Kerr self-cleaning of femtosecond-pulsed beams in graded-index multimode fiber

We observe a nonlinear spatial self-cleaning process for femtosecond pulses in graded-index (GRIN) multimode fiber (MMF). Pulses with similar to 80 fs duration at 1030 nm are launched into GRIN MMF with 62.5 mu m core. The nearfield beam profile at the output end of the fiber evolves from a speckled pattern to a centered, bell-shaped transverse structure with increasing pulse energy. The experimental observations agree well with numerical simulations, which show that the Kerr nonlinearity underlies the process. This self-cleaning process may find applications in ultrafast pulse generation and beam-combining. (C) 2016 Optical Society of America