  The characterization of the complex spatiotemporal dynamics of optical beam
propagation in nonlinear multimode fibers requires the development of advanced
measurement methods, capable of capturing the real-time evolution of beam
images. We present a new space-time mapping technique, permitting the direct
detection, with picosecond temporal resolution, of the intensity from
repetitive laser pulses over a grid of spatial samples from a magnified image
of the output beam. By using this time-resolved mapping, we provide the first
unambiguous experimental observation of instantaneous intrapulse nonlinear
coupling processes among the modes of a graded index fiber.
