Communications Systems

As described above for small a and NA, a fibre is single mode if U < 2.405. Here only one mode, with one group velocity, is possible. This lack of modal dispersion is why single-mode fibre dominates transport media in long-haul communication systems. 

For optical transmission, tire parameters of greatest importance are attenuation (i.e. loss) and material dispersion. In effect tliey define tire limits of tire optical communication system. Loss, due to absorjDtion and scattering, limits tire lengtlis between tire transmission nodes. In transmission quality fibre, tire loss is in units of decibels per kilometre. 

Nonlinear dispersion becomes relevant at sufficient pulse powers. In some fibre stmctures tire interiDlay between tire nonlinear dispersion and tire group velocity dispersion can be used to produce non-dispersive waves called solitons. Solitons, altliough beyond tire scope of tliis treatment, may revolutionize tire communication systems of tire future. A full treatment of soliton tlieory can be found in [4, 261. 

AgraWal G P 1997 Fiber-Optic Communication Systems (New York Wiley-Interscience)... 

The supplanting of germanium-based semiconductor devices by shicon devices has almost eliminated the use of indium in the related ahoy junction (see Semiconductors). Indium, however, is finding increased use in III—V compound semiconductors such as indium phosphide [22398-80-7] for laser diodes used in fiber optic communication systems (see Electronic materials Fiber optics Light generation). Other important indium-containing semiconductors include indium arsenide [1303-11-3] indium antimonide [1312-41 -0] and copper—indium—diselenide [12018-95-0]. 

Laser communication systems based on free-space propagation through the atmosphere suffer drawbacks because of factors like atmospheric turbulence and attenuation by rain, snow, haze, or fog. Nevertheless, free-space laser communication systems were developed for many appHcations (89—91). They employ separate components, such as lasers, modulators, collimators, and detectors. Some of the most promising appHcations are for space communications, because the problems of turbulence and opacity in the atmosphere are absent. 

In the early 1990s, there were more than 9 x 10 km of fiber-optical telecommunication links in practical use in the United States. In addition, many other countries, notably Canada, Japan, and western Europe, have installed extensive fiber-optic communication systems. There are several transoceanic fiber-based telephone cables. Fibers are in use for intracity telephone links, where bulky copper [7440-50-8] wine is replaced by thin optical fibers. This allows crowded conduits in large cities to carry more messages than if copper wine were used. Fiber optics are used for intercity long-haul telephone links, for interoffice tmnk lines, and have replaced many microwave communication links. 

Fig. 16. Applications of fiber-optic communication systems as a function of length between repeater stations and system bandwidth. AppHcations to the...

Pigure 10 shows the typical commercial performance of LEDs used for optical data communication. Both free-space emission and fiber-coupled devices are shown, the latter exhibiting speeds of <10 ns. Typically there exists a tradeoff between speed and power in these devices, however performance has been plotted as a function of wavelength for purposes of clarity. In communication systems, photodetectors (qv) are employed as receivers rather than the human eye, making radiometric power emitted by the devices, or coupled into an optical fiber, an important figure of merit. 

Lithium Niobate. Lithium niobate [12031 -64-9], LiNbO, is normally formed by reaction of lithium hydroxide and niobium oxide. The salt has important uses in switches for optical fiber communication systems and is the material of choice in many electrooptic appHcations including waveguide modulators and sound acoustic wave devices. Crystals of lithium niobate ate usually grown by the Czochralski method foUowed by infiltration of wafers by metal vapor to adjust the index of refraction. 

The relevance of photonics technology is best measured by its omnipresence. Semiconductor lasers, for example, are found in compact disk players, CD-ROM drives, and bar code scaimers, as well as in data communication systems such as telephone systems. Compound semiconductor-based LEDs utilized in multicolor displays, automobile indicators, and most recendy in traffic lights represent an even bigger market, with approximately 1 biUion in aimual sales. The trend to faster and smaller systems with lower power requirements and lower loss has led toward the development of optical communication and computing systems and thus rapid technological advancement in photonics systems is expected for the future. In this section, compound semiconductor photonics technology is reviewed with a focus on three primary photonic devices LEDs, laser diodes, and detectors. Overviews of other important compound semiconductor-based photonic devices can be found in References 75—78. 

We provide more details on inductive interferences in the box below, to make the subject of electrical interference more informative. We also provide a passing reference to communication systems being adopted worldwide. 

In the earlier installations sensitive to such iiuerfercnces (he normal practice was coordination between the generating and power transmission agencies and the authorities of essential services (such as public telephones, defence serv ices and railways), who prov ide (heir (wvn communication systems, lo relocate iheir telephone lines to mitigate this problem at the planning stage,... 

The OREDA Offshore Reliability Data Handbooks covers a variety of components used in offshore oil drilling and platforms, including gas/fire detection systems, process alarm systems, firefighting systems, pressure relieving systems, general alarm and communication systems, evacuation systems, process systems (vessels, valves, pumps, heat exchangers, and compressors), electrical and utility systems, and drilling equipment. 

Extent of emergency power supplies for lighting, communication systems, and key items of equipment (e.g. cooling facilities, reactor agitators, exhaust ventilation) and instruments/alarms. 

Today resource limitations have caused the United States Environmental Protection Agency (USEPA) to reassess schedules for new rules. A 1987 USEPA survey indicated there were approximately 202,000 public water systems in the United States. About 29 percent of these were community water systems, which serve approximately 90 percent of the population. Of the 58,908 community systems that serve about 226 million people, 51,552 were classified as "small" or "very small." Each of these systems at an average serves a population of fewer than 3300 people. The total population served by these systems is approximately 25 million people. These figures provide us with a magnitude of scale in meeting drinking water demands in the United States. Compliance with drinking water standards is not... 

Set up electronic communication systems to receive and transmit data to and from customers. 

Management policies are the source of many of the preconditions that give rise to systems failures. For example, if no explicit policy exists or if resources are not made available for safety critical areas such as procedures design, the effective presentation of process information, or for ensuring that effective communication systems exist, then human error leading to an accident is, at some stage, inevitable. Such policy failures can be regarded as another form of latent human error, and will be discussed in more detail in Section 2.7. 

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