  Nonlinear propagation of signals in single-mode fiber is well understood, and
is typically observed by measuring the temporal profile or optical spectrum of
an emerging signal. In multimode fibers, the nonlinearity has both a spatial
and a temporal element, and a complete investigation of the interactions
between propagating modes requires resolving the output in both space and time.
We report here spatiotemporal measurements of two nonlinear interacting
LP$_{0m}$ modes of a step-index few-mode fiber (FMF). We describe a method to
selectively excite two propagating modes through the use of a phase-mask
directly patterned on the entrance face of the fiber. The output is resolved by
raster-scanning a near-field tapered single-mode optical fiber probe that is
connected to a high-speed detector. The results show that in the presence of
nonlinearity, the output exhibits a spatiotemporal character that cannot be
adequately characterized by a camera image or pulse shape alone.
