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<p>Modulation instability (MI) is a fundamental process that drives pattern formation and soliton generation in nonlinear optics. In this work, we investigate MI in oppositely oriented couplers with a negative-index channel, incorporating the combined effects of nonlocal and high-order non-Kerr nonlinearities (specifically, quintic, septic, and nonic). Using linear stability analysis, we derive the corresponding MI gain spectrum and show that, in the normal dispersion regime, a distinct base band instability emerges, producing non-zero gain even at zero modulation frequency, in contrast to the anomalous dispersion regime. Crucially, we demonstrate that the synergy between nonlocality and higher-order nonlinearities not only creates novel instability bands but also provides a powerful mechanism for the precise control of MI. These emergent bands are shown to facilitate the generation of soliton trains and ultrashort pulses. Furthermore, nonlocality is found to significantly enhance the MI gain spectrum under various normal dispersion and nonlinearity conditions. Our findings establish that tailored non-Kerr nonlocality can profoundly enhance nonlinear excitation in higher-order systems, paving the way for advanced applications in fiber lasers and supercontinuum generation.</p>

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Influence of non-Kerr and nonlocal nonlinearities on instability gain spectrum in oppositely oriented coupler with negative refractive material channel

  • 期刊论文

  • 发表日期：

    2026年4月17日

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  • Emonisha Rajamani,
  • Sivakumar Rajagopalan,
  • Mohanraj Palani

摘要

Modulation instability (MI) is a fundamental process that drives pattern formation and soliton generation in nonlinear optics. In this work, we investigate MI in oppositely oriented couplers with a negative-index channel, incorporating the combined effects of nonlocal and high-order non-Kerr nonlinearities (specifically, quintic, septic, and nonic). Using linear stability analysis, we derive the corresponding MI gain spectrum and show that, in the normal dispersion regime, a distinct base band instability emerges, producing non-zero gain even at zero modulation frequency, in contrast to the anomalous dispersion regime. Crucially, we demonstrate that the synergy between nonlocality and higher-order nonlinearities not only creates novel instability bands but also provides a powerful mechanism for the precise control of MI. These emergent bands are shown to facilitate the generation of soliton trains and ultrashort pulses. Furthermore, nonlocality is found to significantly enhance the MI gain spectrum under various normal dispersion and nonlinearity conditions. Our findings establish that tailored non-Kerr nonlocality can profoundly enhance nonlinear excitation in higher-order systems, paving the way for advanced applications in fiber lasers and supercontinuum generation.