研究目的
To review and extend the study of nonlinear performance of few-mode fiber links in various linear coupling regimes and mode delay maps, aiming to find optimum configurations that minimize nonlinear penalties for practical training sequences and to establish the accuracy of different multimode nonlinear propagation approximations.
研究成果
The proposed distributed linear mode coupling model accurately captures nonlinear performance across various coupling strengths and mode delay maps. Optimum link configurations require coupling strengths above -10 dB/100m to suppress nonlinear distortion below single-mode levels. The distributed model is essential for accurate modeling in intermediate coupling regimes, outperforming lumped and Manakov approximations in terms of precision, especially for high differential mode delay scenarios.
研究不足
The study does not consider mode-dependent loss (MDL), assuming it is negligible due to advancements in spatial division multiplexing devices. The simulations are limited to specific fiber parameters and may not generalize to all fiber types. The step-size constraints in numerical methods could increase computational complexity, and the accuracy of approximations (e.g., Manakov equations) is only valid in extreme coupling regimes.
1：Experimental Design and Method Selection:
The study uses a modified split-step Fourier method (SSFM) for numerical simulations, incorporating semi-analytical solutions for linear mode coupling (LMC) of arbitrary strength. Nonlinear performance is estimated using four-wave-mixing theory and SSFM with symmetric steps for higher accuracy.
2：Sample Selection and Data Sources:
Simulations are based on a graded-index few-mode fiber with six linearly polarized modes, using parameters such as group delay, dispersion, attenuation, and nonlinear coefficients as detailed in the appendix.
3：List of Experimental Equipment and Materials:
The fiber characteristics include specific group delays, dispersion values, attenuation, and nonlinear coefficients for modes like LP01, LP02, etc. No specific external equipment brands or models are mentioned; the focus is on theoretical and numerical modeling.
4：Experimental Procedures and Operational Workflow:
The SSFM involves steps for dispersion, nonlinearity, and LMC, with step-size bounded to ensure accuracy (e.g., local error ≤10^-5, average crosstalk per step ≤-20 dB). System simulations include optical super-channels with 16-QAM signals, mode multiplexing, and digital signal processing for equalization.
5：Data Analysis Methods:
Nonlinear noise power is calculated analytically using generalized FWM theory and numerically via SSFM. Performance metrics like Q-factor are derived from error vector magnitude, with statistical averaging over multiple repetitions.
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