  In conventional OCT, broadband light sources are generally utilized to obtain
high axial resolution due to their low temporal coherence (TC) length. Purely
monochromatic (i.e., high TC length) light sources like laser cannot be
implemented to acquire high resolution optically sectioned images of the
specimen. Contrary to this, pseudo thermal light source having high TC and low
spatial coherence (SC) property can be employed to achieve high axial
resolution comparable to broadband light source. In the present letter, a
pseudo thermal light source is synthesized by passing a purely monochromatic
laser beam through a rotating diffuser. The longitudinal coherence (LC)
property of the pseudo thermal light source is studied as a function of source
size and TC length. The LC length of the synthesized light source decreased as
the source size increased. It is found that LC length of such light source
becomes independent of the parent laser TC length for source size of greater
than or equal to 3.3 mm and become almost constant at around 30 micron for both
the lasers. Thus any monochromatic laser light source can be utilized to obtain
high axial resolution in OCT system irrespective of its TC length. The maximum
achievable axial resolution is found to be equal to 650 nm corresponding to 1.2
numerical aperture (NA) objective lens at 632 nm wavelength. The findings
elucidate that pseudo thermal source being monochromatic in nature can improve
the performance of existing OCT systems significantly.
