電子信息工程專業(yè)英語

1、光纖通信的優(yōu)點(diǎn)利用一根玻璃光纖引導(dǎo)的光載波通信有許多突出的優(yōu)點(diǎn)，其屮的一些優(yōu)點(diǎn)在 最初構(gòu)思這種技術(shù)時就已經(jīng)顯而易見了。然而，當(dāng)今的技術(shù)發(fā)展（advances）已 經(jīng)超越了當(dāng)初最樂觀的預(yù)測，產(chǎn)生了另外一些優(yōu)勢。因此，有必要考慮光纖通信 與許多傳統(tǒng)電子通信相比帶來的優(yōu)點(diǎn)和專業(yè)特色。本文先介紹最瑣碎初預(yù)想的優(yōu) 點(diǎn)，接著介紹隨著科技發(fā)展而出現(xiàn)的另外的（additional）突出優(yōu)點(diǎn)。a）巨大的潛在帶寬：頻率為1013到1016 hz （通常接近大約1014hz紅外線的頻率）的光載波產(chǎn) 牛比金屬電纜系統(tǒng)（如同軸電纜的帶寬最高為500mhz）甚至毫米波無線電系統(tǒng) （例如系統(tǒng)當(dāng)前工作在700mhz調(diào)制帶寬）

2、大得多的潛在傳輸帶寬。目前，光纖 系統(tǒng)可以用帶寬并沒有被完全利用，但是，不需要中繼器而傳輸lookm的幾ghz 的調(diào)制和傳輸300km的幾百mhz的調(diào)制是可能的。光纖系統(tǒng)的信息攜帶能力遠(yuǎn) 優(yōu)于最好的銅電纜系統(tǒng)。比較而言，寬帶同軸電纜系統(tǒng)在1 oomhz以上帶寬內(nèi)的 損耗將傳輸距離限制在只有幾千米的范圍內(nèi)。雖然可以利用的光纖帶寬可以被進(jìn)一步擴(kuò)展到光載波頻率，但是很明顯這一 參數(shù)（可用帶寬）被單個光載波信號所限制。因此，通過在同一個光纖中并行傳 輸幾個工作在不同的屮心波長的光信號可以實(shí)現(xiàn)帶寬利用率的很大提高。利用波 分復(fù)用技術(shù)（wdm）,尤其是密集波復(fù)用（或者說，實(shí)質(zhì)上的頻分復(fù)用）使得光 纖的信息

3、載容量超過電纜或者寬帶無線系統(tǒng)好多個數(shù)量級。b）小尺寸和輕重量：光纖的直徑非常小，通常比頭發(fā)絲的直徑還要細(xì)。因此，就算這些光纖被涂 覆層包裹時，它們都要比銅電纜直徑更小并且重量更輕。這對于緩解城市的管道 擁擠而言占有很大的優(yōu)勢，并且允許在移動體（如飛機(jī)、衛(wèi)星甚至船舶）內(nèi)進(jìn)行 信號傳輸。c）電絕緣：由玻璃或者塑料聚合物制造的光纖是電絕緣體，因此與其他對應(yīng)的金屈物不 同，它們不存在接地循環(huán)與接口問題。而且，該優(yōu)點(diǎn)使得光纖十分適宜于在對電 子比較敏感的危險環(huán)境中的通信，因?yàn)楣饫w在受侵蝕abration或者短路的情況下 不會產(chǎn)牛電弧或者電火花的危險。d）抗干擾和無串話：光纖形成了一種絕緣波導(dǎo)，因此可以

4、避免（free）屯磁干擾、無線電頻譜干 擾及瞬時開關(guān)產(chǎn)生的電磁脈沖。因此在通過電噪聲環(huán)境吋，光纖通信系統(tǒng)的工作 不會受到影響，而且光纖不需要屏蔽電磁干擾。如果工作在高空而不是在地下， 光纖也不容易遭受雷擊。此外，在光纖之間沒有光干擾。與采用電導(dǎo)體通信不同, 即使許多光纖擰合cable在一起，吊話都是可以忽略的。negligible overhead shield|wave guide 一種用在微波波段中傳輸電磁波的裝置，用于無線電通訊、雷 達(dá)、導(dǎo)航等無線電領(lǐng)域。使電磁波的能量在波導(dǎo)內(nèi)部傳輸，使得能力集中電磁波 的能量發(fā)散不到外面去。electro-magnetic interference 電

5、磁干擾emp電磁脈沖e）信號保密：光纖中的光輻射不明顯，因此光纖可以提供高度的信號安全性。和銅電纜情 況不同，被傳輸?shù)墓庑盘柌荒鼙灰苑侨肭值姆绞剑ㄒ簿褪?，不損失光的功率的情 況下）從光纖中獲取。因此從理論上講，任何獲取傳輸?shù)墓庑盘柕钠髨D都可以被 檢測到。該特性在軍事、銀行及基本數(shù)據(jù)傳輸（如計(jì)算機(jī)網(wǎng)絡(luò)）應(yīng)用方面具有明 顯的吸引力。f）低傳輸損耗：在過去的二十年中，光纖的發(fā)展導(dǎo)致了光纜的產(chǎn)生。光纜與最好銅導(dǎo)體相比 具有非常低的衰減或者說是傳輸損耗。生產(chǎn)的光纖的損耗低于0.2b/km,這是光 纖通信的最大的優(yōu)勢。這（低損耗）有利于具有極寬的屮繼距離的（沒有屮間設(shè) 備的情況下實(shí)現(xiàn)很長的傳輸距離）通信鏈

