This Review summarizes the simultaneous transmission of several independent spatial channels of light along optical fibres to expand the data-carrying capacity of optical communications. Recent results achieved in both multicore and multimode optical fibres are documented.

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Space-division multiplexing in optical fibres

Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Space-division multiplexing: the next frontier in optical communication

The theoretical fundamentals of fiber-based optical parametric amplifiers(OPA) are reviewed,and their applications are discussed in this paper.In the end the future research aspects are expected.

Fiber-based Optical Parametric Amplifiers and Their Applications

Multimode fibres (MMFs) are attracting interest in the study of spatiotemporal dynamics as well as in the context of ultrafast fibre sources, imaging and telecommunications. This interest stems from three differences compared with single-mode fibre structures: their spatiotemporal complexity (information capacity), the role of disorder, and their complex intermodal interactions. To date, MMFs have been studied in limiting cases in which one or more of these properties can be neglected. Here, we study a regime in which all these elements are integral. We observe a spatial beam-cleaning phenomenon that precedes spatiotemporal modulation instability. We provide evidence that the origin of these processes is a universal unstable attractor in graded-index MMFs. The self-organization and instability of the attractor are both caused by intermodal interactions characterized by cooperating disorder, nonlinearity and dissipation. Disorder-enhanced nonlinear processes in MMFs have important implications for future telecommunications, and the multifaceted nature of the considered dynamics showcases MMFs as potential laboratories for a variety of topics in complexity science. Spatial beam clean-up and spatiotemporal modulation instability in graded-index multimode fibres are studied in a regime characterized by disorder, nonlinearity and dissipation.

Self-organized instability in graded-index multimode fibres

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

We investigate numerically the influence of fiber splices and fiber connectors to the statistics of mode dependent loss (MDL) and multiple-input multiple-output (MIMO) outage capacity in mode multiplexed multi-mode fiber links. Our results indicate required splice losses much lower than currently feasible to achieve a reasonable outage capacity in long-haul transmission systems. Splice losses as low as 0.03dB may effectively lead to an outage of MIMO channels after only a few hundred kilometers transmission length. In a first approximation, the relative capacity solely depends on the accumulated splice loss and should be less than ≈ 2dB to ensure a relative capacity of 90%. We also show that discrete mode permutation (mixing) within the transmission line may effectively increase the maximum transmission distance by a factor of 5 for conventional splice losses.

Splice loss requirements in multi-mode fiber mode-division-multiplex transmission links.

In this letter, we present a semianalytical approach to calculate the nonlinear interference between mode-multiplexed signals that are transmitted over the eigenmodes of a multimode fiber (MMF). We point out the influence of several fiber parameters on nonlinear mode interaction using four-wave mixing concepts. Finally, we apply the presented theories on a mode-division multiplexed transmission link, which uses a graded index multimode fiber. It is shown, in a configuration, that each of the MMFs 55 modes achieves an optical signal to noise ratio at the receiver at least as high as a standard single-mode fiber, resulting in a higher overall capacity than 55 single-mode fibers.

Analytical Description of Cross-Modal Nonlinear Interaction in Mode Multiplexed Multimode Fibers

We investigate the concept of principal modes and its application for mode division multiplexing in multimode fibers We start by generalizing the formalism of the principal modes as to include mode dependent loss and show that principal modes overcome modal dispersion induced by modal coupling in mode division multiplexing operation, even for multi-mode-fibers guiding a large number of modes, if the product of modulation bandwidth, fiber length and differential group delay is equal or less than one in each transmission channel If this condition is not sustained, modal dispersion and crosstalk at the receiver limit the transmission performance, setting very high constraints towards modal coupling

Perspectives of principal mode transmission in mode-division-multiplex operation

A fast, accurate and simple field coupling model is presented which is capable of describing mode coupling effects due to bends and splices in multimode fibers with parabolic index profile as well as the coupling losses induced by this process. This model is validated numerically by comparing the results to the well-known coupled amplitude theory model yielding the same relative bandwidth increase behavior as long as the coupling losses are the same. It is shown, that the number of discrete segments used in this model can be reduced considerably as long as the coupling losses are kept constant. The effect of mode coupling on the differential group delay, mode dependent loss, bandwidth gain and impulse response width reduction are analyzed. It is shown that the relative bandwidth gain induced in MMF links induced by the coupling process is independent of fiber parameters or number of guided modes; it can be fully characterized by coupling induced losses. The model is compared to well-known results given by power coupling models and a good agreement is observed for high steady state loss values.

Modeling of Mode Coupling in Multimode Fibers With Respect to Bandwidth and Loss

We derive analytical expressions for the performance of a long-haul DWDM communication system employing in-line phase insensitive fiber optic parametric amplifiers (FOPA). In our analysis the impairments of both transmission fibers and FOPAs are taken into account. The analytical model is verified with simulations. The performance of FOPA is compared with EDFA and it is shown that a broad band long-haul DWDM transmission system based on FOPAs as in-line amplifiers could be feasible.

Stefan Warm

Papers

Splice loss requirements in multi-mode fiber mode-division-multiplex transmission links.

Analytical Description of Cross-Modal Nonlinear Interaction in Mode Multiplexed Multimode Fibers

Perspectives of principal mode transmission in mode-division-multiplex operation

Modeling of Mode Coupling in Multimode Fibers With Respect to Bandwidth and Loss

Performance Evaluation of DWDM Communication Systems With Fiber Optical Parametric Amplifiers