Research on space-division multiplexing (SDM) came to prominence in early 2010 being primarily proposed as a means of multiplying the information-carrying capacity of optical fibers at the same time as increasing efficiency through resource sharing. Proposed SDM transmission systems range from parallel single-mode fibers with shared amplifier pump lasers to the full spatial integration of transceiver hardware, signal processing, and amplification around a fiber with over 100 spatial channels comprising multiple cores each carrying multiple modes. In this paper, we review progress in SDM research. We first outline the main classifications and features of novel SDM fibers such as multicore fibers (MCFs), multimode fibers, few-mode MCFs, and coupled-core MCFs. We review research achievements of each fiber type before discussing digital-signal processing, amplifier technology, and milestones of transmission and networking demonstrations. Finally, we draw comparisons between fiber types before discussing the current trends and speculate on future developments and applications beyond optical data transmission.

Space-division multiplexing for optical fiber communications

Survey of resource allocation schemes and algorithms in spectrally-spatially flexible optical networking

Elastic optical networking (EON) with space-division multiplexing (SDM) is the only evident long-term solution to the capacity needs of the future networks. The introduction of space via spatial fibers, such as multicore fibers (MCF) to EON provides an additional dimension as well as challenges to the network planning and resource optimization problem. There are various types of technologies for SDM transmission medium, switching, and amplification; each of them induces different capabilities and constraints on the network. For example, employing MCF as the transmission medium for SDM mitigates the spectrum continuity constraint of the routing and spectrum allocation problem for EON. In fact, cores can be switched freely on different links during routing of the network traffic. On the other hand, intercore crosstalk should be taken into account while solving the resource allocation problem. In the framework of switching for elastic SDM network, the programmable architecture on demand (AoD) node (optical white box) can provide a more scalable solution with respect to the hard-wired reconfigurable optical add/drop multiplexers (ROADMs) (optical black box). This study looks into the routing, modulation, spectrum, and core allocation (RMSCA) problem for weakly-coupled MCF-based elastic SDM networks implemented through AoDs and static ROADMs. The proposed RMSCA strategies integrate the spectrum resource allocation, switching resource deployment, and physical layer impairment in terms of intercore crosstalk through a multiobjective cost function. The presented strategies perform a cross-layer optimization between the network and physical layers to compute the actual intercore crosstalk for the candidate resource solutions and are specifically tailored to fit the type of optical node deployed in the network. The aim of all these strategies is to jointly optimize the switching and spectrum resource efficiency when provisioning demands with diverse capacity requirements. Extensive simulation results demonstrate that 1) by exploiting the dense intranodal connectivity of the ROADM-based SDM network, resource efficiency and provisioned traffic volume improve significantly related to the AoD-based solution, 2) the intercore crosstalk aware strategies improve substantially the provisioned traffic volume for the AoD-based SDM network, and 3) the switching modules grows very gently for the network designed with AoD nodes related to the one with ROADMs as the traffic increases, qualifying AoD as a scalable and cost-efficient choice for future SDM networks.

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Resource Allocation for Space-Division Multiplexing: Optical White Box Versus Optical Black Box Networking

We evaluate the advantages of using the extra dimension introduced by space-division multiplexing (SDM) for dynamic bandwidth-allocation purposes in a flexible optical network. In that respect, we aim to compare spectral and spatial super-channel (Sp-Ch) allocation policies in an SDM network based on bundles of SMFs (to eliminate coupling between spatial dimensions from the study) and to investigate the role of modulation format selection in the blocking probability performance with an emphasis on the spectral efficiency (SE)/reach tradeoff for different multiline-rate scenarios, created either by changing the number of sub-channels (Sb-Ch), or by employing different modulation formats. Our network-performance results show that DP-8QAM —in a multichannel (MC) single-modulation-format system assuming ITU-T 50-GHz WDM Sb-Ch spectrum occupation—offers the best compromise between SE and optical reach for both spectral and spatial Sp-Ch allocation policies. They also reveal that an MC multimodulation-format system improves the network performance, particularly for spectral Sp-Ch allocation with Sb-Ch spectrum occupation of 37.5 GHz on the 12.5-GHz grid. Additionally, as another important contribution of the paper, we investigate, for spatial Sp-Ch allocation, the performance of several SDM switching options: independent switching (InS), which offers highest flexibility, joint-switching (JoS), which routes all spatial modes as a single entity, and fractional-joint switching, which separates out the spatial modes into sub-sets of spatial modes which are routed independently. JoS is proved to offer a similar performance to that of InS for particular network load profiles, while allowing a significant reduction in the number of wavelength-selective switches.

