Волоконная оптика

Министерство Науки и Высшего Образования
Павлодарский Государственный Университет
Кафедра АиВТ
РЕФЕРАТ
Тема: Fiber Optics
Руководитель:
к.т.н.
Потапенко О.Г.
Даниярова Г.Ж.
Выполнили:
Кондратьева Е.Л.
Чепурнов А.А.
ПАВЛОДАР
1999
Введение.
Что такое Волоконная оптика , где используются
оптоволоконные световоды, из чего они сделаны и как работают?
На все эти вопросы вы найдёте ответы в этом материале. А ещё
для вас станет понятно, что человечество никогда не остановится на
достигнутом, то есть не все "паровые двигатели" ещё изобретены, а
компьютер сможет сделать намного больше, если не будет ограничен в
информационных возможностях.
Мы сможем сэкономить большое количество цветного металла, из
которого изготавливаются провода. Почему? Да потомку что, сейчас
появилась реальная возможность и необходимость использования
оптоволоконных световодов.
Ведь оптоволоконные световоды - это просто-напросто очень
чистое стекло, а стекло - это песок. А чего больше на земле: меди или
песка? К тому же на оптоволоконные световоды не действуют ни
электрические, ни магнитные поля, а температура, при которой они
плавятся, равна 2000?С, а эта температура, близкая к околосолнечной.
Волоконная оптика интересна и тем, что носителем информации
является не электромагнитный импульс, а закодированный пучок света.
Если же сравнивать пропускную способность, то оптоволоконный
световод толщиной с человеческий волос равноценен пучку медной
проволоки толщиной с руку человека.
И, наконец, ответьте на вопрос: что это такое, если оно быстрее,
точнее, дешевле? Конечно же, это волоконная оптика!
Contents
1. What is Fiber Optics? 4
2. Where are Optical Fibers used? 6
3. How are Optical Fibers made? 10
4. How do Optical Fibers Work? 12
5. Fiber Optics in the future. 17
1
What is Fiber Optics?
In 1880, four years after he invented the telephone, Alexander Graham Bell
tested another talking device. He called it the photophone.
"Photo" and "phone" come from the Greek words for "light" and "sound."
Bell's telephone used pulses of electricity traveling over copper wires to carry
sound. But the photophone used a beam of sunlight traveling through air to carry
the human voice from one place to another.
Bell was very enthusiastic about the photophone. He wrote to his father, "I
have heard a ray of sun laugh and cough and sing!"
However, the new invention did not prove to be very practical. Sunlight was
only available during the daytime. And even then, bad weather such as fog, rain, or
snow blocked the beam of light.
In spite of these problems, throughout his life, Alexander Graham Bell
thought the photophone was his most promising idea. He felt certain that someday
people would use beams of light to talk to each other.
For nearly one hundred years, scientists like Bell dreamed of using light to
communicate. They knew that light and electricity traveled as vibrations or waves.
And they knew that many more light waves could he transmitted in one second
than electrical waves. For this reason, light could carry more information than
electricity flowing in copper wires.
Not until the 1960s and 1970s did two inventions make the dream possible.
During this time, scientists invented lasers. Lasers are powerful sources of a
special kind of light. Other researchers developed optical fibers.
An optical fiber is a flexible thread of very clear glass—thinner than a cat's
whisker and up to six miles long. Laser light can pass through the length of an
optical fiber and still stay bright. Because optical fibers can serve as pipelines for
light, they also are called lightguides.
In the mid-1970s, these inventions were teamed together. Now pulses of light
flash through optical fibers carrying information and messages over great
distances. This important new technology is called fiber optics.
Glass fibers are replacing copper wires for many reasons. The fibers are not as
expensive for telephone companies to buy and install. They weigh a lot less than
copper wires — making them easier for workers to handle. A single four-and-one-
half-pound spool of optical fiber can carry the same number of messages as two
hundred reels of copper wire that weigh over sixteen thousand pounds!
Optical fibers also take much less space than copper wires. This is very
important in crowded cities where bulging, overloaded telephone cables have little
room for additional lines. Optical fibers can help unsnarl this telephone traffic jam.
The fibers are better, too, because light is nut affected by nearby electrical
generators, motors, power lines, or lightning storms. These often are the cause of
noisy static on telephones or information errors in computer systems connected by
copper wires.
