  We experimentally and theoretically investigate the process of seeded
intermodal four-wave mixing in a graded index multimode fiber, pumped in the
normal dispersion regime. By using a fiber with a 100 micron core diameter, we
generate a parametric sideband in the C band (1530-1565 nm), hence allowing the
use of an Erbium-based laser to seed the mixing process. To limit nonlinear
coupling between the pump and the seed to low-order fiber modes, the waist
diameter of the pump beam is properly adjusted. We observe that the
superimposed seed stimulates the generation of new spectral sidebands. A
detailed characterization of the spectral and spatial properties of these
sidebands shows good agreement with theoretical predictions from the
phase-matching conditions. Interestingly, we demonstrate that both the second
and the fourth-order dispersions must be included in the phase matching
conditions to get better agreement with experimental measurements. Furthermore,
temporal measurements performed with a fast photodiode reveal the generation of
multiple pulse structures.
