Kerr optical frequency combs generated in a coherently driven Kerr nonlinear resonator has the potential for a wide range of applications. However, in a single cavity which is a widely adopted configuration for Kerr optical frequency combs generation, modulation instability is suppressed in normal dispersion regime and the pump-to-comb conversion efficiency is extremely low for a single dissipative Kerr soliton in anomalous dispersion regime. Dual-coupled cavities have been proposed to generate Kerr optical frequency combs in normal dispersion regime, and have potential to remarkably increase conversion efficiency for Kerr optical frequency combs. Here, we investigate modulation instability and Kerr optical frequency-comb formation in dual-coupled cavities. Based on solutions of the continuous-wave steady state, we obtain a quadric algebraic equation describing the modulation instability gain, and we find that it is intensely influenced by the group velocity mismatch between the two cavities. Our numerical simulations demonstrate that platicons can be generated via pump scanning scheme for the case that both the two cavities possess normal dispersion, and a single dissipative Kerr soliton can be generated in the cavity with anomalous dispersion while the dispersion of the other cavity is normal. Our analysis of modulation instability provides a powerful tool for Kerr optical frequency-comb generation via pump modulation and cavity detuning tuning scheme in dual-coupled cavities.
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