As electrical signals pass through copper wires, they become weakened.
Devices called repeaters are used to strengthen the electrical signals about every
mile along each line. In a fiber optic system, repeaters are needed only every six
miles or so to boost the light signals. And experiments have shown that this
distance can be stretched many more miles. This means that installation costs for a
fiber optic system are less now and can be cut further in the future.
However, the most important reason for using glass fibers is that they can
carry much more information than copper wires. A single pair of threadlike glass
fibers can transmit thousands of telephone calls at once. A cable as thick as your
arm and containing 256 pairs of copper wires would be needed to handle the same
number of conversations.
Pairs of fibers (or wires) are used for two-way communication. One fiber
carries your voice to the listener at the other end of the line. The other member of
the pair transmits the other person's reply to you.
Optical fibers are less expensive, easier to install, and more dependable than
copper wires. With tight from a laser, they can transmit thousands of times more
information than electricity in copper wires. The new technology of fiber optics is
a better and faster way to communicate.
2
Where Are Optical Fibers Used?
All over the world, the copper wires of telephone trunk lines are being
replaced by modern glass optical fibers.
One of the first attempts to use an optical fiber system in the United States
was in 1977 in Chicago. There, two offices of the Bell Telephone Company and a
third building for customers were connected successfully by twenty-four light-
carrying glass fibers. The fibers were threaded through telephone cables already
under the city streets. The total length of the fibers was about 1.5 miles.
In 1978, Vista-United Telecommunications at Walt Disney World near
Orlando, Florida was first to use fiber optics commercially in the United States.
Telephones throughout the 28,000-acre park are linked by fiber optic trunk lines.
Video transmissions by glass fibers are made to many individual hotel rooms on
the property from one location. Lighting and alarm systems also use optical fibers.
In EPCOT Center (Experimental Prototype Community of Tomorrow), there
are information booths equipped with television-tike, two-way video screens and
speakers. The screens and speakers are connected by optical fibers to a central
office. A visitor can activate the screen by touching it and select the information
needed. Or the guest can talk to an operator who appears on the screen if requested.
American Telephone and Telegraph has in service a fiber optic trunk line that
connects Boston, New York City, Washington, D.C., and Richmond, Virginia. The
trunk line is part of a project 780 miles long. The light cable used is only about the
thickness of a garden hose. Nevertheless, it can carry eighty thousand calls at once.
By July 1988, American Telephone and Telegraph will have laid a fiber optic
cable beneath the ocean between North America and Europe. The cable is called
TAT-8 because it is AT&T's eighth /transatlantic telephone cable. TAT-1, a copper
cable, was completed in 1956 and could carry fifty-one calls at a rime. TAT-7, the
last copper cable, was laid in 1983. It can handle about eight thousand calls at
once. ТЛТ-8 will transmit forty thousand calls at one time. Even with TAT-8, a
second fiber optic transatlantic cable, TAT-9, probably will be needed by 1991.
Another undersea cable, between California and Hawaii, is planned.
The Japanese telephone company, Nippon Telephone and Telegraph, has
placed glass fiber cables from one end of the country to the other. By 1990, similar
lines will join Japan to Hong Kong, Australia, and New Zealand.
A fiber optic system in Munich and other cities of West Germany is called
Bigfon. It transmits a video picture along with voice. In addition, Bigfon sends and
receives copies of documents and other important papers.
Over fifteen hundred customers in Biarritz, France, use videophones and
television channels made possible by fiber optics.
In the remote countryside of Manitoba, Canada, two towns are part of an
experiment. Elie and St. Eustache have become "glass-wired" communities.
Optical fibers connect keyboards and television sets in homes in these towns to
distant computers. People who live there use the keyboards to get television shows,
radio broadcasts, weather forecasts, news, farm and stock market reports. In
addition, over three hundred items for sale at a large, well-known department store,
Hudson Bay Company, can be viewed on television. To make a purchase, a
customer types an item code, number of items wanted, size, color, and credit card
number on the keyboard. Hudson Bay Company receives the order and ships the
goods directly to the customer.
