Fiber Optics Illustrated Dictionary - Part 27 pdf

Fiber Optics Illustrated Dictionary serve as wavepass filters, bandpass filters that pass a range of wavelengths, or notch filters that reflect a range of wavelengths. Dichroic filter arrays can be assembled by lining up multiple components on a flat or curved base sub- strate and selectively coating the surface with dichroic reflective materials. Thus, the surface could be patterned to selectively filter not only certain frequencies, but wavelengths coming from certain directions over a specified area. Dicke, Robert H. (1916-1997) An inventor and edu- cator, Dicke is known for his modeling of gravita- tional forces, his contributions to aerospace research, and his pioneer work in maser/laser technology. In 1941, he joined MIT's Radiation Laboratory, be- coming a Princeton professor a few years later. In May 1956, Dicke submitted a patent application for "Molecular Amplification and Generation Systems and Methods" describing ways to generate and amplify electromagnetic waves, such as those in the microwave frequencies, through the activity of resonant gases (e.g., ammonia). Dicke's patent anticipates Fabry-Perot interferometers and cavity-resonating gas lasers. Since an infrared light source is described in some of the drawings, it is a pioneer laser device (U.S. #2,851,652). Dicke's invention sought to estab- lish more efficient ways to generate coherent micro- waves or infrared waves and to improve the cavity- resonating (amplification) effect. Dicke was a member of prominent astronomical and physics societies and won numerous awards for his work. See lavan, Ali; laser history; Townes, Charles. Dicke radiometerA device developed by Robert H. Dicke to detect the very subtle radiation residual from the Big Bang, the theoretical cataclysmic expansion .of our universe. DID See Direct Inward Dialing. dielectric Nonconducting material that provides an Microwave (Maser) and Optical (Infrared) Patent Application - 1956 Sept. I. JI58 III H. cm:.Ka 2,8&1 ~ AMPLUtc4nOH All) Q8HBU110N SYSt'JILtJ A!ID asrtiODS 'Un _ 11. 19M a .,.,~ a Sept. 9 t Isse A. H. DlCK£ US1_ IIlLI:DUl.IJ\ AMPLII'ICA'l'.IOII um CifJID.lTlO~ SYS'1'UiS AlIt) M£'!1WQS '1104 _ 1&. 1lMlI a &8,L a.~ ~ These are excerptsfrom the diagrams that accompany Robert Dicke s May 1956patentsubmissionfor "Molecular Amplification and Generation Systems and Methods" describing a means to generate and amplify electromagnetic waves, both in the microwave (maser) and infrared (laser) frequencies through resonant gases such as ammonia acting in an open cavity resonator. The patent is a milestone in the sense that it was rapidlyfollowed by a number of interferometers and optical laser andgas resonating inventions. The Dickepatent was published in September 1958 and may have inspired more scientists than have given Dicke credit for the concepts described. • On the left is an example of apumping system. On the right, a resonating cavity with coherent infrared radiation aimed into the reflective cavity through switching valves (forerunner to Q switches). [U.S. patent #2,851,652.] 252 © 2003 by CRC Press LLC insulating layer by impeding or resisting the passage of current. On an atomic level, a dielectric exposed to an electric field experiences slight changes that result in a bound charge at the surface of the material through polarization (as opposed to movement as occurs in conducting materials). Dielectric materials are often used as cable shield- ings or applied in layers between sheets of conducting materials in condensers. Common dielectrics used over the decades include paper, cloth, air, Bakelite, glass, ceramic, and certain synthetics. Glass and ceramic are the ones used in Leyden jars and utility pole insulators. See air dielectric, insulator. dielectric breakdown The point above which which a dielectric (insulating) material will show conducting/sparking characteristics, usually expressed in volts. dielectric constant DK (symb. - K). The dielectric constant is the degree to which a material can be polarized which determines the bound charge associated with the material. It describes how much an electric field is reduced within a dielectric material. The dielectric constant is related to the properties of a material such as composition, density, homogeneity, and temperature. The bound charge of a dielectric surface can be calculated by using Gauss's law. The dielectric or permitivity property of a material influences the relative speed at which an electrical signal will propagate. The speed of the signal is roughly inversely proportional to the square root of the dielectric constant. Thus, low dielectric constants are associated with higher signal speeds and vice versa. As networking systems become faster and more sophisticated, the dielectric constants of fabrications materials becomes more important. See air dielectric, dielectric, Gauss's law. dielectric feed In communications, a dielectric feed is a type of microwave lens that fits into the mouth of an antenna waveguide. It provides a wideband al- ternative to scalar feeds. dielectric interaction Phenomena resulting from the presence or proximity of dielectric materials relating to the storage and discharge of energy that may cause interference or distortion to signal transmission systems. One solution to reducing undesired dielectric interaction is to use air dielectric cables in which energy-interacting materials are held away from conducting materials by a cushion of air. differential cable A cable commonly implemented as a twisted-pair wireline with two wires transmit- ting the same data at the same time, except that one is transmitted as a positive (+) signal and one is transmitted as a negative (-) signal. It is sometimes also called a balanced cable. Since noise is typically introduced along the transmissions path, especially over longer distances, there will be slight changes in the signal at the receiving end. The "difference" is then taken between the two signals on the two wires in order to eliminate the portion associated with the noise. Twisted pair is used to cause the two separate lines to occupy, as nearly as possible, the same physical space along the transmissions path. This system enables longer cable lengths to be used (e.g., RS-422 serial specification) than in single-ended systems (e.g., RS-232 serial specification). differential GPS DGPS. An implementation of the Global Positioning Service designed to improve local accuracy of the data. One or more high-end GPS receivers are placed at known locations where they receive GPS signals. These become reference stations, which estimate the variations