  We report optical measurements of the spectral width of open transmission
channels in a three-dimensional diffusive medium. The light transmission
through a sample is enhanced by efficiently coupling to open transmission
channels using repeated digital optical phase conjugation. The spectral
properties are investigated by enhancing the transmission, fixing the incident
wavefront and scanning the wavelength of the laser. We measure the transmitted
field to extract the field correlation function and the enhancement of the
total transmission. We find that optimizing the total transmission leads to a
significant increase in the frequency width of the field correlation function.
Additionally we find that the enhanced transmission persists over an even
larger frequency bandwidth. This result shows open channels in the diffusive
regime are spectrally much wider than previous measurements in the localized
regime suggest.
