Fiber Optics Illustrated Dictionary and precision metal components. Lapping film can prepare optical fiber surfaces for bonding by flattening joint surfaces, polishing with progressively finer films, and removing excess bond (e.g., epoxy). Water may be used as a lubricant. laptop computer A low-weight, battery-powered, or combination battery/ AC-powered portable computer. Laptops fit easily on a lap, airline tray, train table, or other support surface in common moving convey- ances. They range in weight from about 3 to 7 pounds. Some people distinguish notebook computers as mid- range between laptops and palmtops, at about 2 to 4 pounds. Larger portable computers are called "luggables" or transportables, and the smallest ones are called palmtops and programmable calculators. They weigh from a few ounces up to about 2 or 3 pounds. A laptop battery usually lasts about 2 to 6 hours, and may be extended by using powers aver functions or turning off the monitor. Car lighter adapters may help power or recharge a laptop battery. Laptops are equipped with flat, light, low power monitors typically incorporating LED, passive ma- trix, active matrix, or gas plasma components. The active matrix screens are brighter and easier to see in dim or bright lighting conditions than screens that de- pend upon optimal ambient lighting. Many laptops are equipped with PCMCIA Type I or Type II card slots so the user can add lower power, compact peripherals such as extra memory cards, fax! modems, network ports, etc. USB and Fire Wire ports are also now common. LArc A data archiving program descended from LZSS, developed by Lempel, Ziv, Storer, and Szymanski and further optimized and extended in the late 1980s by Okumura and Miki. Kazuhiko Miki re- worked Okumura's version ofLZSS with Pascal and Assembler to create the archiving tool called LArc, whose file handling was quick and compact. Huff- man coding was later incorporated into the software by Yoshizaki to create an even faster and more popu- lar version called LHarc. See LHarc. LARe See Livermore Automatic Research Calcu- lator. Large Advanced Mirror Program LAMP. A re- search project involving the design and construction ofa 4-meter-diameter segmented mirror with char- acteristics appropriate for deployment in space, com- pleted in 1989. The segmented design enables large mirrors to be assembled. LAMP is the largest mirror designed for use in space, exceeding the Hubble as- tronomical mirror. See Large Optics Demonstration Experiment. Large Optics Demonstration Experiment LODE. Aproject to research the control oflarge, high-power laser beams, completed in 1987. Since then, LODE data have been used in the development of space- based laser technologies and investigated for defense applications such as missile defense. See Large Ad- vanced Mirror Program. large scale integration LSI. A term describing the evolution in electronics from systems with many 572 separate large-sized components to systems with a smaller number of integrated small-sized compo- nents. Since the distances between various parts of the logic and physical circuits are greatly reduced in LSI, the processing speeds are also much faster. LSI made possible smaller, more powerful electronic components such as calculators, computers, auto- mated appliances, digital watches, clocks, timers, and much more. See very large scale integration. large-core fiber A fiber cable with a core diameter that is broad in relation to most cables. A core of about 200 /lm or greater can be considered large-core though the designation is somewhat dependent upon what is manufactured or commonly used at any par- ticular time. For comparison, multimode fiber gen- erally has a core diameter of about 56 /lID. LASE Laser Applications in Science Education. A workshop of the Optical Society of San Diego that featured a series of diode laser demonstrations activi- ties. laser acronym light amplification by stimulated emis- sion ofradiation. A device that stimulates photons to produce coherent, nonionizing radiation in the vis- ible spectrum and infrared wavelength regions. While lenses and mirrors are commonly used to direct laser beams, the essential components ofa laser are a las- ing medium, a resonant optical cavity, and a pump- ing system (optical, mechanical, or electronic). Gas or semiconductors are used as the active lasing me- dium. The acronym was used by a number of students in the lab environment where C. Townes and his as- sistants worked on masers in 1957. Unlike light from many other sources, such as incan- descent bulbs, a laser beam remains very narrow and straight over long distances, a property called coher- ence. Many people have seen lasers in the form of business presentation pointers or rangefinders on guns to facilitate aiming. The pointer light usually appears as asmall, round, red dot. Lasers make great cat toys, too. Gems such as ruby and garnet (yttrium-aluminum- garnet) are commonly used in the production of la- sers and many familiar laser devices project a red beam. Gallium arsenide is also used. Red lasers used in magneto-optical storage device read heads cur- rently have wavelengths of about 650 urn, but blue lasers may become commercially available and will make it possible to record at higher areal densities. Blue lasers (ca. 410 nm) have a spot incident level almost 40% smaller than red lasers which would en- able higher disc capacities and faster transfer rates. Lasers are used for thousands ofcommercial, indus- trial, and medical applications. They function as high precision surgical cutting tools in medicine, as im- aging components in millions ofconsumer printers, as read/write tools for audio/visual storage technolo- gies, and as signaling tools in a variety ofnetwork- ing tasks, especially through fiber optic cables. A very interesting new type of room-temperature quantum cascade laser technology originated in 1994. QC lasers offer greater control over frequency selec- tion, and have many potential applications in remote © 2003 by CRC Press LLC sensing and industrial environments. Basic Semiconductor Laser Component pigtail or connector An active medium in a laser causes it to emit pho- tons which can be collimated into a coherent beam of light. A laser module typically includes the lenses nec- essary to accomplish this task. A basic laser compo- nent, such as a laser diode module, may emit an ellip- tical beam that is usually correctedwithin the module housing to produce a circular beam. The beam size is the size of the beam as it exits