  The comet-tail-like interference patterns are observed using photons from the
spontaneous parametric down-conversion (SPDC) process. The patterns are caused
by the angular-spectrum-dependent interference and the diffraction of a blazed
grating. We present the theoretical explanation and simulation results for
these patterns, which are in good agreement with the experimental results. The
most significant feature of the patterns is the bright parabolic contour
profile, from which, one can deduce the parameter of the parabolic tuning curve
of the SPDC process. This method could be helpful in designing experiments
based on SPDC.
