# Real-Time Spatial-Division Multiplexing Transmission with Commercial 400 Gb/s Transponders
## 4 Discussions and Conclusions  
In this paper, we propose two real-time SDM transmission approaches which are compatible with commercial 400 G coherent single-mode transponders. In the first scheme, a 60 km single-mode WC-MCF with a pair of fan-in and fan-out devices is employed. In the second experiment, the fiber link is composed of 60 km/150 km WC-FMF and low-modal-crosstalk mode multiplexers, in which only non-degenerate LP\({}_{01}\) and LP\({}_{02}\) modes are adopted. In all transmission tests, the BERs are below the FEC threshold of the 400 G transponders. Therefore, SDM transmission using single-mode WC-MCF or WC-FMF with non-degenerate LP modes are two effective approaches which are compatible with current commercial single-mode coherent transponders. In both MCF and FMF transmissions, the influence of splice points is investigated. For the MCFs, the splice points only affect insertion loss but do not introduce inter-core crosstalk. The experimental results show that the six outer cores have larger splice losses than the center core. This shows that the outer cores should also be angularly aligned rather than simply having cladding alignment. Despite the transmission capacity of the MCF being seven times that of SMF, extra insertion loss leads to a shorter transmission length. Therefore, single-mode WC-MCF is currently more suitable for medium- or short-distance large-capacity transmission. Furthermore, the reduction of the splice loss should be investigated for MCFs by employing CO\({}_{2}\) lasers and more precise alignment techniques. For the FMFs, the experimental results show that the splice loss is much smaller than the average level of MCF, close to the level of commercial SMF. However, weakly coupled FMF transmission over non-degenerate LP modes is suitable for medium or short distance as well. This is because the modal crosstalk--and not the loss introduced by the splice point--is the main issue. Since the length of one span of optical cable is 2-3 km, a long-distance transmission link will contain lots of splice points. The accumulated modal crosstalk will reduce the OSNR of the system. Besides, the distributed modal coupling along the FMF transmission and mode MUX/DEMUX will also inevitably introduce modal crosstalk. Compared to elliptical core FMF transmission using commercial single-mode transceivers, the proposed two SDM transmission schemes increase the transmission distance from 1 km to dozens of kilometers [22]. Although the mode-group division multiplexing could achieve MIMO-free transmission as well, the output fibers are FMFs rather than SMFs, which are not compatible with commercial single-mode coherent transceivers [23]. in addition, employing only non-degenerate LP modes could accelerate use of the SDM technique. Further capacity upgrade of the system could be implemented using degenerate LP modes with 4 \(\times\) 4 real-time MIMO processing when the hardware is available with a low cost. Compared to single-mode WC-MCFs, WC-FMF with 125 \(\upmu\)m cladding diameter has higher spatial channel density, as signals traveling in orthogonal spatial modes share the same fiber core. Since WC-FMFs have a step-index profile similar to commercial SMF, its production process is compatible with the mature plasma chemical vapor deposition technique. Its mechanical reliability and splice accuracy are similar to commercial SMF as well, which is also proved by our experiments. Additionally, although the few-mode multicore fibers can combine two dimensionalities and have a higher spatial density, the length of current single-roll FM-MCF rarely exceeds 10 km due to production yield. Therefore, the two approaches proposed in this paper are more likely to be deployed in the near-term with only one dimensionality extension. Conceptualization, Y.G., L.S. and S.S.; methodology, Y.G. and Y.T. ; software, Y.G. ; validation, J.L. and C.Z. ; writing--original draft preparation, Y.G., L.S. and Y.T. ; writing--review and editing, Y.G. and Y.T. ; supervision, X.Z. and Z.C. ; project administration, X.T. and L.Z.