# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 1 Introduction  
New challenges are envisioned for future optical networks to cope with the exponential and uncertain growth of internet traffic. In particular, the metro segment is emerging as the most challenging, due to the stringent requirements of cost efficiency and reduced power consumption, while offering very high capacity and dynamicity. To address these challenges, optical metro networks need to be equipped with flexible, adaptive, and programmable transmission and switching systems able to efficiently manage the available resources as well as the high peak of traffic and adaptive bit rates with cost- and power-efficient solutions [1; 2]. In particular, to overcome the increasing traffic demand, programmable adaptive transceivers have been proposed as a suitable transmission technology [3]. In dynamic future optical networks, adaptability, flexibility, and programmability are of great importance, especially if deployed fiber infrastructure wants to be exploited. These requirements can be dealt with using Sliceable-Bandwidth Variable Transceivers (S-BVTs) [3]. They allow the generation of multiple flows while suitably and flexibly adapting the transmission in terms of bit rate, modulation formats, bandwidth, and reach. Multiple parameters and S-BVT components can be suitably configured on demand by means of an integration of the transceiver in a Software-Defined Networking (SDN) control plane, following the SDN principles [4]. In particular, an S-BVT based on Orthogonal Frequency Division Multiplexing (OFDM) offers the possibility to manage the capacity/spectrum at the subcarrier level [3,5,6]. The S-BVT architecture can be suitably tailored for the metro network segment [7]. For a cost-effective design Discrete MultiTone (DMT) modulation is preferred as the simplest OFDM implementation [5,6]. Optical networking based on Wavelength Selective Switch (WSS) technology has been studied in [8] for the future adoption of an elastic optical network. In fact, using flexi-WSS at the network nodes the optical spectrum can be used more flexibly and efficiently [8]. However, a significant source of penalty is caused by the transmission through a cascade of nodes. This effect, known as the filter narrowing effect, has been analyzed in [8,9]. In the context of elastic optical networks, Semiconductor Optical Amplifier (SOA) technology can be advantageously used, particularly for the metro segment, where the cost and power consumption are critical issues. In fact, the main advantages of using SOAs are the low power consumption, low cost, small size, and the possibility to be integrated with other optical components [10]. On the other hand, SOAs have high noise value compared to Erbium Doped Fiber Amplifier (EDFA) and have residual polarization-dependent operation <1 dB [11]. However, some studies focus on the reduction of these impairments, as in [12], SOAs can be designed to deploy building blocks for optical switching as well as a booster amplifier, inline amplifier, or preamplifier [13,14]. By carefully adjusting the current injected into the SOA, it can be used as an optical amplifier; meanwhile, by turning on/off the electrical current, the SOA can act as a fast optical gate. Furthermore, SOAs can be used for wavelength selective applications by combining them with wavelength-filtering technologies [10]. In this work, we focus on the role of SOAs as building blocks in optical switching technologies [13]. In particular, we consider an SOA-based switching node with and without filtering elements. In the last case, the switching functionalities are performed by the SOAs and can also find application in filterless optical networks [15]. Filterless optical networks are simple network architectures based on passive splitters and combiners avoiding optical filters. Thus, they are more cost-effective than networks adopting filters; however, some functionalities are limited, such as wavelength reuse or capacity at high utilization rates [8,15].