  Ultrashort light pulse transport and amplification in a 1.3 m long step-index
multimode fiber with gain and with weak coupling has been investigated. An
adaptive shaping of the input wavefront, only based on the output intensity
pattern, has led to an amplified pulse focused both in space (1/32) and in time
(1/10) despite a strong modal group delay dispersion. Optimization of the input
owing to the two-photon detection of the amplified signal permitted to excite
the fastest and more intense principal mode of the fiber and to get an output
pulse duration limited by group velocity dispersion.
