# Future Scenarios for Software-Defined Metro and Access Networks and Software-Defined Photonics
## 4 Photonic Devices and Components
### Technologies for ROADMs  
Remotely reconfiguring ROADMs can be considered one of the basic building blocks to support advanced functionalities in the optical channel layer. ROADMs are already used in core networks: they can be considered key elements for the development of backbone transport networks [68]. For instance, since 2011 multi-degree ROADM are used for remote switch and rerouting in the Telecom Italia Ka\(\lambda\)eidon network, a wholly photonic mesh network capable of transporting 80 channels at 40 Gbit/s (scalable to 100 Gbit/s) with photonic protection and restoration [69]. ROADMs are conventionally based on micro electro-mechanical systems (MEMS) and liquid crystal on silicon (LCoS) technologies. MEMS-based ROADMs use optical gratings to separate wavelengths and micro-mirrors arrays fabricated on a silicon substrate to reflect each spectral component [70]. In the last decade, a new trend has been outlined by market analysts: the so-called \"new ROADM revolution from the core to the edge\" [43; 44]. The ROADM application in metro networks is particularly valuable because DWDM networks are increasingly deployed more extensively in this segment and ROADMs would add the flexibility in software to add/drop wavelengths for information access or to reroute carriers [40]. In other words, ROADMs can help carriers overcome the strict limitations of conventional metro DWDM networks with fixed OADMs. In addition, a migration to metro mesh architectures is a secondary driver for ROADM architectures [45]. Mesh networks make it easier to manage and direct higher volumes of traffic since they allow continuous reconfiguration around blocked paths or by allocating more bandwidth on a fiber as required [46]. Both the drives, i.e., flexibility and mesh networking, are particularly meaningful in an SDN perspective (even if they arenot necessarily directly referred to the original SDN concept) since they enable re-configurability and programmability for a more efficient traffic management. Two general types of ROADMs can be used in metro networks: two-degree ROADMs are used for add/drop functionalities related to local access traffic; multi-degree ROADMs are used for interconnecting DWDM rings or for mesh networking [41]. In recent years, ROADM vendors have made great strides and nowadays are already able to take the ROADMs closer to end users with high-bandwidth applications [42]. Although technologically mature, both MEMS and LCoS technologies are not scalable enough to be applied to metro and access networks where cost, volumes, power consumption, and reliability are extremely significant features [10]. Alternative solutions have been proposed in recent years, such as ROADM based on micro-ring resonators (MRRs) realized on SOI technological platform. [PERSON] et al. [71] have demonstrated thermally a tunable MRR-based ROADM for access networks. These devices have been fabricated in Si\({}_{3}\)N\({}_{4}\)/SiO\({}_{2}\) technology with a footprint of less than 2 mm\({}^{2}\) and they are capable of operating in the second or third telecom window. Four main features are considered necessary for new generation ROADMs in order to rapidly respond to new service innovations made possible by SDN. In legacy architectures, each add/drop port is restricted to be used to transmit/receive only a specific wavelength channel to/from a given output/input fiber, i.e., a specific direction. Recent ROADM architectures should relax this restriction by allowing a single add/drop port to accept any wavelength channel, i.e., the colorless feature, and/or to bridge the signal to/from any direction, i.e., the directionless feature. The usefulness of a third feature, called contention-less, is still the subject of discussion. Contention occurs in case the same wavelength channel is to be used in two or more add/drop ports of the same add/drop structure. Contention-less means that wavelengths can be more easily reused multiple times without any manual configuration.