  We show how the recurrence phenomenon characteristic of the nonlinear stage
of induced modulational instability in a passive fiber is affected by forcing.
An additional linear amplification, even if extremely weak, induces separatrix
crossing in correspondence of critical values of the gain around which the
recurrence process considerably slows down, switching between dynamical orbits
of different kind. We present evidence for such phenomenon in a fiber optics
experiment where the gain is finely tuned by means of Raman amplification. A
theoretical explanation is also provided that matches almost perfectly with our
experimental results.
