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Chapter 1 Optical Fiber Transmission Media - TOMASI

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  1 525 561 605635 641 Optical Fiber Transmission Media --:]TER OUTLINE.:::.rduction -:.tor) of Optical Fiber Communications -,:;ical Fibers versus Metallic Cable Facilities - :;tromagnetic Spectrum : ..k Diagram of an Optical Fiber -- - :'nmunications System ::r.al Fiber Types ' -r:ht Propagation I E Optical Fiber Configurations l-9 Optical Fiber Classifications I l0 Losses in Optical Fiber Cables I ll Light Sources 1- ll Optical Sources I l-l Light Detectors l- 1-1 Lasers I ' I 5 Optical Fiber System Link Bud-set -: -::TIVES | -.-.'-rc optical comnrunications I :-:renr an overview ofthe history ofoptical tibers and optical fiber communications I :-.pare the advantages and disadvantages of optical fibers over metallic cables   :::ne electromagnetic frcquency and wavelenqth spectrunt I - :.:nbe several types of optical fiber construction t : , :.,iin the physics of light and the following terms: velocity of propagation. refraction. refractir e index. critical II  --:-e. acceptance angle, acceptance cone. and numerical aperture -:.-ribe how light waves propagate through an optical fiber cable -.-..te ntocles of propugtttion and irtdex profile: t. ribe the three types of optical fiber configurations: single-mode step index. multimode step index. and mul- 'Je -craded index :..:rbe the various losses incurred in optical fiber cables -: :e liqht source and optical power -:i.rbe lhe following light sources: tight-emitting diodes and injection diodes - ..-:rbe the following light detectors: PIN diodes and avalanche photodiodes :.-::be the operation ofa laser .:..:n ho$ to calculate a link budget for an optical fiber system 1 II II a  1.1 INTRODUCTION 1-2 HISTORY OF Optical fiber cables are the newest and probably the most promising type of guided trans-mission medium for virtually all forms ot'digital and data communications applications' in cluding local, metropolitan' and wide area networks With optical fibers' electromagneticwaves are guided thiough a media composed of a transparent material without using elec- trical cuneirt t'low. With optical fibers, electromagnetic light waves propagate through the media in much the same way that radio signals propagate through Earth's atmosphere' In essence, an oPtic'(tl cotttttlLotic4'i;rts J-) ste'' is one that uses light as the carier of information. Propagating light waves thrcugh Earth's atmosphere is difficult and often im- prr.tl.ul. Cun. quJntly.-opiical fiber communications systems use glass or plastic fiber ca-Lles to 'colralri: the light waves and guide them in a manner similar to the way electro- magnetic \i'aves are guidecl through a metallic transmission medium . The itdbrnutiort-carning cayrcin of any electronic communications system is di- rectly proportional to bandwidth. Optical fiber cables have' for all practical purposes' an in- nnite ilaniwidth. Therefore. they have the capacity to carry much more information than their metallic counterparts or. for that matter, even tAe most sophisticated wireless com-munications sYStems. For comparison purposes. it is common to express the bandwidth of an analog com-munications system as a Percenlage of its carier frequency This is sometimes called the bandtidth utiiizatio,? r4ti;. For inatance. a VHF raclio communications system operating at acarrierfrequencyofl00MHzwithl0-MHzbandwidthhasabandwidthutiliZationratioof 10olr. A microwave rcdio system operating at a carrier frequency of l0 GHz with a l07rhandwirlth utilization ratio would hive I GHz of bandwidth available Obviously' the higher the caniel fiequency. the more bandwidth available' and the greater the information- ca-rrvins caoacitl. Lighr frequencies used in optical fiber communications systems are be- * i,i io'' ir, no+ .l0'rHz(100.000GHzto400,000GHz) A bandwidth utiliza- tion ratio of 107. would be a bandwidth between 10,000 GHz and 40'000 GHz'OPTICAL FIBEB COMMUNICATIONSIn 1880. Alexander Graham Bell experimented u ith an appalatus he called a photophone' The photophone was a device constructed t'rom mirrors and selenium detectors that fians- ,oitt.,i ,ornd *u.'