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By Jeff Hecht
Reproduced from Fiber Optics Technician's Handbook, by Jim Hayes, Delmar
Publishers, Albany, New York.
For the full history of fiber optics, see my book,City of Light: The Story of FiberOptics,Oxford University Press, New York, 1999. (ISBN 0-19-510818-3) A book in
the Sloan Foundation Technology series. For near-immediate gratification,order now
from Amazon.com.
Optical communication systems date back two centuries, to the "optical telegraph"
that French engineer Claude Chappe invented in the 1790s. His system was a series of
semaphores mounted on towers, where human operators relayed messages from one
tower to the next. It beat hand-carried messages hands down, but by the mid-19th
century was replaced by the electric telegraph, leaving a scattering of "Telegraph
Hills" as its most visible legacy.
Alexander Graham Bell patented an optical telephone system, which he called the
Photophone, in 1880, but his earlier invention, the telephone, proved far more
practical. He dreamed of sending signals through the air, but the atmosphere didn't
transmit light as reliably as wires carried electricity. In the decades that followed, light
was used for a few special applications, such as signalling between ships, but
otherwise optical communications, like the experimental Photophone Bell donated to
the Smithsonian Institution, languished on the shelf.
In the intervening years, a new technology slowly took root that would ultimatelysolve the problem of optical transmission, although it was a long time before it was
adapted for communications. It depended on the phenomenon of total internal
reflection, which can confine light in a material surrounded by other materials with
lower refractive index, such as glass in air. In the 1840s, Swiss physicist Daniel
Collodon and French physicist Jacques Babinet showed that light could be guided
along jets of water for fountain displays. British physicist John Tyndall popularized
light guiding in a demonstration he first used in 1854, guiding light in a jet of water
flowing from a tank. By the turn of the century, inventors realized that bent quartz
rods could carry light, and patented them as dental illuminators. By the 1940s, many
doctors used illuminated plexiglass tongue depressors.
Optical fibers went a step further. They are essentially transparent rods of glass or
plastic stretched so they are long and flexible. During the 1920s, John Logie Baird in
England and Clarence W. Hansell in the United States patented the idea of using
arrays of hollow pipes or transparent rods to transmit images for television or
facsimile systems. However, the first person known to have demonstrated image
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transmission through a bundle of optical fibers was Heinrich Lamm, than a medical
student in Munich. His goal was to look inside inaccessible parts of the body, and in a
1930 paper he reported transmitting the image of a light bulb filament through a short
bundle. However, the unclad fibers transmitted images poorly, and the rise of the
Nazis forced Lamm, a Jew, to move to America and abandon his dreams of becoming
a professor of medicine.
In 1951, Holger Mller [or Moeller, the o has a slash through it] Hansen applied for a
Danish patent on fiber-optic imaging. However, the Danish patent office denied his
application, citing the Baird and Hansell patents, and Mller Hansen was unable to
interest companies in his invention. Nothing more was reported on fiber bundles until
1954, when Abraham van Heel of the Technical University of Delft in Holland and
Harold. H. Hopkins and Narinder Kapany of Imperial College in London separately
announced imaging bundles in the prestigious British journal Nature.
Neither van Heel nor Hopkins and Kapany made bundles that could carry light far, buttheir reports the fiber optics revolution. The crucial innovation was made by van Heel,
stimulated by a conversation with the American optical physicist Brian O'Brien. All
earlier fibers were "bare," with total internal reflection at a glass-air interface. van
Heel covered a bare fiber or glass or plastic with a transparent cladding of lower
refractive index. This protected the total-reflection surface from contamination, and
greatly reduced crosstalk between fibers. The next key step was development of glass-
clad fibers, by Lawrence Curtiss, then an undergraduate at the University of Michigan
working part-time on a project to develop an endoscope to examine the inside of the
stomach with physician Basil Hirschowitz, physicist C. Wilbur Peters. (Will Hicks,
then working at the American Optical Co., made glass-clad fibers at about the same
time, but his group lost a bitterly contested patent battle.) By 1960, glass-clad fibers
had attenuation of about one decibel per meter, fine for medical imaging, but much
too high for communications.
Meanwhile, telecommunications engineers were seeking more transmission
bandwidth. Radio and microwave frequencies were in heavy use, so they looked to
higher frequencies to carry loads they expected to continue increasing with the growth
of television and telephone traffic. Telephone companies thought video telephones
lurked just around the corner, and would escalate bandwidth demands even further.
The cutting edge of communications research were millimeter-wave systems, in
which hollow pipes served as waveguides to circumvent poor atmospheric
transmission at tens of gigahertz, where wavelengths were in the millimeter range.
Even higher optical frequencies seemed a logical next step in 1958 to Alec Reeves,
the forward-looking engineer at Britain's Standard Telecommunications Laboratories
who invented digital pulse-code modulation before World War II. Other people
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climbed on the optical communications bandwagon when the laser was invented in
1960. The July 22, 1960 issue of Electronics magazine introduced its report on
Theodore Maiman's demonstration of the first laser by saying "Usable
communications channels in the electromagnetic spectrum may be extended by
development of an experimental optical-frequency amplifier."
Serious work on optical communications had to wait for the continuouswave helium-
neon laser. While air is far more transparent at optical wavelengths than to millimeter
waves, researchers soon found that rain, haze, clouds, and atmospheric turbulence
limited the reliability of long-distance atmospheric laser links. By 1965, it was clear
that major technical barriers remained for both millimeter-wave and laser
telecommunications. Millimeter waveguides had low loss, although only if they were
kept precisely straight; developers thought the biggest problem was the lack of
adequate repeaters. Optical waveguides were proving to be a problem. Stewart
Miller's group at Bell Telephone Laboratories was working on a system of gas lenses
to focus laser beams along hollow waveguides for long-distance telecommunications.
