US Patent 7079256 Interferometric optical apparatus and method for measurements

In order to improve lateral resolution of scanning optical devices, a low-coherence sample beam is divided into beam portions by wavefront division and one of the beam portions is phase delayed relative to the other portions. The sample beam is focused on and reflected by an object. The reflected beam is coupled with a reference beam. The phase retardation is large enough that interference occurs exclusively between the phase delayed beam portion and the reference beam. The phase delayed beam portion can be used as a virtual beam within the sample beam in optical measurements. The virtual beam has a virtual beam spot that is smaller than the sample beam spot.