W-shaped solitons generated from a weak modulation in the Sasa-Satsuma equation

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.