Impact of periodic self-imaging on the nonlinear propagation in GRIN multimode fibers

Multimode optical fibers have recently regained a strong interest in particular because of their ability to deliver a quasi-Gaussian beam profile after nonlinear spatial clean-up of a multimode optical guided wave at high intensity. Indeed, a speckled beam obtained at the end of a multimode fiber in linear propagation regime can be transformed into a quasi-singlemode pattern at high input power. Although such nonlinear dynamic has been mainly obtained in fibers with parabolic index profiles, evidences of nonlinear beam cleaning were also reported in a fiber with a quasi-step index structure. This particular nonlinear spatial reshaping occurs spontaneously above some power threshold before the onset of significant spectral broadening by Kerr self-phase modulation (with hundred picoseconds long pulses). It is impacted by the distribution of power in the excited modal population. Far above threshold, the spatial reshaping is also accompanied by a temporal shortening of the launched pulse and eventually by frequency conversions due to geometric parametric instabilities. At higher power levels supercontinuum generation can be obtained covering all the spectral range between the visible to the IR domain (2.4 um) with all the wavelengths carried by the fundamental transverse mode of the multimode silica fiber.