  Far-field slit-diffraction of circular optical-vortex (OV) beams is efficient
for measurement of the topological charge (TC) magnitude but does not reveal
its sign. We show that this is because in the common diffraction schemes the
diffraction plane coincides with the incident OV waist plane. With explicit
involvement of the incident beam spherical wavefront and based on the examples
of Laguerre-Gaussian modes we show that the far-field profile possesses an
asymmetry depending on the wavefront curvature and the TC sign. These features
enable simple and efficient ways for the simultaneous diagnostics of the TC
magnitude and sign, which can be useful in many OV applications, including the
OV-assisted metrology and information processing.
