# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 1 Introduction  
Passive optical network (PON) technologies find their major deployment in access networks [1; 2; 3; 4; 5; 6; 7] owing to their low requirements on optical distribution networks (ODNs), such as single and shared optical fibers between customers and the central office (CO). This technique uses point-to-multipoint (P2 MP) shared infrastructure, but it should be noted that a shared fiber means some limitations on the customer's side, such as shared bandwidth, and upstream transmission must be secured with another control mechanism [8; 9; 10; 11; 12; 13]. Passive optical networks are able to transmit signals from the optical line terminal (OLT) to optical network unit(s) (ONUs) up to 20 km, but in some cases, this distance limitation has to be broken or extended due to extensions of signal transmission in rural areas, remote offices, remote cities, etc. For these purposes, standardization organizations, such as the International Telecommunication Union (ITU) or Institute of Electrical and Electronics Engineers (IEEE), proposed PONs with longer reach [14; 15; 16; 17; 18; 19]. Furthermore, the extended reach networks require optical amplifiers to extend the distance between the OLT and ONUs [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. In the following sections, the methods for reach extensions are discussed. Optical fiber amplifiers were invented back in 1964, three years after the first fiber laser was developed by [PERSON] and his colleagues. Both the first laser and amplifier used neodymium asa dopant and operated in the spectral window at approximately 1060 nm. Unfortunately, at that time, no low-loss optical fiber was available, and when such fibers were developed in the 1970s, silica and neodymium amplifiers were not suitable for amplification in available spectral windows in silica fibers, which are well-known windows of 850 nm, 1310 nm and 1550 nm. Another version of optical fiber amplifier based on erbium and suited perfectly to work in the 1550 nm spectral window was invented and developed in the late 1980s by Sir [PERSON] and [PERSON] and their colleagues, and the well-known abbreviation \"EDFA\", erbium-doped fiber amplifier, was born [33; 34]. Optical fiber amplifiers, and especially EDFAs, enabled long-distance and high-speed transmissions, especially because optical amplifiers replaced rather expensive electronic repeaters, because one optical amplifier can supersede tens of repeaters and all optical amplifiers are almost indifferent to the speed and modulation of transmitted optical signals [35; 36; 37; 38; 39; 40; 41; 42]. We may compare optical amplifiers to Ethernet technology, which was developed back in the 1970s to connect computers and printers and other networking devices over short distances (for example, one department or one building) with the help of metallic wires. However, in the last 15 years, Ethernet has also adopted optical fibers as transport media; therefore, Ethernet can be deployed not only in local area networks (LANs) but also in metropolitan and wide area networks. Of course, Ethernet can utilize optical amplifiers to achieve these goals. Moreover, not so long ago, there were many people dismissing Ethernet technology as too simple and not suitable for such tasks, but where are Ethernet-contemporary counterparts such as Token Ring and fiber distributed data interface (FDDI)? For the same reasons, optical amplifiers can find their way into networking areas that were considered totally inappropriate for such advanced optical technologies. One of these networking areas is certainly PONs, which are considered to be completely detached from the long-haul and high-speed networks of international Internet providers. However, again, PONs have become increasingly popular, and it has started to become clear that new technologies are needed in these \"cheap\" and completely passive areas of networking. Indeed, new PON standards working with multigigabit speeds at values such as 100 Gbit/s are not only mentioned but seriously considered.