Scattering and absorption mechanisms in an optical fiber cause a progressive attenuation of light signals as they travel along a fiber. At some point the signals need to be amplified so that the receiver can interpret them properly. Traditionally this was done by means of a regenerator that converted the optical signal to an electrical format, amplified this electric signal, and then reconverted it to an optical format for further transmission along the link. The development of an optical amplifier circumvented the time-consuming function of a regenerator by boosting the level of a light signal completely in the optical domain. Thus optical amplifiers now have become indispensable components in high-performance optical communication links.

This chapter classifies the three fundamental optical amplifier types, defines their operational characteristics, and describes their basic applications. The three basic technologies are semiconductor optical amplifiers (SOAs), doped-fiber amplifiers (DFAs), and Raman amplifiers. SOAs are based on the same operating principles as laser diodes, whereas the other two types employ a fiber as the gain mechanism. Among the DFAs, erbium-doped fiber amplifiers (EDFAs) are used widely in the C- and L-bands for optical communication networks. In contrast to an EDFA which uses a specially constructed fiber for the amplification medium, a Raman amplifier makes use of the transmission fiber itself.

This chapter first looks at SOA technology since the same operational principles apply to all types of amplifiers. These discussions include external pumping principles, gain mechanisms, noise effects, and SOA characteristics. Next we look at the operational...