# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 4 BVT with Switching Nodes Adopting SOA Technology: Experimental Setup and Optimization  
We propose to adopt the S-BVT described in Section 2 for optical metro networks adopting SOA-based switching nodes. In particular, for the experiments the simplest BVT architecture using DMT and DD is considered attractive for a cost-effective implementation and, thus, it is envisioned to be used when SOA technology is adopted for the switching nodes. To analyze the performance of this approach, two scenarios are considered as shown in Figure 3. Both scenarios consider spans of 25 km of fiber and different cascading nodes; scenario (a) envisions the use of a simple SOA acting as switching node, while scenario (b) introduces OAD nodes based on SOA technology, as specified in Figure 2 and Section 3. As a reference, the case of multiple (up to 4) fiber spools of 25 km, without any SOA is analyzed as well. The scenario without considering filtering elements (filterless) is analyzed to study the impact on the system performance at different transmission distances with and without SOAs. Then, we analyze the case of SOA-based OAD nodes after each span of fiber. To evaluate the proposed system, the setup of Figure 2 has been considered. The WDM consists of 32 channels with a channel spacing of 100 GHz. However, for the experiments only the 1550.12 nm channel has been studied. It is marked by the red color in Figure 3. Thus, the signal traverses the first WDM acting as a demultiplexer, the SOA and finally the signal is multiplexed by the second WDM. Figure 2: Architecture of a SOA-based OAD node. To experimentally analyze the proposed system, the setup of Figure 3 has been considered. A single BVT, based on DMT and DD, transmits over the cascading of fiber spools of 25 km and SOA-based switching nodes according to scenario (a) or scenario (b). Python software has been used at the DSP module. The IFFT with 512 subcarriers modulates the mapped sequence. Due to the HS, only half of the IFFT subcarriers supports data. The total number of frames is 125 being 5 of them TS. The CP is 1.9% and the Forward Error Correction (FEC) considered is 7%. As it is well-known, the clipping factor can cause distortions and the degradation of the system performance. For this reason, the best clipping level should be selected according to the adopted constellation format. To estimate the channel profile, uniform loading is adopted, in particular 4-QAM. The clipping level recommended for this modulation format is 7 dB, which corresponds to a clipping factor of 2.24. When other modulation formats are used, 7 dB can be not enough. The clipping level must be selected according to the highest modulation format, when BL is used. In our case, the highest modulation format used in this experiment is 16-QAM. Accordingly, we have determined that the best clipping level is 8.5 dB giving a clipping factor of 2.6. The sample rate of the DAC is 28 GS/s. The obtained electrical signal is the input of a Dual Drive Mach-Zehnder Modulator (DD-MZM). The DD-MZM is working in the push-pull operation. The laser driving the DD-MZM is centered at 1550.12 nm with 13 dBm of output power. As the DD-MZM is polarization dependent, a Polarization Controller (PC) is needed to obtain the maximum power of the laser driving the DD-MZM. Another PC is at the output of the DD-MZM. With those PCs we can control the power at the input of the EDFA, obtaining a power value of 1.5 dBm at the input and 13 dBm at the output of the EDFA. Then, the power launched to the network is measured by an optical power monitor (OM). At the receiver, an attenuator followed by an EDFA is used to vary the Optical Signal-to-Noise Ratio (OSNR). After that, another OM  
Figure 3: BVT architecture and experimental setup. OM: optical power monitoring, Att: Attenuator, OSA: Optical Spectrum Analyzer, EQ: Equalizer. The different analyzed scenarios for the optical metro network are indicated: (**a**) with SOAs and 25 km Single Standard Mode Fiber (SSMF) spools, and (**b**) with OAD nodes based on SOAs.