In 1969, several scientists concluded that impurities in the
fiber material caused the signal loss in optical fibers. By
removing these impurities, construction of low-loss optical
fibers was possible. In 1970, Corning Glass Works made a
multimode fiber with losses under 20 dB/km. The same
company, in 1972, made a high silica-core multimode
optical fiber with a 4 dB/km loss [1].