the assembly (the diameter, in the case of acircular beam). The beam divergence is the angle of the beam as it spreads over distance. If it is an elliptical beam (as the beam from a laser diode), this is expressed with two values. The pigtail orstandardized connector en- ables the laser component to be coupled with power sources or other components in a more complex as- sembly The direction in which the beam travels is the propa- gating axis. Ifitfans back andforth (e.g., to produce a line pattern), the fanning angle is described in de- grees. The CDRH classification label warns of laser dan- ger and indicates the power emitted by the laser at a specified wavelength. Most laser light is generated with longer wavelengths in the "hot" color regions (red and yellow), but the quest for an elusive "blue laser" began to bear fruit in the late 1990s. Blue lasers have the potential to revolutionize many aspects of laser storage, scientific, and medical devices because blue light has a very short wavelength and thus the beam from a blue la- ser is very tiny. This enables greater precision in cut- ting, shaping, writing, and reading mechanisms. Laser light is a minimum loss communications tool when proper shielding (cladding) keeps the light beams within the fiber core. In very pure straight fi- ber optic cable, the loss over distances is low, and the transmission cannot be surveilled in the same way that electrical wires can be monitored through emis- sions that extend beyond the cable (a lightguide needs to be bent to read the signals, resulting in loss that can be detected beyond the point of intrusion). Laser light communications are not affected by electromag- netic interference (EMI) in the same way as many other means of transmitting information (there may be some possibility of EM I in long lines electromag- netically amplified at the splices, but optically am- plified lines are being developed that may eliminate this problem). One should never aim a laser pointer at a person or animal, as there is apossibility of harm, especially if the laser shines on a cornea or retina. While some types of light are safer than others, many are capable of causing permanent photochemical or thermal dam- age, even with very brief exposures. See argon laser, cladding, fiber optics, helium-neon laser, laser diode, laser history, overcoat-incident recording, quantum cascade laser, maser, YAG, yttrium. laser class See CDRH classification. Laser Communications Demonstration Experi- ment LCDE. Ajoint project of the National Space Development Agency of Japan (NASDA) and the Optical Space Communications Group (OSCG) of the Tokyo-based Communications Research Laboratory to develop the Communications Demonstration Equipment (LCDE) to be placed aboard the Japanese facility of the International Space Station. The project goal is to demonstrate 2.5-Gbps up- and downlink communications at 1.5 Il wavelength. The experiment is based upon erbium-doped fiber amplifiers and la- sers. Laser Communications Demonstration System LCDS. Established in the early 1990s by NASA through the Jet Propulsion Laboratory (JPL) to dem- onstrate improvements in technology and multidiscipline systems engineering related to space laser communications. See Optical Communications Demonstrator. laser cuttingA laser can be used as a high precision cutting tool for surgical procedures and industriaV commercial applications. Rubber stamps, wood blocks, and stencils cut with lasers have very fine, crisp, clean edges. laser diode A type of semiconductor light-emitting diode (LED) that emits coherent light in response to the application of voltage. Laser diode/lens combinations are used in a wide variety of applications, including alignment and mea- suring systems, interferometers, barcode readers, medical imaging systems, laser printers, and fiber optics communications systems. Near-infrared diodes (NIDs) in the approx. 815±35 nm range are used in sensors and vision systems. Vis- ible laser diodes (VLDs) operate around 662±27 nm, at wavelengths visible to humans, and are practical for use in low-voltage devices. The basic components ofa diode laser module (DLM) are a laser diode, drive circuit, and collimating lens, protected within a compact housing. The drive cir- cuit controls the operating mode of the laser (e.g., con- tinuous wave). It may also provide surge filtering, re- verse-polarity protection, and other maintenance functions. Laser diode components with pumping mechanisms or temperature stabilization will typi- cally be larger than basic DLMs. Cooling components may help extend diode lifetime. Some models per- mit user control of the focusing distance and con- sumer components may include a safety shutter to reduce the possibility of damage from laser light. 573 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Laser diode component specifications include the diameter of the beam hole, the operating and output power, the output wavelength (in nanometers), and recommended operating and storing temperatures. Fiber-coupled laser diodes (FCLDs) combine.small circuits with fiber pigtails for connection to fiber op- tic cables. They are available in continuous-wave and pulsed models, with single- or multimode connectors in a variety of wavelengths, usually from about 800 to 1550 run. Prices per diode currently range from about $200 to $900 depending upon power genera- tion properties. FCLDs are used in scientific research, medical testing and imaging applications, thermal printing, and optical measurement systems. Distrib- uted- feedback laser diodes with wavelength concen- tration are useful for dense wavelength division mul- tiplexing (DWDM) applications. See distributed- feedback laser diode, laser, light-emitting diode. laser fax A combination device that incorporates the printing features ofa laser printer with the scanning and transceiver capabilities ofa facsimile machine. This is a handy tool for small offices where separate Laser Diode Beam Shape, Correction, and Control cladding layers active layer beam-emitting region elliptical beam beam axes elliptical beam correcting lens(es) The emitting region (facet) of a semiconductor laser diode is typically rectangular due to the layered assembly. Thus, the beam is inherently elliptical in cross-section (astigmatic). One or more lenses may be used with the laser diode to collimate and focus the laser beam, correcting the astigmatism and generating a circular beam. fanning beam 1 2 3 This highly simplifieddrawing of a laser diode module (LDM) illustrates how a laser beam can be rapidlyfanned back andforth to produce a line when it hits a surface. Fanning in curves against aprojection screen, fog, or smoke is the basic concept behind laser light shows and the technology is applicable to many other uses such as laser pointers, cutting and etching tools, sensor triggers, defense weaponry, and much more. point line l:;~f ~j! :; , ; .'