Modulational instability induced by mode perturbation in normal-dispersion microresonators

Mode coupling has been proven to be an effective method for initiating modulational instability (MI) in a normal-dispersion microresonator. In various mode-coupling systems, mode loss perturbation occurs alongside mode frequency perturbation. In this study, we present a detailed analysis of MI induced by either loss or frequency perturbations added to the pump mode of a normal-dispersion cavity. By distinguishing the individual contributions of frequency and loss perturbations to MI gain, we demonstrate that, in addition to the widely studied frequency perturbation, the introduction of an additional loss to the pump mode can also induce MI even without frequency perturbation, enabling the spontaneous excitation of frequency combs. The effects of pump mode perturbations in various mode-coupling schemes are compared, revealing that loss perturbation has a more significant impact when the pump power is relatively low or when the auxiliary mode experiences a sufficiently higher loss than the main mode. Our findings provide an alternative perspective on illustrating the underlying mechanisms of normal-dispersion Kerr frequency combs generation aided by mode perturbation.