However, most of the telecommunications industry thought the future belonged to
millimeter waveguides.
Optical fibers had attracted some attention because they were analogous in theory to
plastic dielectric waveguides used in certain microwave applications. In 1961, Elias
Snitzer at American Optical, working with Hicks at Mosaic Fabrications (now Galileo
Electro-Optics), demonstrated the similarity by drawing fibers with cores so small
they carried light in only one waveguide mode. However virtually everyone
considered fibers too lossy for communications; attenuation of a decibel per meter
was fine for looking inside the body, but communications operated over much longer
distances, and required loss no more than 10 or 20 decibels per kilometer.
One small group did not dismiss fibers so easily -- a team at Standard
Telecommunications Laboratories initially headed by Antoni E. Karbowiak, which
worked under Reeves to study optical waveguides for communications. Karbowiak
soon was joined by a young engineer born in Shanghai, Charles K. Kao.
Kao took a long, hard look at fiber attenuation. He collected samples from fiber
makers, and carefully investigated the properties of bulk glasses. His research
convinced him that the high losses of early fibers were due to impurities, not to silica
glass itself. In the midst of this research, in December 1964, Karbowiak left STL to
become chair of electrical engineering at the University of New South Wales in
Australia, and Kao succeeded him as manager of optical communications research.
With George Hockham, another young STL engineer who specialized in antenna
theory, Kao worked out a proposal for long-distance communications over single-
mode fibers. Convinced that fiber loss should be reducible below 20 decibels per
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kilometer, they presented a paper at a London meeting of the Institution of Electrical
Engineers. The April 1, 1966 issue of Laser Focus noted Kao's proposal:
"At the IEE meeting in London last month, Dr. C. K. Kao observed that short-distance runs have shown that the experimental optical waveguide developed
by Standard Telecommunications Laboratories has an information-carrying
capacity ... of one gigacycle, or equivalent to about 200 tv channels or more
than 200,000 telephone channels. He described STL's device as consisting of a
glass core about three or four microns in diameter, clad with a coaxial layer of
another glass having a refractive index about one percent smaller than that of
the core. Total diameter of the waveguide is between 300 and 400 microns.
Surface optical waves are propagated along the interface between the two types
of glass."
"According to Dr. Kao, the fiber is relatively strong and can be easily
supported. Also, the guidance surface is protected from external influences. ...
the waveguide has a mechanical bending radius low enough to make the fiber
almost completely flexible. Despite the fact that the best readily available low-
loss material has a loss of about 1000 dB/km, STL believes that materials
having losses of only tens of decibels per kilometer will eventually be
developed."
Kao and Hockham's detailed analysis was published in the July 1966 Proceedings of
the Institution of Electrical Engineers. Their daring forecast that fiber loss could bereduced below 20 dB/km attracted the interest of the British Post Office, which then
operated the British telephone network. F. F. Roberts, an engineering manager at the
Post Office Research Laboratory (then at Dollis Hill in London), saw the possibilities,
and persuaded others at the Post Office. His boss, Jack Tillman, tapped a new research
fund of 12 million pounds to study ways to decrease fiber loss.
With Kao almost evangelically promoting the prospects of fiber communications, and
the Post Office interested in applications, laboratories around the world began trying
to reduce fiber loss. It took four years to reach Kao's goal of 20 dB/km, and the route
to success proved different than many had expected. Most groups tried to purify thecompound glasses used for standard optics, which are easy to melt and draw into
fibers. At the Corning Glass Works (now Corning Inc.), Robert Maurer, Donald Keck
and Peter Schultz started with fused silica, a material that can be made extremely
pure, but has a high melting point and a low refractive index. They made cylindrical
performs by depositing purified materials from the vapor phase, adding carefully
controlled levels of dopants to make the refractive index of the core slightly higher
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than that of the cladding, without raising attenuation dramatically. In September 1970,
they announced they had made single-mode fibers with attenuation at the 633-
nanometer helium-neon line below 20 dB/km. The fibers were fragile, but tests at the
new British Post Office Research Laboratories facility in Martlesham Heath
confirmed the low loss.
The Corning breakthrough was among the most dramatic of many developments that
opened the door to fiber-optic communications. In the same year, Bell Labs and a
team at the Ioffe Physical Institute in Leningrad (now St. Petersburg) made the first
semiconductor diode lasers able to emit continuouswave at room temperature. Over
the next several years, fiber losses dropped dramatically, aided both by improved
fabrication methods and by the shift to longer wavelengths where fibers have
inherently lower attenuation.
Early single-mode fibers had cores several micrometers in diameter, and in the early
1970s that bothered developers. They doubted it would be possible to achieve themicrometer-scale tolerances needed to couple light efficiently into the tiny cores from
light sources, or in splices or connectors. Not satisfied with the low bandwidth of step-
index multimode fiber, they concentrated on multi-mode fibers with a refractive-index
gradient between core and cladding, and core diameters of 50 or 62.5 micrometers.
The first generation of telephone field trials in 1977 used such fibers to transmit light
at 850 nanometers from gallium-aluminum-arsenide laser diodes.
Those first-generation systems could transmit light several kilometers without
repeaters, but were limited by loss of about 2 dB/km in the fiber. A second generation
soon appeared, using new InGaAsP lasers which emitted at 1.3 micrometer, where
fiber attenuation was as low as 0.5 dB/km, and pulse dispersion was somewhat lower
than at 850 nm. Development of hardware for the first transatlantic fiber cable showed
that single-mode systems were feasible, so when deregulation opened the long-
distance phone market in the early 1980s, the carriers built national backbone systems
of single-mode fiber with 1300-nm sources. That technology has spread into other
telecommunication applications, and remains the standard for most fiber systems.
