Download History of Fiber Optics

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 handcarried 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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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 glassair 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.
History of Fiber Optics
The next key step was development of glassclad 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.
History of Fiber Optics
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.
History of Fiber Optics
Even higher optical frequencies seemed a logical
next step in 1958 to Alec Reeves, the forwardlooking
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."
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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
ElectroOptics), demonstrated the similarity by
drawing fibers with cores so small they
carried light in only one waveguide mode.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
The April 1, 1966 issue of
Laser Focus noted Kao's proposal:
History of Fiber Optics
"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."
History of Fiber Optics
"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."
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
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.
History of Fiber Optics
What are Fiber Optics?
Fiber optics (optical fibers) are
long, thin strands of very pure glass
about the diameter of a human
hair. They are arranged in bundles
called optical cables and used to
transmit light signals over long
distances.
How Fiber Optics Works
Parts of an optical Fiber
•Core - Thin glass center of the fiber where the
light travels
•Cladding - Outer optical
material surrounding the
core that reflects the
light back into the core
Buffer coating - Plastic
coating that protects the
fiber from damage and
moisture
Parts of Fiber Optics
Optical fibers come in two types:
•Single-mode fibers
•Multi-mode fibers
Single-mode fibers have small cores (about 3.5 x 10-4
inches or 9 microns in diameter) and transmit infrared
laser light (wavelength = 1,300 to 1,550 nanometers).
Multi-mode fibers have larger cores (about 2.5 x 10-3
inches or 62.5 microns in diameter) and transmit infrared
light (wavelength = 850 to 1,300 nm) from light-emitting
diodes (LEDs).
Some optical fibers can be made from plastic. These
fibers have a large core (0.04 inches or 1 mm diameter)
and transmit visible red light (wavelength = 650 nm) from
LEDs.
Types of Fiber Optics
How Does an Optical Fiber Transmit Light?
Suppose you want to shine a flashlight beam down
a long, straight hallway. Just point the beam
straight down the hallway -- light travels in
straight lines, so it is no problem. What if the
hallway has a bend in it? You could place a mirror
at the bend to reflect the light beam around the
corner. What if the hallway is very winding with
multiple bends? You might line the walls with
mirrors and angle the beam so that it bounces
from side-to-side all along the hallway. This is
exactly what happens in an optical fiber.
Fiber Optics – light transmission
Fiber Optics – light transmission
The light in a fiber-optic cable travels through the
core (hallway) by constantly bouncing from the cladding
(mirror-lined walls), a principle called total internal
reflection. Because the cladding does not absorb any
light from the core, the light wave can travel great
distances. However, some of the light signal degrades
within the fiber, mostly due to impurities in the glass.
The extent that the signal degrades depends on the
purity of the glass and the wavelength of the
transmitted light (for example, 850 nm = 60 to 75
percent/km; 1,300 nm = 50 to 60 percent/km; 1,550
nm is greater than 50 percent/km). Some premium
optical fibers show much less signal degradation -less than 10 percent/km at 1,550 nm.
Fiber Optics – light transmission
A Fiber-Optic Relay System
To understand how optical fibers are used in
communications systems, let's look at an example from a
World War II movie or documentary where two naval
ships in a fleet need to communicate with each other
while maintaining radio silence or on stormy seas. One
ship pulls up alongside the other. The captain of one
ship sends a message to a sailor on deck. The sailor
translates the message into Morse code (dots and
dashes) and uses a signal light (floodlight with a venetian
blind type shutter on it) to send the message to the
other ship. A sailor on the deck of the other ship sees
the Morse code message, decodes it into English and
sends the message up to the captain.
Fiber Optics – Relay System
Now, imagine doing this when the ships are on either side of
the ocean separated by thousands of miles and you have a
fiber-optic communication system in place between the two
ships. Fiber-optic relay systems consist of the following:
•Transmitter - Produces and encodes the light
signals
•Optical fiber - Conducts the light signals over a
distance
•Optical regenerator - May be necessary to boost
the light signal (for long distances)
•Optical receiver - Receives and decodes the light
signals
Fiber Optics – light transmission
Transmitter
The transmitter is like the sailor on the deck of the
sending ship. It receives and directs the optical
device to turn the light "on" and "off" in the correct
sequence, thereby generating a light signal.
The transmitter is physically close to the optical
fiber and may even have a lens to focus the light into
the fiber. Lasers have more power than LEDs, but
vary more with changes in temperature and are more
expensive. The most common wavelengths of light
signals are 850 nm, 1,300 nm, and 1,550 nm
(infrared, non-visible portions of the spectrum).
