  We experimentally investigate the mode characteristics of multimode radiation
fields propagating through frequency dependent Gaussian channels. After
manipulating the twin beams emitted from a conventional parametric
down-conversion source via spectral filtering, we study the changes in their
mode characteristics, utilizing the joint normalized correlation functions.
While filtering reduces the number of spectral modes, it also leads to an
apparent mode mismatch, which destroys the perfect photon-number correlation
between the twin beams, and influences the mode properties of heralded states.