However, a new generation of single-mode systems is now beginning to find
applications in submarine cables and systems serving large numbers of subscribers.
They operate at 1.55 micrometers, where fiber loss is 0.2 to 0.3 dB/km, allowing even
longer repeater spacings. More important, erbium-doped optical fibers can serve as
optical amplifiers at that wavelength, avoiding the need for electro-optic regenerators.
Submarine cables with optical amplifiers can operate at speeds to 5 gigabits per
second, and can be upgraded from lower speeds simply to changing terminal
electronics. Optical amplifiers also are attractive for fiber systems delivering the same
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signals to many terminals, because the fiber amplifiers can compensate for losses in
dividing the signals among many terminals.
The biggest challenge remaining for fiber optics is economic. Today telephone and
cable television companies can cost-justify installing fiber links to remote sites
serving tens to a few hundreds of customers. However, terminal equipment remainstoo expensive to justify installing fibers all the way to homes, at least for present
services. Instead, cable and phone companies run twisted wire pairs or coaxial cables
from optical network units to individual homes. Time will see how long that lasts.
History of Fiber OpticsAs far back as Roman times, glass has been drawn into fibers. Yet, it was not until the 1790s that the French Chappe
brothers invented the first "optical telegraph." It was a system comprised of a series of lights mounted on towers where
operators would relay a message from one tower to the next. Over the course of the next century great strides were made
in optical science.
John Tyndall, British physicist, demonstrated that light signals could be bent.
In the 1840s, physicists Daniel Collodon and Jacques Babinet showed that light could be directed along jets of water for
fountain displays. In 1854, John Tyndall, a British physicist, demonstrated that light could travel through a curved stream
of water thereby proving that a light signal could be bent. He proved this by setting up a tank of water with a pipe that
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ran out of one side. As water flowed from the pipe, he shone a light into the tank into the stream of water. As the water
fell, an arc of light followed the water down.
Alexander Graham Bell patented an optical telephone system called the photophone in 1880. His earlier invention, the
telephone, proved to be more realistic however. That same year, William Wheeler invented a system of light pipes lined
with a highly reflective coating that illuminated homes by using light from an electric arc lamp placed in the basementand directing the light around the home with the pipes.
Sketch of a telephone systemby Alexander Graham Bell. Bell patented an optical telephone system which assisted in
the advancement of optical technology.
Doctors Roth and Reuss, of Vienna, used bent glass rods to illuminate body cavities in 1888. French engineer Henry
Saint-Rene designed a system of bent glass rods for guiding light images seven years later in an early attempt at
television. In 1898, American David Smith applied for a patent on a dental illuminator using a curved glass rod.
In the 1920s, John Logie Baird patented the idea of using arrays of transparent rods to transmit images for television andClarence W. Hansell did the same for facsimiles. Heinrich Lamm, however, was the first person to transmit an image
through a bundle of optical fibers in 1930. It was an image of a light bulb filament. His intent was to look inside
inaccessible parts of the body, but the rise of the Nazis forced Lamm, a Jew, to move to America and abandon his dream
of becoming a professor of medicine. His effort to file a patent was denied because of Hansell's British patent.
In 1951, Holger Moeller applied for a Danish patent on fiber-optic imaging in which he proposed cladding glass or
plastic fibers with a transparent low-index material, but was denied because of Baird and Hansell's patents. Three years
later, Abraham Van Heel and Harold H. Hopkins presented imaging bundles in the British journalNature at separate
times. Van Heel later produced a cladded fiber system that greatly reduced signal interference and crosstalk between
fibers.
Also in 1954, the "maser" was developed by Charles Townes and his colleagues at Columbia University. Maser stands
for "microwave amplification by stimulated emission of radiation."
The laser was introduced in 1958 as a efficient source of light. The concept was introduced by Charles Townes and
Arthur Schawlow to show that masers could be made to operate in optical and infrared regions. Basically, light is
reflected back and forth in an energized medium to generate amplified light as opposed to excited molecules of gas
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amplified to generate radio waves, as is the case with the maser. Laser stands for "light amplification by stimulated
emission of radiation."
A helium-neon gas laser (He-Ne) is tested in a laboratory setting. The laser tube is made from lead glass- the same glass
used in neon signs. Image courtesy of J&K Lasers.
In 1960, the first continuously operating helium-neon gas laser is invented and tested. That same year an operable laser
was invented which used a synthetic pink ruby crystal as the medium and produced a pulse of light.
In 1961, Elias Snitzer of American Optical published a theoretical description of single mode fibers whose core would be
so small it could carry light with only one wave-guide mode. Snitzer was able to demonstrate a laser directed through a
thin glass fiber which was sufficient for medical applications, but for communication applications the light loss became
too great.
Charles Kao and George Hockham, of Standard Communications Laboratories in England, published a paper in 1964
demonstrating, theoretically, that light loss in existing glass fibers could be decreased dramatically by removing
impurities.
In 1970, the goal of making single mode fibers with attenuation less then 20dB/km was reached by scientists at Corning
Glass Works. This was achieved through doping silica glass with titanium. Also in 1970, Morton Panish and Izuo
Hayashi of Bell Laboratories, along with a group from the Ioffe Physical Institute in Leningrad, demonstrated a
semiconductor diode laser capable of emitting continuous waves at room temperature.
Military scientists have utilized laser technology for variety of military applications.
In 1973, Bell Laboratories developed a modified chemical vapor deposition process that heats chemical vapors and
oxygen to form ultra-transparent glass that can be mass-produced into low-loss optical fiber. This process still remains
the standard for fiber-optic cable manufacturing.
