Internal motions of harmonically mode-locked soliton molecules in a NPR based fiber laser

Abstract Ultrafast fiber laser as a dissipative nonlinear system is capable of generating various dynamical soliton molecules, providing an abundant platform for investigating the transient soliton dynamics. Here, we report on the real-time internal motions of harmonically mode-locked soliton molecules (HMLSMs) in a passively mode-locked fiber laser with nonlinear polarization rotation (NPR). Assisted with time-stretch dispersive Fourier transform (DFT), various real-time scenarios towards multi-pulse states are revealed. Particularly, both the stationary and evolving HMLSMs are respectively observed, indicating that each soliton molecule in one roundtrip possesses identical intra-molecular motions. These vivid internal motions are mainly dominated by the underlying interactions and energy flow between the adjacent constituents. All these findings provide an intriguing perspective in dynamical intra-molecular evolution of ultrafast nonlinear phenomena and further enrich the research of fundamental science.