# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 3 Optical Amplifiers in Telecommunications Networks
### Amplifiers for PONs  
Long-reach optical access is a promising technology for future access networks. This technology can enable broadband access for a large number of customers in access/metro areas while decreasing capital and operational expenditures for the network operator. Almost all the described optical amplifiers can also be used in passive optical networks (see Table 2), with the notable exception of Brillouin amplifiers (which are not suitable due to a very small gain spectrum, as previously described). Prospects for PONs with reaches of 100 km and 10 Gb/s speeds are being investigated, but these devices are not commercially available. A typical PON can reach 20 km with a maximum split ratio of 64. For example, the GPON standard established optical budgets of 28 dB with 2.488 Gb/s for downstream transmission and 1.244 Gb/s for upstream transmission. This standard is the current standard of PONs. In long-haul systems, optical amplifiers are extensively employed to extend the reach of systems to hundreds or thousands of kilometers. The cost of optical amplifiers is sufficiently low; thus, we can consider their use in PONs. The cost of amplifiers can be shared among numerous customers. The GPON protocol can support a logical reach of 60 km and a split ratio of 1:128. Optical amplifiers may be used to extend the reach. The transparency of optical amplifiers indicates their use for GPONs and gigabit Ethernet PONs (GEPONs). Optical amplifiers are a main technology for next-generation access (NGA) PONs. Several benefits of extended-reach PONs exist. First, customers are located as far from the CO as they can be connected. Second, where customers are sparsely distributed over a large area, optical amplifiers can be used to ensure good utilization of the shared PON. Third, depending on an end-to-end network design, extending the reach of a PON can enable node consolidation, which entails reducing the number of PON head-end locations that must be managed by the operator [76]. Four options of optical amplifiers for PONs are available:  
* erbium-doped fiber amplifiers,
* thulium (1490 nm downstream) and praseodymium (1310 nm upstream) fiber-doped amplifiers,
* semiconductor optical amplifiers,
* Raman amplifiers. In metro and long-haul networks, EDFAs are extensively employed because they provide a high gain, output power and noise figure from 1530-1565 nm. Existing PON standards apply EDFAs for \begin{table}
\begin{tabular}{c c c c} \hline \hline
**PROPERY** & **EDFA** & **RAMAN** & **SOA** \\ \hline Gain [dB] & \textgreater{}40 & \textgreater{}30 & \textgreater{}30 \\ \hline Wavelength [nm] & 1530–1625 & 1280–1650 & 1280–1650 \\ \hline Bandwidth (3 dB) [nm] & 30–60 & up to 100 & 60 \\ \hline Max. Saturation [dBm] & 30 & 0.75 \(\times\) pump power & 18 \\ \hline Polarization Sensitivity & No & No & Yes \\ \hline Noise Figure [dB] & \textgreater{}3.5 & 5 & 8 \\ \hline Pump power & 25 dBm & \textgreater{}30 dBm & \textless{}400 mA \\ \hline Time constant [s] & 1.00\(\times 10^{-1}\) & 1.00\(\times 10^{-14}\) & 2.00\(\times 10^{-9}\) \\ \hline Size & \multicolumn{1}{c}{Rack-mounted} & Bulk module & Compact \\ \hline Switchable & No & No & Yes \\ \hline Cost factor & \multicolumn{1}{c}{Medium} & High & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of amplifiers. analog video broadcast (overlay PON). An alternative to fiber amplifiers is the SOA. While SOAs do not provide gain and noise figures that are comparable to those of EDFAs, their advantage is that they can operate at any wavelength. The gain dynamics of SOAs are also substantially faster than those of EDFAs, so SOAs can be used for bursts upstream [76]. While the Raman amplifier can be theoretically used downstream, we have to consider the high price and necessity of high-power, dangerous pumps. FEC is another important technology for extending the capability of PONs. While FEC is specified in GPONs and GEPONs, an enhanced version of FEC could be used in future PONs.