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The first non-experimental fiber-optic link was installed by the Dorset (UK) police in 1975. Two years later, the first live
telephone traffic through fiber optics occurs in Long Beach, California.
In the late 1970s and early 1980s, telephone companies began to use fibers extensively to rebuild their communications
infrastructure.
Sprint was founded on the first nationwide, 100 percent digital, fiber-optic network in the mid-1980s.
The erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by eliminating the need for
optical-electrical-optical repeaters, was invented in 1986 by David Payne of the University of Southampton and
Emmanuel Desurvire at Bell Labratories. Based on Desurvire's optimized laser amplification technology, the first
transatlantic telephone cable went into operation in 1988.
In 1991, Desurvire and Payne demonstrated optical amplifiers that were built into the fiber-optic cable itself. The all-
optic system could carry 100 times more information than cable with electronic amplifiers. Also in 1991, photonic
crystal fiber was developed. This fiber guides light by means of diffraction from a periodic structure rather then total
internal reflection which allows power to be carried more efficiently then with conventional fibers therefore improving
performance.
The first all-optic fiber cable, TPC-5, that uses optical amplifiers was laid across the Pacific Ocean in 1996. The
following year the Fiber Optic Link Around the Globe (FLAG) became the longest single-cable network in the world and
provided the infrastructure for the next generation of Internet applications.
Today, a variety of industries including the medical, military, telecommunication, industrial, data storage, networking,
and broadcast industries are able to apply and use fiber optic technology in a variety of applications.
The Birth of Fiber OpticsFiber optics is the contained transmission of light through long fiber rods of either
glass or plastics.
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ByMary Bellis
In 1854, John Tyndall demonstrated to the Royal Society that light
could be conducted through a curved stream of water, proving
that a light signal could be bent.
In 1880,Alexander Graham Bellinvented his 'Photophone', which transmitted a voice signal on
a beam of light. Bell focused sunlight with a mirror and then talked into a mechanism that
vibrated the mirror. At the receiving end, a detector picked up the vibrating beam and
decoded it back into a voice the same way a phone did with electrical signals. Many things -
- a cloudy day for instance -- could interfere with thePhotophone, causing Bell to stop any
further research with this invention.
In 1880, William Wheeler invented a system of light pipes lined with a highly reflective
coating that illuminated homes by using light from an electric arc lamp placed in the
basement and directing the light around the home with the pipes.
In 1888, the medical team of Roth and Reuss of Vienna used bent glass rods to illuminate
body cavities.
In 1895, French engineer Henry Saint-Rene designed a system of bent glass rods for
guiding light images in an attempt at early television.
In 1898, American David Smith applied for a patent on a bent glass rod device to be used
as a surgical lamp.
In the 1920's, EnglishmanJohn Logie Bairdand American Clarence W. Hansell patented the
idea of using arrays of transparent rods to transmit images for television and facsimiles
respectively.
Fiber optics is the contained transmission of light through long fiber rods of either glass
or plastics. The light travels by a process of internal reflection. The core medium of therod or cable is more reflective than the material surrounding the core. That causes thelight to keep being reflected back into the core where it can continue to travel down
the fiber. Fiber optic cables are used for transmitting voice, images and other data atclose to the speed of light.
In 1930, German medical student, Heinrich Lamm was the first person to assemble a bundle
of optical fibers to carry an image. Lamm's goal was to look inside inaccessible parts of the
body. During his experiments, he reported transmitting the image of a light bulb. The image
was of poor quality, however. His effort to file a patent was denied because of Hansell's
British patent.
n 1954, Dutch scientist Abraham Van Heel and British scientist Harold. H. Hopkins
More on Fiber OpticsFiber OpticsTo read or research more onfiber optics, history, timelines,glossary of fiber optic terms,fiber optics industry magazinesand associations, FAQs, andbiographies of the inventors.
http://inventors.about.com/mbiopage.htmhttp://inventors.about.com/mbiopage.htmhttp://inventors.about.com/mbiopage.htmhttp://inventors.about.com/library/inventors/bltelephone.htmhttp://inventors.about.com/library/inventors/bltelephone.htmhttp://inventors.about.com/library/inventors/bltelephone.htmhttp://inventors.about.com/library/inventors/bltelephone3.htmhttp://inventors.about.com/library/inventors/bltelephone3.htmhttp://inventors.about.com/library/inventors/bltelephone3.htmhttp://inventors.about.com/library/inventors/blbaird.htmhttp://inventors.about.com/library/inventors/blbaird.htmhttp://inventors.about.com/library/inventors/blbaird.htmhttp://inventors.about.com/library/inventors/blfiberoptics.htmhttp://inventors.about.com/library/inventors/blfiberoptics.htmhttp://inventors.about.com/library/inventors/blfiberoptics.htmhttp://inventors.about.com/library/inventors/blfiberoptics.htmhttp://inventors.about.com/library/inventors/blbaird.htmhttp://inventors.about.com/library/inventors/bltelephone3.htmhttp://inventors.about.com/library/inventors/bltelephone.htmhttp://inventors.about.com/mbiopage.htm7/28/2019 fber
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separately wrote papers on imaging bundles. Hopkins reported on imaging bundles of
unclad fibers while Van Heel reported on simple bundles of clad fibers. He covered a bare
fiber with a transparent cladding of a lower refractive index. This protected the fiber
reflection surface from outside distortion and greatly reduced interference between fibers.
At the time, the greatest obstacle to a viable use of fiber optics was in achieving the lowest
signal (light) loss.
