The interplay between nonlinearity, disorder and interference exhibits fascinating and rich physics. Multimode optical fibers are a perfect testbed for studying such phenomena, as they exhibit strong nonlinearity and random mode mixing. We launch into the fiber ultrashort optical and explore the role of mode-mixing in nonlinear processes such as white light generation, spontaneous four-wave mixing and self-phase modulation.

Nonlinear interactions in multimode fibers exhibit fascinating dynamics that can remarkably be utilized for applications. For example, we use nonlinear modal interference to make a saturable absorber for mode-locked fiber lasers that generate ultrafast pulses. In this scheme, light occupying a single guided mode is coupled to a multimode fiber, whose output facet is spliced to a single-mode fiber. The intensity at the splicing point depends on the interference of the modes excited in the multimode fiber. At high powers, the interference is modified by Kerr nonlinearity, making the transmittance at the splicing point power-dependent. This allows us to modify the properties of the saturable absorber, such as linear loss, saturated loss, and saturation power, by adjusting the conformation of the multimode fiber. As a result, we were able to obtain a wide range of soliton configurations, exhibiting both short-range and long-range interactions in the laser cavity.