of the satellite range measurements, forming corrections for GPS satellites within current view and then broadcasting the correction information to local users. See local differential GPS. differential modulation A means of relative modulation based upon the detected state of the previous instant, rather than on an absolute predefined parameter. See delta modulation for an example ofa commonly used, simple type ofdifferential modulation. differential phase shift keying DPSK. A means of relative modulation in which the previous state of the carrier signal phase is detected, and the subsequent state is based on the previous, rather than on an absolute predefined parameter. See phase shift keying. differential polar relayA telegraphic transmissions relay in which the armature is polarized by contact with a permanent magnet and is operated by the difference in the strength of the currents. The direction of the currents constantly changes and can be controlled with a pole changer. Differential PowerAnalysis DPA. A powerful analytical tool for extracting secret keys from cryptographic devices using statistical analysis and error correction techniques. Since a large percentage of sensitive computer communications is stored on media that may be carried about (portable computers, smart cards, etc.) or are transmitted over wireless networks, which can be intercepted by eavesdroppers, analytical tools for evaluating cryptographic integrity are important for developing and testing new techno logies. DPA can be used to attack a system to try to discern encoded information using hardware that is readily available. While many technologies are resistant to Simple Power Analysis (SPA), Differential Power Analysis can break many systems that are immune to SPA attacks. As is often the case, there may be a trade-off in time and power; a DPA attack may take longer than a SPA attack. DPA has been put into prac- tical application by Paul Kocher and Cryptography Research. differential pulsecode modulation DPCM. A means of sampling a signal, subdividing it, and assigning values to the individual parts (quantization) in order to add this information to a carrier signal. This modulation can be done in a number of ways, and not all PCM transmissions are compatible. PCM is a very common means of converting analog to digital signals and is widely used in telecommunications. In differential PCM, a transmitted digital signal is used to represent the difference between consecutive analog signals. These differences are obtained by using a fixed quantization step size. See quantization, pulse code modulation. 253 :;:II~ , j~ © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary differential quadrature phase shift keying DQPSK. In general, quadrature phase shift keying (QPSK) is a modulation scheme in which four signals are used, each shifted by 90 degrees, with each phase representing two data bits per symbol, in order to carry twice as much information as binary phase shift keying. DQPSK is a subclass in which the difference between the current value of the phase and the previous value of the phase are used instead of the absolute value of the phase. See modulation, quadrature phase shift keying. differential Ziv-Lempel difiZL. A text compressor combining Lempel-Ziv compression and arithmetic coding with a form of vector quantization, described in 1995 by Peter Fenwick. This compression scheme is similar to Lempel and Ziv's original LZ77 scheme but without explicit phrase lengths or coding for lit- erals. It combines dictionary compression and vector quantization by using a standard scan to detennine the longest earlier phrase to match ensuing text to create a reference phrase. A phrase includes a posi- tion code, sequence of zero symbols, and terminating nonzero symbol which is processed through arithmetic coders for displacement and data. Coding pro- ceeds until an unexpected character is encoded; displacement coding accounts for most of the compressed output stream. Performance of diftzL compares reasonably well against LZB and LZ3VL. The unique characteristic of diffZL is that it has no explicit phrase length or li teral encoding. The development of diffZL leads, in part, to the suggestion that the limits ofLZ77 compression may be about 3.0 bitslbyte, a limit that has very nearly been reached. See Lempel-Ziv. Diffie-Hellman A fairly fast public key encryption system described by Whitfield Diffie and Martin Hellman in a 1976 IEEE issue of Transactions on Information Theory entitled "New Directions in Cryptography." This concept has since been incorporated into many encryption schemes, including some Cellular Digital Packet Data (CDPD) systems and the well-known Pretty Good Privacy (PGP) program developed by Philip Zimmermann. While the inventors patented the system, it came under dis- pute because of its public disclosure prior to the patent application. See Hellman-Merkle, Pretty Good Privacy. diffraction The reflection of wave or wavelike phenomena as they encounter an obstacle that is in their line of travel can cause a complex interaction called diffraction as the incoming waves are reflected away from the obstacle and somewhat towards the incoming wave (depending upon the angle of incidence). If the obstacle causing the diffraction is fixed and ordered and the diffraction phenomenon is narrow and homogenous, the diffraction pattern may be more easily studied and exploited. As an example, some of the sound waves emanating from an audio speaker component may hit the interior of the speaker cabinet and reflect back across the original sound waves, causing a complex pattern in the sound waves where they interact. Some of the waves hitting the cabinet interior will be absorbed and 254 some will be diffracted, depending upon the shape and composition of the cabinet, the distance from the speaker to the cabinet interior, and the reflectivity/ absorbancy of its construction materials. Diffraction may have both positive and negative con- sequences. In acoustics, diffraction can sometimes create a more complex or interesting sound, but it may also cause unwanted interference, depending upon the nature and magnitude of the diffraction. Similar effects occur in the movement and interaction of electromagnetic phenomena. When light energy encounters an obstacle, a fringe pattern may result from the light diffracting from the obstacle as it reflects back from the reflecting obstacle. This fringe pattern may be seen with scientific instruments and can provide information about the character of the light and the obstacle. When viewed with optical instruments, diffraction patterns tend to appear