In 1961, Elias Snitzer of American
Optical published a theoretical
description of single mode fibers, a
fiber with a core so small it could
carry light with only one wave-guide
mode. Snitzer's idea was okay for a
medical instrument looking inside the human, but the fiber had a light loss of one decibel
per meter. Communications devices needed to operate over much longer distances and
required a light loss of no more than 10 or 20 decibels (measurement of light) per
kilometer.
In 1964, a critical (and theoretical) specification was identified by Dr. C.K. Kao for long-
range communication devices, the 10 or 20 decibels of light loss per kilometer standard.
Kao also illustrated the need for a purer form of glass to help reduce light loss.
In 1970, one team of researchers began experimenting with fused silica, a material capable
of extreme purity with a high melting point and a low refractive index. Corning Glass
researchersRobert Maurer, Donald Keck and Peter Schultzinvented fiber optic wire or "Optical
Waveguide Fibers" (patent #3,711,262) capable of carrying 65,000 times more information
than copper wire, through which information carried by a pattern of light waves could be
decoded at a destination even a thousand miles away. The team had solved the problemspresented by Dr. Kao.
In 1975, the United States Government decided to link the computers in the NORAD
headquarters at Cheyenne Mountain using fiber optics to reduce interference.
In 1977, the first optical telephone communication system was installed about 1.5 miles
under downtown Chicago, and each optical fiber carried the equivalent of 672 voice
channels.
Today more than 80 percent of the world's long-distance traffic is carried over optical fibercables, 25 million kilometers of the cable Maurer, Keck and Schultz designed has been
installed world wide.
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The Nineteenth Century
Figure 1 - John Tyndall's Experiment.
In 1870, John Tyndall, using a jet of water that flowed from one container to another and a beam of light, demonstrated thatlight used internal reflection to follow a specific path. As water poured out through the spout of the first container, Tyndall
directed a beam of sunlight at the path of the water. The light, as seen by the audience, followed a zigzag path inside the
curved path of the water. This simple experiment, illustrated in Figure 1, marked the first research into the guided
transmission of light.
William Wheeling, in 1880, patented a method of light transfer called "piping light". Wheeling believed that by using mirrored
pipes branching off from a single source of illumination, i.e. a bright electric arc, he could send the light to many different
rooms in the same way that water, through plumbing, is carried throughout buildings today. Due to the ineffectiveness of
Wheeling's idea and to the concurrent introduction of Edison's highly successful incandescent light bulb, the concept of
piping light never took off.
That same year, Alexander Graham Bell developed an optical voice transmission system he called the photophone. The
photophone used free-space light to carry the human voice 200 meters. Specially placed mirrors reflected sunlight onto a
diaphragm attached within the mouthpiece of the photophone. At the other end, mounted within a parabolic reflector, was a
light-sensitive selenium resistor. This resistor was connected to a battery that was, in turn, wired to a telephone receiver. As
one spoke into the photophone, the illuminated diaphragm vibrated, casting various intensities of light onto the selenium
resistor. The changing intensity of light altered the current that passed through the telephone receiver which then converted
the light back into speech. Bell believed this invention was superior to the telephone because it did not need wires to
connect the transmitter and receiver. Today, free-space optical links find extensive use in metropolitan applications.
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The Twentieth Century
Figure 2 - Optical Fiber with Cladding.
Fiber optic technology experienced a phenomenal rate of progress in the second half of the twentieth century. Early successcame during the 1950's with the development of the fiberscope. This image-transmitting device, which used the first practicalall-glass fiber, was concurrently devised by Brian O'Brien at the American Optical Company and Narinder Kapany (who firstcoined the term 'fiber optics' in 1956) and colleagues at the Imperial College of Science and Technology in London. Early all-glass fibers experienced excessive optical loss, the loss of the light signal as it traveled the fiber, limiting transmissiondistances.
This motivated scientists to develop glass fibers that included a separate glass coating. The innermost region of the fiber, orcore, was used to transmit the light, while the glass coating, or cladding, prevented the light from leaking out of the core byreflecting the light within the boundaries of the core. This concept is explained by Snell's Law which states that the angle atwhich light is reflected is dependent on the refractive indices of the two materials ' in this case, the core and the cladding.The lower refractive index of the cladding (with respect to the core) causes the light to be angled back into the core asillustrated in Figure 2.
The fiberscope quickly found application inspecting welds inside reactor vessels and combustion chambers of jet aircraftengines as well as in the medical field. Fiberscope technology has evolved over the years to make laparoscopic surgery oneof the great medical advances of the twentieth century.
The development of laser technology was the next important step in the establishment of the industry of fiber optics. Onlythe laser diode (LD) or its lower-power cousin, the light-emitting diode (LED), had the potential to generate large amounts oflight in a spot tiny enough to be useful for fiber optics. In 1957, Gordon Gould popularized the idea of using lasers when, as
a graduate student at Columbia University, he described the laser as an intense light source. Shortly after, Charles Townesand Arthur Schawlow at Bell Laboratories supported the laser in scientific circles. Lasers went through several generationsincluding the development of the ruby laser and the helium-neon laser in 1960. Semiconductor lasers were first realized in1962; these lasers are the type most widely used in fiber optics today.
Because of their higher modulation frequency capability, the importance of lasers as a means of carrying information did notgo unnoticed by communications engineers. Light has an information-carrying capacity 10,000 times that of the highest radiofrequencies being used. However, the laser is unsuited for open-air transmission because it is adversely affected byenvironmental conditions such as rain, snow, hail, and smog. Faced with the challenge of finding a transmission mediumother than air, Charles Kao and Charles Hockham, working at the Standard Telecommunication Laboratory in England in1966, published a landmark paper proposing that optical fiber might be a suitable transmission medium if its attenuationcould be kept under 20 decibels per kilometer (dB/km). At the time of this proposal, optical fibers exhibited losses of 1,000dB/ km or more. At a loss of only 20 dB/km, 99% of the light would be lost over only 3,300 feet. In other words, only 1/100thof the optical power that was transmitted reached the receiver. Intuitively, researchers postulated that the current, higheroptical losses were the result of impurities in the glass and not the glass itself. An optical loss of 20 dB/km was within thecapability of the electronics and opto-electronic components of the day.