. i:,::,: ,";" ": ; ;;c:. ~j~ crosshair matrix Depending upon how the lenses and rotors are designed and organized, it is possible to control the shape of the laser beam. With the addition offanning and multiple beams, many configurations are possible, as shown here. 574 © 2003 by CRC Press LLC high-capacity devices are not needed and space is at a premium. Ideally, a laser fax machine should be networkable so users can send and receive faxes with- out printing each one, and then select to print only the ones to be distributed and filed as hard copies. This saves paper and lineups at the fax machine. laser history Laser history is a fundamental aspect of fiber optics history. Events leading up to the de- velopment of lasers and laser-based fiber optic trans- missions include the invention of telescopes, televi- sion lenses, and the channeling of light through vari- ous media, including water (1840s), quartz rods (1880s), and bundles of fibers (1920s). In the late 1930s, scientists in Europe were on the verge of put- ting together the concepts fundamental to the laser, but the turbulent times and outbreak of the war shifted priorities and impinged on many areas of scientific research. It was not until the 1940s that solid theory and practical applications coalesced into modem la- ser technology. Lasers were developed in part because scientists wanted a source of coherent light from which to carry out other types of research, such as the carrying of signals and "bending" of light. Traditional light sources would spread and dissipate too quickly for most experimental applications. In 1841, J D. Colladon attempted to direct the bright light from an arc lamp into a stream of water. J. Tyndall demon- strated a similar idea in 1870. In 1888, Roth and Reuss extended the concept to medical imaging, using glass rods to illuminate internal body structures. Einstein's theories in stimulated emission had, by 1917, set a theoretical foundation for laser technology. 1. Baird was experimenting with television technol- ogy in the early 1920s and received a patent for trans- mitting images using glass rods in 1926. C. Hansell extended this idea to bundled rods, work that was later referenced to deny H. Hansen a patent application. By the 1930s, H. Lamm had successfully transferredd an image using a fiber bundle. Basic laser science was described by A. Prokhorov, a Russian physicist, in the late 1940s and early 1950s, and Prochorov continued to make important contri- butions to the field of masers and lasers over the next several decades. Another Soviet scientist, V.A. Fabrikant, described population inversion in the 1940s and coauthored apatent application submitted in 1951. Experimentation with fiber bundles continued in the 1950s through the work ofH. Meller Hansen, H. van Heel, H. Hopkins, and N. Kapany, but the practical- ity of light transmission without further developments in laser technology was limited. In June 1952, Joseph Weber lectured on how Einsteinian coefficients could be used to amplify by stimulated electromagnetic emission. A practical em- bodiment of early lasers was invented by Arthur L. Schawlow, a Bell Laboratories scientist, and Charles H. Townes, a consultant to Bell, who first demon- strated and described microwave-based ammonia- beam lasers (masers) in 1954. The laser research was written up by Townes, Zeiger, and Gordon in Physical Review V. 95. Building upon this research, Townes and Shawlow were applying the concepts to optically- pumped lasers by the late 1950s and Shawlow made later contributions to the development of laser spec- troscopy. In 1956, R. Dicke applied for a patent on improved maser/laser resonating chambers (essen- tially the forerunner to Fabry-Perot interferometers and open-cavity lasers). Prochorov and Manenkov suggested the use of ruby in lasers in 1957. By 1957, the idea of using longer wavelengths, per- haps in the optical region (lasers rather than masers) was beginning to occur to a number of researchers, including Townes, and perhaps also to Gordon Gould, an ambitious graduate student at Columbia who con- sulted Townes on patent issues before leaving school to accept ajob at TRG. Dicke proposed the idea of infrared open cavity amplification in his patent ap- plication of 1956, so the fundamental idea of optical spectrum stimulated emission was not original to Townes nor Gould, but since Dicke's patent wasn't published until September 1958, it's difficult to know how many people were inspired by Dicke's work. Townes continued his maser/laser research in collabo- ration with Schawlow at Bell Laboratories Dicke's patent was awarded and published in September 1958. When Townes and Schawlow circulated their laser design, in December 1958, Gould showed re- newed interest in the idea, constructed a prototype laser in collaboration with TRG, and submitted a patent in 1959. The laser patent wars had begun. Gould's application was initially refused due to the Townes/Schawlow/Bell patent. TRG disputed the patent and lost because Gould was unable, at the time, to prove due diligence or produce substantial notes from the year 1958. Later, the Gould patent claim was reopened when la- ser technology caught on and there was a strong po- tential for royalties. Gould was awarded the patent in 1977 for some of the claims in his 1959 applica- tion. Gould's conversations with Townes and his fa- miliarity with the scientific papers circulated by Townes/Schawlow appear to have been strong fac- tors stimulating his interest in laser technologies. How much Dicke's September 1958 patent contrib- uted is still an open question. Most ofGould's engi- neering contributions were refinements of basic la- ser concepts rather than groundbreaking technologies, such as those developed in the early 1950s by We- ber, Schawlow, Townes, and Dicke. In 1958, G. Goubau applied for a patent for a means to control laser light with a lightguide and lenses, the beginning of refinements that would coalesce into modem fiber optics when lasers, lenses and fiber op- tic filaments reached a level where their capabilities could be built into a cohesive system for communi- cations. Also