  We introduce a new type of states for light in multimode waveguides featuring
strongly enhanced or reduced spectral correlations. Based on the experimentally
measured multi-spectral transmission matrix of a multimode fiber, we generate a
set of states that outperform the established "principal modes" in terms of the
spectral stability of their output spatial field profiles. Inverting this
concept also allows us to create states with a minimal spectral correlation
width, whose output profiles are considerably more sensitive to a frequency
change than typical input wavefronts. The resulting "super-" and
"anti-principal" modes are made orthogonal to each other even in the presence
of mode-dependent loss. By decomposing them in the principal mode basis, we
show that the super-principal modes are formed via interference of principal
modes with closeby delay times, whereas the anti-principal modes are a
superposition of principal modes with the most different delay times available
in the fiber. Such novel states are expected to have broad applications in
fiber communication, imaging, and spectroscopy.
