  Coherence properties and wavelength of light sources are indispensable for
optical coherence microscopy/tomography as they greatly influence the signal to
noise ratio, axial resolution, and penetration depth of the system. In the
present letter, we investigated the longitudinal spatial coherence properties
of the pseudo-thermal light source (PTS) as a function of spot size at the
diffuser plane, which is controlled by translating microscope objective lens
towards or away from the diffuser plane. The axial resolution of PTS is found
to be maximum ~ 13 microns for the beam spot size of 3.5 mm at the diffuser
plane. The change in the axial resolution of the system as the spot size is
increased at the diffuser plane is further confirmed by performing experiments
on standard gauge blocks of height difference of 15 microns. Thus, by
appropriately choosing the beam spot size at the diffuser plane, any
monochromatic laser light source depending on the biological window can be
utilized to obtain high axial-resolution with large penetration depth and
speckle-free tomographic images of multilayered biological specimens
irrespective of the source temporal coherence length. In addition, PTS could be
an attractive alternative light source for achieving high axial-resolution
without needing chromatic aberration corrected optics and
dispersion-compensation mechanism, unlike conventional setups.
