By the 1970s, all telephone cables and microwave links on the planet were saturated. The solution came when Charles Kao and George Hockham of the British company Standard Telephones and Cables promoted the idea that the attenuation in the existing optical fibers could be reduced below 20 decibels per kilometer (dB/km), making fibers a practical communication medium. They proposed that the extremely high attenuation in fibers available at the time was caused by impurities that could be removed by chemical processes. They correctly and systematically theorized the light-loss properties for optical fiber, and they identified the right material for such fibers: silica glass with high purity. This discovery earned Kao the Nobel Prize in Physics in 2009.
The crucial attenuation limit of 20 dB/km was first achieved in 1970 by researchers at Corning Glass Works (an American glass maker, now Corning Incorporated). They demonstrated a fiber with 17-dB/km attenuation by doping silica glass with titanium. A few years later, they produced a fiber with only 4-dB/km attenuation using germanium dioxide as the core dopant. Such low attenuation allowed optical fiber to be used in telecom from the 1980s until now; current fibers have an attenuation of 0.25 dB/km at the telecom C-band.
Figure 1.1 shows the fiber optic attenuation spectrum with three telecom windows. The three lines represent the progress of optical fiber attenuation from the early 80s, late 80s, and the present. In the early 1980s, only the first window at 850 nm was known and used with multimode fibers. By the 1990s, the second window at 1300 nm was explored with specific lasers and a photodetector because the old silicon photodetector was useless at this wavelength. It was a promising window with a much lower attenuation. Finally, by the late 1990s, the third window at 1550 nm started being used with an attenuation that today is around 0.25 dB/km.
