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 l
Trang 1Fiber 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 computerA 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 powersaver functions or
turning offthe 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 FireWire ports
are also now common
LArcA 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
LAReSee Livermore Automatic Research
Calcu-lator
Large Advanced Mirror Program LAMP A
re-search project involving the design and construction
of a 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 ExperimentLODE
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
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 Acore 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
LASELaser Applications in Science Education A workshop of the Optical Society of San Diego that featured a series of diode laser demonstrations activi-ties
laseracronymlight amplification by stimulated emis-sion of radiation 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 of a 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 ofstudents
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 a small, 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 of commercial, indus-trial, and medical applications They function as high precision surgical cutting tools in medicine, as im-aging components in millions of consumer printers,
as read/write tools for audio/visual storage technolo-gies, and as signaling tools in a variety of network-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
Trang 2Basic 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 corrected within the module
housing to produce a circular beam.
The beam size is the size ofthe beam as it exits the
assembly (the diameter, in the case ofa circular 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 or standardized 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 oflaser
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 oflaser 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, wricut-ting, 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 ofintrusion) 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 ofEMI in long lines
electromag-netically amplified at the splices, but optically
am-this problem)
One should never aim a laser pointer at a person or animal, as there is a possibility ofharm, especially if the laser shines on a cornea or retina While some types oflight are safer than others, many are capable ofcausing 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 classSee CDRH classification
Laser Communications Demonstration Experi-mentLCDE A joint project of the National Space Development Agency of Japan (NASDA) and the Optical Space Communications Group (OSCG) ofthe Tokyo-based Communications Research Laboratory
to develop the Communications Demonstration Equipment (LCDE) to be placed aboard the Japanese facility ofthe 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 diodeA type of semiconductor light-emitting diode (LED) that emits coherent light in response to the application ofvoltage
Laser diode/lens combinations are used in a wide variety ofapplications, 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 ofthe laser (e.g., con-tinuous wave).Itmay 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
Trang 3Fiber 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 Acombination device that incorporates the printing features of a laser printer with the scanning and transceiver capabilities of a 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) ofa 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 andfocus the laser beam, correcting the astigmatism and generating a circular beam.
fanning
beam
This highly simplified drawing ofa 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 a projection 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.
l:;~f ~j!
:; ,;
.'.
i:,::,:
,";" ":
; ;;c:.
~j~
Depending upon how the lenses and rotors are designed and organized, it is possible to control the shape ofthe laser beam With the addition offanning and multiple beams, many configurations are possible, as shown here.
Trang 4a 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 historyLaser history is a fundamental aspect
of fiber optics history Events leading up to the
de-velopment oflasers and laser-based fiber optic
trans-missions include the invention of telescopes,
televi-sion lenses, and the channeling oflight 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
ofput-ting together the concepts fundamental to the laser,
but the turbulent times and outbreak ofthe 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 ofcoherent 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 ofmasers and lasers over the next
several decades Another Soviet scientist, V.A
Fabrikant, described population inversion in the
1940s and coauthored a patent 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 oflight transmission without further developments
inlaser technology was limited
In June 1952, Joseph Weber lectured on how
Einsteinian coefficients could be used to amplify by
stimulated electromagnetic emission Apractical
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
writtenupby Townes, Zeiger, and Gordon inPhysical
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 ofruby 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 a job 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 of Gould'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 A practical embodiment of the gas laser, a dif-ferent approach from optically pumped lasers, was invented by a Bell researcher, Ali Javan Javan
Trang 5Fiber 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 researchinquantum 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 patentinMarch 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 ofthe 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 TheKazarinovlAlferov/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 aNobel Prize in 1997 for their
workinthe 1980s on developing methods to cool and
trap atoms with laser light See laser, maser See in-dividuallistings for many ofthe scientists mentioned above
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 sightinseveral 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
assem-bly based upon a collimate diode laser(eDL).See
Subnanosecond- Pulse Laser Schematic
I
J:,/11
This schematic illustrates some ofthe basic components ofa subnanosecond laser assembly, as described in1971
by~Simmons ofTRW, Inc Until this time, it had been difficult to fabricate lasers ofsuffiCient power that could emit light pulses that were exceptionallyfast, as there was a limiting relationship between the light pulse 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 ofthe radiation contained in the cavity The remaining radiation is then ampli-fied by laser action, exploiting the Fourier components ofthe pulse which match the Fabry-Perot resonances ofthe 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 ofemitting 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, October1972.}
Trang 6laser pointer Apopular 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 project a pinpoint beam but some
are designed to oscillate back and forth to project a
line The extent ofthe oscillation, called thefan 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 beam to render images Laser printers typically
use a specially treated drum that is influenced by the
light ofthe laser.Animpression is made on the drum
by aiming very fine, high precision laser light beams
at the coating, so the electrical charge is selectively
altered.Anelectrostatic 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 adryprinting process,
as opposed to offset printing on a press that uses wet
inks.Itis not advisable to use recharged toner
car-tridges in laser printers In most consumer laser
print-ers, the toner cartridge also includes part ofthe drum
mechanism, which has a limited term ofuse 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 ofthe printer See dot matrix
printer, dye sublimation, inkjet printer, thermal wax
printer
illumi-nate and sense a phenomenon, process, or specimen
distance through the use of a beam of coherent light
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 1fre-quencies that are not considered dangerous, it is wise
to be cautious
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 ofthe 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 optical fiber to illuminate and image a specimen mounted on a test stand at the NASA/Langley Research Center [NASA/
Langley, 1993.J
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
:.
