Optical communications

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. 

The primary driver for the expansion of optoelectronic teclmologies is optical communications [2]. It was realized in the second-half of the 20th century that an increase of several orders of magnitude in bandwidth would be possible if optical waves were used as the carrier for telephone signals. The basic configuration of an optical communication... 

Figure C2.15.18. The basic components of an optical communications link.

Palais J C 1992 Fiber Optic Communication 3rd edn (Englewood Cliffs, NJ Prentice Hall) Tanenbaum AS 1996 Computer Networks 3rd edn (Upper Saddle River, NJ Prentice Hall)... 

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

Quaternary Ga In j.As jPj, grown on InP is of major importance to fibre-optic communications. In quaternary compounds, both the gap and the lattice constant can be tailored by changing the chemical composition. In thick layers, in order to avoid the generation of strain-induced defects, care must be taken in adjusting the ratio of x and v to maintain the lattice-matched composition x = 2.2v. The available gaps range from 1.34 eV in InP to -0.75 eV in... 

In additions to improvements in Si, a variety of devices based on compound semiconductors can be expected. Blue lasers witli high brightness and long operating lifetimes already exist in tlie laboratory. LEDs are likely to be used for all lighting purjDoses. The bandwidtli of optical communications will continue to increase witli ever faster semiconductor lasers. 

PCTFEin pLUORINECOMPOUNDS,ORGANIC - POLYCm.OROTRIFLUOROETTTVLENE] (Volll) Fiber optic communications... 

Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

In photodetectors (qv) for photocouplers, optical switches, fiber optics, communication, and imaging, the materials used are InGaAs, InGaAsP, and... 

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]. 

Communications. The advent of the laser improved prospects for optical communications enormously. The coherence of the laser meant that techniques developed in the radio portion of the electromagnetic spectmm could be extended to the optical portion of the spectmm. Because lasers operate at frequencies near 10 Hz, they offer a potentially wide bandwidth, equal to about 10 television channels of width (ca 10 Hz). It has not proved possible to take advantage of this full bandwidth because devices such as modulators capable of operating at 10 Hz are not available. 

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. 

However, optical fiber communications are not useful only for long-distance communication links. Fiber-optic data links are also used in a variety of short-distance systems, for example in computer—computer links and for internal communications on ships and aircraft. Figure 16 shows some possible appHcations for fiber-optic communications, with respect to length and bit rate. The common carrier appHcations, like telephone links. He to the upper right of the dashed line labeled 100 MHzkm. However, a wide variety of other lower performance appHcations, illustrated to the lower left of the dashed line, are in use or under development. 

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

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. 

Photodiode (LED) Pin photodiode Cameras, strobes, illuminators, remote controls, IR sensors Fiber-optic communications, liber links... 

Optical communications have an large capacity for the transmission of information and, in this respect, is far superior to... 

Heterodyne is a very efficient tool for detecting the phase of a "coherent" signal i.e. a signal which has a stable phase relation to the local oscillator. The detector is only limited by the quantum fluctuation of vacuum. This property is common use in coherent lidar. Satellite to satellite optical communications using laser as a local oscillator are under development (Fig. 3). 

Light wave technologies provide a number of special challenges for polymeric materials. Polymer fibers offer the best potential for optical communications in local area network (LAN) applications, because their large core size makes it relatively cheap to attach connectors to them. There is a need for polymer fibers that have low losses and that can transmit the bandwidths needed for LAN applications the aciylate and methacrylate polymers now under study have poor loss and bandwidth performance. Research on monomer purification, polymerization to precise molecular-size distributions, and weU-controlled drawing processes is relevant here. There is also a need for precision plastic molding processes for mass prodnction of optical fiber connectors and splice hardware. A tenfold reduction in the cost of fiber and related devices is necessaiy to make the utilization of optical fiber and related devices economical for local area networks and tlie telecommunications loop. 

In the past few years, erbium doped materials gained much attention in the field of optical communications, since the Er ion shows a broad optical emission at 1540 nm [111], within the main wavelengths window in the telecommunication technology. For this reason Er can be suitable as an active element for the generation and amplification of light in optical devices [112,113], also if limitations for the realization of an efficient planar amplifier are related to the small cross section for Er excitation (typically 10 -10 cm according to the matrix). In order to enhance Er ion pumping efficiency, a possible... 

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