Near Tokyo, in Japan, there is an optical fiber communications network
known as HI-OVIS (Highly Interactive Optical Visual Information System). With
this two-way system people can take an active part in educational classes such as
piano lessons. They also can learn about schedules for airlines, trains, and concerts,
and get up-to-the-minute news and weather reports.
New installations for communications at Kennedy Space Center in Florida use
fiber optics. These include the Space Shuttle control center and operations building
for Launch Complex 39. In addition, the Space Center's fiber optic system is used
to check out experiments, such as those on board Skylab, before launch.
Eventually, all of the facilities for the Shuttle at Kennedy Space Center will use
fiber optic systems.
There arc many other uses for fiber optics. A medical instrument known as an
endoscope is made from bundles of optical fibers packed inside a long, slim,
bendable tube. A doctor slips this medical "spyglass" into a patient's throat,
stomach, lungs, or intestines to look for anything abnormal. One bundle of fibers
carries light to the tip of the probe. Another bundle of fibers transmits pictures
back to an eyepiece. This allows a doctor to see inside the human body without
surgery. And sometimes it locates early stages of serious diseases, such as cancers,
that X-rays may miss. Miniature tools within a separate channel in the endoscope
tube can remove samples of tissue for a closer look.
Veterinarians examine horses, cats, clogs, and other animals with similar fiber
optic scopes. Pets sometimes choke on foreign objects. With the probe of the
scope, the animal doctor can locate the object, snare it, and quickly remove it.
People peer into dangerous or hard-to-see places with industrial fiber optic
scopes too. Workers can look inside and check radioactive reactors in nuclear
power plants, the jet engines of airplanes, turbines, boilers, pipelines, gear boxes,
and many other types of machinery.
Image conduits are large pipelines for light. They are formed from thousands
of optical fibers that have been bundled and fused together into one unit. They can
directly transmit images or pictures from one place to another. If the conduit is
tapered on one end, it can be used to make an image larger or smaller. And if the
fibers in the conduit are twisted, the picture can be turned upside down.
Wafer-thin plates sliced from fused bundles of optical fibers are used to make
night-vision goggles or scopes. The plates are treated with chemicals that enable
them to magnify moonlight, starlight, or any other available light thousands of
times. With the goggles, U.S. Forest Service helicopter crews can spot even small
embers on the ground that could start a fire.
Individual optical fibers guide light from one source to many switches and
displays on the dashboard of a late model automobile or the instrument panel of a
recently built jet fighter. The fibers are small and lightweight. And they are not
bothered by other electrical equipment fitted closely behind the dash or panel. In
some cars, optical fibers monitor parts of the car. They signal the driver if a light
burns out or if a door is ajar.
Many kinds of sensors are made with optical fibers. These devices can detect
changes in temperature, pressure, or the presence or absence of something.
Different sensors can check for a wide range of things at factories—from missing
caps on soda bottles to toxic fumes. They help guide robots or other automatic
machinery to manufacture items as intricate as electronic circuits or as large as
automobiles.
Glass fibers are ideal for military defense. In addition to their other
advantages, the fibers are easy to hide from an enemy. Metal detectors cannot
locate them, for example. Also, the fibers are almost impossible to secretly tap or
jam. Thus, vital messages are more likely to get through. Light-carrying fibers
usually are not affected by radiation. And they can be used safely near ammunition
storage areas or fuel tanks because they do not create sparks as electricity can in
copper wires.
The North American Air Defense Command is located deep inside Cheyenne
Mountain in Colorado. Its computers, linked by optical fibers, process radar
information from around the globe. Army field communications systems also
depend on optical fibers.
Optical fibers are being used by the University of Pittsburgh to connect school
computers. A college student or teacher will be able to get information from any
connected computer, library, or classroom on campus. Other schools are installing
similar networks.
The new technology of fiber optics has grown quickly in the past decade. In
the next ten to fifteen years, the copper wire telephone trunk lines in most of the
world will be replaced with glass "wires." These slender strands will harness pulses
of light to transmit the human voice and vast amounts of information in a
twinkling. More and more, people will use beams of light to communicate with
each other.
Imagine how excited Alexander Graham Bell would be to know that his
dream has come true.
3
How Are Optical Fibers Made?
The glass used to make optical fibers must be very pure. Light must be able to
pass through the length of the fiber without being scattered, or losing brightness.
