Modulation instability generation with blue-detuned pump laser in coupled microcavities

Optical frequency combs based on microcavities with Kerr nonlinearity are promising frequency comb sources for many applications. A typical Kerr soliton comb is generated in a nonlinear microcavity with anomalous dispersion pumped by a red-detuned continuous-wave laser. Modulation instability (MI) is the basis for Kerr soliton comb generation. In a microcavity with nearly zero dispersion, the first pair of MI modes can grow only with a red-detuned pump laser. In this paper, we find that MI generation is possible with blue-detuned pump lasers for coupled microcavities with nearly zero dispersion. We study a microcavity with Kerr nonlinearity coupled with an auxiliary microcavity, which has negligible nonlinearity. By theoretical analysis, we show that the coupled microcavities can create a region supporting MI generation in the blue-detuned side of the resonances of the nonlinear main cavity, whereas there is no blue-detuned MI generation in a single nonlinear microcavity. The properties of the blue-detuned MI region are determined by the coupling coefficient between the two microcavities, the loss of the auxiliary cavity, and the detuning between the modes of the two microcavities. The size and location of the blue-detuned MI region can be varied by tuning these parameters. Numerical simulations of MI generation based on the blue-detuned MI region in the coupled microcavities are presented. By considering more modes, MI comb generations with coupled microcavities having anomalous and normal dispersion are also numerically simulated.