., over a beam of light The photophone was awkward and unreliable and hacl no real practical application. Actuall\. Iisual light was a primary means ofcommuni- cating long ;efore eleciionic communications came about Smoke signals and minors were ur.d-ag.r-ago to conrel shon. simple messages Bell's contraption' however' was the tirst attempt at using a beam of light for carrying information'Transmission of light waves for any useful distance through Earth's atmosphere is impractical because water vapor, oxygen. and particulates in the air absorb and attenuate the signals at light frequencie.s. Consequently. the only practical type of optical communi- catiois system is one that uses a fiber guide ln 1930' J' L. Baird' an English scientist' and c. w. Hansell. a scientist from the u;ircd States, were granted patents for scanning and transmitting television images through uncoated fiber cables A few years later' a German scientist named H. Lamm successfully transmitted images through a single glass fiber At that time, most people considered fiber optics more of a toy or a laboratory stunt and' con-sequently. it was not until the early 1950s that any substantial breakthrough was'.qnade in the field of tiber oPtics. In t951. A. b. S. van Heel of Holland and H H Hopkins andN' S Kapany ofEn- glandexperimentedwithlighttransmissionthroughDundlesoffibers.Theirstudies]edto t-h. d.r iop..rt of the fle;ible fberscope, which is used extensively in the medical held'It uas Kapany who coined the teIm fiber optics in 1956' Chapter 1  ns- in- :ric rhe iof im- ln 1958. Charles H. Townes. an American. and Afthur L. Scharvlou. a Canadian.wrote a paper describing how it was possible to use stimulated emission for amplifying light waves (laser) as well as microwaves (maser). Two years later. Thcodore H. Maintan. a sci entist with Hughes Aircraft Company, built the first optical maser The laser (Iight amplification by .rtimulated emission of radiatir)n) was invented in 1960. The laser's relatively high output po*'er. high tiequcncy of operation. and capabilitlof carrying an extremely wide bandwidth signal make it ideally suited for high-capacitycommunications systems. The invention of the laser -sreatly accelerated research efforts infiber-optic coinmunications. although it was not until I967 that K. C. Kao and C. A. Bock- hanr of the Standard Telecommunications Laboratory in England proposed a new conrmu-nications medium using c larlded fiber cables. The fiber cables available in the 1960s were extrenre)y 1tr.r.ir' (more than 1000 dB/km), which limited optical transmissions to short distances. ln 1970. Kapron. Keck. and Maurer of Corning Glass Works in Corning, New York. developed an optical tiber with losses less than 2 dB/km. That was the big breakthrough needed to pcrnlit practical flber optics communications systems. Since l9?0, fiber optics technology has grown exponen- tially. Recently. Bell Laboratories succ'essfully transmitted I billion bps thlough a fiber ca- ble for 600 miles without a regenerator In the late 1970s and early 1980s. the refinement ofoptical cables and the development ofhigh-quality, affordable light sources and detectors opened the door to the development of high-quality, high-capacity, etficient, and affordable optical fiber communications systems. By the late 1980s, losses in optical fibers were reduced to as low as 0.16 dB/krn. and in 1988 NEC Corporation set a new long-haul transmission record by transrnitting I0 -uigabytes per second over 80.1 kilometers ofoptical fiber Also in 1988, the American National Standards Institute (ANSI) published th e St trchntnous Opricdl Nenrork (.SON ET). By the mid- I 990s. opticnl voice and data networks were commonplace throughout the United States and much ofthe world. ca-IIO- di- L in- han  rll1-  )In- . the lg at -atio l0 ,q( the :ion- : be- iiza- lt)tle. rans- : and nuni- \\ ere | first rre isnuate nuni- r. and i and ,I11AN er. At . con-rde in ri En-led tofield. OPTICAL FIBERS VERSUS CABLE FACILITIESCommunications through glass or plastic fibers has several advantages ovel conven-tional metallic transmission media for both telecommunication and computer rretworkingapplications. 1-3-1 Advantages of Optical Fiber Cables The advantagcs of using optical fibers include the tbllou ing: l. Wider bandridtlt and grcdter iDformLltiott ('lPttit\'. Optical fiberr hirr e treater in-formation capacity than metallic cabies becalrse of lhe inherentl) s idel bands idth: lr ail-able with optical t'requencies. Optical libers ure arailable \\ith band\\idlh\ up lo \e\eral thousand gigahertz. The pri,ran eleclritttl tottslunrs (lesi\tan -e. inductance. and capaci-tance) in metallic cables cause them to act like lo\\ -prss iille[s. $ hich lintit iheir triir]\nlis- sion frequencies, band$,idth. bit rate. and intbrmttion-carq ing clpircil). \lode:n opticalfiber communications systems arc capable of transmitting ser elal gigrbitr per second over hundreds of miles, allowing literally millions of indi\ idLral \ oice .1nd clata channels to be combined and propagated over one optical tiber cable. 