1、Unit12 Optical Fiber Communication Unit12 Optical Fiber Communication PassageA The General SystemPassageB Advantages of Optical Fiber CommunicationPassageC Fiber-optics CommunicationsUnit12 Optical Fiber Communication PassageA The General SystemCommunication may be broadly defined as the transfer of
2、 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
3、acts as a carrier for the information signal.Unit12 Optical Fiber Communication 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
4、 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.Unit12 Optical Fiber Communication An optic
5、al 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 co
6、mmunication 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
7、which is not electrical(e.g.sound),Unit12 Optical Fiber Communication 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 previou
8、sly,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
9、 the destination.Unit12 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 mediu
10、m 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 ampli
11、fiers at intervals,both to remove signal distortion and to increase signal level before transmission is continued down the link.Unit12 Optical Fiber Communication For optical fiber communications system shown in Figure12.1(a)may be considered in slightly greater detail,as given in Figure12.1(b).In t
12、his 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 em
13、itting diode(LED).Unit12 Optical Fiber Communication 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 so
14、me 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
15、.Unit12 Optical Fiber Communication Figure 12.1 The General Communication System and The Optical Fiber Communication SystemUnit12 Optical Fiber Communication The optical carrier may be modulated using either an analog or digital information signal.In the system shown in Figure12.1(b)analog modulatio
16、n 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 les
17、s efficient,requiring a far higher signal to noise ratio at the receiver than digital modulation.1Unit12 Optical Fiber Communication Also,the linearity needed for analog modulation is not always provided by semiconductor optical sources,especially at high modulation frequencies.For these reasons,ana
18、log optical fiber communication links are generally limited to shorter distances and lower bandwidths than digital links.Unit12 Optical Fiber Communication Figure 12.2 shows a block schematic of a typical digital optical fiber link.Initially,the input digital signal from the information source is su
19、itably 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 amplifi
20、er and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction.2Unit12 Optical Fiber Communication Finally,the signal obtained is decoded to give the original digital information.However,at this stage it is instructive to consider the advantages provided
21、 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.Unit12 Optical Fiber Communication Figure12.2 A Typical Digital Optical Fiber Link BlockUnit12 Op
22、tical 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
23、为信噪比。Unit12 Optical Fiber Communication 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。Unit12 Optical Fiber Communica
24、tion 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
25、)The optical source which provides the conversion may be either a semiconductor or LED.Unit12 Optical Fiber Communication(4)The laser drive circuit directly modulates the of the semiconductor laser with the encoded digital signal.(5)The APD detector is followed by a amplifier to provide gain.(6)Much
26、 more enhanced bandwidth utilization for an optical fiber can be achieved by transmitting several optical signals,each at different ,in parallel on the same fiber.Unit12 Optical Fiber Communication(7)Optical fibers have very small diameters which are often than the diameter of a human hair.(8)The li
27、ght from optical fibers does not radiate significantly and therefore they provide a degree of signal security.Unit12 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
28、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 tha
29、n analog modulation.()Unit12 Optical Fiber Communication(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 syst
30、ems.()(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.()Unit12 Optical Fiber Communication(7)Like the situation with copper cables,a t
31、ransmitted 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 lea
32、st comparable strength and durability.()Unit12 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 limite
33、d,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.()Unit12 Optical Fiber Communication(11)In theory,the greater the carrier frequency,the smaller the available transmission band
34、width and thus the information-carrying capacity of the communication system.()Unit12 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
35、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 aboveUnit12 Optical Fiber Communication(3)are not uti
36、lized 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
37、 fibers create no .a.arcing b.spark hazard at abrasionsc.short circuits d.any of aboveUnit12 Optical Fiber Communication(5)Optical fibers form a dielectric wave guide but not free from .a.electromagnetic interference b.radio frequency interferencec.mechanical damage d.electromagnetic pulse(6)Fibers
38、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/kmUnit12 Optical Fiber Communication(7)To obtain both the low loss and low dispersion at the same operating wavelength,which kind of new advan
39、ced single-mode structure has been realized:.a.namely b.dispersion shiftedc.dispersion flattened d.all of aboveUnit12 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 attrac
40、tive 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.Unit12 Optical Fiber Communication Hence it is useful to consider the merits
41、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.Unit12 Optical Fiber
42、 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 mi
43、llimeter 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 i
44、ntervening electronics(repeaters)is possible.Unit12 Optical Fiber Communication 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
45、to only a few kilometers at bandwidths over one hundred megahertz.Unit12 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
46、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
47、 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.Unit12 Optical Fiber Communication (b)Small size and weight.Optical fibers have very small diameters which ar
48、e 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 expansi
49、on of signal transmission within mobiles such as aircraft,satellites and even ships.Unit12 Optical Fiber Communication (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
50、 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.Unit12 Optical Fiber Communication (d)Imm
