Dynamics of soliton molecules in a normal-dispersion fiber laser

Soliton molecules have been extensively investigated in mode-locked anomalous-dispersion fiber lasers by virtue of real-time measurement technologies. However, in the normal-dispersion regime, it is unclear how a soliton molecule evolves. Here, we first present, to the best of our knowledge, experimental investigations on the dynamics of soliton molecules in the normal-dispersion regime. We observe aperiodical evolutions of two dissipative solitons within a soliton molecule. They exchange energies during propagation. In particular, the large dissipative soliton splits when energy exchange occurs. Rogue waves are first revealed in the spectral domain of soliton molecules. High-resolution time-stretch technique, field autocorrelation analysis, and spatiotemporal intensity measurements allow us to reveal these critical behaviors of soliton molecules.