## 1 Introduction
The reduced number of cables benefitting from the high port capacity may facilitate the deployment and management of the network. Several optical switching schemes have been proposed and investigated [10; 11]. Optical switches based only on space have inefficient bandwidth utilization and inflexible connection. On the other  
Figure 1: Target hardware built from commodity servers, I/O blades, and switches. hand, only time domain fast optical switches can offer sub-wavelength granularity for on-demand and high-degree connectivity by exploiting statistical multiplexing. To fully reap the profits from the statistical multiplexing, the switch should support fast scheduling and reconfiguration at the nanosecond scale. Nevertheless, the optical switch featuring with nanoseconds reconfiguration time has been only demonstrated with limited port-count [12; 13; 14]. An innovative optical interconnect network architecture for data center network architecture based on fast flow-controlled optical cross-connect (OXC) switches have been employed as the distributed switching elements in [15]. The capability of switching the aggregated traffic in both time and wavelength domain at the nanosecond scale has further improved the flexibility and feasibility of the system. However, no application to optical metro network node architecture has been reported, and experimental assessment of the WDM optical cross-connect switch has been performed only for non-return to zero signals and for a limited optical power dynamic range. In this work we present a novel optical metro node architecture that exploits the WDM optical cross-connect nodes for interconnecting network elements, as well as computing and storage resources. The experimental assessment of a photonic WDM cross-connect node based on semiconductor optical amplifiers (SOA) as a main building functionality for implementing the interconnected network of the metro node is also reported. The advantages of using an SOA to realize the WDM cross-connect switch in terms of switching speed, photonic integrated amplification for loss-less operation, and gain equalization will be verified experimentally. Experimental results on the assessment of a \(4\times 4\) photonic integrated WDM cross-connect confirmed the capability of the cross-connect chip to switch WDM signal in space and wavelength. Experimental results show lossless operation and cross-talk \(<\!-30\) dB. Compensation of the losses is a good indication that the modular architecture could scale to a larger number of ports. Moreover, the operation of the cross-connect switch with multiple WDM channels and diverse modulation formats is also investigated. Experimental results confirmed the capability of the cross-connect chip to dynamically switch, within a few nanoseconds, the WDM data packets in space and wavelength. Error-free operation with less than a 2 dB power penalty for a single channel, as well as WDM input operation, has been measured for multiple 40 Gb/s NRZ-OOK, 20 Gb/s PAM4, and data-rate adaptive DMT traffic. The paper is organized as follow. In Section 2, a brief description of the optical metro node architecture comprising of the WDM optical cross-connects based on SOAs will be presented. The design and operation principle of the SOA-based photonic WDM cross-connect will be discussed in detail in Section 3. The photonic WDM cross-connect chip fabrication and characterization will be described in Section 4. The experimental assessment of the photonic switch under single and multiple channel operation, and multiple modulation formats, will be reported in Section 5. Finally, conclusions will be summarized in Section 6.