ellipti- cal. In Fresnel lenses, diffraction through successively angled prism-shaped projections serves to concen- trate light. In rear projection systems, diffraction helps display an image over a large surface. Diffraction patterns in crystal structures have interesting properties due to the lattice arrangement of the atoms. When hit with a stimulus (e.g., collimated X- rays), some of the atoms in the upper surface will reflect back the beams, while some beams travel through the upper surface to reflect from the next level in the lattice, etc., creating a complex but pre- dictable diffraction pattern that was first described mathematically by W. Lawrence Bragg in the early 1900s. In some cases, diffraction can be acoustically or op- tically controlled to produce complex effects or filtering mechanisms. See Bragg's law, diffusion, diffraction grating, dispersion, Fresnel lens, spot of Arago, Wood anomaly. diffraction gratingA component designed to diffract electromagnetic or acoustic phenomena as they im- pact a grating component, which is often a corrugated structure. In optics, a diffraction grating can help reveal some of the characteristics of light and can be harnessed to provide some control over the direction of diffracted light and the wavelengths affected by its shape, its period (distance between corrugations), and its materials. This is useful in filtering wavelengths and is important in the development of narrow-wavelength lasers and fiber optic filament switching assemblies. See Bragg grating, diffraction, dispersion, grating equation. diffraction orders An ordered set of reflection patterns associated with an ideal diffraction grating that diffracts incident radiant energy in discrete directions at angles that can be calculated. See grating equation. diffraction, Fraunhofer Diffraction in situations where the factors that influence the complexity of the diffraction patterns are somewhat controlled. For example, a light source should be a sufficient distance from the reflecting obstacle and the beams monochro- matic and parallel in order to create planar waves that can be easily observed (somewhat like surface waves © 2003 by CRC Press LLC hitting the side of a fixed dock). Hence, it is also known as far-field diffraction. Because laser beams can be collimated more readily than other wave sources, they are favored for this type of study. Diffraction Types and Light Sources A I~~§: =:::===: I B I~~§:::::::=:::::::::=::::::::::~"o/ Several basic concepts in diffraction light sources are illustrated here. Imagine light beams generated by lasers that are aimed at obstacles set at distances that are far or nearrelatedto the breadth of the beam. A collimatedlaserbeam canprovideplanarlightsuit- able for diffraction studies (A and B). When the parallel beams strike a planar obstacle sufficiently dis- tant, they are reflectedbackin ways that are relatively straightforward to observe and predict with mathematical calculations. This is called far-field or Fraunhofer diffraction. (As with most things, its actually a little more complicated. Since laser light is readily planar, you can place the obstruction closer and still get good results, but you will probably have to compensate by putting the viewing optics farther away.) Laser light can also be usedtoprovide apointlight source thatcreatesa spherical lightbeam when a lens is interposed to focus the beam on a small hole (C) through which itexits. When the sphericallight beams encounter an obstacle, especially one close to the light source, the reflection pattern and its interaction with incoming lightwaves is more complex (D), as is the resulting diffraction pattern. This situation is called near-field or Fresnel diffraction. diffraction, Fresnel Diffraction that is more chal- lenging to differentiate and calculate than basic Fraunhofer diffraction due to factors that complicate the interactions of the incoming waves and those that are reflected by an obstacle, such as nonparallel waves, or the close proximity of source and refracted waves is called Fresnel diffraction or near-jield dif fraction. diffraction, selected area (SAD). The optical viewing of diffraction patterns in a selected area ofa speci- men, usually as viewed through an aperture for this purpose. This is a means to narrow the field of inter- est, for example, to view asingle-crystal pattern. For even smaller views, such as a single particle, it may be necessary to use microdiffraction, with tradeoffs in precision due to optical limitations. See diffraction. diffuser, LambertianAn ideal diffuser, with a spatially uniform lossless reflectance over a wide spectrum equal to unity (one). While no physical diffuser has perfect diffusing characteristics, there are some materials that have excellent diffusing properties and thus are useful as references for calculations, lab work, etc. Barium sulfate (BaS0 4 ) is a material with excellent diffusing properties useful as a reference ~:::g ;~~~~~~~a~:~~~~~~o~~;~~:ei~~~~~11 Lambertian diffusers to study their influences on sur- rounding objects or to create certain lighting effects. See diffusion, isotropic antenna. diffusing glass Glass that has been shaped, coated, laminated with plastic film, or sand blasted, to "break up" the incoming light in terms of its direction of travel such that the exiting light has a smoothly vary- ing quality. Perceptually, diffused light looks "soft" and even rather than bright and narrowly focused. Sometimes a second type of clear or colored glass is used as a laminating layer in the fabrication of diffusing glass. Thus, the exiting light emanates in many directions as it passes through the diffusing layer associated with the glass. Small-diameter glass fiber filament arrays may be used in place ofa sheet of glass, the filaments acting as prisms to diffuse light evenly. Diffusing glass is useful for scientific instruments, lab experiments, projectors, interior lighting, windows (especially skylights, which are typically small), and other applications where a more uniform distribution of light is desired. Diffusing plastic is sometimes used in place of diffusing glass (e.g., in photo finishing) to reduce cost or weight. Light diffusion tends to in- crease scattering loss and glare. Scattering losses can be minimized by placing a reflector around the diffused light to reflect back up to about 90% of the light in the desired direction, as in a car headlight. Materials can be tested for their diffusion characteristics with a spectrophotometer or modeled with ray- tracing computer algorithms. Either way has benefits. Physical testing gives a real-life measurement but may be cumbersome in terms of time and assessment, especially when evaluating unevenly diffusing materials over a wide surface area. Algorithmic testing can save time if many measurements are required or the surface area area is large or uneven, but is only as accurate as the theory and programming inherent in the software. The fabrication of diffusing glass must be carried out such that laminated structures or coatings are tightly bonded to the glass with materials with matched (or othelWise appropriate) refractive indexes to minimize loss at laminating seams. Bonding materials must also be transparent, to let the light pass into the next layer of the laminate, and must be applied in a way that keeps out air bubbles or particles. Since loss at joints and through materials with vary- ing refractive indexes is characteristic of layered components, there have been suggestions for ways to incorporate the diffusing structures into the lens itself, whether this be a glass lens or a fiber optic lightguide. 