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

In this work we investigate the optimum design parameters of a bit-true computationally-efficient RF-Pilot-tone implementation. For a 23.9-Gb/s receiver an implementation penalty of less than 0.6dB is observed using only 27% of Virtex5 resources.

Computationally efficient hardware design of RF-pilot tone based phase noise compensation for optical OFDM

We present a phase noise canceler to improve the accuracy of transmitter I/Q skew estimation for high baud-rate optical transponders using high-order modulation formats. The proposed technique reduces the probability of failure due to estimation errors by an order of magnitude.

Accurate Estimation of Transmitter I/Q Skew for High-Rate Spectrally Efficient Optical Transponders

We analyze a novel strategy to reduce the computational complexity of the back-propagation method with negligible system performance degradation considering different fiber types.

Reduced complexity for back-propagation method algorithm

In We model the third-order material susceptibility $\overleftrightarrow \chi $[3] in silicon waveguides for integrated optics. Analysis of four- wave mixing in these waveguides requires an in-depth study of material nonlinearity - in contrast to modeling light propagation in fibers with the optical nonlinear Schrodinger equation. We include electronic and atomic lattice (Raman) responses of the material and present a relatively easy-to-use representation of the material susceptibility.

Modeling Material Susceptibility in Silicon for Four-Wave Mixing Based Nonlinear Optics

Ultra-Broadband Wavelength Conversion is one of the key issues of future high-capacity, flexible optical networks. Using optimized Multi-Modal Optical Waveguides with high cubic nonlinearity, broadband wavelength conversion between extreme optical wavelength-bands has been achieved. In this contribution, the physical background of ultra-broadband optical wavelength-conversion in a highly nonlinear multi-modal Silicon-On-Insulator (SOI-) waveguide and methods to model and optimize its functionality are outlined. Finally, experimental results are presented that prove ultra-broadband conversion of data signals from the C-to O-band of silica fibers at an OSNR-penalty of less than 0.4 dB.

Norbert Hanik

Papers

Computationally efficient hardware design of RF-pilot tone based phase noise compensation for optical OFDM

Accurate Estimation of Transmitter I/Q Skew for High-Rate Spectrally Efficient Optical Transponders

Reduced complexity for back-propagation method algorithm

Modeling Material Susceptibility in Silicon for Four-Wave Mixing Based Nonlinear Optics

Ultra-Broadband Optical Wavelength-Conversion using Nonlinear Multi-Modal Optical Waveguides