in the late 1950s, Theodore Maiman followed the line of research evolving from the optically- pumped laser and developed and patented the ruby laser. Apractical embodiment of the gas laser, a dif- ferent approach from optically pumped lasers, was invented by a Bell researcher, Ali Javan. Javan 575 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary developed a helium-neon laser capable of emitting a pure, continuous beam by 1960. At the atomic level, Russian physicist N. Basov con- ducted pioneer research in quantum radiophysics that was to become important in laser technology. Rus- sian physicist and engineer R. Kazarinov lectured on homojunction semiconductor lasers in 1962 and co- authored a patent in March 1963, with Alferov, for a double heterostructure laser. Dye lasers were described in 1966 by F. Schmidt et al. in Gennany and P. Sorokin and J. Lankard in the U.S. By 1967, B. Soffer and B. McFarland had de- scribed how these new lasers could be tuned to dif- ferent frequencies. While dye lasers were not as pow- erful as some of the other pioneer laser technologies, there were many potential uses for tunable lasers. By the early 1970s, Kazarinov and R.A. Suris had de- scribed a distributed feedback double heterostructure laser. The Kazarinovl Alferov/Suris lasers are now im- portant components in communications technologies and compact disc players. For their contributions to laser technology, Prokhorov, Townes, and Basov were co-awarded a Nobel Prize in 1964. Chu, Cohen-Tannoudji, and Phillips were awarded a Nobel Prize in 1997 for their work in the 1980s on developing methods to cool and trap atoms with laser light. See laser, maser. See in- dividuallistings for many of the scientists mentioned above. laser level A laser-based alignment tool that uses a straight, coherent laser light beam to sight along a plane. Laser levels in the $500 range are compact, usually water-resistant, self-leveling, and convenient for contractors, cable installers, and other personnel who require quick plumb lines and reference points. Laser levels have many advantages over traditional liquid-and-gravity-based (bubble vial) levels. They can instantly sight over longer distances (30 to 60 m, depending upon model), are pocket-sized, and may sight in several directions at once. Less expensive laser levels in the $100 to $200 range resemble traditional levels and incorporate bubble vials. They are a transitional technology that fill a niche in the lower price range of laser levels. As the price of true laser levels decreases, these will prob- ably become obsolete. Low-end pocket-sized, single-beam laser levels in the under-$100 range are similar to laser pointers and are useful for spot-checking and may include a magnetic base for attachment to steel girders. laser light pen A compact cylindrical laser assem- bly based upon a collimate diode laser (eDL). See Subnanosecond- Pulse Laser Schematic I J:,/11 This schematic illustrates some of the basic components of a subnanosecond laser assembly, as described in 1971 by ~ Simmons of TRW, Inc. Until this time, it had been difficult tofabricate lasers of suffiCient power that couldemit lightpulses that were exceptionallyfast, as there was a limiting relationship between the lightpulse and the length of the laser cavity. This design overcomes that limitation by generating laser pulses until the optical laser cavity is mostlyfilled and then removing most of the radiation containedin the cavity. The remaining radiation is then ampli- fied by laser action, exploiting the Fourier components of the pulse which match the Fabry-Perot resonances of the cavity. The light pulse is mode-locked in phase such that the pulse retains its shape during amplification (while reflected back andforth in the cavity). There is also the option of emitting a pulse train (a sequence offast pulses). Subnanosecond-pulse capabilities are essential to many applications, including atomic research and telecommuni- cations. [U.S. patent#3,701,956, October 1972.} 576 © 2003 by CRC Press LLC laser diode, laser pointer. laser pointer A popular adaptation of laser light for use as a pointing mechanism. Remember the point- ing sticks teachers and sales representatives used for lectures and visual presentations (and to wake up dozing students)? The laser pointer is the modem equivalent (with the exception that dozing students are no longer prodded). The beam is typically red or green and may project to about 150 m. Pointers come in a variety of shapes and sizes from small cylindri- cal models resembling AA batteries, to executive pen- and mouse-shaped models. While many laser point- ers are sold as novelty or presentation items, they are also useful for installation, construction, and align- ment of components, cables, and other communica- tions-related equipment. Most laser pointers proj ect a pinpoint beam but some are designed to oscillate back and forth to project a line. The extent of the oscillation, called the fan angle, is usually about 30°. Some laser pointers include pulsed beams that can be used to create special ef- fects, especially when combined with fog. Most laser pointers are designed to emit laser light in ranges that are not harmful to eyes, but it is best to be cautious and never aim a laser pointer at anyone's face or at a reflective surface that might reflect the light beam into someone's face. See laser diode, laser level. laser printer Aprinter that uses a computer-directed laser beam to render images. Laser printers typically use a specially treated drum that is influenced by the light of the laser. An impression is made on the drum by aiming very fine, high precision laser light beams at the coating, so the electrical charge is selectively altered. An electrostatic process then attracts the toner to the imaged areas (areas altered by the beam), and heat fuses the toner onto the printing medium, which is usually paper or card stock. Most laser printers range from resolutions of 300 to 1200 dpi, although some can print at higher resolu- tions with special papers or plates capable of hold- ing a very fine image. Many are enhanced with Adobe PostScript page description interpreters. Higher print- ing speeds and PostScript capabilities require addi- tional memory. Laser printing is considered a dry printing process, as opposed to offset printing on a press that uses wet inks. It is not advisable to use recharged toner car- tridges in laser printers. In most consumer laser print- ers, the toner cartridge also includes part of the