# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks  
Optical amplifiers are an essential part of any optical transmission system and are not limited to long-haul systems, such as submarine systems. There are excellent books that address optical amplifiers, for example, [60; 61; 62], which is used as the basic reference for the following paragraphs. For 1 Gb/s and 10 Gb/s transceivers, the maximum fiber distance is usually 80 km; some transceivers can reach 120 km. While 80 km distances can be overcome without any correction control, for longer distances, forward error correction (FEC) mechanisms must be implemented. This situation changed with the emergence of coherent systems in 2008 [63]. In 2019, coherent systems with maximum transmission rates of 200 Gb/s are very common, and those with rates of 400 Gb/s are also available; however, these systems are expensive, with the optical reach limited to a few hundred kilometers. A new generation of silicon electronics--digital signal processing (DSP)--will be able to increase this rate to 600 Gb/s, with the potential to extend the optical reach to 400 km compared with the current situation. The first concepts of optical amplifiers were introduced in the early 1960s, and the first optical amplifier was invented in 1964 by Professor [PERSON], who used neodymium and worked in a 1060 nm spectral window. Professor [PERSON] also demonstrated the first erbium glass laser. Other experiments  
Figure 4: Principle of synchronous modulation optical regeneration. with neodymium followed in 1970, but it was too early for real deployment. These principles were also applied for the first single-mode fibers in early 1980 at Bell Laboratories. Erbium was used for amplification at the University of Southampton and AT&T Bell Laboratories in 1985. The key advantage was the capability of erbium to work at 1550 nm--the most important part of the spectrum in silica fibers [60]. Optical amplifiers are referred to as all-optical (OOO) compared with OEO regenerators. We note that optical amplifiers are referred to as \"regenerators\" in the submarine world, which may be confusing for readers from the terrestrial telco world. The main advantage of optical amplifiers is that one device is able to amplify many optical signals at once. This feature is in sharp contrast with OEO regenerators, where one regenerator can be used for only one signal and expensive multiplexing and demultiplexing techniques are necessary. Optical amplifiers amplify optical signals by stimulated emission; this mechanism is the same mechanism used for lasers. Optical amplifiers are sometimes described as lasers without feedback. An optical amplifier is pumped (fed with energy) optically or electrically to achieve population inversion of the dopant elements. Population inversion indicates that some parts of the system--photons in the case of optical amplifiers--are in higher energy or excited states than would be possible without pumping. These excited states are unstable and revert to normal states with population relaxation times that are approximately in the range 1 ns to 1 ms (other limits are possible and are discussed in more focused resources on optical amplifiers) [60]. Figure 5 shows different configurations of optical amplifiers used in practical applications. A configuration with a booster only is typically used for shorter distances of 150 km (see Figure 5a). A configuration with a preamplifier only is used when we want to avoid high optical powers produced by boosters; in this configuration, it is often necessary to use an optical filter to suppress noise (see Figure 5b). When distances are longer, for example, 250 km, it is necessary to use a configuration with both a booster and preamplifier (see Figure 5c). For longer cascaded optical spans, it is necessary to deploy inline amplifiers (see Figure 5d). Optical filters may be necessary for all configurations with preamplifiers to reduce noise; usually, this is not needed for booster and/or inline amplifiers.