ABSTRACT
The field of erbium-doped fiber amplifiers (EDFAs) has, in a relatively short time, come to the forefront of research in fiber-optic technology. The abundant literature published every month and the growing number of conference papers on this subject indeed reveal a worldwide interest and an intense research effort in a domain that was little known only a few years ago. Not surprisingly, this recently appearing technology rests on concepts and achievements dating as far back as 1964, with the first amplification experiments in rare earth doped fiber lasers [1, 2]. As early as 1974, following the development of GaAs semiconductor diode lasers, the first laser-diode-pumped fiber lasers were developed to achieve compact sources for optical communications [3]. The reduction of fiber transmission loss and the progress in reliability of InGaAs and InGaAsP diode lasers made in the 1970s led to the development of modern 1.3-and 1.55-µm fiber communication systems, thus leaving the field of rare earth doped fiber devices in the background. Semiconductor optical amplifiers were also investigated experimentally for increasing receiver sensitivity (as preamplifiers) or transmission length (as repeaters) and for multichannel amplification in communication systems. During the same period, however, much attention was given to nonlinear effects in single-mode fibers (e.g., Raman and Brillouin scattering, four-photon mixing) because of their potential for signal amplification, the standard silica-based optical fiber being the gain medium itself [4]. During the years 1982-1985, studies were continuing in the characterization of rare earth doped fibers, for lasers and amplifier applications [5] or for the investigation of photon echoes [6].
