  We make a source of entangled photons (SEP) using spontaneous parametric
down-conversion (SPDC) in a non-linear crystal and study the spatial
distribution of photon pairs obtained through the down-conversion of different
modes of light including higher order vortices. We observe that for the
Gaussian pump, the thickness of the SPDC ring varies linearly with the radius
of pump beam. However, in case of vortex carrying beams, two concentric SPDC
rings are formed for beams above a critical radius. The full width at half
maximum (FWHM) of SPDC rings increase with increase in the order of optical
vortex beams. The presence of a critical beam width for the vortices as well as
the observed FWHM of the SPDC rings are supported with our numerical results.
