Period-doubling bifurcation in spatiotemporal mode-locked lasers

Period-doubling bifurcation is a universal dynamic of nonlinear systems, which has been extensively investigated in laser systems with a single transverse mode. This study presents an experimental observation and theoretical investigation of the period-doubling bifurcation in spatiotemporal mode-locked (STML) multimode fiber lasers. In the period-doubling state, it is observed that the pulse train modulation varies with the transverse-mode, and the output beam profile fluctuates rapidly and periodically. The numerical simulations conducted in this study are in good agreement with the experimental observations. Furthermore, a simple iterative model is proposed by considering the mode-dependent saturable absorption effect, and the experimental results can be qualitatively interpreted by this model. Based on these results, spatiotemporal saturable absorption is believed to be the key factor for the unique spatiotemporal characteristic of period-doubling bifurcation 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.