Intrigued by Kao and Hockham's proposal, glass researchers began to work on the problem of purifying glass. In 1970, Drs.Robert Maurer, Donald Keck, and Peter Schultz of Corning succeeded in developing a glass fiber that exhibited attenuationat less than 20 dB/km, the threshold for making fiber optics a viable technology. It was the purest glass ever made.
The early work on fiber optic light source and detector was slow and often had to borrow technology developed for otherreasons. For example, the first fiber optic light sources were derived from visible indicator LEDs. As demand grew, lightsources were developed for fiber optics that offered higher switching speed, more appropriate wavelengths, and higheroutput power. For more information on light emitters see Laser Diodes and LEDs.
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Figure 3 - Four Wavelength Regions of Optical Fiber.
Fiber optics developed over the years in a series of generations that can be closely tied to wavelength. Figure 3 shows three
curves. The top, dashed, curve corresponds to early 1980's fiber, the middle, dotted, curve corresponds to late 1980's fiber,and the bottom, solid, curve corresponds to modern optical fiber. The earliest fiber optic systems were developed at an
operating wavelength of about 850 nm. This wavelength corresponds to the so-called 'first window' in a silica-based optical
fiber. This window refers to a wavelength region that offers low optical loss. It sits between several large absorption peaks
caused primarily by moisture in the fiber and Rayleigh scattering.
The 850 nm region was initially attractive because the technology for light emitters at this wavelength had already been
perfected in visible indicator LEDs. Low-cost silicon detectors could also be used at the 850 nm wavelength. As technology
progressed, the first window became less attractive because of its relatively high 3 dB/km loss limit.
Most companies jumped to the 'second window' at 1310 nm with lower attenuation of about 0.5 dB/km. In late 1977, Nippon
Telegraph and Telephone (NTT) developed the 'third window' at 1550 nm. It offered the theoretical minimum optical loss for
silica-based fibers, about 0.2 dB/km.
Today, 850 nm, 1310 nm, and 1550 nm systems are all manufactured and deployed along with very low-end, short distance,
systems using visible wavelengths near 660 nm. Each wavelength has its advantage. Longer wavelengths offer higher
performance, but always come with higher cost. The shortest link lengths can be handled with wavelengths of 660 nm or
850 nm. The longest link lengths require 1550 nm wavelength systems. A 'fourth window,' near 1625 nm, is being
developed. While it is not lower loss than the 1550 nm window, the loss is comparable, and it might simplify some of the
complexities of long-length, multiple-wavelength communications systems.
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The Nineteenth Century
The earliest attempts to communicate via light
undoubtedly go back thousands of years. Early long
distance communication techniques, such as "smoke
signals", developed by native North Americans and the
Chinese were, in fact, optical communication links. Alarger scale version of this optical communication
technique was the "optical telegraph" deployed in
France and elsewhere in the late 18th century. The"optical telegraph" was a series of tall towers that
passed along messages at a rate of a few words per
minute by means of large semaphore flags that could bemanipulated to spell out words. However, thedevelopment of fiber optic communication awaited the
discovery of TIR (Total Internal Reflection) and a host ofadditional electronic and optical innovations.
Jean-Daniel Colladon, a 38-year-old Swiss professor atUniversity of Geneva, demonstrated light guiding or TIRfor the first time in 1841. He wanted to show the fluid
flow through various holes of a tank and the breakingup of water jets. However, in the lecture hall theaudience could not see the flowing water. He solved the
problem by collecting and piping sunlight through a tube
to the lecture table. The light was focused through thewater tank and was made to incident on the edge of thejet at a glancing angle. TIR trapped the light in the
liquid forcing it to follow the curved path until the waterjet broke up. Instead of traveling in a straight line, thelight followed the curvature of the water flow. Colladon
later on wrote:"I managed to illuminate the interior of astream in a dark space. I have discovered that thisstrange arrangement offers in results one of the mostbeautiful, and most curious experiments that one can
perform in a course on Optics." ( Comptes Rendes, 15,800-802 Oct. 24, 1842). Colladon demonstrated lightguiding in water jets through a number of public
performances to the urban intelligentsia of Paris.
Auguste de la Rive, another Geneva Physicist,duplicated Colladon's experiment using electric arc light.Colladon designed a spectacular device using arc light
for Conservatory of Arts and Science of Paris in 1841,
Oct..
Colladon sent a paper to his friend Francois Arago whoheaded the French Academy of Sciences and edited its
journal Comptes Renedes. Arago recalled that Jacques
Babinet, a French specialist in Optics had made similardemonstrations in Paris. He focused candle light on tothe bottom of a glass bottle as he poured a thin stream
of water from the top. TIR guided the light along the
Figure 1 - Jean-Daniel Colladon
Figure 2 - Jacques Babinet
Figure 3 - John Tyndall
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jet. Arago asked Babinet to write down his work, butBabinet did not think that the work is very important.
Yet he made a comment that "the idea also works verywell with a glass shaft curved in what ever manner andI had indicated that (it could be used) to illuminate theinside of the mouth (Comptes Rendes 15, Oct. 24,
1842). After sending his letter to Arago, Babinet neverreturned to guiding of light before he died in 1872.