Though the glass in an eyeglass lens looks perfect, a three-foot-thick piece of
this kind of glass would stop a beam of ordinary light. Tiny particles of iron,
chromium, copper, and cobalt absorb or scatter the light.
The glass in an optical fiber is nearly free of impurities and so flawless that
light travels through it for many miles. If ocean water were as pure, we would be
able to see the bottom of the Mariana Trench, over thirty-two thousand feet down,
from the surface of the Pacific.
An optical fiber has a glass inner core. Light travels through this highly
transparent part of the fiber.
The core of an optical fiber is surrounded by an outer covering called the
cladding. The cladding is made of a different type of glass from the core of the
fiber. For this reason, the cladding acts like a mirror. Light traveling through the
core of the fiber is reflected back into the core by the cladding—much like a ball
bouncing off the inside wall of a long pipe. In this way, light entering one end of
an optical fiber is trapped inside the core until it comes to the other end.
How do people make these gossamer threads of glass that can carry light
around curves and corners and over long distances?
Optical fibers are manufactured in "clean rooms." The air in these rooms is
filtered to keep out the tiniest particles of dust. Even the smallest specks of dirt
could ruin the fiber as it is made. Workers in these areas usually wear jump suits or
lab coats and caps made from lint-free fabric.
An optical fiber starts out as a hollow glass tube. The tube is mounted on a
machine that rotates it. A special gas is fed into the tube. A naming torch moves
back and forth along the tube, heating it to nearly 1,600° С. With each pass of the
torch, some of the hot gas inside forms a fine layer of glass on the inner wall of the
tube. A series of different gases can be fed into the tube. With this method, layers
of several different kinds of glass are added to the inside wall. When the addition
of glass is complete, gas still inside the tube is gently sucked out.
Now, the heat from the torch is increased to 200U° C-The hollow tube
collapses into a solid glass rod called a preform. The preform is the size of a
broomstick—about as big around as a fifty-cent piece and a yard long.
The preform is cooled and carefully inspected. Light from a laser is used to
make sure the core and cladding of the glass preform are perfect.
Next, the preform is placed in a special furnace where it is heated to 2,200°
С. At this temperature, the tip of the preform can be drawn or pulled like taffy into
a wisp of an optical fiber—thinner than a human hair.
Usually, as soon as it is drawn, the fiber passes through a tiny funnel where it
is coated with fast-drying plastic. The coating protects the fiber from being
scratched or damaged.
The fiber from a draw may be up to six miles long. It is wound onto a spool
for ease of handling and storage.
Glass is usually thought to be brittle, unbendable, and easily broken.
Amazingly, optical fibers arc flexible and strong as threads of steel. The fibers can
be tied into loose knots without breaking and light still passes through from end to
end.
4
How Do Optical Fibers Work?
Whenever you talk to someone else the sound of your voice travels to their
ears as a pattern of vibrations or waves in the air. Light and electricity also move in
waves.
To get an idea what waves look like, tie one end of a long rope to a post or
tree. Hold the other end of the rope and walk away until the rope is stretched out,
but still slightly slack. Now yank the free end of the rope up and down repeatedly.
A series of bumps or waves travels down the rope.
You can change the pattern of the waves. You can make small waves by
giving weak, up-and-down yanks on the rope. Or you can make big waves by
giving strong, up-and-down yanks on the rope. The height or tallness of the waves
depends on the strength you use to yank the rope up and down.
The distance between the top of one wave and the top of the next wave is
called the wavelength.
Another way to vary the waves is to change their speed. You can yank the
rope up and down only once in a second or many times in a second. The number of
waves reaching the tree or post each second is the frequency of the waves.
Why do pulses or waves of light streaking through an optical fiber go farther,
better, and faster than electricity pulsing through copper wires?
Lasers used in fiber optic systems are made from tiny crystals of a material
called gallium arsenide. These lasers are as small as a single grain of salt and easily
could fit through the eye of a needle. Nevertheless, they can produce some of the
world's most powerful pinpoints of light.
Light from a laser is unlike ordinary light. Laser light is all of the same
frequency and wavelength. And all of it is traveling together in the same
direction—like bullets aimed from the barrel of a gun at one target. The result is a
brilliant source of very pure light. Laser light can shine through miles of optical
fiber without being boosted as often as an electrical signal.
