  We study linear and nonlinear wave scattering by an optical vortex in a
self-defocusing nonlinear Kerr medium. In the linear case, we find a splitting
of a plane-wave front at the vortex proportional to its circulation, similar to
what occurs in the scattered wave of electrons for the Aharonov-Bohm effect.
For larger wave amplitudes, we study analytically and numerically the
scattering of a dark-soliton stripe (a nonlinear analog of a small-amplitude
wavepacket) by a vortex and observe a significant asymmetry of the scattered
wave. Subsequently, a wavefront splitting of the scattered wave develops into
transverse modulational instability, ``unzipping'' the stripe into trains of
vortices with opposite charges.
