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

Dispersion-managed long-haul optical WDM-links operated at high channel-data-rates are ultimately limited in non-regenerated reach due to accumulated ASE-noise and eventually due to polarisation-mode-dispersion (PMD) of the optical fiber material. In this contribution a method is presented that permits a relatively fast assessment of the worst-case performance of optical transmission systems subject to PMD.

Worst case simulation of polarisation mode dispersion in long-haul optical transmission

We study the feasibility and robustness of soliton-based fiber optic transmission systems, with a maximum of five discrete eigenvalues, over Raman amplified optical fiber. The realistic system-level simulation takes the following perturbations into account: Quantization distortion of DAC, non-ideal MZM, IQ imbalance, filters, laser phase noise, residual fiber loss with distributed Raman amplification, polarization coupling and polarization mode dispersion. The receiver DSP includes: Channel estimation, MIMO equalization and phase noise compensation. Taking into account all the system impairments stated above, we realize a robust eigenvalue on-off keying (OOK) transmission scheme for transmission over up to 1000km SSMF, with a net data rate of 1.30Gbps. We subsequently characterize the system performance by the resulting BER and the variance of the detected eigenvalues.

Soliton Transmission with 5 Eigenvalues over 2000km of Raman-Amplified Fiber

Solving the time-dependent power-flow equation using the Crank-Nicholson scheme

Fiber nonlinearity compensation via expectation maximization is investigated for PDM CO-OFDM. An improvement in Q-factor of 0.33 dB is observed for 1400-km 100 Gb/s single channel and 1.44 dB for 1000-km 700 Gb/s WDM transmission.

Fiber nonlinearity compensation for dispersion unmanaged PDM 8-QAM CO-OFDM using expectation maximization

Optical transmission is mainly influenced by attenuation, chromatic dispersion, nonlinearities and polarization mode dispersion of the fibre. For repeater-free high speed and long haul WDM transmission systems a broadband compensation of chromatic dispersion is required. One of the most effective methods to perform this, is the use of dispersion compensating fibres (DCF) 4/spl times/2.5 Gbit/s field transmission experiments were demonstrated over 135 km and a single channel transmission over 4000 km was performed in a recirculating loop testbed. A replica of the Berlin-Darmstadt field transmission line was realized in a laboratory experiment and four 2.5 Gbit/s WDM channels with directly modulated transmitter lasers were transmitted over 750 km. We present the results of field installation of four WDM channels on the Berlin-Darmstadt fibre link, with full dispersion compensation. The link is transparent for all applied data rates up to 10 Gbit/s and wavelengths in the amplifying bandwidth of fibre amplifiers. A 1.3 /spl mu/m supervisory channel was installed and processed by a line service unit within the fibre amplifiers (EDFA). The link is designed for test applications of commercially available WDM-systems and components.

Norbert Hanik

Papers

Worst case simulation of polarisation mode dispersion in long-haul optical transmission

Soliton Transmission with 5 Eigenvalues over 2000km of Raman-Amplified Fiber

Solving the time-dependent power-flow equation using the Crank-Nicholson scheme

Fiber nonlinearity compensation for dispersion unmanaged PDM 8-QAM CO-OFDM using expectation maximization

Testbed for WDM field trials over 750 km with full compensation of chromatic dispersion