The laser light used in fiber optic telephone or communications systems is
infrared. The frequency of infrared light is just below what people can see with
their eyes unaided. Infrared light is used in communications systems because it can
travel long distances through optical fibers with less loss of power.
Another source of light that also is used with optical fibers for communication
is a light emitting diode or LED. LEDs are less costly than gallium arsenide lasers.
However, lasers can transmit more information at higher speeds than LEDs.
Copper wires can carry a few million electrical pulses each second. But the
number of light pulses an optical fiber can carry is much greater. It is limited by
how many pulses of light each second today's best lasers can produce. Recent
experiments done at AT&T Bell Laboratories combined the output of several lasers
to achieve as many as 20 billion pulses per second! This far outshines the number
transmitted by copper wires.
How do telephones connected by optical fibers work?
In the mouthpiece of a telephone, the pattern of sound waves of your voice is
first changed into a pattern of waves of electricity moving through copper wire. In
a fiber optic system, a special electronic device called an encoder measures
samples of the waves of electricity eight thousand times each second. Then, each
measurement of the waves is changed into a series of eight ON-OFF pulses of
light.
The pulses of light are a code that stands for the strength or height of the
waves of electricity. This is called a binary code because it uses only two signals
or digits; zero for when the light is OFF and one for when the light is ON. The
word "binary" means two. Each zero or one is called a binary digit or bit. And each
pulse of ON-OFF light stands for one piece or bit of information. Fight bits
grouped together are a byte.
The specks of ON-OFF light flash like tiny comets through optical fiber
carrying your message in binary code.
At the other end of the line is another device called a decoder. The decoder
changes the pulses of light back into electrical waves. The receiver of the
telephone then changes the electrical waves back into the sound waves of your
voice.
The coded pulses of light in a fiber optic system can carry so much
information so rapidly that many telephone conversations can be stacked in an
optical fiber. They are then unscrambled at the other end of the line.
Because a fiber optic system uses coded pulses of ON-OFF light, it is ideal to
link together computers. Computers "speak" this binary language. They not only
count in binary, computers also store and handle huge amounts of information as a
code of zeros and ones. The entire 2,700 pages of Webster's Unabridged
Dictionary can be transmitted from one computer to another over optical fibers in
six seconds'
Morse Code is a binary code you may already know. Instead of zeros and
ones, Samuel Morse used dots and dashes to send any message by telegraph. The
dots and dashes can stand for any letter of the alphabet or any decimal number.
Here are two binary codes. One is international Morse Code and the other is a
computer code known as the American Standard Code for Information Interchange
or ASCII-8.
Character Morse Code ASCII-8
0 ----- 01010000
1 .---- 01010001
2 .. --- 01010010
3 …-- 01010011
4 ….- 01010100
5 ….. 01010101
6 -…. 01010110
7 --… 01010111
8 ---.. 01011000
9 ----. 01011001
Can you figure out what the following message says? First it is given in
International Morse Code; then in ASCII-8. Alexander Graham Bell said these
words in the first telephone message to his assistant, Watson, on March 10, 1876.
.-- .- - … --- -. --..--
-.-. --- -- . …. . .-. . .-.-.-
.. .-- .- -. - -.-- --- ..- ---.
10110111 11100001 11110100 1111001I 11101111
11101110 01001100 11100011 11101111 11101101
11100101 11101000 11100101 11110010 11100101
01001110 10101001 11110111 11100001 11101110
11110100 11111001 11101111 11110101 01000001
ANSWER: Watson, come here. I want you!
Morse code and ASCI1-8 may seem awkward. But Morse code made possible
sending messages quickly by telegraph over long distances as early as 1845.
Today, computers linked by optical fibers can send vast amounts of any kind of
information, including pictures. And they can do it faster than the human mind can
think.
5
Fiber Optics in the Future
Many scientists think that the technology of fiber optics wilt lead to an
enrichment of life like that following the invention of the steam engine, light bulb,
and computer.
Only a small number of homes, businesses, school, hospitals, and libraries in
the world are connected by optical fibers now. But as fiber optic technology
develops there will be an enormous expansion of use. In the future, fiber optics
wilt make affordable a wide range of services that may be too expensive for most
people or businesses now.