Trang 7Fiber 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
a practical 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 Atype oftunable 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, green Atype oflaser useful 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.Itcan 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
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 itself with 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 ofa former slave, Latimer had great draft-ing skills and an inventive mind that earned him pro-fessional positions in some ofthe preeminent technol-ogy labs ofthe time [Image ca.1882,courtesy ofthe 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 Eagertodo more, the ambitious young man self-studied mechanical drawing and convinced
Trang 8forts, 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
Latimerjoined the American Electric Light Company
and became a pioneer in the development ofthe 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 ofthe 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 ofthe 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
ofLatvia in cooperation with the Riga Technical
Uni-versity LATNET utilizes leased lines and radio links,
operating with TCP/IP
lattice model Aflow control-related network access
security model based upon the lattice format that
arises from the ordering offinite 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 ofways, such as double-clicking on icons or
typ-ing the name ofthe program on a command line
Pro-grams may also be launched automatically from
pre-programmed script files, or transparently from within
other programs
launch, productInmanagement, 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 of classified 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
string See Clipper chip
LAWN See local area wireless network and wireless local area network
laws of electric charges Stated simply: bodies with
unlikecharges 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 of virtual 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 ofprocesses 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 a brief over-view of some of the common types of layers Layer Two Forwarding Protocol L2F ACisco Sys-tems Layer 2 application tunneling protocol in the TCPlIP 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, inturn,enables virtual private networks (VPNs) 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 multilink 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
Trang 9Fiber 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 laser A type oflaser that 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 of a transmission
or transmissions medium 2 The first few centime-ters on a magnetic tape (audio, video, etc.); the leader attaches and feeds the tape onto the spool It is not intended for recording, and may be made ofnonmag-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 beimpend-ing a component ofthe 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 Enforcement Access Field
Common Layer Hierarchy in Layered Computer Networks
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, if needed, and clears the communication when done
transport layer A generalized, network-independent means of interlayer
communication between the high application-oriented layers and the lower level layers, supporting different types of connections
network layer Low-level network connection, routing, and flow-control functions linklayer 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
Trang 10located at the end of a branch, so that only one
con-nection is between the leaf node and the rest of the
network
leakageInelectrical circuits, particularly those which
are not well shielded, leakage ofthe 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 memorySee memory leak
leaky bucketLB 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 ofthe options
There is more than one way to implement a leaky
bucket Cisco Systems, Inc suggests usingdual leaky
buckets,so a preconfigured queue depth threshold is
set according to an agreed class of service (CoS) and
quality ofservice (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 lineA 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 RoutingLCR 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
LEC1 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,
least significant bitLSB 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 ofones,
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 ofthe 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 byteLSB 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) AFrench engineer who invented a type of electrolytic battery cell later refined and used for signal bells and telegraph ser-vices Leclanche studiedinEngland 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 hisdry
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 ofportable 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 cellA historic primary electrolytic cell developed by Georges Leclanche in the mid-1860s This was an important time in battery history as tech-nology - a transition from wet todrycells 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 fordrycells and