Real-time access to the coexistence of soliton singlets and molecules in an all-fiber laser.

Recent progress in time-stretch spectroscopy accelerates the exploration of ultrafast dynamics in mode-locked lasers through mapping the spectral information into time domain. Here, we report on real-time access to the coexistence of soliton singlets and molecules in an all-fiber laser mode-locked by a 45° tilted fiber grating. By virtue of the dispersive Fourier transform process, spectral information of the pulse trains under multi-pulse states can be resolved. It is identified that soliton singlets and soliton molecules coexist in one cavity roundtrip with different assembling forms. In addition, consecutive recordings of the shot-to-shot spectra further enable insight into the transient dynamics of soliton molecules. Particularly, varieties of internal motions, including the diverging/oscillating phase evolution and the temporal separation vibration, are validated loosely bound intra-soliton molecules. All of these findings unveil the scenarios of multi-soliton phenomena in fiber lasers, as well as highlight the significance of pulse interaction towards both scientific research and practical applications.