Revelation of the birth and extinction dynamics of solitons in SWNT-mode-locked fiber lasers

The dispersive Fourier transform (DFT) technique opens a fascinating pathway to explore ultrafast non-repetitive events and has been employed to study the build-up process of mode-locked lasers. However, the shutting process for the mode-locked fiber laser seems to be beyond the scope of researchers, and the starting dynamics under near-zero dispersion remains unclear. Here, the complete evolution dynamics (from birth to extinction) of the conventional soliton (CS), stretched pulse (SP), and dissipative soliton (DS) are investigated by using the DFT technique. CS, SP, and DS fiber lasers mode locked by single-walled carbon nanotubes (SWNTs) are implemented via engineering the intracavity dispersion map. The relaxation oscillation can always be observed before the formation of stable pulse operation due to the inherent advantage of SWNT, but it exhibits distinct evolution dynamics in the starting and shutting processes. The shutting processes are dependent on the dispersion condition and turn-off time, which is against common sense. Some critical phenomena are also observed, including transient complex spectrum broadening and frequency-shift interaction of SPs and picosecond pulses. These results will further deepen understanding of the mode-locked fiber laser from a real-time point of view and are helpful for laser design and applications. (C) 2019 Chinese Laser Press