255 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Monolithic Lens/Diffusing Components Diffusion Configurations FIG. 1 /7 ~ ~~J_ilH : ~~:~ =.~ <\ ~- -=- - _ - - - - _. I~ I~~-::::": I- /4 J4~JI FIG 2b AG3b 6' , , ~I \: :t::r Some examples of monolithic diffusion lenses, that is, lenses that incorporate the diffusing microsurfaces in the lenses themselves rather than relying upon laminated coatings or plastics to provide a separate dif- fUsing layer. The advantages of monolithic components include longer lifespan (coatings can peel away) and lower light losses from bubbles, particles, or differ- ing defractive indexes. [Shie et al., U.S. patent #6,266,476, July 2001.} Shie et al. have developed a number of designs for embodiments of monolithic diffusing elements that overcome some of problems of layered fabrications. diffusion At the molecular level, a net transfer of mass due to random molecular motion caused by concentration gradients in the diffusing material. Mol- ecules will move from regions of high concentration to low, a process that continues (unless interrupted) until the concentration reaches a state of equilibrium. As an example, sugar stirred into a hot cup of coffee diffuses through a process called osmosis until there is an even suspension of the sugar dissolved through- out the liquid. Certain ions are known to have higher diffusing properties than others due to their molecular motion. These properties can be exploited to create materials for facilitating diffusion in scientific experiments or commercial products. An isotropic diffusing medium is one that is not con- sidered dependent upon the direction of motion or Some example configurations for incorporating a speciallyfabricated lens with a diffusing microstruc- ture as it might be used in the path of a light beam to homogenize (diffuse) andpropagate the beam in the desired direction(s). In this case the lens and diffusion surface are integrated, but the general concepts may apply to certain laminated structures, as well. [Shie et al., U.S. patent #6,259,562, July 2001.J orientation of its motion and thus would diffuse evenly in all directions from the point of reference (see isotropic antenna). An anisotropic diffusing medium is one in which direction or orientation would influence its diffusion properties. In the broader sense, diffusion refers to changes in the direction of travel ofa constrained or point radiant energy source (e.g., a beam of light) such that it spreads over a wider area. The amount of diffusion that occurs in any given situation is dependent upon the angle of incidence of the radiant energy source, 256 © 2003 by CRC Press LLC Laminated Diffusing Glass Digital and Analog Clock Examples On the alarm clock on the left, a digital readout shows the time incremented in minutes. On the right, an analogdisplay with a 'sweep hand'travels through the minutes and seconds in its arc in a continuous movement. information can be achieved by combining, organiz- ing, and variously encoding this digital information. Most communications systems prior to the 1970s were analog, but the trend is strongly towards converting analog signals into digital signals. Digital systems allow a far greater degree of control, security, compression, noise control, and modifiability compared to analog systems. A simplistic explanation of the difference between digital and analog is often illustrated with watches. An analog watch has a hand that sweeps around in a 360 0 arc, showing hours and minutes and the posi- tions in between. A digital watch has a readout that displays the time incrementally, usually in one second or one minute increments. Another example is an analog dial on an AM radio that allows the tuner to be gradually adjusted through adjacent stations. As the dial moves, the signal vol- ume increases and decreases, and there may even be periods where multiple radio signals overlap. This type of radio dial does not allow the listener to jump directly from a low frequency station to a high frequency station. With digital pushbuttons on AM and FM radios, the tuner can be set to jump to a specific frequency, and stations can be selected in any order, even if they are not in adjacent frequency ranges. In an analog phone system, the phone equipment con- verts sound waves into electrical signals that are sent through the lines between the conversing parties, but it is also possible to encode the conversation as a digital signal and send it through a computer network or computerized phone system. Digital encoding allows the information to be compressed, modified, stored for later retrieval, or sent in conjunction with other data signals (such as a computer data file transfer) at the same time. See analog, ISDN, quantization, voice over ATM. Digital 8A video recording/playback system developed by Sony. It is a hybrid transitional format that records digital video signals onto Hi8mm tapes. See D- VHS, Hi8mm. Digital Access and Cross-connect System DACS. A technology for reconfiguring a circuit, without coating glass the diffusing materials, the power of the initial radiant energy, and ambient environmental conditions (e.g., humidity). Diffusion is characteristic of other propagating sources besides radiant acoustic or electromagnetic energy. A drop of water hitting a flat surface straight on will diffuse in a more or less even pattern, as it spreads from the initial point of contact. See diffraction, diffusing glass, Fick's first law. By coating a glass surface with chemicals or plastic film laminates, it is possible to alter the course of travel of a light beam to spread it over a wider area, thusproviding more even