【正文】 gh protective coatings are essential, 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, pact and extremely rugged. 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 parable strength and durability.[3] Unit One Optical Fiber Communication (h) System reliability and ease of maintenance. 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 generally enhanced in parison with conventional electrical conductor systems. Furthermore, the reliability of the optical ponents is no longer a problem with predicted lifetimes of 20 to 30 years now quite mon. Both these factors also tend to reduce maintenance time and costs. Unit One Optical Fiber Communication (i) Potential low cost. The glass which generally provides the optical fiber transmission medium is made from sand— not a scarce resource. So, in parison with copper conductors, optical fibers offer the potential for low cost line munication. Although over recent years this potential has largely been realized in the costs of the optical fiber transmission medium which for bulk purchases is now being petitive with copper wires (. twisted pairs), it has not yet been achieved in all the other ponent areas associated with optical fiber munications. For example, the costs of high performance semiconductor lasers and detector photodiodes are still relatively high, as well as some of those concerned with the connection technology (demountable connectors, couplers, etc). Unit One Optical Fiber Communication Overall system costs when utilizing optical fiber munication on longhaul links, however, are substantially less than those for equivalent electrical line 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 gain for longhaul links it is not always the case in shorthaul applications where the additional cost incurred, due to the electricaloptical conversion (and vice versa), may be a deciding factor. Nevertheless, there are other possible cost advantages in relation to shipping, handing, installation and maintenance, as well as the features indicated in (c) and (d) which may prove significant in the system choice. Unit One Optical Fiber Communication The reducing costs of optical fiber munications has not only provided strong petition with electrical line transmission systems, but also for microwave and millimeter wave radio transmission systems. Although these systems are reasonably wideband the relatively short span “ line of sight” transmission necessitates expensive aerial towers at intervals no greater than a few tens of kilometers. Hence optical fiber is fast being the dominant transmission medium within the major industrialized societies. Many advantages are therefore provided by the use of a lightwave carrier within a transmission medium consisting of an optical fiber. The fundamental principles giving rise to these enhanced performance characteristics, together with their practical realization, are described in the following chapters. However, a general understanding of the basic nature and properties of light is assumed. Unit One Optical Fiber Communication KEY WORDS metallic (像 )金屬的，含金屬的， (聲音 )刺耳的 parameter 參數(shù)，參量 congestion 充滿，擁擠，阻塞 hazardous 危險(xiǎn)的 arc 電弧 ? abrasion 擦傷，磨損 ? immunity 抗擾，免除，免疫性 dielectric 電介質(zhì)，絕緣材料；電解質(zhì)的，絕緣的 waveguide 波導(dǎo) transient 瞬時(shí)的 susceptible (to) 敏感的，易受 … 的 noninvasive 非侵略的，非侵害的 Unit One Optical Fiber Communication attenuation 衰減 implementation 實(shí)施，實(shí)現(xiàn)，執(zhí)行，敷設(shè) ruggedness 結(jié)實(shí) tensile 拉力的，張力的 demountable 可拆卸的 substantial 多的，大的，實(shí)際上的 aerial 空氣的，空中的，無形的 , 虛幻的；天線 enpass 圍繞，包圍，造成，設(shè)法做到 Unit One Optical Fiber Communication ※ ※ ※ wavelength division multiplexed 波分復(fù)用 in parallel on 并聯(lián)到，合并到 in excess of 超過 no greater than 不大于 take…into account 考慮，重視 … stem from… 起源于；由 … 發(fā)生 in parison with… 與 … 比較 give rise to 引起，使產(chǎn)生 Unit One Optical Fiber Communication NOTES [1] ?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. 目前，傳輸 100 km的幾 GHz的調(diào)制信號(hào)和傳輸 300 km的幾百 MHz的調(diào)制信號(hào)都是可能的，因此，光纖系統(tǒng)的可用帶寬并沒有 得到充分利用。 but引導(dǎo)并列從句“ modulation …is possible ”。 “ at several gigahertz over a hundred kilometers”中的“ over a hundred kilometers”為介詞短語，修飾介詞短語“ at several gigahertz”。 Unit One Optical Fiber Communication 當(dāng)各光載波之間間隔比較大時(shí)，用波長描述其間隔，稱波分復(fù)用；當(dāng)各光載波之間間隔比較小時(shí)，用頻率描述其間隔，稱頻分復(fù)用。 “ which is many orders of magnitude in excess of that obtained using copper cables of a wideband radio system”為 which引導(dǎo)的“ fiber informationcarrying capacity”的定語從句。 “ taking the size and weight advantage into account”為現(xiàn)在分詞短語作狀語，“ take into account”為習(xí)慣用法，意為“考慮”，其賓語可放在 take后 ,也可放在 account后。 superior to意為“優(yōu)于”，這里狀語“ in terms of… ”插在了superior與 to