Interaction of strong pulse laser radiation with underdense plasma may show strong reflectivity sometimes modulated with time. Stimulated ion-Compton or Brillouin (in the strong ion damping limit) backscattering explains the red-shifted backscattered spectrum observed. We show that the general nonlinear and spectral dependent problem may also explain the satu-ration of the reflectivity at finite values by pump depletion, as well as time modulation. Space, time and frequency evolutions of the incident and backscattered radiation spectra are governed by nonlinear integrodifferential equations. Numerical computation for a finite homogeneous or inhomogeneous plasma slab are performed and compared to some analytical approaches.