Spatiotemporal Period-Doubling Bifurcation in Mode-Locked Multimode Fiber Lasers

Spatiotemporal mode-locked (STML) lasers are ideal platforms for investigating high-dimensional nonlinear dynamics. This study presents an experimental observation and theoretical investigation of a novel high-dimensional nonlinear dynamics, which we term spatiotemporal period-doubling bifurcation (SPB), in the STML lasers. In the SPB state, not only the temporal shape but also the spatial beam (i.e., the spatiotemporal structure) of the pulses exhibit period-doubling bifurcation. As a result, it is observed that the pulse train modulation varies with the transverse mode, and the output beam profile fluctuates rapidly and periodically. Our numerical simulations are in good agreement with the experimental observations. In addition, a simple iterative model is presented to underline the physical insights of SPB, by considering the mode-dependent saturable absorption effect. Based on these results, spatial-dependent saturable absorption is believed to be the key factor for the unique spatiotemporal characteristic of SPB in multimode lasers. This study contributes to the understanding of the complex spatiotemporal dynamics in STML multimode lasers and to the discovery of novel dynamics in high-dimensional nonlinear systems.