drum mechanism, which has a limited term of use. Even if the recharge toner is of good quality, it is still pos- sible for the aging drum to stress the printer, perhaps even damaging it. The money saved on toner car- tridges may be offset by the potential loss due to dam- age or reduced lifespan of the printer. See dot matrix printer, dye sublimation, inkjet printer, thermal wax printer. laser probe A laser-based instrument used to illumi- nate and sense a phenomenon, process, or specimen. laser range finder LRF. A device for determining distance through the use ofa beam of coherent light and associated sensing systems to calculate the range. Gun sights, binoculars, camera autofocusing systems, and a number of security and surveillance systems use laser range finders. Depending upon the device, the range may be indicated symbolically with graph- ics or numbers. For example, the distance to an ob- ject may be displayed on the viewing screen in yards or meters, superimposed over the scene being viewed. Some laser frequencies can be harmful to the eyes; while many consumer devices use Laser Class 1 fre- quencies that are not considered dangerous, it is wise to be cautious. laser show A laser show is an entertainment display created with colored laser lights crossing through the air, sometimes in a darkened room and sometimes falling on a display screen, domed theater surface, or fog/smoke medium. Some of the vector-based computer arcade games from the 1980s that successively drew colored lines around the screen give a general idea ofa laser light show, except that it's three-dimensional and bigger. Laser Fiber Optic Probe A laser light is projected through opticalfiber to illuminate and image a specimen mounted on a test stand at the NASA/Langley Research Center. [NASA/ Langley, 1993.J laser-using communications equipment LUCE. Communications technologies based upon laser light beams emissions through air or through waveguides such as fiber optic cables. When transmitted through air, the light can be used as a traditional visible sig- nal light or may trigger an electronic sensor. When transmitted through a waveguide, it functions much as traditional wired technologies but offers greater 577 :. © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary bandwidth capacity and lower susceptibility to elec- tromagnetic interference. laser, blue One of the more recent laser technologies, researched in the early 1980s and developed in the 1990s, blue lasers lagged behind others due to fabri- cation difficulties. Researchers had to find a way to generate more quantum "holes" in order to create the p-n junctions that were needed to stimulate photon emission in the blue wavelengths. Wide bandgap ma- terials (e.g., zinc selenide - ZnSe) were needed to pro- duce short wavelength blue light. Once some of the hurdles were overcome, blue lasers began to become apractical reality. Shuji Nakamura is a significant blue laser inventor. He created high quality gallium nitride crystals with double the previous hole mobility, in 1991, and com- mercial blue LEDs in 1993 using a custom-built re- actor and novel strategies. Meanwhile many well- known firms were advancing ZnSe-based blue and green laser technology, with practical implementa- tions appearing in the mid-1990s. Blue lasers provide a very precise, tiny laser beam with high potential for precision fabrication and high- density storage devices. Blue diode-based lasers emit light at approx. 460±30 urn, but were initially shorter- lived and more difficult to fabricate than conventional red lasers. However, as fabrication technology im- proves and the price drops, blue lasers can potentially increase the early 2000 storage capacity of red or in- frared laser-based optical technologies by 4 to 10 times. By 2002, commercial blue/red DVD players for the Japanese market were being announced, with global distribution projected by 2004 or 2005. laser, dye A type of tunable laser based upon injected organic dye molecules developed in the mid-1960s in Germany and the U.S. This laser has a broad emis- sions band and may be tuned by means of an adjust- able diffraction grating incorporated into the laser resonating cavity. laser, gallium arsenide GaAs laser. A type of laser commonly used in consumer CD and DVD players that emits light in the approx. 820 urn range. GaAs- based lasers commonly have a bandgap energy of about 1.45 electron volts. laser, greenA type oflaseruseful for interferometry, holography, and laser pointers; green lasers have wavelengths of around 527 nm and may send a beam 100 m or more. A compact diode-pumped green la- ser module may be small enough to fit in a hand. LAT See Local Area Transport. LATA See Local Access and Transport Area. latency Delay or period of dormancy. The speed of acquisition or perception ofa thought, object, or com- munication in relation to the desired speed of acqui- sition or perception. Latency can result from the intrinsic properties of the communications medium or the communication it- self. It can arise from the effects of the time it takes for information to transmit, or from the physical or logical pathways associated with the transmission. It can also result from crowding, congestion, misalign- ment, mistuning, unanticipated effects or traffic, and 578 a large number of other possible factors. Response time is related to latency, with reduced latency usu- ally desirable in this context. Every aspect of com- munications has to concern itselfwith latency. In networking, there has been a lot of research and quantification oflatency in order to design, evaluate, and tune computer architectures to carry out desired tasks. Here, latency is usually expressed in small units called milliseconds (e.g., latency in ISDN systems is ca. 10 msec). With slower communications pathways, such as slow modems over phone lines, latency may instead be expressed in seconds. There are many ways to reduce latency: better algo- rithms, wider bandwidth, better transmissions media, more efficient hardware, different topologies, and new technologies. Latency is sometimes intended in a dynamic system where connections change and the topology cannot always be anticipated. A queuing system is a means of using latency to good effect, as in email systems, which will hold messages until the intended recipi- ents or routers along the communications path are available to receive them. Latency may also be used as a signaling system; in other words, delays are taken into consideration and used to convey information. See realtime. Lewis Latimer - Inventor in Electricity The son of aformer slave, Latimer had great draft- ing skills and an inventive mind that earned him pro- fessionalpositions in some of thepreeminenttechnol- ogy labs of the time. [Image ca. 1882, courtesy of the National Historical Society.