In 1870, John Tyndall, using a jet of water that flowedfrom one container to another and a beam of light,
demonstrated that light used internal reflection to follow
a specific path. As water poured out through the spoutof the first container, Tyndall directed a beam ofsunlight at the path of the water. The light, as seen by
the audience, followed a zigzag path inside the curvedpath of the water. This simple experiment, illustratedinFigure 4, marked the first research into the guidedtransmission of light.
William Wheeling, in 1880, patented a method of lighttransfer called piping light. Wheeling believed that by
using mirrored pipes branching off from a single source
of illumination, i.e. a bright electric arc, he could sendthe light to many different rooms in the same way that
water, through plumbing, is carried throughout buildings
today. Due to the ineffectiveness of Wheelings idea andto the concurrent introduction of Edisons highlysuccessful incandescent light bulb, the concept of piping
light never took off.
Figure 4 - Typical Early TIR (Total Intern
Reflection) Demonstration
That same year, Alexander Graham Bell developed an optical voice transmission system he called th
photophone. The photophone used free-space light to carry the human voice 200 meters. Specially
placed mirrors reflected sunlight onto a diaphragm attached within the mouthpiece of the photophoAt the other end, mounted within a parabolic reflector, was a light-sensitive selenium resistor. This
resistor was connected to a battery that was, in turn, wired to a telephone receiver. As one spoke in
the photophone, the illuminated diaphragm vibrated, casting various intensities of light onto theselenium resistor. The changing intensity of light altered the current that passed through the telephreceiver which then converted the light back into speech. Bell believed this invention was superior t
the telephone because it did not need wires to connect the transmitter and receiver. Today,free-spaopticallinks find extensive use in metropolitan applications.
The Twentieth Century
Fiber optic technology experienced a phenomenal rateof progress in the second half of the twentieth century.
Early success came during the 1950s with thedevelopment of the fiberscope. This image-transmittingdevice, which used the first practical all-glass fiber, wasconcurrently devised by Brian OBrien at the American
Optical Company and Narinder Kapany (who first coinedFigure 5 - Optical Fiber with Cladding
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the term fiber optics in 1956) and colleagues at the
Imperial College of Science and Technology in London.
Early all-glass fibers experienced excessive optical loss,the loss of the light signal as it traveled through the
fiber, limiting transmission distances.This motivated scientists to develop glass fibers that included a separate glass coating. The innermo
region of the fiber, orcore, was used to transmit the light, while the glass coating, orcladding, prevented thelight from leaking out of the core by reflecting the light within the boundaries of the core. This concis explained by Snells Law which states that the angle at which light is reflected is dependent on threfractive indices of the two materials - in this case, the core and the cladding. The lowerrefractiveindexof the cladding (with respect to the core) causes the light to be angled back into the core as illustrated in Fig5.
The fiberscope quickly found application inspecting welds inside reactor vessels and combustion
chambers of jet aircraft engines as well as in the medical field. Fiberscope technology has evolved othe years to make laparoscopic surgery one of the great medical advances of the twentieth century
The development oflasertechnology was the next important step in the establishment of the industry of fiber
optics. Only thelaser diode(LD) or its lower-power cousin, thelight-emitting diode(LED), had the potential togenerate large amounts of light in a spot tiny enough to be useful for fiber optics. In 1957, Gordon Gould popularizthe idea of using lasers when, as a graduate student at Columbia University, he described the laser as an intense source. Shortly after, Charles Townes and Arthur Schawlow at Bell Laboratories supported the laser in scientificcircles. Lasers went through several generations including the development of the ruby laser and the helium-neonlaser in 1960. Semiconductor lasers were first realized in 1962; these lasers are the type most widely used in fiberoptics today.
Because of their higher modulation frequency capability, the importance of lasers as a means of
carrying information did not go unnoticed by communications engineers. Light has an information-
carrying capacity 10,000 times that of the highest radio frequencies being used. However, the laserunsuited for open-air transmission because it is adversely affected by environmental conditions sucas rain, snow, hail, and smog. Faced with the challenge of finding a transmission medium other tha
air, Charles Kao and Charles Hockham, working at the Standard Telecommunication Laboratory inEngland in 1966, published a landmark paper proposing that optical fiber might be a suitabletransmission medium if itsattenuationcould be kept under 20 decibels per kilometer (dB/km). At the time of thproposal, optical fibers exhibited losses of 1,000 dB/ km or more. Even at a loss of only 20 dB/km, 99% of the lightwould still be lost over only 3,300 feet. In other words, only 1/100th of the optical power that was transmitted reachthe receiver. Intuitively, researchers postulated that the current, higher optical losses were the result of impurities ithe glass and not the glass itself. An optical loss of 20 dB/km was within the capability of the electronics andoptoelectronic components of the day.
Intrigued by Kao and Hockhams proposal, glass researchers began to work on the problem of
purifying glass. In 1970, Drs. Robert Maurer, Donald Keck, and Peter Schultz of Corning succeeded developing a glass fiber that exhibited attenuation at less than 20 dB/km, the threshold for makingfiber optics a viable technology. It was the purest glass ever made.
The early work on fiber optic lightsourceanddetectorwas slow and often had to borrow technology developfor other reasons. For example, the first fiber optic light sources were derived from visible indicator LED's. As demgrew, light sources were developed for fiber optics that offered higher switching speed, more appropriatewavelengths, and higher output power. For more information on light emitters seeLaser DiodesandLED's.
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er/fiber-history.htm#Figure_5http://www.olson-technology.com/mr_fiber/fiber-history.htm#Figure_5http://n_window%28%27glossary-r.htm/#Refractive_Index')http://n_window%28%27glossary-r.htm/#Refractive_Index')http://n_window%28%27glossary-c.htm/#Cladding')http://n_window%28%27glossary-c.htm/#Core')7/28/2019 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Fiber optics developed over the years in a series of
generations that can be closely tied towavelength.Figure 6shows three curves. The top,
dashed, curve corresponds to early 1980s fiber, the
middle, dotted, curve corresponds to late 1980s fiber,
and the bottom, solid, curve corresponds to modern
optical fiber. The earliest fiber optic systems weredeveloped at an operating wavelength of about 850 nm.
