In this paper, we review space division multiplexing (SDM) transmission experimental demonstrations and associated technologies. In past years, SDM achieved high capacity transmission through increased spatial multiplicity, and long-haul transmission through improved transmission performance. More recently, dense SDM (DSDM) with a large spatial multiplicity exceeding 30 was demonstrated with multicore technology. Various types of multicore and multimode SDM fibers, amplification, and spatial multi/demultiplexers have helped achieve high-capacity DSDM transmission. We describe recent progress in space division multiplexed (SDM) transmission, and our proposal and demonstration of dense space-division multiplexing (DSDM), which offers the possibility of ultra-high capacity SDM transmission systems with high spatial density and spatial channel count of over 30 per fiber. We introduce the SDM transmission matrix, which cross indexes the various types of multi-core multi-mode transmissions according to the type of light propagation in optical fibers and how the spatial channels are handled in the network. For each category in the matrix, we present the latest advances in transmission studies, and evaluate their transmission performance by spectral and spatial efficiencies. We also expound on technologies for multi-core and/or multi-mode transmission including optical fiber, signal processing, spatial multi/demultiplexer, and amplifier, which will play key roles in configuring DSDM transmission systems, and review the first DSDM transmission experiment over a 12 core × 3 mode fiber. 

Spatial multiplexing in optical fibers

We have previously defined an SDM transmission matrix, which classifies the type of light transmission in SDM fibers [17]. Fig. 4 shows a cross-section of the SDM fibers used in each category of the matrix consisting of (IA) multicore single-mode, (IIA) groups of coupled-core and (IIIA) multicore multimode elements, which are the parallel forms of (IB) single-mode, (IIB) coupled-core, and (IIIB) multimode transmission. Here, n designates the number of spatial channel groups, and m shows the number of spatial channels in each spatial channel group.

For transmission in category B, n is equal to 1, since all the spatial channels are handled as a single group. Transmission in category A consists of multiple spatial channel groups, and nP2. Typically, uncoupled n-core single-mode MCF, n groups of mcoupled core MCF [42], and n-core m-mode MC-FMF [43] are used in category IA, IIA, and IIIA transmissions, respectively, and m-coupled core MCF and m-mode MMF are used in category IIB and IIIB transmissions, respectively.

Fig. 5 shows the spatial multiplicity versus cladding crosssectional area of various SDM fibers. The tilted dotted line shows spatial multiplicity per cladding area, which we have already defined as spatial efficiency (gspatial = spatial multiplicity/cladding area) [17]. Plots on the same dotted line are equivalent regarding the usage of space in a fiber. In reliability terms, the cladding diameter should be within 125–250 lm. SDM has attempted to raise spatial efficiency by utilizing core and mode multiplexing. The multicore multimode approach is most effective in enhancing spatial efficiency, while the multimode approach requires more advanced transmission technology than the single-mode approach to utilize this space efficiency, and achieve high-capacity longdistance transmission. The multicore single-mode approach has moderate efficiency in terms of spatial usage. On the other hand, it has the advantage of high spectral efficiency values comparable to that of a conventional SMF, and thus has successfully demonstrated high-capacity and long-haul SDM-WDM transmissions.

For single-mode transmission (I), m is equal to 1 with each core containing a single spatial channel. The transmission medium can be bundled SMF [44] or uncoupled MCF. For coupled-core (II) and multimode (III) transmissions, each spatial channel group contains multiple spatial channels within the group, and mP2. The m spatial channels in each spatial channel group mix during transmission, and 2m 2m MIMO can be used at the receiver to separate the coupled signals and also dual polarizations. On the other hand, the n spatial channel groups must be handled individually in an optical network. Therefore, it is important to develop MCF technology that can reduce the crosstalk from different spatial channel groups.

CONCLUSION

We have reviewed recent progress on high capacity dense space division multiplexing (DSDM) transmission over multicore and multimode fibers. Various spatial multiplexing approaches for SDM systems, and recent transmission performances were reviewed. SDM studies have made rapid progress as regards increasing the transmissible capacity, distance and spatial multiplicity over SDM fibers. When the large spatial multiplicity provided by current DSDM fibers is taken into account, a higher capacity is possible by improving the fibers and components used in transmission lines, and SDM amplification technologies. Further advances in SDM technology should make future optical transport systems capable of ultra-high capacity long-haul transmission.