An example of this is teleconferencing. Rather than travel far from their
companies and homes, business people will more commonly meet by
teleconference. They will send live television pictures of themselves to each other
and talk as though they were in the same room. AT&T, Western Union, and some
hotels already have teleconferencing rooms for rent in many major cities.
However, in 1984, the cost was over $2,000 an hour. But soon it may cost as little
as $30 an hour. Further in the future, fiber optics may be used with a method called
holography for teleconferences. Holography uses lasers to project three-
dimensional images of people or things into thin air—no viewing screen is needed.
Laser images transmitted by glass fibers will be so lifelike they will be hard to tell
from the real person or object.
Some researchers dream of building an optical computer. The "brains" of
today's computers are microchips. These tiny electronic devices are only as thick
as a thumbnail and one-quarter inch on a side. Within, they are a maze of miniature
metal circuits and thousands of special switches known as transistors. Pulses of
electricity passing through the microchip's circuits and switches process all of the
computer's information.
An optical computer will operate using pulses of light passing through optical
switches. The transistors in a microchip are fast— they can switch ON or OFF
millions of times each second. But scientists have built experimental optical
switches that are ten thousand times faster. They can switch ON or OFF an
incredible one trillion times each second!
Supercomputers of the future operating at faster speeds would make possible
automatic translation of foreign language telephone calls (such as English to
Japanese). Optical computers also would be the best way to transmit or process
highly detailed visual information such as photographs or maps.
In an optical computer, switches will be able to process many bits of
information at the same time—something electronic computers usually do not do.
Because of this and their faster speed, optical computers would be far more
powerful than the computers we have now.
With fiber optics, individual homes and businesses will have new, improved
services available. The future will bring routine use of videophones that allow
callers to see and hear each other. Telephone consoles may also be computer
terminals. And there will be two-way television reception.
Fiber optic sensors will send information to automatic controls for lights,
heat, air conditioners, appliances, or industrial machinery. Police and fire fighters
will give better security to homes and businesses that have sensors connected
directly by optical fibers to monitors at headquarters.
Someday you may work in an "intelligent" office building. The building itself
may look much like other offices. But inside will be a world of difference.
The first of these office buildings is City Place in Hartford, Connecticut.
Others that already have been built include Tower Forty-Nine in New York, LTV
Center and Lincoln Plaza in Dallas, Texas, and Citicorp Center in San Francisco.
By 1990, over 300 million square feet of "high I.Q." office space is expected to be
in use.
An "intelligent" office building has fiber optic detectors that "see" if people
are in a room before turning lights on or off. The detectors are connected to a main
computer that regulates heat, ventilation, air conditioning, and lighting in each
office of the building. Such automatic controls in large buildings can save as much
as one-half on energy usage.
Just as important is that businesses in an "intelligent" office building share the
benefits and costs of the most modern computer information networks, electronic
mail, word processing, and telephone service. These services have been designed
into the building's fiber optic system.
Security in "intelligent" office buildings also is improved. If, for example, a
sensor detects a fire, its signals automatically ring alarms, call the fire department,
activate sprinklers, exhaust smoke to the outside, and broadcast emergency
instructions.
Fiber optics is lighting the way to an astonishing information age. Home
computers will be "wired" to the world. Information from libraries and other
sources will be available to us instantly. Banking and shopping will be done from
home as well- Electronic newspapers, magazines, and mail will become
commonplace. Telephones will be fitted with sockets to plug in computers,
printers, television screens, and other information transmitting or receiving
devices.
Away from Earth, new uses for fiber optics also will be found. In the 1990s,
the National Aeronautics and Space Administration will build a permanent space
station. It will be in orbit about three hundred miles up. The space station will use
on-board fiber optic systems for communications, computer processing,
monitoring, and controls. The station also will establish factories in the near-zero
gravity of space. Some of these factories will manufacture glass more flawless and
free of impurities than can be made on Earth. This ultrapure glass will be brought
back to Earth by the Space Shuttle to be made into even better optical fibers and
other products.
Sometime in the next century, people will live in space colonies. They will
process information and communicate using optical fibers and light. And they
probably will find uses for fiber optics that hasn't yet been imagined.
Alexander Graham Bell's brightest idea will have become a reality reaching
far beyond his most fantastic dreams.