# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 2 3R--Reamplification, Reshaping, and Retiming
Figure 2 shows the principle of 3R amplifiers, and Figure 3 shows the block scheme of a reach extended passive optical network (RE-PON). Regeneration of 3R can occur in two ways: Inline 3R regeneration and in-node regeneration. Inline 3R regeneration is usually implemented when the physical distance between the end points exceeds the maximal power budget of the optical network. In-node regeneration can occur in the optical cross-connect nodes, where some OEO regenerators are usually deployed [56]. Note that OEO 3R regenerators are dependent on the signal waveform (modulation formats). If the waveform is changed, the 3R regenerator must be adapted to it. A second significant limitation of 3R regeneration is the bit rate. The maximal bit rate for OEO 3R regenerators is approximately 40 Gb/s. Both problems are solved in all-optical 3R regenerators. The standard for a GPON optical reach extension was ratified by ITU-T G.984.6 in 2008. This standard includes the architecture and interface parameters for GPON systems with extended reach using a physical layer mid-span extension between the OLT and the ONU that uses an active device in the remote node. The GPON reach extender enables operation over a maximum of 60 km of fiber with a maximum split ratio of 1:128 [48]. Two ways to amplify a signal are presented in ITU-T G.984.6. The first method is based on optical amplification of the optical signal: Bidirectionally. This principle is based on 1R regeneration. This kind of amplifier can be based on an EDFA, Raman amplifier or semiconductor optical amplifier (SOA). Figure 3: Block scheme of an RE-PON. Figure 2: Principle of 3R in a reach extended passive optical network (RE-PON). The second approach is to use an OEO regenerator, as shown in Figure 2. The regenerator consists of a couple of branches for each way using diplexers. In both branches, the receiver and the transmitter are dimensioned for the wavelength band, which explains why the optical signal must be converted to an electrical signal. The electrical signal is recovered and converted to the optical domain. The important function of this part is to recover the clock signal. This step is resumed by the receiver downstream-continual mode-but upstream, the burst mode is used. ITU-T G.984.6 also considers the combination of both systems, e.g., the OEO regenerator downstream and the SOA amplifier upstream. All-optical 2R is also possible; however, it is not transparent to modulation of the input signal [57]. Full optical 3R regeneration is not considered in standardized PONs but is suggested for future networks [58]. Full optical 3R regeneration with a real function of retiming requires clock recovery, which can be achieved either electronically or all-optically. The main difference between both types of retiming is that electronic functions are narrowband compared with broadband optical clock recovery [59]. Full optical 3R regeneration can be realized in two different ways:  
1. Data-driven 3R regenerator--nonlinear optical gate. This scheme mainly consists of an optical amplifier, that is, a clock recovery block providing an unjittered short pulse clock stream, which is then modulated by a data-driven nonlinear optical gate block [50]. 2. Synchronous modulation 3R regenerator--this technique is particularly efficient with pure soliton pulses. It consists of combining the effects of a localized \"clock-driven\" synchronous modulation of data, filtering, and line fiber nonlinearity, which results in both timing jitter reduction and amplitude stabilization (see Figure 4). The high-dispersion fiber first converts the amplified pulse into a pure soliton. The filter blocks the unwanted ASE but also has an important role in stabilizing the amplitude in the regeneration span. Data are then synchronously and sinusoidally modulated through an intensity or phase modulator, driven by the recovered clock [50].