  Research on multimode optical fibers is arousing a growing interest, for
their capability to transport high-power laser beams, coupled with novel
nonlinear optics-based applications. However, when beam intensities exceed a
certain critical value, optical fiber breakdown associated with irreversible
modifications of their refractive index occurs, triggered by multiphoton
absorption. These processes have been largely exploited for fiber material
microstructuration. Here we show that, for intensities slightly below the
breakdown threshold, nonlinear absorption strongly affects the dynamics of a
propagating beam as well. We experimentally analyze this sub-threshold regime,
and highlight the key role played by spatial self-imaging in graded-index
fibers for enhancing nonlinear optical losses. We characterize the nonlinear
power transmission properties of multimode fibers for femtosecond pulses
propagating in the near-infrared spectral range. We show that an effective
N-photon absorption analytical model is able to describe well the experimental
data.
