Fiber History
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 signaling 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 ultimately solve 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 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 Møller [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 Møller 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, but their 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
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 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 be
reduced 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 the compound 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 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 the
micrometer-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 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 remains
too 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.
Acknowledgments Thanks to the
Alfred P. Sloan Foundation for research support. This is a much expanded
version of an article originally published in the November 1994 Laser
Focus World.
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