Contents 1. Introduction
Space-division multiplexing (SDM) [1] is a promising approach to overcome the input-power limit of standard single-mode fiber and to realize over-1Pb/s capacity transmission systems. Figure 1(a) plots the transmission capacity versus transmission distance of the four SDM transmission experiments that have demonstrated over 1-Pb/s capacity [2]–[5]. However all the transmission lines were unrepeated and the maximum reach was restricted to 52 km [2]. Furthermore, in the case over-1Pb/s experiments using single-mode multi-core fibers, which is highly-compatible with conventional single-mode fiber transmission systems, both high-order modulation formats (≥ 32 levels) and the use of -band were required [2]–[4]. One key to realizing cost-efficient SDM systems is to reduce the system complexity caused by the number of transceivers, which increases in proportion to the spatial multiplicity. The number of total channels versus channel speed per wavelength is shown in Figure 1 (b). The total number of channels, which corresponds to the number of transceivers, is defined as the product of SDM multiplicity and the number of WDM channels. As the channel speeds per lambda were less than 300 Gb/s as in recent experiments [3]–[5], the required number of transceivers for over-1-Pb/s capacity SDM transmission systems can easily become as high as 10,000. One solution to simplify SDM systems would be to employ higher baud rate transceivers [6], [7] to decrease the number of WDM channels in each SDM channel.