Fiber Optics – light transmission
Optical Regenerator
As mentioned above, some signal loss occurs when the light is
transmitted through the fiber, especially over long distances
(more than a half mile, or about 1 km) such as with undersea
cables. Therefore, one or more optical regenerators is spliced
along the cable to boost the degraded light signals.
An optical regenerator consists of optical fibers with a special
coating (doping). The doped portion is "pumped" with a laser.
When the degraded signal comes into the doped coating, the
energy from the laser allows the doped molecules to become
lasers themselves. The doped molecules then emit a new,
stronger light signal with the same characteristics as the
incoming weak light signal. Basically, the regenerator is a laser
amplifier for the incoming signal.
Fiber Optics – light transmission
Optical Receiver
The optical receiver is like the sailor on
the deck of the receiving ship. It takes
the incoming digital light signals,
decodes them and sends the electrical
signal to the other user's computer, TV
or telephone (receiving ship's captain).
The receiver uses a photocell or
photodiode to detect the light.
Fiber Optics – light transmission
Advantages of Fiber Optics
Why are fiber-optic systems revolutionizing telecommunications?
Compared to conventional metal wire (copper wire), optical fibers
are:
•Less expensive - Several miles of optical cable can be made
cheaper than equivalent lengths of copper wire. This saves your
provider (cable TV, Internet) and you money.
•Thinner - Optical fibers can be drawn to smaller diameters than
copper wire.
•Higher carrying capacity - Because optical fibers are thinner
than copper wires, more fibers can be bundled into a givendiameter cable than copper wires. This allows more phone lines
to go over the same cable or more channels to come through the
cable into your cable TV box.
Advantages of Fiber Optics
•Less signal degradation - The loss of signal in optical fiber is
less than in copper wire.
•Light signals - Unlike electrical signals in copper wires, light
signals from one fiber do not interfere with those of other
fibers in the same cable. This means clearer phone
conversations or TV reception.
•Low power - Because signals in optical fibers degrade less,
lower-power transmitters can be used instead of the highvoltage electrical transmitters needed for copper wires. Again,
this saves your provider and you money.
•Digital signals - Optical fibers are ideally suited for carrying
digital information, which is especially useful in computer
networks.
•Non-flammable - Because no electricity is passed through
optical fibers, there is no fire hazard.
Advantages of Fiber Optics
•Lightweight - An optical cable weighs less than a
comparable copper wire cable. Fiber-optic cables
take up less space in the ground.
•Flexible - Because fiber optics are so flexible and
can transmit and receive light, they are used in
many flexible digital cameras for the following
purposes:
•Medical imaging - in bronchoscopes, endoscopes,
laparoscopes
•Mechanical imaging - inspecting mechanical welds
in pipes and engines (in airplanes, rockets, space
shuttles, cars)
•Plumbing - to inspect sewer lines
Advantages of Fiber Optics
How Are Optical Fibers Made?
Now that we know how fiber-optic systems work
and why they are useful -- how do they make
them? Optical fibers are made of extremely pure
optical glass. We think of a glass window as
transparent, but the thicker the glass gets, the
less transparent it becomes due to impurities in
the glass. However, the glass in an optical fiber
has far fewer impurities than window-pane glass.
One company's description of the quality of glass
is as follows: If you were on top of an ocean that
is miles of solid core optical fiber glass, you could
see the bottom clearly.
How Fiber Optics are made
Making optical fibers requires the
following steps:
1.Making a preform glass cylinder
2.Drawing the fibers from the
preform
3.Testing the fibers
How Fiber Optics are made
Making the Preform Blank
The glass for the preform is made by a process called
modified chemical vapor deposition (MCVD).
How Fiber Optics are made
In MCVD, oxygen is bubbled through solutions of
silicon chloride (SiCl4), germanium chloride
(GeCl4) and/or other chemicals. The precise
mixture governs the various physical and optical
properties (index of refraction, coefficient of
expansion, melting point, etc.). The gas vapors
are then conducted to the inside of a synthetic
silica or quartz tube (cladding) in a special
lathe. As the lathe turns, a torch is moved up
and down the outside of the tube. The extreme
heat from the torch causes two things to
happen:
How Fiber Optics are made
•The silicon and germanium react with oxygen, forming
silicon dioxide (SiO2) and germanium dioxide (GeO2).
•The silicon dioxide and germanium dioxide deposit on
the inside of the tube and fuse together to form glass.
