翻译：On top of the physical deployment of computational, storage and network resources, known as a Data Center, a wide range of application is running, from financial and e-commerce to scientific operations. The latest trend is towards “Cloud Computing”, where end-users are given the opportunity to run any of the above applications remotely inside a DC. It is obvious that this can only be realized with the support of high-performance networks inside and between datacenters. As a result, Big Data and massive storage clouds are the critical points that future data centers need to consider. Requirements: Emerging Data Centers will need to accommodate from 10 s to 100 of thousands of servers to provide the necessary computational power and storage space needed for the operation of mainly cloud-based functions. It is obvious that these servers, which are usually organized in racks and clusters, need to communicate vastly, either for long or for very short periods of time, always depending on the type of application [140]. From the above it is also obvious that high capacity, low power consumption, fiber complexity, scalability and low latency are important requirements for intra Data Center Networks [141], [142]. Current DCNs utilize optical fibers (mostly MMF), but not in the most efficient way. Some intra-DCN physical connections are really hard to manage due to the extreme fiber complexity and fiber count. Another essential aspect usually to consider is the restricted space inside a data center. While everything is organized in racks, the area and volume for each server, switch or any other network component is finite and pre-arranged. That is because the data center needs to be strictly systematic in order to ease thermal management and central control. Currently, for transmission rates of 10 GBd and short distances (1–2 km) intra data center, direct modulation with On-Off Keying (OOK) is used, since it is simple, low-power and cost-effective. However, for future DC interconnection with requirements of more than 100 Gb/s (10 × 10 G or 4 × 25 G) and 1 Tb/s (32 × 25 G or 10 × 100 G) [143], novel modulation schemes, a mix of TDMSDM-WDM and digital signal processing (DSP) might be unavoidable. At last, from a financial perspective, the purchase cost of the infrastructure along with the maintenance expenses, seems to be an equally deciding factor for the developing of future DCs. How Can SDM Meet Those Requirements?: SDM can address these requirements to a large extent and improve the interconnection between the servers of the datacenter in quite a few ways. Firstly, with a single SDM fiber, MCF or other, the same and even higher capacity and spectral-spatial efficiency can be achieved, than deploying multiple single-core fibers. Recently, an all-optical scalable intra-DCN interconnect using SDM (Multi-Element Fibers) in collaboration with TDM (PLZT-based, SOA-based switches) for finer granularity was  experimentally demonstrated, offering ToR-to-ToR intra- and inter-cluster communication [144]. In that way, cable density between racks is decreased dramatically too, resulting in a more relaxed spacing and finally easier cooling of the whole network system. Furthermore, highly-integrated SDM network devices, like TxRx that enable direct coupling with SDM fibers, save useful space inside the DC racks and also resources, leading in an increase of energy efficiency [145], without compromising performance. Low power consumption could be achieved if a SDM-only architecture was adopted, instead of WDM-SDM. Then, firstly no WDM mux/demux would be necessary, and secondly with the use of low-cost low-power consumption grey interface VCSELs in different SDM channels of an SDM fiber, the required capacity and energy efficiency targets could be met (Fig. 21). For latency sensitive DCN cases, for instance High Performance Computing (HPC), all-optical switches in Top of the Rack (ToR) and in other places of the DCN could be utilized along with HC-PBGF, in order to avoid O/E/O conversion and provide ultra-low latency light-paths between processing, memory and storage racks. In addition, TDM ultrafast switches of the nanosecond scale could cooperate greatly with SDM fibers too and add the necessary granularity for switching short-live data bursts. Finally, as far as cost-efficiency in concerned, we need to examine whether plain cost is a good metric for designing a DCN or if it would be better to consider cost per Gb/s of network/link capacity. In that case, SDM could prove quite cost-efficient as well.