Modulation instabilities in nonlinear two-core optical fibers with fourth order dispersion.

We study in detail the modulation instabilities (MIs) in nonlinear two-core optical fibers (TCFs) in the presence of fourth-order dispersion (FOD) by considering the symmetric and antisymmetric CW states. The analytical instability gain spectra with the FOD are shown explicitly. For the symmetric CW state, in the normal (anomalous) dispersion regime, the positive (negative) FOD reduces the original MI band, and the negative (positive) FOD can generate a completely new dominate MI band in the very high frequency range even with quite small amplitude of FOD for typical TCF parameters. For the antisymmetric CW state, in the normal dispersion regime, there is no MI without FOD. With the negative FOD, MI is generated at any given total powers. In the anomalous dispersion regime, being similar with the behaviors for symmetric CW state, the positive (negative) FOD effect is to reduce the original MI band, and the negative (positive) FOD can generate a completely new dominate MI band in the very high frequency range. We also verify our MI analysis by solving the coupled-mode equations numerically. [ABSTRACT FROM AUTHOR]