Seeded intermodal four-wave mixing in a highly multimode fiber

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-μm 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 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.