illumination overa broader area. This is usefUl for microscope stages,for example, where even lighting aids in reducing contrastypoint- light shadows, thus facilitating sample observation and imaging. diffusion coefficient A mathematical means of ex- pressing the transfer rate of diffusing atoms through molecular movement from the point at which they diffuse. This provides a way to evaluate or model the different diffusing characteristics of media and energy stimuli. See Fick's first law. diffusion, semiconductor 1. Diffusion of dopant atoms in a semiconductor upon stimulation by heat. 2. The process by which dopants are introduced into a semiconductor medium to create p-n junctions, con- duction channels, and other useful structures. digest access authentication See authentication, digest access. dig A Unix command providing information about computer network domain names. digital A means of representing information in discrete units, rather than as a continuous stream. In communications technologies, information is typically represented in terms of binary units. There are many ways to represent information in a binary system: on/off, high/low, large/small, changed from previous state, loud/soft, fast/slow, lit/unlit, up/down, present/absent, etc. In digital computing, the binary units are usually ones and zeros. In electronic circuits, the units are often represented electrically by on/off, high/low, or change from previous state. Despite the simplicity ofa binary system, it is powerful and flex- ible, and extremely sophisticated processes and 257 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary manually changing the interconnections. It is similar to a multiplexer, except that changes can be made with software, rather than through physical rewiring. Digital Advanced Mobile Phone Service See DAMPS. digital audio broadcasting DAB. A transmission modulation technique that sends digital rather than analog audio signals. One of the more interesting developments in DAB is that impediments that cause interference to analog signal transmissions (e.g., mountains) are used as reflectors in digital signal transmissions to improve reception. In the process, the best regional transmitter is automatically selected to forward the signal to local listeners. Thus, a regu- lar antenna, like the kind commonly mounted on a car, can be used to receive distortion-free programming with excellent sound quality. DAB receivers have been commercially available since 1998, and as of the turn of the century there were more than 200 million DAB users worldwide, a number that is expected to grow substantially. DAB signals are individually coded at the source, error protected, and time-interleaved in the channel coder. The services are then multiplexed in the Main Service Channel (MSC), combined with Multiplex Control and Service information, and transmitted through the Fast Information Channel (FIC). Trans- mission frames are formed in the Transmission Mul- tiplexer. The DAB signal is shaped through Orthogo- nal Frequency Division Multiplexing (OFDM) and transposed to the selected radio frequency, where it is amplified and transmitted. At the receiving end, the digital signal is demodulated and channel decoded to process and error-correct the signal and then fed to the listener's radio device. Left and right audio channels are produced by processing through an audio decoder, data information is processed as is appropriate if textual or other enhanced features are in- cluded, and the final product presented to the user. DAB is not restricted to audio broadcasting; the concept is applicable to a variety of types of programming, which means car radios, for example, could display images and textual information (song titles, artist names, lyrics, etc.) in conjunction with the radio programming. Given these characteristics, it's not surprising that there is much excitement in the radio industry about the potential of this new digital broadcasting technology. Within the next few years, it is likely to revolutionize the face and form of radio programming. Currently DAB is established as a standard for terrestrial broadcasting, but it has been developed with an eye toward future satellite broadcasting as well. See Digital Audio Broadcasting Forum. DigitalAudio BroadcastingForum, DAB Forum, World DAB Forum A consortium dedicated to the commercial implementation of the DAB Standard through international coordination and cooperation among official governmental and regulatory authori- ties, manufacturers, network providers, and data broadcasters. World DAB was initiated in Europe, but the technology developed by the consortium is being imple- 258 mented worldwide. In June 2000, the Asian DAB Committee was established during a conference in Singapore, and a cooperative Memorandum of Understanding was signed between the WorldDAB and the Singapore Broadcasting Authority. http://www.worlddab.org/ http://www.asiadab.org/ DigitalAudio BroadcastingStandard,DAB Stan- dard An ITU standard for digital broadcasting developed by the Eureka 147 Consortium, an international group of broadcasters, consumer electronics developers and scientists, and radio network opera- tors. The work on the DAB Standard culminated in January 2000 and went onto the next stage, commercial implementation with the Technical and Commer- cial Committee of the Digital Audio Broadcasting Forum. See digital audio broadcasting. http://www.eurekadab.org/ digital audio radio DAR. A new audio broadcast technology that provides high quality sound over the airwaves and a wider selection of regional programming. It also integrates with various news, paging, and email services. There has been talk of putting DAR in the S-band, but a number of technical characteristics of DAR indicate this may not be the best choice. In 1995, the Federal Communications Com- mission (FCC) assigned a frequency spectrum for DAR use. See digital audio broadcasting. Digital Audio Radio Service DARS. A high-quality-audio satellite-delivered radio programming service. Thus, listeners in remote areas who are without terrestrial broadcasting services or those who wish to receive higher-quality audio than might be available through a local analog-based station can receive high-quality radio transmissions. Technologies change, so these numbers may change as capabilities improve, but during the mid-1990s, at least 12.5 MHz of radio frequency spectrum was needed to support a commercial DARS system. Since only 25 MHz of spectrum had been allocated to DARS in 1997 (2320 to 2345 MHz), when licenses were auctioned by the Wireless Telecommunications Bureau (WTB) of the Federal