# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 4 Conclusions  
In this paper we focus on reach extension in passive optical networks whereas applications in access and passive optical networks are being considered. Achieving longer distances without amplifiers or repeaters is not possible so the article explains both the basic principles of repetition and amplification, as well as the optical fiber amplifiers themselves. History, the general principles of operation and the basic configurations are explained for all types of amplifiers. While many standards for high-speed PONs exist and additional standards are being prepared, there are also new trends that have been barely documented. However, the lack of standards should not hinder the creation of new approaches, for example, deployment of optical amplifiers in PONs, mainly EDFAs, Ramans and SOAs. An evaluation measurement to verify the dependence of the power level of the Raman amplifier on the saturation power was performed. Measurements have shown that even a relatively small powers of the pump diodes of the Raman amplifier (\(\approx\)300 mW) can amplify the transmitted signal. In addition to explaining the basics of amplification and measuring the amplification itself with a Raman amplifier, the article provides a comprehensive overview of the current state of research in the use of optical fiber amplifiers in PON networks. Both simple solutions that would be easily implementable in practice and complex solutions with signal regeneration are presented. New trends of open networking promoted by hyperdata center companies should be considered for new trends in PON deployment to avoid any undesirable vendor dependencies and lock-ins. Open networking can ensure that technologies are replaced or migrated to new equipment as needed, especially when deploying out-of-box optical equipment, whether 2R or 3R, in PON ecosystems. These new open trends are yet not standardized in many cases but should not be disregarded because they are emerging in many parts of the world, especially in North America and Asia. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**PROJECT** & **STANDARD** & **Reach [km]** & **Wavelengths** & **Down/Upstream [Gb/s]** & **End-Users** \\ \hline ACTS-PLANET & APON & 100 & 1 & 2.5/0.311 & 2048 \\ \hline British Telecom & GPON & 135 & 40 & 2.5/1.25 & 2560 \\ \hline WDM-TDM & & 100 & 17 & 10/10 & 4352 \\ \hline PIEMAN & & 100 & 32 & 10/10 & 16384 \\ \hline WE-PON & GPON/EPON & 100 & 16 & 2.5/2.5 & 512 \\ \hline SARDANA & GPON/EPON & 100 & 32 & 10/2.5 & 1024 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Realized long-reach passive optical network (LR-PON) projects [84; 85; 86; 87; 88; 89; 99; 100; 101; 11; 102; 103; 104; 105; 106; 107]. Additionally, we believe that, with optical amplification, the support of new applications, such as accurate time transfer or distributed fiber sensing, could be important for PON end-users. This new class of applications may not appear to be appropriate for a PON environment at first, but future user requirements and new open approaches are to be utilized here. Conceptualization, T.H., J.R., P.M., and N.-H.B. ; methodology, T.H. and J.R.; formal analysis, P.M. and N.-H.B. ; resources, T.H. and P.M.; writing--original draft preparation, T.H., J.R., P.M., and N.-H.B. ; visualization, T.H., J.R., P.M., and N.-H.B. ; supervision, T.H. and J.R.; project administration, T.H. ; funding acquisition, T.H. and P.M. All authors have read and agreed to the published version of the manuscript. The research described in this paper was financed by grants from the Ministry of the Interior of the Czech Republic, Program of Security Research, VI20172020110, PID VI2 VS/422 \"Reduction of security threats at optical networks\" and the National Natural Science Foundation of China (NSFC 61671092). [PERSON] would like to give thanks to [PERSON] for his support at the University in memorial. [PERSON] taught and inspired him regarding many things in his personal life. Acknowledgment is also given to CESNET for technical support and the equipment used for the measurement.