Introduction

Traffic on the internet backbone has been increasing exponentially at a rate of 10 times every 5 years. Digital coherent optical communication is a success story that translated research into commercial deployment in a time span of five short years. As impressive as the development of digital coherent transmission technology may have been, it can only provide additive growth, on the order of a few b/s/Hz, to existing WDM networks. Space-division multiplexing (SDM), including both mode-division multiplexing (MDM) and core multiplexing has been promoted as the next technology for the multiplicative growth in optical transmission capacity and dominated research in optical fiber communication in the last few years.

Since the publication of the first focus issue [Opt. Express 19(11), 2011], single-fiber transmission capacity has eclipsed the 1 Pb/s mark. All aspects related to space-division multiplexing including fiber, passive components [(de)multiplexer, couplers], active components (EDFA and Raman amplifiers), switching and routing elements (ROADM and WSS), as well as transmission and networking have progressed rapidly. This focus issue is intended to bring together the most up-to-date research in space-division multiplexing, including fibers, passive and active components, transmission systems and networking.

Coupling between space channels is an important characteristic that impacts detection and recovery of SDM signals. For mode-division multiplexing, mode coupling accentuates modal dispersion. The modal dispersion delay spread, however, is strongly dependent on the nature of mode coupling which in turn depends on fiber parameters and external perturbations. The contribution from Antonelli theoretically investigates this effect while the contribution from Nakazawa et. al. measures distributed mode coupling using a multi-channel OTDR. An analogous study for multicore fibers is presented in the contribution by Ohashi et. al. Just as for single-mode fibers, SDM transmission is affected by fiber nonlinearity. Nonlinearity in SDM transmission is dependent on modal dispersion, which is covered by a contribution from Rademacher et. al. It turns out mode coupling has a strong effect, and in certain cases reduces the impact of nonlinearities. Intermodal four-wave mixing with random linear mode coupling in few-mode fibers is presented in the contribution from Essiambre et. al..

SDM systems require new passive and active components. Mode multiplexers and demultiplexers based on three-dimensional structured directional couplers using direct laser writing and lithography are described by contributions from Withford et. al., and Hanzawa et. al, respectively. In the contribution from Chen et. al., an integrated-optics version of the spot multiplexer was fabricated, while a free-space multiplexer based on phase plates eliminated combining losses using image inversion, see the paper by Igarashi et. al. Bin et. al. demonstrate mode-group-selective photonic lanterns that use graded-index multimode fibers as input fibers, achieving better mode-group selectivity and insertion loss compared with using single-mode input fibers.

Amplifiers have been considered as an area where integration would allow SDM to realize energy efficiency and cost reduction. Jung et. al. demonstrate the first cladding-pumped few-mode EDFA. Krummrich el. al. present different pump concepts to reduce amplifier cost and power consumption for both MDM and core multiplexing. The dynamic mode power equalizer presented by Blau et. al. can be used for gain equalization for SDM amplifiers.

In the area of SDM transmission, the added dimensionality in SDM allows new coding concepts and modulation formats, which are described by Okonkwo et. al. for MDM, and Puttman et. al. for core multiplexing. It has been well-known that mode-dependent loss (MDL) reduces MDM transmission capacity. Antonelli et. al. show that the impact on MDM transmission capacity cannot be described by a generic MDL value but depends on the operation mode of inline MDM amplifiers. This Focus Issue also features major advances in SDM transmission in a contribution by Ryf et. al. on MDM in conventional graded-index multimode fibers, as well as a contribution by Morita et. al. on ultra long-haul core-multiplexed transmission over trans-oceanic distances.

Research on optical networking either compatible with SDM transmission or exploiting the additional spatial dimension has just began. In this Focus Issue, Arik et. al. investigate the effect of spectral and spatial aggregation on network scaling and Xia et. al. demonstrate the world’s first few-mode GPON.

In conclusion we hope this focus issue provides the readers with a helpful state-of-the-art snapshot from the rapidly developing research area of spatial division multiplexing and transmission.