## 1 Introduction  
Optical metro networks face significant challenges supporting ever-increasing bandwidth demands and ever increasing service expectations [1]. High-performance next-generation dynamic optical metro networks should efficiently support a variety of access applications with dynamic traffic patterns (LTE and 5G backhaul and fronthaul, multi-technology Passive Optical Networks (PON), data center interconnects, enterprises, etc.) as well as multi-Tbit/s interfaces with core networks by leveraging the latest advances in optical transmission and switching. Moreover, applications, such as 5G with deployment of multiple antennas and MIMO radio configurations, require not only large bandwidth beyond 100 Gb/s, but also the computing and storage resources for processing the signals from the radio antennas. Therefore, next generation metropolitan nodes will co-allocate telecom network elements and compute and storage resources to cope with such applications. In addition, new developments in network virtualization could partition the optical data layer to be able to accommodate a wide range of use cases, from the vertical industries and other infrastructure users with different requirements (e.g., latency, resiliency, and bandwidth) allocating logical networks and infrastructures, optimally tailored for each specific use case. This requires the metro node network architecture to be a flexible infrastructure that can be adapted and scaled on demand according to the applications. On the other hand, power consumption and costs of such infrastructures should be sustainable as the infrastructure scales with data rate and network elements, as well as computing and storage resources. Several programmable optical metro node architectures based on simplified, flexible, and cost-efficient optical switching and transmission technology, with high capacity and faster network reconfiguration and with efficient exploitation of the available optical spectrum, have been currently investigated [2; 3; 4; 5; 6]. Novel node architectures based on the \"whitebox\" concept, including \"distributed DC\" capabilities, i.e., using disaggregated hardware in a vendor-neutral approach and having local processing and storage resources, is highly promising for integrating commodity hardware with telecom network elements. In the target architecture of Central Office Re-architected as a Datacenter (CORD) [3] shown in Figure 1, it includes a collection of commodity servers interconnected by a fabric constructed from electrical whitebox switches. The switching fabric is organized in a leaf and spine topology to optimize for traffic flowing east to west between the access network that connects customers to the Central Office and the upstream links that connect the Central Office to the operator's core network. Relying on the implementation of a high-bandwidth electronic switch node is limited by the switch ASIC I/O bandwidth due to the scaling issues of the ball grid array (BGA) package [7]. Higher bandwidth is achievable by stacking several ASICs in a multi-tier structure, but at the expense of larger latency, higher cost, and power consumption. Moreover, power consumption is an additional issue for such switches. In addition to the power-hungry electronic switching fabrics, the E/O and O/E conversions at the switching node actually dissipate a large portion of the consumed power [8]. Switching the data signals in the optical domain has the potential to overcome the scaling issues of electronic switches [9]. The advantages of deploying optical switching technologies in an optical metro node are multi-fold:  
Transparency to data-rate and data-format allows for extremely high I/O bandwidth without implementing signal-dependent interfaces. 1. The high capacity helps to flatten the network topology, avoiding bandwidth bottleneck and large latency caused by hierarchical structures. 2. Massive O/E/O conversions can be eliminated, improving the energy-efficiency and cost-efficiency. 3.