## 6 Conclusions  
This paper presented a novel protection scheme for working EDFAs on both downstream and upstream paths. A backup optical amplifier based on a reconfigurable optical amplifier was implemented in the optical line terminal to enable intelligent protection switching in practical operation scenarios. Our scheme has three main advantages. (1) The proposed amplifier has the function of switching between the post-amplifier and pre-amplifier to serve as a backup optical amplifier for the working EDFA on the downstream or upstream path. (2) The response time of the proposed amplifier is lower than 10 ms; therefore, it can meet the requirement of most network management and surveillance. (3) The backup optical amplifier was optimized using simulation software; furthermore, it can achieve the output power and gain requirement of 20 dBm and 30 dB via the same EDF. The proposed scheme can realize in a cost-effective and energy-efficient manner. In addition, the feasibility of the scheme was verified through a simulation using 40-channel 10 Gb/s downstream and upstream WDM-PON transmission systems. In the simulation results, the optimized EDFA in ROA\({}_{3}\), whose length of EDF is 7.5 m long with a 1480 nm pump laser, has a backward- or forward-pumping configuration with the corresponding pump power of 200 mW or 50 mW to achieve the system margin of about 7 dB for a transmission distance of 50 km. The proposed ROAs can be applied to WDM-PONs of higher data rates per channel. Therefore, the proposed scheme can minimize deployment cost and space while maintaining a high degree of reliability and system performance.  
Conceptualization, H.-S.L. and C.-L.Y.; validation, H.-S.L. and C.-H.C.; writing--original draft preparation, H.-S.L.; writing--review and editing, C.-L.Y. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by the Ministry of Science and Technology (formerly National Science Council) in Taiwan under grant number MOST 105-2218-E-011-006 and NSC 96-2219-E-032-001.  
Not applicable.  
We would like to thank [PERSON] for supporting the simulation platform in this study work.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Channel Power (Channel No.)** & **Link Loss** & **Minimum ROP for a BER of 10\({}^{-9}\)** & **System Margin** \\ \hline \(-3\) dBm (CH1) & 5 dB (AWG) & \(-31.817\) dBm & 12.277 dB \\ \(-3\) dBm(CH20) & 10 dB (50 Km SSMF) & \(-35.91\) dBm & 16.37 dB \\ \(+\) & & & \\ \(-3\) dBm(CH40) & 1.54 dB (OC + OS) & \(-35.09\) dBm & 15.55 dB \\  & \(=16.54\) dB & & \\ \hline \hline \end{tabular}
\end{table}
Table 6: System margins of the protection scheme for ROA\({}_{2}\).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Channel Power (Channel No.)** & **Link Loss** & **Minimum ROP for a BER of 10\({}^{-9}\)** & **System Margin** \\ \hline \(2.96\) dBm (CH1) & 1.54 dB (OC + OS) & \(-21.92\) dBm & 8.34 dB \\ \(+\) & & & \\ \(7.708\) dBm (CH20) & 10 dB (50 Km SSMF) & \(-22.06\) dBm & 13.228 dB \\ \(+\) & & & \\ \(1.335\) dBm (CH40) & 5 dB (AWG) & \(-22.19\) dBm & 6.985 dB \\  & \(=16.54\) dB & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: System margins of the protection scheme for ROA\({}_{1}\).