  We investigate the up-conversion process of broadband light from parametric
down-conversion (PDC), focusing on the spatio-temporal spectral properties of
the sum-frequency generated (SFG) radiation. We demonstrate that the incoherent
component of the SFG spectrum is characterized by a skewed geometry in
space-time, which originates from a compensation between the group-velocity
mismatch and the spatial walk-off of the fundamental and the SFG fields. The
results are illustrated both by a theoretical modeling of the optical system
and by experimental measurements.