Lathe
used
in preparing
the preform
blank
How Fiber Optics are made
The lathe turns continuously to make an
even coating and consistent blank. The
purity of the glass is maintained by using
corrosion-resistant plastic in the gas
delivery system (valve blocks, pipes, seals)
and by precisely controlling the flow and
composition of the mixture. The process
of making the preform blank is highly
automated and takes several hours. After
the preform blank cools, it is tested for
quality control (index of refraction).
How Fiber Optics are made
Drawing Fibers from
the Preform Blank
Once the preform blank has
been tested, it gets loaded
into a fiber drawing tower.
The blank gets lowered into a
graphite furnace (3,452 to
3,992 degrees Fahrenheit or
1,900
to
2,200
degrees
Celsius) and the tip gets
melted until a molten glob falls
down by gravity. As it drops,
it cools and forms a thread.
How Fiber Optics are made
The operator threads the strand through a series
of coating cups (buffer coatings) and ultraviolet
light curing ovens onto a tractor-controlled spool.
The tractor mechanism slowly pulls the fiber from
the heated preform blank and is precisely
controlled by using a laser micrometer to measure
the diameter of the fiber and feed the information
back to the tractor mechanism. Fibers are pulled
from the blank at a rate of 33 to 66 ft/s (10 to
20 m/s) and the finished product is wound onto the
spool. It is not uncommon for spools to contain
more than 1.4 miles (2.2 km) of optical fiber.
How Fiber Optics are made
Testing the Finished Optical Fiber
The finished optical fiber is tested for the following:
•Tensile strength - Must withstand 100,000 lb/in2 or more
•Refractive index profile - Determine numerical aperture as
well as screen for optical defects
•Fiber geometry - Core diameter, cladding dimensions and
coating diameter are uniform
•Attenuation - Determine the extent that light signals of
various wavelengths degrade over distance
How Fiber Optics are made
•Information carrying capacity (bandwidth) - Number
of signals that can be carried at one time (multi-mode
fibers)
•Chromatic dispersion - Spread of various wavelengths
of light through the core (important for bandwidth)
•Operating temperature/humidity range
•Temperature dependence of attenuation
•Ability to conduct light underwater - Important for
undersea cables
How Fiber Optics are made
Once the fibers have passed the
quality control, they are sold to
telephone
companies,
cable
companies and network providers.
Many companies are currently
replacing their old copper-wirebased systems with new fiberoptic-based systems to improve
speed, capacity and clarity.
How Fiber Optics are made
Physics of Total Internal Reflection
When light passes from a medium with one
index of refraction (m1) to another medium
with a lower index of refraction (m2), it
bends or refract away from an imaginary
line perpendicular to the surface (normal
line). As the angle of the beam through m1
becomes greater with respect to the normal
line, the refracted light through m2 bends
further away from the line.
Physics of Total Internal Reaction
At one particular angle (critical angle), the refracted
light will not go into m2, but instead will travel along
the surface between the two media (sin [critical angle]
= n2/n1 where n1 and n2 are the indices of refraction
[n1 is less than n2]). If the beam through m1 is
greater than the critical angle, then the refracted
beam will be reflected entirely back into m1 (total
internal reflection), even though m2 may be
transparent!
In physics, the critical angle is described with respect
to the normal line. In fiber optics, the critical angle is
described with respect to the parallel axis running down
the middle of the fiber. Therefore, the fiber-optic
critical angle = (90 degrees - physics critical angle).
Physics of Total Internal Reaction
Physics of Total Internal Reaction
In an optical fiber, the light travels
through the core (m1, high index of
refraction) by constantly reflecting
from the cladding (m2, lower index of
refraction) because the angle of the
light is always greater than the critical
angle. Light reflects from the cladding
no matter what angle the fiber itself
gets bent at, even if it's a full circle!
Physics of Total Internal Reaction
Because the cladding does not absorb any light
from the core, the light wave can travel great
distances. However, some of the light signal
degrades within the fiber, mostly due to
impurities in the glass. The extent that the
signal degrades depends upon the purity of the
glass and the wavelength of the transmitted light
(for example, 850 nm = 60 to 75 percent/km;
1,300 nm = 50 to 60 percent/km; 1,550 nm is
greater than 50 percent/km). Some premium
optical fibers show much less signal degradation - less than 10 percent/km at 1,550 nm.
Physics of Total Internal Reaction
Computer Designs and Programming
David Aaron Paul Aranas
Co-researchers
Mickel Lo and Quito