Modulation instability, Cherenkov radiation, and Fermi-Pasta-Ulam recurrence

We study, numerically, the influence of third-order dispersion (TOD) on modulation instability (MI) in optical fibers described by the extended nonlinear Schrödinger equation. We consider two MI scenarios. One starts with a continuous wave (CW) with a small amount of white noise, while the second one starts with a CW with a small harmonic perturbation at the highest value of the growth rate. In each case, the MI spectra show an additional spectral feature that is caused by Cherenkov radiation. We give an analytic expression for its frequency. Taking a single frequency of modulation instead of a noisy CW leads to the Fermi-Pasta-Ulam (FPU) recurrence dynamics. In this case, the radiation spectral feature multiplies due to the four-wave mixing process. FPU recurrence dynamics is quite pronounced at small values of TOD, disappears at intermediate values, and is restored again at high TOD when the Cherenkov frequency enters the MI band. Our results may lead to a better understanding of the role of TOD in optical fibers.