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

A sampling frequency offset compensation based on pilot-aided phase noise mitigation scheme is proposed. SFO tolerance up to 1000 ppm for CO-OFDM is shown. An estimation deviation of 0.5 ppm is observed.

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Pilot-aided sampling frequency offset compensation for coherent optical OFDM

We use a global optimization algorithm to find the optimum fiber launch powers and dispersion map of single-channel RZ-DPSK and RZ-DQPSK systems at data rates ranging from 5 Gbit/s to 130 Gbit/s.

Global optimization of RZ-DPSK and RZ-DQPSK systems at various data rates

Different pulse shapes are presented as alternative to traditional (N)RZ pulses in communication systems using direct detection. The pulses are efficiently generated in the frequency domain using the discrete Fourier transform. The trade-off between spectral occupancy and eye opening is discussed and performance is evaluated by means of simulations.

Pulse shaping using the discrete fourier transform for direct detection optical systems

In optical wavelength-division multiplexed (WDM) transmission, nonlinear coupling of co-propagating modulated light waves due to cross phase modulation (XPM) is one of the main sources of signal degradation. The strength of the XPM effect, in turn, is strongly influenced by the evolution of light-polarisations of the WDM-carriers. In this contribution, physical models that numerically evaluate system degradation due to XPM, assuming various scenarios of the evolution of light polarisations, are introduced and applied to WDM transmission in an optimised optical link. Finally these system simulations are compared to re-circulating loop experiments.

Norbert Hanik

Papers

Pilot-aided sampling frequency offset compensation for coherent optical OFDM

Global optimization of RZ-DPSK and RZ-DQPSK systems at various data rates

Pulse shaping using the discrete fourier transform for direct detection optical systems

Influence of polarisation mode dispersion on the effect of cross phase modulation in optical WDM transmission

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