} Latimer, Lewis Howard (1848-1928) A skilled American drafter and inventor, Latimer worked in some of the most prestigious labs in America. In 1865, Latimer received a Union Navy honorable discharge and became an office boy for a patent so- liciting firm. Eager to do more, the ambitious young man self-studied mechanical drawing and convinced © 2003 by CRC Press LLC his employers to evaluate his drawings. For his ef- forts, he earned a promotion to drafting work and a dramatic increase in salary. He was later an assistant to Alexander Graham Bell and prepared drawings and descriptions for Bell's telephone patent. He subse- quently received several patents of his own, begin- ning in 1874. Latimer joined the American Electric Light Company and became a pioneer in the development of the light bulb. He received a patent for an improved electric lamp in 1881, and one for a process for manufactur- ing carbon filaments which he co-developed with Joseph V. Nichols in 1882. The filament patent was commercially successful, with carbon filaments re- placing the short-lived bamboo paper filaments com- mon at the time. In 1884, he became a member of Thomas A. Edison's research team. In 1890, he authored Incandescent Electric Lighting: A Practi- cal Description of the Edison System, which became an engineering handbook. He was appointed as an expert witness on the Board of Patent Control of the company that evolved into General Electric (GE) and, in 1918, became a member of the Edison Pioneers. LATNET A Latvian network service, concentrated mainly in Riga, where most of the scientific commu- nity is located. LATNET provides services to the RTD community and some businesses. It is operated by the Department of Computer Science at the University of Latvia in cooperation with the Riga Technical Uni- versity. LATNET utilizes leased lines and radio links, operating with TCP/IP. lattice model A flow control-related network access security model based upon the lattice format that arises from the ordering of finite security levels within a system. This is usually one of the first models dis- cussed in courses related to security models because lattice-based access control is important for confiden- tiality and, to some extent, integrity. Lattice-based access control models were described by Sandhu in the early 1990s arising out of research in the 1970s (e.g., Denning, 1976). Sandhu's contributions were based upon work supported by a National Science Foundation grant and a National Security Agency (NSA) contract. launch 1. To start, to set into operation. 2. To start, activate, or begin a computing process, operating sys- tem, or application. Programs are launched in a vari- ety of ways, such as double-clicking on icons or typ- ing the name of the program on a command line. Pro- grams may also be launched automatically from pre- programmed script files, or transparently from within other programs. launch, productIn management, to begin a new pro- gram, project, or marketing plan, sometimes with a lot of fanfare in order to attract the attention of po- tential customers and the media. New software pack- ages are often launched at industry trade shows. Law Enforcement Access Field LEAF. In computer security, a section ofclassified data created in asso- ciation with a Clipper chip or Capstone, and sent along with the encrypted message. The LEAF in- cludes the session and unit keys concatenated with the sender's serial number and an authentication string. See Clipper chip. LAWN See local area wireless network and wireless local area network. laws ofelectric charges Stated simply: bodies with unlike charges will attract one another; bodies with like charges will repel one another; bodies with no charges will neither attract nor repel one another. layer architecture Layer architectures are common in computer networks. Asynchronous transfer mode (ATM) is the most broadly implemented layer net- work architecture. Defining a number ofvirtual and physical layers al- lows communications paths to be organized and ad- ministered so that many different developers and manufacturers can create processes and devices in- dependently of one another, yet still apply them to the same system once standards and protocols for the various layers are established. Layers also provide a means to optimize the characteristics of the layer to the type of processes that occur within that layer. The layer architecture is usually described and diagramed horizontally, from bottom (physical or low-level lay- ers) to top (virtual or user interface and applications layers) with variations depending upon the specific organization of the architecture. Layers typically communicate with adjacent layers directly above or below, or may pass through an intervening layer. The Common Layer Hierarchy chart shows abrief over- view of some of the common types of layers. Layer Two Forwarding Protocol L2F. A Cisco Sys- tems Layer 2 application tunneling protocol in the TCP lIP suite introduced in 1996 and submitted to the IETF for consideration as a standard. The advantage ofL2F was that it enabled virtual dialup connections with multiple protocols and unregistered IP addresses. L2F uses UDP port 1701. In 1997, Compuserve adapted L2F for establishing securable private net- working services for dialup customers. In 1998, Valencia, Littlewood, and Kolar described the pro- tocol as a Historic Request for Comments (RFC). See Layer Two Tunneling Protocol, RFC 2341. Layer Two Tunneling Protocol L2TP. A securable network protocol that extends the Point-to-Point Pro- tocol (PPP) model to enable it to tunnel over Internet Protocol (IP) which, in turn, enables virtual private networks (VPN s) to operate over public packet- switched networks such as the Internet. L2TP can project a PPP network connection to a location other than the point at which the transmission was physi- cally received, enabling multi link operation across distinct physical Network Access Servers (NASs). L2TP incorporates characteristics of Point-to-Point Tunneling Protocol (pPTP) and Cisco Systems' Layer Two Forwarding Protocol (L2F) to provide an exten- sible control environment for the dynamic setup, maintenance, and teardown of multiple Layer 2 tun- nels established between logical endpoints in a trans- mission path. See RFC 2661. Layer Two Tunneling Protocol extensions L2TPext. Extensions to the Layer 2 Tunneling Pro- tocol including links, multicast, etc. The IETF has a 579 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary working group responsible for the orderly develop- ment of extensions to L2TP, in addition to separat- ing out the components of RFC 2661 for greater modularity. The IETF has submitted L2TP over Frame Relay and L2TP Security to the IESG as Pro- posed Standards. See Layer 2 Tunneling Protocol. LB See leaky bucket. LBA See Logical Block Address. LBS See load-balancing system. LC 1. lead channel. 