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Single Mode Fiber's physical capacity boundaries will soon be reached, so alternative solutions are much needed to overcome the multiplying and remarkably large bandwidth requests. Space division multiplexing (SDM) using multicore fibers (MCF), multi-element fibers (MEF), multimode fibers (MMF) and their combination; fewmode multicore fibers (FM-MCF) or fibers based on orbital angular momentum (OAM), are considered to be the propitious stepping-stones to overcome the capacity crunch of conventional single-core fibers. We critically review research progress on SDM fibers and network components and we introduce two figures of merit (FoM) aiming in quantitative evaluation of technologies such as amplifiers, fanin/fan-out multiplexers, transmitters, switches, SDM nodes. Results show that SDM fibers achieve an 1185fold (18-fold) Spectral-Spatial Efficiency increase compared to the 276-SMF bundle (single-core fiber) currently installed on the ground. In addition, an analysis of crosstalk in MCFs shows how SDM concepts can be exploited further to fit in various optical networks, such as core, metro and especially future intra-datacenter optical interconnects. Finally, research challenges and future directions are discussed.reinforce diverse future optical network types, such as datacenters and metro/core networks, are demonstrated. Furthermore, we critically review other major networking aspects considering SDM network and node concepts and components, such as SDM Reconfigurable Optical Add/Drop Multiplexers (ROADMs), Self-homodyne detection and Multiple Input Multiple Output -Digital Signal Processing (MIMO-DSP) on SDM receivers, routing and core allocation complexity in MCFs as well as multidimensionality and granularity in switching. A study on the inter-core crosstalk interference constraint of MCFs for SDM networks is included. The outcomes of our analysis are specific MCF design rules considering the crosstalk vs fiber core-pitch relation and network link distance with regards to the end application; i.e. 10m to 1km for Intra-DC or tens to hundreds of kilometers for metro/core networks. Different network classes are examined in the light of the outcomes of this analysis, resulting in a complete report on the challenges and the role of SDM in future high-capacity scalable optical networks. Finally, section V discusses challenges, potential use-cases and future directions of SDM.

We propose a sliceable bandwidth variable transceiver (S-BVT) architecture suitable for metro/regional elastic networks and highly scalable data center (DC) applications. It adopts multicarrier modulation (MCM), either OFDM or DMT, and a cost-effective optoelectronic front-end. The high-capacity S-BVT is programmable, adaptive and reconfigurable by an SDN controller for efficient resource usage, enabling unique granularity, flexibility and grid adaptation, even in conventional fixed-grid networks. We experimentally demonstrate its multiple advanced functionalities in a four-node photonic mesh network. This includes SDN-enabled rate/distance adaptive multi-flow generation and routing/switching, slice-ability, flexibility and adaptability for the mitigation of spectrum fragmentation as well as for a soft migration towards the flexi-grid paradigm.

In this work we detail the strategies adopted in the European research project IDEALIST to overcome the predicted data plane capacity crunch in optical networks. In order for core and metropolitan telecommunication systems to be able to catch up with Internet traffic, which keeps growing exponentially, we exploit the elastic optical networks paradigm for its astounding characteristics: flexible bandwidth allocation and reach tailoring through adaptive line rate, modulation formats, and spectral efficiency. We emphasize the novelties stemming from the flex-grid concept and report on the corresponding proposed target network scenarios. Fundamental building blocks, like the bandwidth-variable transponder and complementary node architectures ushering those systems, are detailed focusing on physical layer, monitoring aspects, and node architecture design

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