Communications Com- mission (FCC), only two licenses were made available to four applicants that filed by 1992. The auc- tion bids for these licenses approached $90 million each and were finally awarded to Satellite CD Ra- dio, Inc. and American Mobile Radio Corporation. The FCC stipulated that winning applicants must have a satellite operational within four years, with the system as a whole operational within six years. digital audio tape DAT. A high-quality, high-capacity digital audio recording format suitable for high quality digital audio recordings and computer data storage. For audio recordings, the sound is sampled, quantized, and converted to a specified encoded format. The encoding includes error checking mechanisms and tracking information to facilitate search- ing for aparticular location on the tape. DAT became popular in Europe in the early 1990s, but American vendors were so concerned about audio piracy on DATs that they effectively blocked the spread of the technology in the United States. DAT is now used to © 2003 by CRC Press LLC Some technologies incorporated into DAVIC are the intellectual property of the contributors; they have agreed to make the technology available to anyone for free or for reasonable royalty fees. http://www.davic.org/ digital bank, digital channel bank See D bank. digital bearing discriminator DBD. Adigital RF- based system used in direction-finding applications, often with microwave frequencies. In conjunction with aradio-frequency receiver, it enables the direction of arrival ofincoming signals to be estimated. A DBD can be configured by creating a circular array of open direction finding receivers, thus enabling a full 360-degree field of view and a high probabil- ity of intercept, without the need to rotate the antenna or to guess at the incoming direction in order to ori- ent the antenna correctly. Each element in the array feeds individually into the network where the incoming data can be processed for spatial modeling. digital camcorderA digital camera capable of cap- turing and storing information at a rate that is fast enough to create a series of digital frames which, when played back, show full motion video. See cam- corder, digital video, dry camera. some extent for computer tape backup systems. digital audio-visual DAV. Digitized audio/video data that typically bypasses a computer's main bus. Digital Audio-Video Council DAVIe. Anonprofit association established in 1994 in Geneva, Switzer- land, to promote global open interfaces and protocol specifications (DA VIC specifications) in audio-visual applications and services. There are over 200 member companies from more than 25 countries worldwide, representing manufacturing, service, research, and government agencies. DAVIC concerns itself with the specification and development of tools rather than systems, with a focus on identifying and specifying components which are relocatable on a specific platfonn, and which are also cross-platform. The DAVIC 1995 specification recommends SDH! SONET as the core network physical layer to which ATM cells, as standardized by various international bodies, can be mapped. Timing involves the use ofa transmit clock derived from the network. Jitter is also managed with the network clock as the reference clock. There are five main entities within the specification, as shown in the chart. Abbrev. Notes digital cellular service A digital version of mobile cellular telephone communications in which the voice conversations are sampled, quantized, and encoded for transmission. This permits increased security, privacy, capacity, better handling of noise interference, and corrective processes when roaming across cells. digital certificate See certificate. Digital CommandControl Standards, DeC Stan- dards A set of National Model Railroad Association (NMRA) standards for packet-based digital command control. The DCC standards encompass the format of information that is sent via digital command stations to digital encoders using NMRA digital signals. Digital Communications Conference DCC. An an- nual communications conference jointly held by the ARRL and TAPR radio associations. It is an international forum for beginning and expert amateur radio operations to discuss digital communications technologies and their applications. digital cross-connect DXC. A centralized network component for aggregating and interconnecting a variety ofdigital signal links, ranging from a few to several thousand. For example, a multi bank cross- connect running at 44 Mbps can carry almost 700 telephone voice channels per line. DXCs are used in wired, fiber optic, and digital radio systems. SONET ring-based optical interfaces for DXCs were introduced in the mid-1990s, by Lucent Technologies. DXCs typically have interface ports for remote access and monitoring. Smaller units are often rack mountable. Larger units may be floorstanding and may include cable management enclosures and prewired fuse panels. See add/drop multiplexer. Digital Data Communications Message Protocol DDCMP. A station-to-station, byte-oriented, link- layer protocol developed by Digital Equipment Cor- poration (DEC) in the early 1970s that was used to develop DEC's network architecture as a processor- to-processor system (DECnet). DDCMP continued to evolve and had reached Phase IV by the mid-1980s. DDCMP functions over half- and full-duplex syn- chronous and asynchronous channels in point-to- point and multipoint modes. It provides management of the physical channel, message sequencing, and data integrity. DDCMP has been implemented on a wide variety of types ofsystems, including worksta- tions, personal computers, and robots. Digital Equipment Corporation DEC. A well- known computer hardware/software/services com- panywhich was established in the 1950s by Kenneth H. Olsen. DEC is perhaps best known for its PDP minicomputer series, the subsequent VAX series (VMS and UNIX operating systems), and the DEC Alpha. Many universities are equipped with VAX machines. In 1998, DEC was bought by Compaq, one of the leading makers of desktop computers. See Compaq. Digital European Cordless Telecommunications DECT. Now called Digital Enhanced Cordless Telecommunications. An organization and wireless standard developed in Europe and adopted by the Eu- ropean Telecommunication Standard Institute (ETSI) Connects CPS to SPS Connects SPS to SCS Content Provider System CPS Service Provider System SPS Service Consumer System SCS CPS-SPS Delivery System - SPS-SCS Delivery System - Entity 259 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionanj in 1992. It was originally proposed as a unifying digital radio