Реферат: Волоконная оптика
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.