# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 3 SOA Technology for Optical Metro Network Nodes  
SOA technology can be adopted for implementing switching devices. There are other optical switching technologies based on Electro-Optic, Acousto-Optic, Thermo-Optic or Opto-Mechanical switching. The main advantages of SOAs with respect to these other switching technologies are the loss compensation, the high speed, and the scalability [13]. When several switches are in cascade, SOAs can control the gain with the injected current and, depending on the node architecture, overcome the limitation due to the power decay. As losses due to the network elements can be compensated thanks to the presence of SOAs, this switching option can be envisioned also for applications in filterless optical networks. This would represent a cost-effective solution [15], as active reconfigurable components are eliminated or minimized using passive optical elements as combiners or/and splitters to interconnect the fiber links and to add or drop channel wavelengths at the nodes. Some advantages are expected for these networks such as simplified maintenance or reconfigurability. However, in a filterless optical network we must consider the related drawbacks and limitations. In this scenario, the use of SOA-based switching nodes combined with BVT technology can be advantageous. SOA-based switching nodes can be useful to compensate the attenuation, due to the passive elements and the transmission over the fiber, and BVT with loading capabilities, to combat the CD.  
On the other hand, a generic filtered optical mesh network is equipped with OAD multiplexer, placed at the nodes, enabling to dynamically configure the dropping or adding of different wavelengths  
Figure 1: S-BVT architecture based on adaptive DSP using DMT for optical metro networks with SOA-based switching nodes. The S-BVT is programmable and composed by an array of BVT modules.  
through the fiber, remotely. Typically, the reconfigurable OAD nodes adopt WSS technology. Particularly, in the context of elastic optical networks, flexible WSS are needed to deploy a more flexible and bandwidth efficient network [8]. The main disadvantage of this solution is the penalties in signal degradation due to the filter narrowing effect caused by the crossing through a cascade of WSS [8], as it was mentioned in Section 1. The use of S-BVT can flexibly adapt the transmission.  
In this work, SOA-based wavelength selectors are considered as the key element in OAD nodes to drop and add traffic in the network [13; 23]. WDM optical cross-connect based on SOA has also been presented in [10] for interconnecting network elements, computing, or storage resources in a metro network architecture. It has been experimentally demonstrated the potential lossless, low cross-talk or multicasting operation of these network elements.  
In our study, we analyze the performance when the OAD nodes with SOA-based wavelength selectors are included, being the target the metro segment. The OAD node consists of one splitter, one combiner, the SOA array for gating one or more wavelengths and two wavelength selectors. One acting as a demultiplexer and the other acting as a multiplexer. In the proposed system, the filter narrowing effect is also present due to the concatenation of the wavelength selectors [8]. The architecture of the SOA-based OAD node can be seen in Figure 2. In this case, losses are generated by the transmission over the fiber links and the filtering elements as well. Adjusting the SOA bias current, the filtering effects mainly due to the wavelength selectors and the losses due to the fiber, can be compensated.