standard for European cordless phones. It has since been adopted by other countries, including Britain and some Asian countries. The DECT standard improves on previous technologies by support- ing two-way calling as well as better mobility. Open Systems Interconnection (OSI) principles have been incorporated into DECT in the sense that it con- sists ofa physical layer, a data link layer, and a network layer. DECT is implemented with transceiving base stations and mobile handsets. As it is optimized for capabilities different from those developed for cellular, it re- quires more cells to be used in a manner similar to cellular, due to the low power signals of DECT, but higher densities are then also possible. DECT incorporates hand over capabilities and Dy- namic Channels Allocation (DCA) instead of fixed channels, with the hand unit scanning for the best signals. Digital HDTV Grand Alliance A consortium of major U.S. and European entities with a stake in consumer electronics and broadcast television technologies. The Alliance was founded in May 1993 as a result of work by the Advisory Committee on Advanced Television Service (ACATS). In order to streamline the process of development and testing of advanced television (ATV) systems, the Alliance was tasked with taking the best of the best proposals evaluated and researched by ACATS and combining them into one superior technology, without making the specification too rigid or industry-centric. ACATS continued to advise the Alliance through the ACATS Tech- nical Subgroup which was divided into six Expert Groups. Based on the work of the Alliance and ACATS, an advanced system proposal was tendered in late 1993 and early 1994 which was subsequently approved for prototyping, construction, and laboratory and field testing in 1995. The results were communicated in the ACAT Final Technical Report. Requests from the Federal Communications Commission resulted in the addition of standard definition television (SDTV) multiple stream scanning formats being adopted for inclusion in the ATSC Digital Television Standard. See Advisory Committee on Advanced Television Service, ATSC Digital Television Standard. Digital Loop Carrier DLC. Similar to a Local Loop Carrier, which provides a physical connection between subscribers and a main distribution switching frame, except that the DLC is committed to digital services over twisted-pair copper phone wires. The DLC is a system of switches and multiplexers which concentrates low-speed services prior to distribution through a local central switching office or controlled environment vault (CEV). By multiplexing signals up to a local terminal where it then splits to provide service to subscriber pairs, the cost ofwiring can be reduced. DLC systems were developed in the early 1970s. See Next Generation Digital Loop Carrier. Digital Micromirror Device DMD. A spatial light modulator semiconductor technology that has been 260 incorporated into high-resolution Digital Light Pro- cessing (DLP) display devices. Tiny mirrors, only 16 microns square, can be organized in rows and col- urnns to form the basis for a high-resolution display. DMD displays from Texas Instruments combine a CMOS SRAM with a movable micromirror mounted over each memory cell corresponding to a pixel on the display. The light is pulse-width modulated incident to the mirror by electrostatic forces controlled by the cell data. Filters or color wheels can be used to create color. DMD was developed by Texas Instruments (II) and first demonstrated by II and Sony in the mid-1990s as a digital high-definition display system. DMD- based projection technologies are designed to be scal- able and intended to provide images superior to cath- ode-ray tube (CRT) and liquid crystal diode (LCD). DMD technology also has applications in the digital color printing field. Digital Millennium Copyright Act DMCA. A U.S. act enacted in 1998 through acomprehensive reform of U.S. copyright law to encompass changes brought about by evolutions in electronics that effect the cre- ation oforiginal digital works. The DMCA was also a step in the preparation for ratification of the international World Intellectual Property Organization (WIPO) treaties. digital multiplexer A system for aggregating or in- terleaving two or more digital signals, so they can be carried over fewer transmission lines, and sometimes also to aid in synchronization of multimedia applications that may require more than one signal (e.g., audio and video for videoconferencing). The signal is frequently demultiplexed at the receiving end in order to separately handle the various component signals. See digital cross-connect. Digital NetworkArchitecture DNA. I.An architecture that incorporates many aspects of the Open Sys- tems Interconnection (OSI) model used by Digital Equipment Corporation (DEC) to develop applications. 2. A commercial network system from Network Development Corporation. Digital Performance Archive DPA. A means for tracing the rapid developments in digital technologies as they pertain to performance arts activities. The archive is managed within the Arts and Humanities Research Board of the University of Salford, Manchester, U.K. Digital Private Network Signaling System DPNSS. A standard for integrating private branch systems with E1 lines. DPNSS was originally developed as an open standard by British Telecom pic and U.K. PINX manufacturers, in the 1980s. In the early 1990s, DPNSS and ISDN internetworking was described, followed by DPNSS and Signaling System No. l. Open documents are available from British Telecom. Digital Research Inc. DR. Originally called Inter- Galactic Digital Research, Digital Research was founded by Gary Kildall and his wife at the time, Dorothy McEwen. Gary was the developer of CP/M (Control Program for Microcomputers), a popular text-based operating system for microcomputers. DR © 2003 by CRC Press LLC produced a line of good quality products starting with CP/M-80. GEM, the DR graphical operating system predated working versions ofMicrosoft Windows by several years, and DR-DOS was often described by reviewers and users as superior to MS-DOS. DR's efforts were not limited to software. In 1984, the company released an expansion board for Intel 8088-based personal computers that allowed four ter- minals to be networked to a PC using standard RS- 232. With Concurrent PC-DOS, it provided the user the ability to run up to four MS-DOS or CP/M-86 applications concurrently, along with the program running on each individual terminal. Over the years, Digital Research introduced many basic desktop computing and networking tools that have become intrinsic to