength,which kind of new advanced single-mode structure has been realized:.a.namely b.dispersion shiftedc.dispersion flattened d.all of aboveUnit One Optical Fiber Communication Passage B Advantages of Optical Fiber CommunicationCommunication using an optical carrier wave guided along a glass fiber has a number of extremely attractive features,several of which were apparent when the technique was originally conceived.Furthermore,the advances in the technology to date have surpassed even the most optimistic predictions,creating additional advantages.Hence it is useful to consider the merits and special features offered by optical fiber communications over more conventional electrical communications.In this context we commence with the originally foreseen advantages and then consider additional features which have become apparent as the technology has been developed.Unit One Optical Fiber Communication(a)Enormous potential bandwidth.The optical carrier frequency in the range 1013 to 1016 Hz(generally in the near infrared around 1014Hz or 105GHz)yields a far greater potential transmission bandwidth than metallic cable systems(i.e.coaxial cable bandwidth up to around 500MHz)or even millimeter wave radio systems(i.e.systems currently operating with modulation bandwidths of 700 MHz).At present,the bandwidth available to fiber systems is not fully utilized but modulation at several gigahertz over a hundred kilometers and hundreds of megahertz over three hundred kilometers without intervening electronics(repeaters)is possible.Therefore,the information-carrying capacity of optical fiber systems has proved far superior to the best copper cable systems.By comparison the losses in wideband coaxial cable systems restrict the transmission distance to only a few kilometers at bandwidths over one hundred megahertz.Unit One Optical Fiber Communication Although the usable fiber bandwidth will be extended further towards the optical carrier frequency,it is clear that this parameter is limited by the use of a single optical carrier signal.Hence much enhanced bandwidth utilization for an optical fiber can be achieved by transmitting several optical signals,each at different center wavelengths,in parallel on the same fiber.This wavelength division multiplexed operation,particularly with dense packing of the optical wavelengths(or,essentially,fine frequency spacing),offers the potential for a fiber information-carrying capacity which is many orders of magnitude in excess of that obtained using copper cables or a wideband radio system.Unit One Optical Fiber Communication(b)Small size and weight.Optical fibers have very small diameters which are often no greater than the diameter of a human hair.Hence,even when such fibers are covered with protective coatings they are far smaller and much lighter than corresponding copper cables.This is tremendous boon towards the alleviation of duct congestion in cities,as well as allowing for an expansion of signal transmission within mobiles such as aircraft,satellites and even ships.(c)Electrical isolation.Optical fibers which are fabricated from glass,or sometimes a plastic polymer,are electrical insulators and therefore,unlike their metallic counterparts,they do not exhibit earth loop and interface problems.Furthermore,this property makes optical fiber transmission ideally suited for communication in electrically hazardous environments as the fibers create no arcing or spark hazard at abrasions or short circuits.Unit One Optical Fiber Communication(d)Immunity to interference and cross-talk.Optical fibers form a dielectric wave-guide and are therefore free from electromagnetic interference(EMI),radio frequency interference(RFI),or switching transients giving electromagnetic pulse(EMP).Hence the operation of an optical fiber communication system is unaffected by transmission through an electrically noisy environment and the fiber cable requires no shielding from EMI.The fiber cable is also not susceptible to lightning strikes if used overhead rather than underground.Moreover,it is fairly easy to ensure that there is no optical interference between fibers and hence,unlike communi