Nondegenerate Akhmediev breathers and abnormal frequency jumping in multicomponent nonlinear Schr\"odinger equations

Nonlinear stage of higher-order modulation instability (MI) phenomena in the frame of multicomponent nonlinear Schr\"odinger equations (NLSEs) are studied analytically and numerically. Our analysis shows that the $N$-component NLSEs can reduce to $N-m+1$ components, when $m(\leq N)$ wavenumbers of the plane wave are equal. As an example, we study systematically the case of three-component NLSEs which cannot reduce to the one- or two-component NLSEs. We demonstrate in both focusing and defocusing regimes, the excitation and existence diagram of a class of nondegenerate Akhmediev breathers formed by nonlinear superposition between several fundamental breathers with the same unstable frequency but corresponding to different eigenvalues. The role of such excitation in higher-order MI is revealed by considering the nonlinear evolution starting with a pair of unstable frequency sidebands. It is shown that the spectrum evolution expands over several higher harmonics and contains several spectral expansion-contraction cycles. In particular, abnormal unstable frequency jumping over the stable gaps between the instability bands are observed in both defocusing and focusing regimes. We outline the initial excitation diagram of abnormal frequency jumping in the frequency-wavenumber plane. We confirm the numerical results by exact solutions of multi-Akhmediev breathers of the multi-component NLSEs.