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Lasers and Fiber Optics History Part 5 - Gossamer Web


By the mid-1970s all the necessary ingredients for fiber-optic communications were ready, and operational trials got under way. The first commercial service was launched in Chicago in 1977, with 1.5 miles of underground fiber connecting two switching stations of the Illinois Bell Telephone Company. Improvements in both lasers and fibers would keep coming after that, further widening light's already huge advantage over other methods of communication.

Any transmission medium's capacity to carry information is directly related to frequency—the number of wave cycles per second, or hertz. The higher the frequency, the more wave cycles per second, and the more information can be packed into the transmission stream. Light used for fiber-optic communications has a frequency millions of times higher than radio transmissions and 100 billion times higher than electric waves traveling along copper telephone wires. But that's just the beginning. Researchers have learned how to send multiple light streams along a fiber simultaneously, each carrying a huge cargo of information on a separate wavelength. In theory, more than a thousand distinct streams can ride along a single glass thread at the same time.

Toward the 20th century's end, one of the few lingering constraints was removed by a device that is both laser and fiber. For all the marvelous transparency of silica glass, light inevitably weakens as it travels along, requiring amplification from time to time. In the early years of fiber optics, the necessary regeneration was done by devices that converted the light signals into electricity, boosted them, and then changed them back into light again. This limited the speed of transmission because the electronic amplifier was slower than the fiber. But the 1990s saw the appearance of vastly superior amplifiers that are lasers themselves. These optical amplifiers consist of short stretches of fiber, doped with the element erbium and optically energized by an auxiliary "pump" laser. The erbium-doped amplifiers revive the fading photons every 50 miles or so without the need for electrical conversion. The amplification can occur for a relatively broad range of wavelengths, allowing roughly 40 different wavelengths to be amplified simultaneously.

For the most part the devices that switch messages from one fiber to another (as from one router to another on the Internet) still must convert a message from light to electricity and back again. Yet even as researchers and engineers actively pursue the development of all-optical switches, this last bottleneck scarcely hampers the flow of information carried on today's fiber-optic systems. Flashing incessantly between cities, countries, and continents, the prodigious torrent strains the gossamer web not at all.


     Lasers and Fiber Optics
     The Idea
     Working Lasers
     New Applications
     Optical Fiber
     Gossamer Web
     Essay - Charles H. Townes

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