A telecommunications system having erbium-doped fiber optical amplifiers for transmitting signals including at least one signal at a wavelength of about 1550 nm, wherein the amplifiers are fibers whose cores are doped with erbium and germanium and are free from aluminum; and wherein for each of the amplifiers,...http://www.google.se/patents/US5251061?utm_source=gb-gplus-sharePatent US5251061 - Telecommunications system having fiber optical amplifiers for transmitting signals over long distances
