【文章內(nèi)容簡介】 a. electromagic interference b. radio frequency interference c. mechanical damage d. electromagic pulse Unit One Optical Fiber Communication (6) ?Fibers have been fabricated with losses as low as and this feature has bee a major advantage of optical fiber munication. a. 5 dB/km b. dB/km c. 1 dB/km d. ?dB/km (7) ?To obtain both the low loss and low dispersion at the same operating wavelength, which kind of new advanced singlemode structure has been realized: . a. namely b. dispersion shifted c. dispersion flattened d. all of above Unit One Optical Fiber Communication Passage B Advantages of Optical Fiber Communication Communication 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 munications over more conventional electrical munications. In this context we mence with the originally foreseen advantages and then consider additional features which have bee 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 1014?Hz or 105?GHz) yields a far greater potential transmission bandwidth than metallic cable systems?(. coaxial cable bandwidth up to around 500?MHz) or even millimeter wave radio systems?(. 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 , the informationcarrying capacity of optical fiber systems has proved far superior to the best copper cable systems. By parison 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 munication 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 crosstalk. Optical fibers form a dielectric waveguide and are therefore free from electromagic interference (EMI), radio frequency interference (RFI), or switching transients giving electromagic pulse(EMP).Hence the operation of an optical fiber munication 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 munication using electrical conductors, crosstalk is negligible, even when many fibers are cabled together. Unit One Optical Fiber Communication (e) Signal security. The light from optical fibers does not radiate significantly 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 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 (. puter work) applications. Unit One Optical Fiber Communication (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 parison with the best copper conductors. Fibers have been fabricated with losses as low as dB/km and this feature has bee a major advantage of optical fib