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

We demonstrate transmission of 10 Gbit/s over 220 m perfluorinated graded-index polymer optical fiber (PFGI-POF) with a core diameter of 120 µm using a pulse amplitude modulation with 4 levels (PAM-4) and feed-forward (FFE) or decision-feedback (DFE) equalization.

10 Gbit/s transmission over 220 m perfluorinated graded-index polymer optical fiber using PAAM-4 modulation and simple equalization schemes

http://ieeexplore.ieee.org/iel5/5757121/5757122/05758497.pdf

Numerical and Experimental Investigation of the Effect of Dispersion on Nonlinear Phase Noise in RZ-DPSK Systems

One of the major impairments in high-speed optical transmission links is Polarization-Mode Dispersion (PMD). We propose the method of electronic predistortion (EPD) for the mitigation of PMD. This approach has already been successfully applied for the compensation of Chromatic Dispersion (CD) and Fiber-Nonlinearities. The advantage of this method is that impairments can efficiently be mitigated without the need for coherent reception. The proposed scheme is based on the possibility to control the optical field at the transmitter by using two complex modulators for the modulation of two orthogonally polarized optical signals. If the physical origin of PMD is exactly known then the ideal predistorted field and the corresponding electrical driving signals can be computed accurately. In practice, however, this information is not available. Therefore it is shown how to determine appropriate driving signals for a set of measured PMD parameters. Measurements will be communicated through a feedback channel in practice. We suggest a possible strategy for application of this technique in scenarios, in which the adaptation speed is intrinsically limited due to the round-trip delay. Numerical simulations reveal that the use of EPD can significantly increase the tolerance towards PMD in comparison to a system without compensation.

Electronic predistortion for compensation of polarization-mode dispersion

Digital Subcarrier Multiplexing allows simple direct detection of multicarrier signals. We show, by means of simulations, that by differentially encoding in symbol-wise manner, the tolerance to both, nonlinearities and dispersion, is remarkably improved.

Block- vs. Symbol-wise Differential Encoding in Spectrally Efficient Digital Subcarrier Multiplexing for Direct Detection

OFDM is an interesting candidate to meet the requirements of high flexibility and speed for the next generation optical access networks. We show an OFDM system utilizing a timing advance tracking algorithm to realize a synchronized upstream transmission. Therefore, a dynamic bandwidth allocation can be used in the upstream and an interleaved assignment is made possible. We then demonstrate the feasibility in a field trial with four individual ONUs using a 37.5 km field deployed feeder fiber of the Deutsche Telekom AG. Power loading and the trellis coded modulation was used to achieve a power budget supporting 32 ONUs for up- as well as downstream transmission.

http://www.tf.uni-kiel.de/etit/NT/download/publications/ruprecht_icton2014a.pdf

Norbert Hanik

Papers

10 Gbit/s transmission over 220 m perfluorinated graded-index polymer optical fiber using PAAM-4 modulation and simple equalization schemes

Numerical and Experimental Investigation of the Effect of Dispersion on Nonlinear Phase Noise in RZ-DPSK Systems

Electronic predistortion for compensation of polarization-mode dispersion

Block- vs. Symbol-wise Differential Encoding in Spectrally Efficient Digital Subcarrier Multiplexing for Direct Detection

Field trial of an OFDMA-PON system