  We report the experimental demonstration of modulation instability process
assisted by a dispersion grating in an optical fiber. A simple analytical model
is developed to further analyze and explain the complex dynamics of this
process, showing that each of the multiple spectral components grows thanks to
a quasi phase-matching mechanism inherent to the periodicity of the waveguide
parameters. This model is confirmed by numerical simulations and it is
successfully used to tailor the multi-peak modulation instability spectrum
shape. These theoretical predictions are confirmed by experiments.
