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In this paper we theoretically and experimentally demonstrate a novel adaptation of independent component analysis (ICA) for compensation of both cross-polarization and inter-symbol interference in a direct-detection link using Stokes vector modulation (SVM). SVM systems suffer from multiple simultaneous impairments that can be difficult to resolve with conventional optical channel DSP techniques. The proposed method is based on a six-dimensional adaptation of ICA that simultaneously de-rotates the SVM constellation, corrects distortion of constellation shape, and mitigates inter-symbol interference (ISI) at high symbol rates. Experimental results at 7.5 Gb/s and 15Gb/s show that the newly developed ICA-based equalizer achieves power penalties below ∼1 dB, compared to the ideal theoretical bit-error rate (BER) curves. At 30-Gb/s, where ISI is more severe, ICA still enables polarization de-rotation and BER < 10⁻⁵before error correction. 

A novel non-cubic constellation for eightfold Stokes vector modulation improves modulation loss, link budget, and intersymbol interference at high speed, while using simpler drive signals. Experiments confirm 5.2 dB improvement at 30 Gb/s. 

Abstract: A novel adaptation of independent component analysis controls both cross-polarization and inter-symbol interference in a direct-detection link using Stokes vector modulation. 30-Gb/s experiments confirm polarization de-rotation and near-error-free transmission. 

The nonlinear Gaussian-noise (GN) model is a useful analytical tool for the estimation of the impact of distortion due to Kerr nonlinearity on the performance of coherent optical communications systems with no inline dispersion compensation. The original nonlinear GN model was formulated for coherent optical communications systems with identical single-mode fiber spans. Since its inception, the original GN model has been modified for a variety of link configurations. However, its application to coherent optical communications systems with hybrid fiber spans, each composed of multiple fiber segments with different attributes, has attracted scarcely any attention. This invited paper is dedicated to the extended nonlinear GN model for coherent optical communications systems with hybrid fiber spans. We review the few publications on the topic and provide a unified formalism for the analytical calculation of the nonlinear noise variance. To illustrate the usefulness of the extended nonlinear GN model, we apply it to coherent optical communications systems with fiber spans composed of a quasi-single-mode fiber segment and a single-mode fiber segment in tandem. In this configuration, a quasi-single-mode fiber with large effective area is placed at the beginning of each span, to reduce most of the nonlinear distortion, followed by a single-mode fiber segment with smaller effective-area, to limit the multipath interference introduced by the quasi-single-mode fiber to acceptable levels. We show that the optimal fiber splitting ratio per span can be calculated with sufficient accuracy using the extended nonlinear GN model for hybrid fiber spans presented here.