Emission of multiple resonant radiations by spatiotemporal oscillation of multimode dark pulses

In this paper, we illustrate how the periodically modulated nonlinear parameter induced by the spatial beam oscillation can be used to generate broadband resonant radiations, through a train of dark pulses in normally dispersive graded-index multimode fibers under the efficient quasi-phase-matching schemes. More precisely, we demonstrate that two co-propagating waves with equal intensities and certain temporal delays can induce the formation of a train of dark solitons, with each emitting multiple resonant radiation lines, which can possibly form multiple radiation continuums based on vast amount of excited dark solitons. The nonlinear-interaction-aided excitation of dark pulses and their radiations appear to occur through a deterministic pathway, in sharp contrast to the situation for bright pulses in the anomalous dispersion region. The quasi-phase-matching condition via periodic oscillation of spatial beam in the normal-dispersion regime adds a unique dimension to the physical design of multimode waveguides, allowing the spectrum to be engineered for specific applications.