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

Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

Differential-phase-shift keying (DPSK) has recently been used to reach record distances in long-haul lightwave communication systems. This paper will review theoretical, as well as implementation, aspects of DPSK, and discuss experimental results.

Optical phase-shift-keyed transmission

We present theoretical and simulation results for the performance of asymmetrically-clipped optical OFDM (ACO-OFDM) and DC-biased optical OFDM (DCO-OFDM) in AWGN for intensity-modulated direct-detection systems. Constellations from 4 QAM to 1024 QAM are considered. For DCO-OFDM, the optimum bias depends on the constellation size which limits its performance in adaptive systems. ACO-OFDM requires less optical power for a given data rate than DCO-OFDM for all but the largest constellations and is better suited to adaptive systems as the same structure is optimum for all constellations.

Comparison of Asymmetrically Clipped Optical OFDM and DC-Biased Optical OFDM in AWGN

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

Advanced Modulation Formats for Electronic Predistortion of Intrachannel Nonlinearities at 40 Gbit/s

Long-Haul 10 Gb/s WDM transmission over standard singlemode fiber, NZDSF and mixed fiber infrastructure is experimentally demonstrated. For optimized operation parameters 2500 km error-free transmission is feasible for all scenarios without use of FEC and Raman-amplification.

https://ieeexplore.ieee.org/iel5/10668/33653/01601249.pdf

WDM-Transmission Over Mixed Fiber Infrastructures

We experimentally study different four-dimensional demappers in a dispersion-managed fiber system. The proposed blind algorithm is shown to offer gains of 0.2 bits per 4D symbol for DP-16QAM.

Improved achievable information rates by optimized four-dimensional demappers in optical transmission experiments

Probabilistic shaping of quadrature amplitude modulation (QAM) is used to enhance the sensitivity of an optical communication system. Sensitivity gains of 0.43 dB and 0.8 dB are demonstrated in back-to-back experiments by shaping of 16QAM and 64QAM, respectively. Further, numerical simulations are used to prove the robustness of probabilistic shaping to a mismatch between the constellation used and the signal-to-noise ratio (SNR) of the channel. It is found that, accepting a 0.1 dB SNR penalty, only four shaping distributions are required to support these gains for 64QAM.

Demonstration of Sensitivity Gains by Probabilistic Shaping for Optical Communication Systems

OFDM requires the lowest equalizer complexity for crosstalk compensation in a mode-division-multiplexing receiver. For a 2000-km transmission distance and less than 10% OFDM-specific overhead, the modal dispersion must be below 6 ps/km for 10×10 MIMO.

Norbert Hanik

Papers

Advanced Modulation Formats for Electronic Predistortion of Intrachannel Nonlinearities at 40 Gbit/s

WDM-Transmission Over Mixed Fiber Infrastructures

Improved achievable information rates by optimized four-dimensional demappers in optical transmission experiments

Demonstration of Sensitivity Gains by Probabilistic Shaping for Optical Communication Systems

DSP requirements for MIMO spatial multiplexed receivers