6、路的實(shí)現(xiàn)，從而降低了系統(tǒng)成本和復(fù)雜 性。低損耗再加上光纜的大容量調(diào)制帶寬使得大部分長途電信應(yīng)用都采用光纖通 信的方式。compelling case 說月良力 applicationcomplexity cost thus facilitate as fabricatedg）結(jié)實(shí)（ruggedness）而柔韌：雖然光纖的保護(hù)性外層是必不可少的（essential）,但是光纖本身具有非常大 的拉伸強(qiáng)度?？赡芊浅Ａ钊顺泽@的是，作為一種玻璃類物質(zhì)，光纖可以被彎曲成 非常小的半徑(radii)的圓圈或者被扭曲(twist)而不會受到損壞。此外，光纜 結(jié)構(gòu)被開發(fā)出來，并證實(shí)光纜的結(jié)構(gòu)很柔韌、很緊密以及很

7、結(jié)實(shí)(nigged)。考 慮到數(shù)量和重量，一般來說光纜在存儲、運(yùn)輸、處理及安裝方面都優(yōu)于相應(yīng)的銅， 而其強(qiáng)度和耐用性至少與銅纜相當(dāng)。dielectricdurability whilst install in terms of rugged compact flexible radiih) 系統(tǒng)可靠和維護(hù)方便：這些特性最主要來源于光纜的低損耗特性，低損耗減少了提高信號強(qiáng)度的中 繼器或者線性放大器的數(shù)量。因此，在使用較少中繼器的情況下，系統(tǒng)的可靠性 比傳通電子導(dǎo)體系統(tǒng)普遍提高。此外，隨著20至30年預(yù)計(jì)壽命的普及，光學(xué)元 件的可靠性已經(jīng)不成問題了。這兩方面因素使得很容易降低維護(hù)的時間及成本。i)

8、 潛在的低成本：通常作為光纖傳輸?shù)拿劫|(zhì)的玻璃是由沙子制造的沙子并不是一種稀有 資源。因此，相較于銅導(dǎo)體，光纖提供了低成本線路通信的潛能。雖然近幾年這 種潛能己經(jīng)在光纖傳輸介質(zhì)的成本上實(shí)現(xiàn)了，通過批量購買的方式光纖傳輸介質(zhì) 已經(jīng)能夠和銅電線(如雙絞線)相競爭，然而在和光纖通信相關(guān)聯(lián)的其他元件部 分，這種低成本潛能并沒有得以實(shí)現(xiàn)。比如，高性能半導(dǎo)體激光器、光電二極管 檢測器以及一些與連接技術(shù)相關(guān)的器件(可拆卸連接器，耦合器等)的成本仍然 相對較高。coupler connection technology concern withas well as detector photodiode ass

9、ociatefractions分?jǐn)?shù)通常將分子讀為基數(shù)，將分母讀為序數(shù)。1/2 = a (or one) half1/3 = a (or one) third1/4 = a quarter or one fouth1/5 = a (or one) fifth2/3 = two thirds9/10 = nine tenths53/4 = five and three quarters15/64= fifteen over (or by) sixty-four15% = fifteen per cent4%o = four per mill1010=the tenth power of ten 或

10、者 ten to the tenth power 第四十 fortieth 其他 sixtieth fiftieth 等等然而，由于光纖傳輸媒質(zhì)的低損耗特性和寬帶特性，在長途鏈路上采用光纖 通信時整個光纖通信系統(tǒng)的成本要遠(yuǎn)遠(yuǎn)低于相應(yīng)的電線系統(tǒng)的成本。如(f)段 所指出的，中繼器和相關(guān)電子器件的需要減少了，從而產(chǎn)生了巨大的成本優(yōu)勢。 雖然在長途鏈路屮這種成本的優(yōu)勢獲得了凈利潤，但在短途鏈路上并非總是這 樣。這是因?yàn)樵诙掏炬溌分?，光電轉(zhuǎn)換(反之亦然即電光轉(zhuǎn)換)帶來的附加成本 起到了決定性的作用。然而，在運(yùn)輸、管理、安裝與維護(hù)以及在(c) (d)中所 提到的優(yōu)點(diǎn)上存在其他可能的成本優(yōu)勢，這些優(yōu)勢在系