2. local channel. 3. local com- pany. LCD I. linear collider detector. 2. See liquid crystal display. LCDE Laser Communications Demonstration Equipment; Laser Communications Demonstration Experiment. See Laser Communications Demonstra- tion System. LCDS See Laser Communications Demonstration System. LCI International An American telecommunica- tions system providing international voice and data services through owned and leased fiber optic net- works. LCI is known as the first long-distance pro- vider to bill both business and residential calls in I-second increments, a service known under the Ex- act Billing service mark. LCP See Link Control Protocol. LCS I. See Lan Channel Station. 2. See Laboratory for Computer Science. LCU Lightweight Computer Unit. LCV See line code violation. LDAP See Lightweight Directory Access Protocol. LDIP See Long Distance Internet Provider. LDMC See Loop Data Maintenance Center. LDMS See Local Multipoint Distribution Service. LDU 1. local distribution utility. 2. See load distri- bution unit. LE light-emitting. lead-salt diode laserAtype oflaserthat was experi- mental in the early 1980s and difficult to operate. Improvements in the theory and the increased avail- ability of semiconductor technologies led to diode- based systems in the early 1990s that could be used to frequency modulate signals in a number of types of devices, including spectroscopes. Lead-salt diode lasers are now used in semiconductor processing, detection, and fiber optic communication applica- tions. They can be tuned by controlling diode current at a constant cooled temperature. Lead-salt diodes have some performance and temperature limitations compared to quantum cascade lasers. See quantum cascade laser. leader I. The first segment, or part ofa transmission or transmissions medium. 2. The first few centime- ters on amagnetic tape (audio, video, etc.); the leader attaches and feeds the tape onto the spool. It is not intended for recording, and may be made of nonmag- netic material. 3. A packet, cell, segment, or other leading part of a data transmission, which contains information about data following, without including the data. It is a space or a signal to indicate impend- ing information, rather than being acomponent of the information itself. See header. leadership priority In an ATM network, an organi- zational function of a logical node assigning it prior- ity which, in tum, enables it to be designated as the peer group leader (PGL). LEAF See Law EnforcementAccess Field. Common Layer Hierarchy in Layered Computer Networks Layer Notes application layer A high-level layer at which the user interacts with the network applications programs and utilities. Various types of text or graphical user interfaces may be implemented at this layer. The application may also include remote access mechanisms and information messaging and transfer services. presentation layer Data security and data representation during transfer. session layer A traffic directing layer that sets up a communication between applications, adjusts synchronization, ifneeded, and clears the communication when done. transport layer A generalized, network-independent means ofinterlayer communication between the high application-oriented layers and the lower level layers, supporting different types ofconnections. network layer Low-level network connection, routing, and flow-control functions. link layer Low-level connectionless or connection-oriented data transfer. physical layer Low-level electrical connections and interfaces between the computing platforms and the network cables and connections, and the link layer data transmissions. 580 © 2003 by CRC Press LLC leaf node A type of connecting point in a network, located at the end ofa branch, so that only one con- nection is between the leaf node and the rest of the network. leakage In electrical circuits, particularly those which are not well shielded, leakage of the electromagnetic radiation outside the boundaries of the physical me- dium can occur. This may interfere with other trans- missions and devices. The Federal Communications Commission (FCC) provides guidelines and regulations for shielding vari- ous radio and computer devices in order to minimize interference from leakage. leaking memory See memory leak. leaky bucket LB. Acongestion-related conformance checking cell flow concept in ATM networking, an implementation of the Generic Cell Rate Algorithm (GCRA). Think of the bucket as a point in the net- work where cells may accumulate, depending upon varying rates of inflow and outflow. If cells are en- tering and leaving in equilibrium, that is, maintain- ing a sustainable cell rate (SCR), then the bucket will never be filled. If, however, inflow exceeds outflow, as the network experiences congestion, then the bucket may become full. There are various strategies for dealing with a full bucket, although prevention is advised. If, when the bucket becomes full, there are no further incoming cells, then it can be emptied in "bucket depth/SCR rate" amount of time. Bucket depth, the tolerance to cell bursting, can be set in re- lation to cell flow and retransmission timing. If, how- ever, the incoming cells continue to accumulate, the bucket will overflow and must be handled in some manner, with cell discard as one of the options. There is more than one way to implement a leaky bucket. Cisco Systems, Inc. suggests using dual leaky buckets, so a preconfigured queue depth threshold is set according to an agreed class of service (CoS) and quality of service (QoS). The first bucket can be con- figured to provide a service algorithm based on peak cell rate (PCR) and cell delay variation tolerance (CDVT) service parameters. The second bucket is based on sustainable cell rate (SCR) and maximum burst size (MBS). Nonconformant cells can be con- figured as cell discard, tag, or no change for each bucket. Dual discard thresholds can be supported to provide a delay mechanism for congestion cell dis- card rates. See cell rate. leased line A line whose use is rented over a period of time from the entity that owns and manages the physical connection. Long-distance companies, spe- cialized services, and businesses with direct private lines often lease lines from the local primary tele- phone carrier rather than installing their own. Least Cost Routing LCR. A phone service that au- tomatically seeks and selects the line through which to send a call with the least cost. See Automatic Route Selection. LEC 1. See Local Exchange Carrier. 