  Ultrafast fiber lasers play important roles in many aspects of our life for
their various applications in fields ranging from fundamental sciences to
industrial purposes. Passive mode-locking technique is a key step to realize
the ultrafast soliton fiber lasers. However, the booting dynamics of the
soliton fiber lasers have not yet been well understood. Herein, we reveal the
soliton buildup dynamics of ultrafast fiber lasers operating both in anomalous
and net-normal dispersion regimes. Based on the advanced experimental
methodologies of spatio-temporal reconstruction and dispersive Fourier
transform (DFT), the soliton booting dynamics are analyzed in the time and
spectral domains. It was found that the booting dynamics of conventional and
dissipative solitons operating in the anomalous and net-normal dispersion
regimes, respectively, are different from each other due to the different pulse
shaping mechanisms. In particular, the spectral interference pattern with
strong relaxation oscillation behavior was observed near the mode-locking
transition for conventional soliton, while no relaxation oscillation of
spectral pattern was obtained for dissipative soliton. We firstly revealed that
the spectral pattern distributions are induced by the transient structured
soliton formation during the pulse shaping from the noise background. The
experimental results were verified by the theoretical simulations. The obtained
results would provide a general guideline for understanding the soliton booting
dynamics in ultrafast fiber lasers, and will prove to be fruitful to the
various communities interested in solitons and fiber lasers.