This wavelength corresponds to the so-called firstwindow in a silica-based optical fiber. This windowrefers to a wavelength region that offers low opticalloss. It sits between several large absorption peaks
caused primarily by moisture in the fiber andRayleighscattering.
Figure 6 - Four Wavelength Regions of OpticFiber
The 850 nm region was initially attractive because the technology for light emitters at this wavelenghad already been perfected in visible indicator and infrared (IR) LED's. Low-cost silicon detectors coalso be used at the 850 nm wavelength. As the technology progressed, the first window became lesattractive because of its relatively high 3 dB/km loss limit.
Most companies jumped to the second window at 1310 nm with lower attenuation of about 0.5
dB/km. In late 1977, Nippon Telegraph and Telephone (NTT) developed the third window at 1550nm. It offered the theoretical minimum optical loss for silica-based fibers, about 0.2 dB/km.
Today, 850 nm, 1310 nm, and 1550 nm systems are all manufactured and deployed along with verlow-end, short distance, systems using visible wavelengths near 660 nm. Each wavelength has itsadvantage. Longer wavelengths offer higher performance, but always come with higher cost. The
shortest link lengths can be handled with wavelengths of 660 nm or 850 nm. The longest link lengtrequire 1550 nm wavelength systems. A fourth window, near 1625 nm, is being developed. Whileis not lower loss than the 1550 nm window, the loss is comparable.Applications in the Real World
The U.S. military moved quickly to use fiber optics for improved communications and tactical system
In the early 1970s, the U.S. Navy installed a fiber optic telephone link aboard the U.S.S. Little Rock
The Air Force followed suit by developing its Airborne Light Optical Fiber Technology (ALOFT) prograin 1976. Encouraged by the success of these applications, military R&D programs were funded to
develop stronger fibers, tactical cables, ruggedized, high-performance components, and numerousdemonstration systems ranging from aircraft to undersea applications.
Commercial applications followed soon after. In 1977, both AT&T and GTE installed fiber optictelephone systems in Chicago and Boston respectively. These successful applications led to the
increase of fiber optic telephone networks. By the early 1980s, single-mode fiber operating in the1310 nm and later the 1550 nm wavelength windows became the standard fiber installed for these
networks. Initially, computers, information networks, and data communications were slower toembrace fiber, but today they too find use for a transmission system that has lighter weight cable,resists lightning strikes, and carries more information faster and over longer distances.
The broadcast industry also embraced fiber optic transmission. In 1980, broadcasters of the Winter
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Olympics, in Lake Placid, New York, requested a fiber optic video transmission system for backup v
feeds. The fiber optic feed, because of its quality and reliability, soon became the primary video fee
making the 1980 Winter Olympics the first fiber optic television transmission. Later, at the 1994 WiOlympics in Lillehammer, Norway, fiber optics transmitted the first ever digital video signal, anapplication that continues to evolve today.
In the mid-1980s the United States government deregulated telephone service, allowing smalltelephone companies to compete with the giant, AT&T. Companies like MCI and Sprint quickly went
work installing regional fiber optic telecommunications networks throughout the world. Takingadvantage of railroad lines, gas pipes, and other natural rights of way, these companies laid miles foptic cable, allowing the deployment of these networks to continue throughout the 1980s. Howeve
this created the need to expand fibers transmission capabilities.
In 1990, Bell Labs transmitted a 2.5 Gb/s signal over 7,500 km without regeneration. The system ua soliton laser and an erbium-doped fiber amplifier (EDFA) that allowed the light wave to maintain its shapand density. In 1998, they went one better as researchers transmitted 100 simultaneous optical signals, each at adata rate of 10 gigabits (giga means billion) per second for a distance of nearly 250 miles (400 km). In thisexperiment, dense wavelength-division multiplexing (DWDMtechnology, which allows multiple wavelengths to becombined into one optical signal, increased the total data rate on one fiber to one terabit per second (10
12bits per
second).The Twenty-first Century and Beyond
Today, DWDM technology continues to develop. As the
demand for data bandwidth increases, driven by thephenomenal growth of the Internet, the move to optical
networking is the focus of new technologies. At this
writing, over 800 million people have Internet accessand use it regularly. That's over 12% of the entireworld's population of 6.4 billion people. The world wide
web already hosts over 350 million domain names, 8billion web pages (that's only the visible, indexed,
Internet, the invisible Internet is estimated to be up to100 times larger), and according to estimates people
upload more than 3.5 million new web pages everyday.The Internet dominates traditional voice communicationas shown inFigure 7.
Figure 7 - Projected Internet Traffic Increase
The important factor in these developments is the
increase in fiber transmission capacity, which has grown
by a factor of 200 in the last decade.Figure 8illustratesthis trend.
Because of fiber optic technologys immense potential
bandwidth, 50 THz or greater, there are extraordinary
possibilities for future fiber optic applications. Already,the push to bring broadband services, including data,
audio, and especially video, into the home is wellunderway.
Broadband service available to a mass market opens upa wide variety of interactive communications for bothconsumers and businesses, bringing to reality
interactive video networks, interactive banking and
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shopping from the home, and interactive distance
learning. The last mile for optical fiber goes from the
curb to the television set top, known asfiber-to-the-home(FTTH) andfiber-to-the-curb(FTTC),
allowingvideo on demandto become a reality.
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