Comparison of Spectral and Spatial Super-Channel Allocation Schemes for SDM Networks

Space division multiplexing (SDM) and elastic optical networking (EON) have been proposed to increase the transmission capacity and flexibility of optical transport networks. The problem of allocating available resources over the EON network is called routing and spectrum assignment (RSA); it is called routing, modulation level, and spectrum assignment (RMLSA) if modulation adaptivity is enabled. Considering SDM adds more flexibility to the resource allocation problem. In this paper, we formulate the routing, modulation level, space, and spectrum assignment (RMLSSA) as integer linear programming (ILP) in a path-based manner for static traffic. Next, the stepwise greedy algorithm (SGA) and four different sorting policies to initiate the algorithmare proposed as a heuristic method to find a near-optimal solution of the RMLSSA problem. Finally, the paper evaluates the effectiveness of sorting policies and the SGA algorithm with different metrics. The performance results show that the SGA initiated with connection demands sorted based on increasing the number of possible space and spectrum assignment layouts (SAL) can achieve the best near-optimal solution for a small network experiment. Moreover, the ascending SAL number (ASN) policy shows the best performance for realistic networks.

Adaptive modulation and flexible resource allocation in space-division- multiplexed elastic optical networks

Fiber optic communication networks in the era of large data centers are required to provide large capacity, often orders of magnitude larger than the capacity of a single-mode fiber. The transport capacity must also be rapidly reconfigurable to adapt to the applications and services provided by the data centers. This article explores new transponders capable of providing larger capacities and making optimal use of the optical transmission spectrum, new optical switches capable of flexible routing of wavelength and multiple spatial channels, and new amplifiers supporting multiple spatial channels and broad optical bandwidth.

Physical layer transmission and switching solutions in support of spectrally and spatially flexible optical networks

Links in an IP network are unidirectional but currently are routed over bidirectional optical circuits with identical A → Z and Z → A capacities. Potentially there could be savings if the two directions of an optical circuit could be treated independently. In this study of possible savings, we have quantified the asymmetry of traffic on a current large IP backbone. A theoretical greenfield network with similarly asymmetric traffic is modeled, and it is shown that the use of unidirectional circuits to satisfy traffic demands provides significant equipment savings. We consider this study of the benefits of this approach to be a first step in deciding whether it makes economic sense to tackle the hurdles that would face such a major change in network design and operations.

Asymmetric optical connections for improved network efficiency

I outline key challenges of implementing optical routing in SDM networks, review recent research developments, and suggest some possible directions for future work.

Optical routing for SDM networks

We report crosstalk measurements among spatially-multiplexed subchannels in a ROADM supporting spatial superchannels. Aggregate crosstalk can reach -23dB when express paths are routed on adjacent fibers, but >6dB improvement results when paths are interleaved.

Crosstalk in a ROADM Supporting Spatial Superchannels for Space Division Multiplexing

Polarization-shift keying (PolSK) and Stokes vector modulation (SVM) offer multi-dimensional signaling for high-throughput data links in terabit-class data center networks, using low-cost, direct detection (DD) receivers. In this paper, we develop and characterize a system based on a cubic constellation for 8-SVM, using an off-the-shelf integrated modulator driven with simple bias points and data waveforms. Symbol error rates (SER) and bit error rates (BER) are measured up to 7.5 Gb/s, and analysis of the symbol errors reveals a significant effect of inter-symbol interference.

Mark D. Feuer

Papers

Physical layer transmission and switching solutions in support of spectrally and spatially flexible optical networks

Asymmetric optical connections for improved network efficiency

Optical routing for SDM networks

Crosstalk in a ROADM Supporting Spatial Superchannels for Space Division Multiplexing

Symbol Error Rate Analysis of 8-state Stokes Vector Modulation for Large Capacity Data Centers