We study analytically and numerically the modulation instability (MI) of two incoherently co-propagating optical pulses in nonlinear fiber, taking into account high-order nonlinearity, walk-off effects, delay response time, and cross-phase modulation in several dispersion regimes. We show that higher-order nonlinearity is responsible for superposition of the secondary MI spectrum. We realize that there is a peak located in the emerged Raman band that we call “Raman peak” due to the combined effects of third-order dispersion, delay response time, and the opposite sign of the fourth-order dispersion parameter. These peaks are shifted toward higher frequencies when the walk-off increases.

中文翻译：

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在非瞬时Kerr介质中由于走离而产生的拉曼峰移动具有更高阶的影响

我们在分析和数值上研究了非线性光纤中两个非相干共传播的光脉冲的调制不稳定性（MI），同时考虑了高阶非线性，走离效应，延迟响应时间和几种色散状态下的交叉相位调制。我们表明，高阶非线性是次生MI谱叠加的原因。我们意识到，由于三阶色散，延迟响应时间以及四阶色散参数的相反符号的共同作用，出现在出现的拉曼频带中的一个峰被称为“拉曼峰”。当步距增加时，这些峰值将移向更高的频率。