2. LAN Emu- lation Client. A LAN software client that keeps ad- dress translation and connection information for com- munication through an ATM network. See LANE, LECS. 3. Loop Electronic Coordinator. least significant bit LSB. The LSB is the lowest or- der bit in a binary value. This is an important con- cept in computer data storage and programming that applies to the order in which data are organized, stored, or transmitted. As an example, in the binary value 110 (as it is nor- mally written with the larger values to the left), the zero (0) on the right, representing the number of ones, is the least significant bit (LSB) and the one (1) on the left, representing the number of fours (4) is the most significant bit. One of the reasons data are not directly transportable among different systems is that some file formats or operating systems are standardized on LSB priorities and some are standardized on MSB priorities. In other words, some systems code/decode a binary value from smaller to larger (little endian) and some from larger to smaller (big endian). least significant byte LSB. The least sigpificant or lower-order value in a multibyte word. More often than not, a byte represents eight bits. See least sig- nificant bit. Leclanche, Georges (1839-1882) A French engineer who invented a type ofelectrolytic battery cell later refined and used for signal bells and telegraph ser- vices. Leclanche studied in England and returned to France to continue his studies. In Belgium he was encouraged in his endeavors by Mourlon and estab- lished a small laboratory. After developing his dry cell, he opened a factory to produce batteries and elec- trical devices. Shortly before the death of his father, Leclanche returned to France and, with his own health in decline, made a tour of Europe, Egypt, and other countries (to collect Italian furniture). The Leclanche S.A. company of Switzerland, established in 1909, was a pioneer producer of portable lighting, paper ca- pacitors, rechargeable batteries, and other battery- related products. It was associated with other com- panies in the late 1990s to become the Leclanche Group. See Gassner, Jr., Carl; Leclanche cell. Leclanche cell A historic primary electrolytic cell developed by Georges Leclanche in the mid-1860s. This was an important time in battery history as tech- no logy - a transition from wet to dry cells and from cumbersome hard-to-move batteries to those that were encased and thus more practical and portable. In its original form, the Leclanche battery was en- cased in a porous pot with a positive manganese di- oxide and carbon electrode on the top and a zinc nega- tive electrode on the bottom. The pot and zinc rod were immersed in a solution of [electrolytic] ammo- nium chloride. The electrolyte penetrated the porous container to reach the cathode. Moulin and Leclanche made commercial improvements to the battery and established a factory to create and distribute the in- vention. Later, Georges Leclanche's son, Max- Georges, made some changes to the commercial bat- tery, replacing the porous container. The battery was used in many aspects of the emerging telecommuni- cations industry and as a power source for bells and automotive lights. The TIS Standard for dry cells and 581 © 2003 by CRC Press LLC . LLC Fiber Optics Illustrated Dictionary Laser diode component specifications include the diameter of the beam hole, the operating and output power, the output wavelength (in nanometers), and recommended operating and storing temperatures. Fiber- coupled laser diodes (FCLDs) combine.small circuits with fiber pigtails for connection to fiber op- tic cables. They are available in continuous-wave and pulsed models, with single- or multimode connectors in a variety of wavelengths, usually from about 800 to 1550 run. Prices per diode currently range from about $200 to $900 depending upon power genera- tion properties. FCLDs are used in scientific research, medical testing and imaging applications, thermal printing, and optical measurement systems. Distrib- uted- feedback laser diodes with wavelength concen- tration are useful for dense wavelength division mul- tiplexing (DWDM) applications. See distributed- feedback laser diode, laser, light-emitting diode. laser fax A combination device that incorporates the printing features ofa laser printer with the scanning and transceiver capabilities ofa facsimile machine. This is a handy tool for small offices where separate Laser Diode Beam Shape, Correction, and Control cladding. history Laser history is a fundamental aspect of fiber optics history. Events leading up to the de- velopment of lasers and laser-based fiber optic trans- missions include the invention of telescopes, televi- sion lenses, and the channeling of light through vari- ous media, including. Press LLC Fiber Optics Illustrated Dictionary developed a helium-neon laser capable of emitting a pure, continuous beam by 1960. At the atomic level, Russian physicist N. Basov con- ducted pioneer research in quantum radiophysics that was to become important in laser technology. Rus- sian physicist and engineer R. Kazarinov lectured on homojunction semiconductor lasers in 1962 and co- authored a patent in March 1963, with Alferov, for a double heterostructure laser. Dye lasers were described in 1966 by F. Schmidt et al. in Gennany and P. Sorokin and J. Lankard in the U.S. By 1967, B. Soffer and B. McFarland had de- scribed how these new lasers could be tuned to dif- ferent frequencies. While dye lasers were not as pow- erful as some of the other pioneer laser technologies, there were many potential uses for tunable lasers. By the early 1970s, Kazarinov and R.A. Suris had de- scribed a distributed feedback double heterostructure laser. The Kazarinovl Alferov/Suris lasers are now im- portant components in communications technologies and compact disc players. For