applied sciences Article Optical Amplifiers for Access and Passive Optical Networks: A Tutorial Tomas Horvath 1,* , Jan Radil 2, Petr Munster 1 and Ning-Hai Bao 3 1 Department of Telecommunication, Brno University of Technology, Technicka 12, 616 00 Brno, Czech Republic; [email protected] 2 Independent Consultant, 16 000 Prague, Czech Republic; [email protected] 3 School of Communication and Information Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China; [email protected] * Correspondence: [email protected]; Tel.: +420-541-146-923 Received: 20 July 2020; Accepted: 22 August 2020; Published: 26 August 2020 Abstract: For many years, passive optical networks (PONs) have received a considerable amount of attention regarding their potential for providing broadband connectivity, especially in remote areas, to enable better life conditions for all citizens. However, it is essential to augment PONs with new features to provide high-quality connectivity without any transmission errors. For these reasons, PONs should exploit technologies for multigigabit transmission speeds and distances of tens of kilometers, which are costly features previously reserved for long-haul backbone networks only. An outline of possible optical amplification methods (2R) and electro/optical methods (3R) is provided with respect to specific conditions of deployment of PONs. We suggest that PONs can withstand such new requirements and utilize new backbone optical technologies without major flaws, such as the associated high cost of optical amplifiers. This article provides a detailed principle explanation of 3R methods (reamplification, reshaping, and retiming) to reach the extension of passive optical networks. The second part of the article focuses on optical amplifiers, their advantages and disadvantages, deployment, and principles. We suggest that PONs can satisfy such new requirements and utilize new backbone optical technologies without major flaws, such as the associated high cost. Keywords: reamplification; reshaping; retiming; optical amplifiers; raman amplifiers; EDFA; SOA 1. Introduction Passive optical network (PON) technologies find their major deployment in access networks [1–7] 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 (P2MP) 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 [8–13]. 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 [14–19]. Furthermore, the extended reach networks require optical amplifiers to extend the distance between the OLT and ONUs [20–32]. 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 Elias Snitzer and his colleagues. Both the first laser and amplifier used neodymium as Appl. Sci. 2020, 10, 5912; doi:10.3390/app10175912 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 5912 2 of 28 a 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 David Payne and Emmanuel Desurvire 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 [35–42]. 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. Even Ethernet PONs are serious candidates for asynchronous transfer mode (ATM)-based PONs, and it is clear that optical amplifiers should be part of such activities, especially in countries such as Canada, the USA, Norway and Sweden, where the distances to be overcome are certainly longer than those proposed years ago in PON standards (this situation is very similar to that for Ethernet then and now). Fortunately for PONs, optical amplifiers have come a long way. Today, optical amplifiers with small form factors are widely available, and high-quality EDFAs can be bought even with form factors known from the pluggable transceiver market. This means that optical amplifiers are no longer expensive parts of optical systems and that the power consumption is very promising. Of course, no active elements can be compared to totally passive optical elements such as splitters, but optical amplifiers and Ethernet switches have lower power consumption than OLTs and ONUs (of course, OLTs and ONUs are increasingly better in every generation). Optical amplifiers are perhaps the great unknown for people working in the PON environment, as many of us who work in these areas know, and therefore, it is very desirable to provide such papers as our attempt to bridge long-haul high-speed networks with PONs. The rest of this paper is organized as follows. Section2 provides details about the 3R reamplification, reshaping, and retiming methods for signal reconstruction after optical fiber transmission. Section3 introduces optical amplifiers for telecommunications networks and their usage in passive optical networks along with principle details. Section4 concludes this paper. 2. 3R—Reamplification, Reshaping, and Retiming The specification of gigabit passive optical networks (GPONs) considers two scenarios for reach extension. The first specification is based on optical-electrical-optical (OEO) conversion, and the second specification uses full optical signal processing and amplification. We provide a basic principle of amplifiers based on OEO conversion. In general, these amplifiers can be divided into Appl. Sci. 2020, 10, 5912 3 of 28 three categories: 1R, 2R, and 3R. While the current research interest is full optical amplifiers, we discuss all three categories due to the potential usage of 3R amplifiers in xPONs [43–49]. The main signal degradation in fiber optic systems arises from amplified spontaneous emission (ASE) due to optical amplifiers, pulse spreading due to group velocity dispersion (GVD), which can be corrected by passive dispersion compensation schemes, and polarization mode dispersion (PMD). Nonlinear distortions are attributed to Kerr nonlinearity, such as cross-phase modulation, which can be responsible for time jitter in wavelength division multiplexing (WDM), or Raman amplification, which can induce channel average power discrepancies [50]. The 1R category represents the simplest amplifier of an optical signal. Only the input signal is amplified and transferred to the output. Note that an input signal is not recovered (the shape, position, and phase are exactly the same as those of the input signal). However, 1R amplifiers are simple, which presents some advantages. For example, a processed signal does not depend on the modulation format, transmission speed, or other parameters of a signal. The basic principle of 1R amplifiers is shown in Figure1. The input signal is degraded, but the output signal is only