Published online by Cambridge University Press: 03 May 2011
Modern optical fibers exhibit very low losses (≈ 0.2 dB/km) in the 1.55 μm wavelength region that is of interest for telecommunications applications. Even though light at wavelengths in this region can be transmitted over more than 100 km before its power degrades considerably, an optical amplifier is eventually needed for any telecommunications system to restore the signal power to its original level. Since a fiber-based amplifier is preferred for practical reasons, such amplifiers were developed during the 1980s by doping standard optical fibers with rare-earth elements (known as lanthanides), a group of 14 elements with atomic numbers in the range from 58 to 71. The term rare appears to be a historical misnomer because rare-earth elements are relatively abundant in nature. When these elements are doped into silica or other glass fibers, they become triply ionized. Many different rare-earth elements, such as erbium, holmium, neodymium, samarium, thulium, and ytterbium, can be used to make fiber amplifiers that operate at wavelengths covering a wide range from visible to infrared. Amplifier characteristics, such as the operating wavelength and the gain bandwidth, are determined by the dopants rather than by the fiber, which plays the role of a host medium. However, because of the tight confinement of light provided by guided modes, fiber amplifiers can provide high optical gains at moderate pump power levels over relatively large spectral band-widths, making them suitable for many telecommunications and signal-processing applications [1–3].