  Nonlinear optical wave propagation manifests in a multitude of frequencies
generated from quantum-noise, and selecting desired nonlinear products usually
requires seeding the medium with extraneous waves, employing spatial or
spectral filters, or operation in resonant cavities. This is especially true
for multimode systems because of their high density of states. Here we report
the discovery of a self-action effect, originating from quantum noise, leading
to complete nonlinear optical conversion of an ultrashort soliton pulse between
two distinct, spatially coherent eigenmodes that are frequency-separated by one
Raman Stokes shift. That systematic nonlinear spatial reconfiguration occurs in
fibers with mode counts exceeding 10,000, which are often deemed to be chaotic,
points to the fundamental role of intermodal group velocity dispersion in the
selection rules for multimode nonlinear optics. We demonstrate wideband
spectral translations of ~70% of the carrier frequency, and the generation of
record, Megawatt peak-power pulses in the biologically crucial 1300-nm spectral
window, directly out of a flexible optical fiber. More generally, this novel
nonlinear coupling mechanism may be applied to fibers or on-chip waveguides,
and facilitate, power-scalable spatially coherent, ultrashort pulse generation
from the visible to the mid-IR.
