 associate primarily with the invention of the tele- phone, demonstrated the use of a membrane to modulate an optical signal in response to varying sound. Bell's photophone represented a free-space transmission system and not a guided optical system; however, it paved the way for further effort. Although AT&T obtained a patent on guided optical communica- tions over glass in 1934, at that time the glass manufacturing process did not provide the capability needed to produce fiber-optic cable with an attenuation level low enough to make guided optical communications a reality. Instead, approximately 30 more years passed until researchers were able to better understand how light attenuates in glass and how optical fiber should be manufactured to provide a practical method for supporting optical communications. The efforts of various researchers resulted in a reduction in the attenuation of glass-fiber optic cable from over 1000 dB/km to under 20 dB/km. In 1970, Corning Glass Works patented its fabrication process, which made it possible to manufacture fiber with a loss of 20 dB/km. That level of loss was originally consid- ered as a Rosetta stone for optical communications. This is because a loss of 20 dB/km is equivalent to receiving 1 percent of the original light power after traveling a distance of 1 km. Today you can obtain optical fiber whose attenuation can range to below 0.5 dB/km, illustrating the progress that has occurred in the manufacture of optical fiber since the late 1960s or the early 1970s. Of course, as you might expect, the attenua- Understanding Optical Fiber tion of an optical fiber governs the fiber's production cost and the resulting retail price. Companies today, ranging in scope from small- business LAN operators to multinational telecommunications giants, can select from a wide range of optical fibers with attenuation limits suit- able for different applications. Fabrication Two basic methods are used to manufacture optical fiber. One method requires the deposit of chemical vapor or gas wit