2. Inmwtitv to <rossr4lt. Optical fiber cables are inmune to crosstalk becaLlse glassand plastic fibers are nonconductors ofelectrical curent. Therelbre. fiber cables are not sur rounded by a changing magnetic tield. which is the prima4 cause ol crosstalk between metallic conductors located physical)y close to each other. 3. lmmufiit\'tu stciic interferefice. Because optical tiber cables are nonconductors of electrical current, they are immune to static noise due to electromagnetic interference (EMI) caused by lightning, electric motors. relays. fluorescent lights. and other electricalFiber Transmission Media  noise sources (most of which are man-made). For the same reason' fiber cables do not ra-diate electromrg.netic energl 4. Entirotlma t.tl inrlrlnin. Optical fiber cables are more resistant to environmen- ul extremes (including weather variations) than metallic cables Optical cables also oper- ate over a wider temperature range and are less aftected by corrosive liquids and gases' 5. Sa/en anrl contefiien('e. Oplicdl fiber cables are sat'er and easier to install and maintain than metallic cables. Because glass and plastic fibers are noncondrrdors- there are no electrical currents or voltages associated with them Optical fibers can be used around volatile liquids and gasses without worying about their causing explosions or fires Opti-cal tibers are also smaller and much more lightweight and compact than metallic cables' Consequently. they are more f-lexible. easier to work with' require less storage space' cheaper to transport. and easier to install and maintain. 6. Lrnter trctnsmi.isiorr /oss Optical libers have considerably less signal loss than their metallic counterparts. Optical tibers are cuffently being manufactured with as lit- tle as a few tenths-of-a-decibel loss per kilometer. Consequently' optical regenerators anit amplifiers can be spaced considerably farther apart than $ith metallic transmission lines. 7. Secrrill. Optical fiber cables are more secure than metallic cables lt is virtuall) impossible to tap into a fiber cable without the user's knowledge' and optical cables cannot be detected with metal detectors unless they are reintbrced with steel for strength' 8. Durat:tilitl (tnd rcliabilitt Optical fiber cables last longer and are more reliable than metallic facilities because fiber cables have a higher tolemnce to changes in environ- mental conditions and are immune to colrosive materials' 9. Econontics. The cost of optical fiber cables is approximately the same as metalli' cables. Fiber cables have less loss and require fewer repeaters' \ 'hich equates to lower in-stallation and overall systen costs and improved reliability' 1-3-2 Disadvantages of Optical Fiber Cables Although the advantag;s of optical tiber cables far exceed the disadvantages it is impor-tant to know rhe limirations of the fiber. The disad antages of optical fibeIS include Ihe following: l. lntetf(kittg cost.t Optical fiber cable s) stems are virtually useless by themseh e Tobepracticalanduseful.the},muslbeconnectedtostandardelectronicfacilities.whic]: often require expensir e interf'aces 2. Strengih. Optical ilbers bl themsehes have a significantly lower tensile sffensti than coaxial cabie. This can be improred by coating the fiber with standard Ker'lar and protective jacket of PVC. In addition. glass fiber is much more tiagile than copper \\ iri' making fiber less attractive where hardwarc portability is required' i. Renu)te electrical por|er Occasionally. it is necessary to provide electrical po* e: to remote interface or regenerating equipment. This cannot be accomplished with the opt:' cal cable. so additional metallic cables must be included in the cable assembly' 4. OptiutlJiber utbles are more susceptible to Losses iriroducetl by bending tlte c':' b1c. Electromagnetic waves propagate through an optical cable by either refraction or re'flection. Thereibre. bending the cable causes irregularities in the cable dimensions' rcsu::- ing in a loss of signal power. Optical fibers are also more prone to manufacturing defec:' as even the most minor detect can cause excessive loss of signal power' 5, Speciali:ed kx s. equiltnent. trnd truining' Optical fibcr cables require spec:'turls to splice anrl repair cables and special test equipment to make routine measuremen:' Not only is repairing fiber cables difficult and expensive, but technicians working on op:-- cal cables also require special skills and training. tn addition' sometimes it is difTicult to - cate taults in optical cables because there is no electrical continuity' Chapter'l :l i i'l j
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