the industry. The company was purchased in the 1990s by the Novell Corpora- tion, who subsequently transferred DR-DOS to Caldera who released it as OpenDOS. Unfortunately, Kildall, who pioneered so many fundamental contributions to the microcomputer industry, was found dead at the age of 52. See CP/M; Graphics Environ- ment Manager; Kildall, Gary. digital selective calling DSC. Asynchronous transmissions system developed by the International Ra- dio Consultative Committee (CCIR Recommenda- tion 493). DSC is the basis for the Digital Selective Calling (DSC) communications service that provides automated access to coastal stations and marine craft. F our priority levels have been established, from rou- tine to distress, with distress calls receiving priority handling and, in the U.K., routing to a Rescue Coor- dination Centre (RCC). Alarms may be associated with incoming distress calls received on marine craft. DSC calls include the caller identity, phasing signals, and error-checking signals. A dot pattern alerts scanning receivers ofa call about to be received. Ships at sea are required to maintain continuous DSC watches at designated frequencies. The GMDSS Master Plan and the lTU List ofCoast Stations list the DSC distress and safety call frequencies. Digital Short-Range Radio DSRR. Initially per- ceived as an easy-to-license evolutionary heir to public domain Citizen's Band radio services, DSRR has since become a commodity for offering commercial services within the business community in North America and a venue for offering remote public services in Europe. It incorporates digital radio communications technologies for small low-range portable radio sets and has become an ETSI and TETRA standard for low-cost radio communications. DSSR op- erates in single- and double-frequency repeater modes in 933-935-MHz and 880-890-MHz bands. In North America and Australia, the double-frequency band is reserved for AMPS cellular services. Enhanced Digital Short-Range Radio E-DSRR was introduced through the RACE MOEBIUS project, designed to make use of the INMARSAT HSD Sat- ellite mode. Initially the system is being tested in remote European sites for applications such as the edu- cation of the children ofitinerant travelers or person- nel stationed in inaccessible areas. It is also being evaluated for its use in telemedicine applications. digital signalhierarchy DS A North American time division multiplex (TDM) signal hierarchy, which is used in connection with data communications proto- cols. See DS-O through DS-4, TI. digital signalprocessor DSP. A specialized computer processor designed to work with digitized wave- forms, often audio and video samples, in order to speed execution and provide more complex operations. Their computing power and flexibility allow them to be used for a wide variety ofapplications, such as the compression of voice and video signals, multimedia applications, medical imagery, combina- tion phone/fax/modem devices, etc. digital signal cross-connect panel DSX panel. A type ofelectrical cross-connect wiring bay or closet to facilitate the interconnection or patching of digital telecommunication facilities and equipment. This facilitates rearrangement, restoration, or monitoring of circuits. Bantamjacks are common in DSX panels. DSX panels connect a wide variety of equipment, depending upon the type of service (e.g.,' Tl). Cir- cuit connections may include channel banks, multiplexers, switches, repeater bays, and terminating con- nectors or circuits. See digital cross-connect, tap. digital signatureA type ofdigital identification associated with an individual or association that is sufficiently unique, secure, and resistant to forgery that it can be used for confidential and commerce-related online messages and transactions. Adigital signature is essentially the electronic equivalent of a handwritten signature that can be traced to the person who cre- ated it. Adigital signature was initially seen as a digitized version of this handwritten signature (somewhat like arubber stamp signature), but it was quickly re- alized that adigitized version did not have the same verification characteristics (pressure, direction, speed, etc.) that were inherent in a handwritten signature, and other more abstract versions of the digital signature were developed that not only were more unique but were amenable to strong encryption techniques to ensure security. (The author feels there may still be some merit in developing algorithms that actually encode the pressure, speed, and other characteristics that can be measured in aperson's signature through a special pressure-sensitive pad. There is more research that can be done in this area and specialized circumstances in which it could be applied, as in encoding signatures for local legal transactions to help prevent fraud.) In a more technical sense, for the purpose of imple- menting software, a digital signature has been described as a value generated from an application via a cryptographic algorithm that embodies data integrity, message authentication, and/or signer authentication. Anumber ofdigital signature schemes are al- ready in use for stock-related transactions, contracts, and general messaging. Digital signatures typically employ key encryption methods. In Aug. 2001, the W3C described a Proposed Rec- ommendation for XML digital signature processing rules and syntax to provide integrity, message 261 © 2003 by CRC Press LLC . LLC Fiber Optics Illustrated Dictionary Monolithic Lens/Diffusing Components Diffusion Configurations FIG. 1 /7 ~ ~~J_ilH : ~~:~ =.~ < ~- - =- - _ - - - - _. I~ I~ ~-: :::": I- /4 J4~JI FIG 2b AG3b 6' , , ~I : :t::r Some examples of monolithic. public services in Europe. It incorporates digital radio communications technologies for small low-range portable radio sets and has become an ETSI and TETRA standard for low-cost radio communications. DSSR op- erates in single- and double-frequency repeater modes in 93 3-9 35-MHz and 88 0-8 90-MHz bands. In North America and Australia, the double-frequency band is reserved for AMPS cellular. organization and wireless standard developed in Europe and adopted by the Eu- ropean Telecommunication Standard Institute (ETSI) Connects CPS to SPS Connects SPS to SCS Content Provider System CPS Service Provider System SPS Service Consumer System SCS CPS-SPS Delivery System - SPS-SCS Delivery System - Entity 259 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionanj in 1992. It was originally proposed as a unifying digital radio standard for European cordless phones. It has since been adopted by other

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