# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks
### Erbium-Doped Fiber Amplifiers  
The real revolution with optical amplification started in late 1980 when amplifiers based on rare earth elements became commercially available. The most significant research was performed by [PERSON] and [PERSON]. A detailed description of EDFAs is available in [65], and a detailed theory of EDFAs is provided in [66]. These fiber-doped amplifiers were investigated in the 1960s; however, techniques such as fabrication were not sufficiently mature. Many rare earth elements can be used as dopants in fibers, for example, neodymium, holmium, thulium or ytterbium; these amplifiers can operate in the wavelength range from 500 nm to 3500 nm. However, some combinations of rare earth elements and fibers (fiber is host medium only) can be produced at reasonable prices, and some nonsilica fibers are not easily produced and maintained. Statements about explosive and exponential growth in data traffic and worldwide fiber networks are almost cliche. Twenty years ago, these networks carried telephone traffic and cable television signals; however, the real explosion started with the World Wide Web (WWW). At that time, deployment of optical amplifiers in local networks was expensive; this situation has changed in the last few years. The first EDFA was demonstrated in 1989, and the initial users of these new devices were submarine (or undersea) systems because all-optical amplifiers could replace expensive and unreliable electronic regenerators. Trans-Atlantic transmission (TAT) systems are usually cited as the first long-haul systems to fully utilize the strength of EDFAs in 1996. Other systems followed (US and Japan); note that the amplifier spacing range is 30 km to 80 km. Terrestrial communication systems followed their aquatic counterparts for the same reason--to replace electronic regenerators. It is interesting to note that the first transmission systems supported only a single-channel configuration, and even in early 1990, top commercial transport systems could transport a maximum of 16 channels on a single fiber (with speeds of 2.5 Gb/s, with the latest step to 10 Gb/s), with predictions to support a maximum of 100 channels in the future [60]. The most important of all rare earth elements for telecommunication fiber networks is erbium because it can amplify signals in the most important frequency spectrum in silica fiber: The third  
Figure 5: (a) Booster-only configuration, for shorter distances (100–150 km), (b) preamplifier-only configuration, when it is necessary to avoid high-power boosters, (c) booster and preamplifier configuration for distances of approximately 200 km, (d) booster, inline and preamplifier configuration for longer distances (cascaded fibers). window or the conventional C-band near 1550 nm. EDFAs started a new era of optical communication. For example, the usual spacing of EDFAs is 80 km but may be longer; in some links where the total distance is shorter, the spacing can exceed 200 km. This fact is in sharp contrast to the spacing of OEO regenerators, where the spacing was typically 10 km, and as previously mentioned, regenerators can be used for one signal only. EDFAs can amplify a maximum of 100 signals in the C-band, which covers 1530-1565 nm. Almost all optical dense wavelength division multiplexing (DWDM) transmission systems operate in the C-band. However, if the capacity is not sufficient, EDFAs can be customized to amplify signals in the long L-band, which covers 1565-1625 nm [60]. EDFAs must be pumped to achieve gain by population inversion (refer to Figure 6). Figure 6 shows energy levels for erbium atoms. Electrons are pumped from the low energy level to the high energy level, with a relatively short lifetime of 1 microsecond. On the metastable level, with a lifetime of 10 milliseconds, electrons \"wait\" for incoming photons and amplify them via a radiative transition process. The levels are described by the well-known Russell-Saunders notation, and a detailed description is beyond the scope of this text.