11、統(tǒng)的選擇中都是有意義 的。光纖通信成本的降低不但能夠與電線傳輸系統(tǒng)相競爭competition,而且還能 與微波和毫米波無線傳輸系統(tǒng)相競爭。雖然微波和毫米波無線通信系統(tǒng) reasonably具有相當(dāng)寬的帶寬，但是相對較短跨度的“視距"傳輸使得必須每隔不 到兒十千米的距離安裝昂貴的天線塔。因此在大部分工業(yè)社會中，光纖迅速成為 主要的傳輸媒質(zhì)。因此，由光纖組成的傳輸媒質(zhì)內(nèi)的光波載波的應(yīng)用提供了很多的益處。產(chǎn)生 這些改進(jìn)的性能特性的基本原理將以及基本原理的實(shí)現(xiàn)將在以下章節(jié)描述。在此 仍然假設(shè)己經(jīng)對光的基木原理及特性有了一定的了解。advantages of optical fiber co

12、mmunicationcommunication 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 pred

13、ictions, 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

14、which have become apparent as the technology has been developed(a) enormous potential bandwidth.the optical carrier frequency in the range 1013 to 1016 hz (generally in the near infrared around 1014 hz or 105 ghz) yields a far greater potential transmission bandwidth than metallic cable systems (i.e

15、. coaxial cable bandwidth up to around 500 mhz) 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

16、 hundreds of megahertz over three hundred kilometers without intervening electronics (repeaters)is possible.therefore, the in formation-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 rest

17、rict the transmission distance to only a few kilometers at bandwidths over one hundred megahert乙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 enh

18、anced 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 wavele ngths (or, essentially,

19、 fine frequency spaci ng), offers the pote ntial 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(b) small size and weight. optical fibers have very small diameters which are often no greater than th

20、e 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 transmiss

21、ion 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.

22、 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(d) immunity to interference and cross-talk.optical fibers form a dielectric wave-guide an

23、d 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 fib

24、er cable requires no shielding from emi. the fiber cable is also not susceptible to lightning strikes if used overhead rather than undergrounci. moreover, it is fairly easy to ensure that there is no optical interference between fibers and hence, unlike communication using electrical conductors, cro

25、ss-talk is negligible, even when many fibers are cabled togethe匚(e) signal security.the light from optical fibers does not radiate significan什y and therefore they provide a high degree of signal security unlike the situation with copper cables, a transmitted optical signal cannot be obtained from a

26、fiber in a noninvasive manner (i. e. without drawing optical power from the fiber). therefore, in theory, any attempt to acquire a message signal transmitted optically may be detected this feature is obviously attractive for military, banking and general data transmission (i.e. computer network) app

27、lications(f) low transmission loss.the development of optical fibers over the last twenty years has resulted in the production of optical fiber cables which exhibit very low attenuation or transmission loss in comparison with the best copper conductors. fibers have been fabricated with losses as low

28、 as 0.2 db/km and this feature has become a major advantage of optical fiber communications. it facilitates the implementation of communication links with extremely wide repeater spacing (long transmission distances without intermediate electronics), thus reducing both system cost and complexity. to

29、gether with the already proven modulation bandwidth capability of fiber cable this property provides a totally compelling case for the adoption of optical fiber communication in the majority of long-haul telecommunication applications(g) ruggedness and flexibility.although protective coatings are es

30、sential, optical fibers may be manufactured with very high tensile strengths perhaps surprisingly for a glassy substance, the fibers may also be bent to quite small radii or twisted without damage furthermore, cable structures have been developed, which have proved flexible, compact and extremely ru

31、gged. taking the size and weight advantage into account, these optical fiber cables are generally superior in terms of storage, transpoliation, handling and installation to corresponding copper cables, whilst exhibiting at least comparable strength and durability(h) system reliability and ease of ma

32、intenance.these features primarily stem from the low loss property of optical fiber cables which reduces the requirement for intermediate repeaters or line amplifiers to boost the transmitted signal strength. hence with fewer repeaters, system reliability is gen erally en hanced in comparison with c

33、onv enti onal electrical conductor systems. furthermore, the reliability of the optical components is no longer a problem with predicted lifetimes of 20 to 30 years now quite common. both these factors also tend to reduce maintenance time and costs.(i) potential low cos匸the glass which generally pro

34、vides the optical fiber transmission medium is made from sand一not a scarce resource. so, in comparison with copper conductors, optical fibers offer the potential for low cost line communication. although over recent years this potential has largely been realized in the costs of the optical fiber tra

35、nsmission medium which for bulk purchases is now becoming competitive with copper wires (i.e. twisted pairs), it has not yet been achieved in all the other comp on ent areas associated with optical fiber communications. for example, the costs of high performanee semiconductor lasers and detector pho

36、todiodes are still relatively high, as well as some of those concerned with the connection technology (demountable connectors, couplers, etc).overall system costs when utilizing optical fiber communication on long-haul links, however, are substantially less than those for equivalent electrical line

37、systems because of the low loss and wideband properties of the optical transmission medium. as indicated in (f), the requirement for intermediate repeaters and the associated electronics is reduced, giving a substantial cost advantage. although this cost benefit gives a net gain for long-haul links it is not always the case in shoil-haul applications where th