The increasing demand for higher bandwidth data services is pressuring optical communications networks to expand capacity cost-effectively [1–4]. Nevertheless, single-mode fibre transport systems are rapidly approaching the Shannon limit with a maximum capacity that does not answer to the future demands. This limit points to a SMF capacity of no more than 100 Tbit/s of data, at a spectral efficiency of approximately 10 bits/s/Hz [5], having the ultimate limit imposed by fibre nonlinearity [6,7]. Over the last decades, the steady progress in optical transmission systems was enhanced by the exploitation of the several properties of electromagnetic waves, namely time, phase, frequency and polarization. Since 2010, the research community has generally agreed that optical transport systems will have to resort to the last remaining physical dimension (space) by employing spatial division multiplexing (SDM). Therefore, to avoid a network “capacity crunch”, SDM appears as a new milestone in the evolution of fibre-optic communication, with high-capacity SDM systems demonstrations starting only around 2011 [8].
Nevertheless, technological improvements need to be considered to guarantee the efficient use of SDM systems, and several impairments need to be addressed to reduce the energy/cost per bit despite the system capacity increase [9]. Therefore, and extensive study of the subject is necessary. Thus, this study aimed to provide an overview on the main fundaments of SDM optical systems, by addressing the available technologies for this type of systems, the main challenges and inherent physical impairments (e.g., nonlinearities and crosstalk) to its effective employment, the newest research achievements (e.g., nonlinear mitigation based on phase conjugated twin waves - PCTW) in the field and future perspectives on how to optimise SDM systems (e.g., the use PCTW to mitigate nonlinearities; the employment of spatial light modulator (SLM) and computer generated holograms (CGH) to perform channel compensation; and the application of advanced modulation formats to increase the aggregate data rate and the spectral efficiency).