Using femtosecond upconversion we investigate the time and
wavelength structure of infrared supercontinuum generation. It is shown
that radiation is scattered into higher order spatial modes (HOMs) when
generating a supercontinuum using fibers that are not single-moded, such as
a step-index ZBLAN fiber. As a consequence of intermodal scattering and
the difference in group velocity for the modes, the supercontinuum splits up
spatially and temporally. Experimental results indicate that a significant part
of the radiation propagates in HOMs. Conventional simulations of supercontinuum generation do not include scattering into HOMs, and including
this provides an extra degree of freedom for tailoring supercontinuum
sources.