翻译：So far, SDM theoretical and experimental research was based on total bandwidth, capacity and aggregate spectral efficiency, without considering the space domain at all. It is essential to have a reference point in order to analyze and evaluate SDM technologies. To this end, appropriate Figures of Merit along with their metrics to quantify SDM features are necessary. Two such new metrics are proposed in this paper. The first one aims to measure Spectral Efficiency (SE) per cross-sectional area of the fibers (Spectral-Spatial Efficiency—S.S.E.) and the second focuses in evaluating components used for optical networks with regards to their footprint area and/or volume (Components Performance per Footprint Area/Volume—CPFA/CPV). The figures of merit (FoM) introduced in this paper are numerical expressions based on spatial effectiveness, representing measures of efficiency and performance for SDM fiber technologies and devices. Our proposed metrics lead to effective comparison and categorization of SDM approaches. That offers a two-fold benefit; the evaluation of current and upcoming technologies as well as the connection of network systems with technology, considering capacity, S.E. and space. The rest of this section is organized as follows: first we introduce the SDM FoM and we use them to assess the available technologies. Then we present briefly the requirements  of metro/core networks and Data-center Networks (DCN), and how SDM could address those in terms of fibers and network devices. Furthermore, we focus in the MCF’s greatest impairment, i.e. inter-core crosstalk, and we study the limitations that this imposes to an SDM network. A. Spectral-Spatial Efficiency (SSE) In order to identify SSE as a metric we propose a simple formula (Eq. (2)), expressing the aggregate SE of the whole fiber divided by the area of its cross-section. SSE = SE · SM Across . (2) In Eq. (2), SE is the Spectral Efficiency (b/s/Hz) of each spatial mode, SM the number of discrete Spatial Modes, and Across (mm2) the area of the cross-section of the fiber. SM could be the number of cores in a MCF, the number of LP modes (single or dual polarization) in a FMF, the amount of elements in a Multi-Element Fiber, the number of multiplexed modes carrying Orbital Angular Momentum in a Vortex Fiber or the number of cores multiplied by the number of LP modes in a FM-MCF. Using the above mathematical formula, we calculated the Spectral-Spatial Efficiency for 10 fiber structures used in SDM in various ways. These fibers were reviewed qualitatively in the first part of this paper, and here we evaluate them quantitatively. Fig. 9 illustrates the SSE of these SDM fiber technologies for three discrete SE values, 1, 4 and 8 b/s/Hz. This figure shows that fibers with more spatial channels and less cross-section area, use spectrum much more space-efficiently. Remarkable distinction is found between the SMF ribbon and the FM-MCF (5 and 5928 b/s/Hz/mm2), as shown in Fig. 9. This is due to the large difference between the Across of the two technologies, although SMF-bundle outnumbers FM-MCF in Spatial Modes (Table I). The fibers’ detailed specifications, cladding diameter, number of spatial modes along with their SSE values for 8 b/s/Hz SE can be found in Table I. In the cases of the fiber-bundle and MEF, coating diameters have been used, since the fibers and the elements respectively do not share the same cladding, so taking their cladding diameter as a reference would be inaccurate. In the same table, the last two entries represent theoretical fiber designs extrapolated from existing MCF and FM-MCF designs (core pitch, cladding diameter, etc.) and also from the centered hexagonal number (see Appendix), in alignment with the mostly-used hexagonal core-arrangement scheme. Offering 169 and 222 spatial modes, these designs show excellent SSE in comparison with the real implementations, 5,694 and 18,469 b/s/Hz/mm2 respectively. Although this is an encouraging fact for the future SDM fibers, one needs to consider the practical challenges on drawing large cladding diameter, i.e. 550 μm, MCFs. The outcome of the above evaluation using SSE, shows that there is enough room for future improvement in SDM networks. Especially in reducing cable complexity and conventional fiber mesh by having fewer SDM fibers still offering the same and higher spectral efficiency and capacity services. Nevertheless,  an important subject that has to be addressed is the relation and possible trade-offs between the SSE, crosstalk and link reachability when different SDM networking approaches are used. A potential way to investigate this is by associating OSNR and the various modulation formats with SSE and reachability of signals propagating in SDM systems as well as considering any crosstalk constraints.