  Holographic optical coherence tomography (OCT) is a powerful imaging
technique, but its ability to reveal low-reflectivity features is limited. In
this study, we performed holographic OCT by incoherently averaging volumes with
changing diffuse illumination of numerical aperture (NA) equal to the detection
NA. While the reduction of speckle from singly scattered light is only modest,
we discovered that speckle from multiply scattered light can be arbitrarily
reduced, resulting in substantial improvements in image quality. This technique
also offers the advantage of suppressing noises arising from spatial coherence,
and can be implemented with a partially spatially incoherent light source for
further mitigation of multiple scattering. Finally, we show that although
holographic reconstruction capabilities are increasingly lost with decreasing
spatial coherence, they can be retained over an axial range sufficient to
standard OCT applications.
