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Lasers and Fiber Optics History Part 4 - Optical Fiber




 

Whatever the future holds, the laser's status as a world-changing innovation has already been secured by its role in long-distance communications. But that didn't happen without some pioneering on another frontier—fiber optics. At the time lasers emerged, the ability of flexible strands of glass to act as a conduit for light was a familiar phenomenon, useful for remote viewing and a few other purposes. Such fibers were considered unsuitable for communications, however, because any data encoded in the light were quickly blurred by chaotic internal reflections as the waves traveled along the channel. Then in 1961 two American researchers, Will Hicks and Elias Snitzer, directed laser beams through a glass fiber made so thin—just a few microns—that the light waves would follow a single path rather than ricocheting from side to side and garbling a signal in the process.

This was a major advance, but practical communication with light was blocked by a more basic difficulty. As far as anyone knew, conventional glass simply couldn't be made transparent enough to carry light far. Typically, light traveling along a fiber lost about 99 percent of its energy by the time it had gone just 30 feet. Fortunately for the future of fiber optics, a young Shanghai-born electrical engineer named Charles Kao was convinced that glass could do much better.

Working at Standard Telecommunications Laboratories in England, Kao collected and analyzed samples from glassmakers and concluded that the energy loss was mainly due to impurities such as water and minerals, not the basic glass ingredient of silica itself. A paper he published with colleague George Hockham in 1966 predicted that optical fibers could be made pure enough to carry signals for miles. The challenges of manufacturing such stuff were formidable, but in 1970 a team at Corning Glass Works succeeded in creating a fiber hundreds of yards long that performed just as Kao and Hockham had foreseen. Continuing work at Corning and AT&T Bell Labs developed the manufacturing processes necessary to produce miles of high quality fiber.

At about the same time, researchers were working hard on developing a light source to partner with optical fibers. Their efforts were focused on semiconductor lasers, sand-grain-sized mites that could be coupled to the end of a thread of glass. Semiconducting materials are solid compounds that conduct electricity imperfectly. When a tiny sandwich of differing materials is electrically energized, laser action takes place in the junction region, and the polished ends of the materials act as mirrors to confine the light photons while they multiply prolifically.

 In 1967 Morton Panish and Izuo Hayashi at Bell Labs spelled out the basic requirements for a semiconductor laser for fiber optic communications. It would have to generate a continuous beam rather than pulses. It would need to function at room temperature and operate for hundreds of thousands of hours without failure. Finally, the laser's output would have to be in the infrared range, optimal for transmission down a fiber of silica glass.  In 1970 Panish and Hayashi, and Zhorez Alferov at the Ioffe Institute, demonstrated the double hetrostructure concept which led to such lasers.  The same basic lasers later proved essential for the light source in CD and DVD players, bar code scanners and other devices.


 


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





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