  We revisit on rational solution of Sasa-Satsuma equation, which can be used
to describe evolution of optical field in a nonlinear fiber with some
high-order effects. We find a striking dynamical process which involves both
modulational instability and modulational stability regimes, in contrast to the
rogue waves and W-shaped soliton reported before which involves modulational
instability and stability respectively. It is demonstrated that stable W-shaped
solitons can be generated from a weak modulation signal on continuous wave
background. This provides a possible way to obtain stable high-intensity pulse
from low-intensity continuous wave background.
