# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 5 Results and Discussion
Please note that the maximum net bit rate of 37.32 Gb/s is achieved for 25 km of fiber when the SOAs are not included. On the other hand, the net bit rate penalty between the cases of 25 km of fiber and 100 km of fiber is greater (12.25 Gb/s of difference), when the SOAs are not included than when the SOAs are added (10.7 Gb/s of difference). Therefore, there is a reduction of the penalties, when SOAs are included. Therefore, also in this case, the current injected into the SOAs is a key factor to control the penalties when multiple fiber links and SOAs are in cascade. In case of considering SOA-based OAD nodes, the net bit rate decrease is greater than the case of using only SOAs. However, the maximum net bit rate achievable after 25 km of fiber is similar: 34.57 Gb/s and 34.77 Gb/s, with a simple additional SOA or including an OAD node, respectively. When 2 OAD nodes and 50 km of fiber links are included, the maximum net bit rate is 27.97 Gb/s that is similar to the maximum net bit rate obtained for 75 km of fiber link and 3 SOAs (the difference is 0.29 dB). In Figure 8a,b the net bit rate versus the OSNR for 25 km and 50 km of fiber are presented. It can be observed that the overlapping between the results of the different scenarios is higher when 2 spans (50 km) of fiber are considered. This is due to the increase of the number of elements in the optical network, either SOAs acting as switching nodes or OAD nodes. In fact, this enables the possibility to \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Fiber Link w/o SOAs** & **Fiber Link with SOAs** & **Fiber Link with OAD** \\ \hline
**Reach (km)** & **Net Bit Rate (Gb/s)** & **Net Bit Rate (Gb/s)** & **Net Bit Rate (Gb/s)** \\ \hline
25 & 37.32 & 34.57 & 34.77 \\
50 & 33 & 31.78 & 27.97 \\
75 & 31.4 & 27.68 & - \\
100 & 25.07 & 23.87 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Achievable net bit rate and maximum OSNR for different transmission distances using RA BL/PL algorithm, for different fiber link with and without SOAs and considering SOA-based OAD. Figure 7: Net bit rate versus fiber link (with and without SOAs and including SOA-based OAD nodes), using RA BL/PL algorithm, for an OSNR of 33 dB at a target BER of \(3.8\times 10^{-3}\). suitably manage (and carefully adjust) the SOAs bias current. Consequently, the system performance can be better optimized. For 50 km transmission distance, the penalties when the SOAs/OAD nodes are included, are minimum (<1 dB). Unfortunately, in this experiment, it has not been possible to further increase the cascading of more OAD nodes. Due to the different filtering elements and the associated noise accumulation, the maximum achievable OSNR was limited [21]. Thus, it was not possible to retrieve results after 75 km with the OAD nodes. However, to target metro network distances, the system can be improved considering other alternatives such as DMT with SSB or VSB modulation [20; 21] with DD. In [24], a multi-band SSB-DD system has been studied for a metro-regional network. With this system, 545 km of transmission distance has been demonstrated. Furthermore, DD can be replaced with coherent detection for improving the performance. In [25], DMT transmission and coherent detection has been demonstrated in flexgrid metro networks covering distances up to 150 km. More recently, in [26] two different transceiver configurations have been experimentally validated. One is based on intensity modulation with DD and the other on amplitude modulation with coherent reception for the metro networks. Thus, in future work, different configurations would be addressed to improve the performance.