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° Ñ. 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° Ñ. 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.
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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 com¬munications 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 sys¬tems 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. How¬ever, 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 infor¬mation. 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. Com¬puters "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.
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