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.

/pdf/space-division-multiplexing-in-optical-fibres-9acl86j916.pdf

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

The invention relates to a method for monitoring the transmission quality of an optical transmission system, notably of an optical wavelength-division multiplex network in which an amplitude histogram of an optical signal transmitted via said transmission system is recorded and classified by means of a neuronal net by bit error rates and/or cause of interference. The invention does not make demands on the transmission system as regards the transmission mode, format and/or cycle but can be carried out with any signals. The invention also makes it possible to attribute interference to causes of interference which cannot be detected by conventional bit rate classification.

Method for monitoring the transmission quality of an optical transmission system, notably an optical wavelength-division multiplex network

A dual-link self-homodyne receiver is used to measure the phase correlations introduced by the interplay of dispersion and fiber nonlinearities. The dependence of the memory on the WDM setup, signal power and transmission distance is experimentally demonstrated.

http://publications.lib.chalmers.se/records/fulltext/246177/local_246177.pdf

Experimental Analysis of Correlations in the Nonlinear Phase Noise in Optical Fiber Systems

Mutual information is experimentally investigated for long-haul coherent transmission. Receivers that consider memoryless four-dimensional noise distributions can achieve significantly higher rates than receivers assuming two-dimensional symmetric distributions.

https://publications.lib.chalmers.se/records/fulltext/223906/local_223906.pdf

Four-dimensional estimates of mutual information in coherent optical communication experiments

We describe how simulations based on the Split-Step Fourier Method can be significantly accelerated by using commodity graphics hardware. Results for a benchmark scenario are provided to highlight performance and accuracy.

Fast implementation of the Split-Step Fourier Method using a graphics processing unit

This study reviews key technologies of next generation wavelength division multiplexing passive optical networks (WDM-PONs). The authors have studied WDM-PONs with centralised lightwave source and direct detection, where a wavelength-reuse system is employed to transmit the uplink data by using a colourless transmitter at the optical network unit (ONU). WDM-PON system was demonstrated using a Fabry-Perot laser diode as a colourless transmitter at the ONU for differential phase-shift keying (DPSK) downlink signals and on-off keying uplink signals. The constant-amplitude feature of the DPSK modulation could retain a great extinction ratio of the downlink signal and decrease the noise to the uplink signal. The proposed scheme offers a promising solution for future PONs.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-opt.2018.5056

Norbert Hanik

Papers

Method for monitoring the transmission quality of an optical transmission system, notably an optical wavelength-division multiplex network

Experimental Analysis of Correlations in the Nonlinear Phase Noise in Optical Fiber Systems

Four-dimensional estimates of mutual information in coherent optical communication experiments

Fast implementation of the Split-Step Fourier Method using a graphics processing unit

Technologies for future wavelength division multiplexing passive optical networks