Near-core structure of a propagating optical vortex

We study the propagation of a charge-1 vortex beam using the angular spectrum method. While vortex beams are commonly assumed to have a helical wavefront, it is well known that the phase of the vortex in the near-core region varies arbitrarily quickly. In order to explain the wavefront behavior in the near-core region, ideas such as evanescent fields or superoscillatory functions have been used before. Our study using the angular spectrum method can inherently take into account the propagating as well as evanescent spatial frequencies and is able to provide the detailed wavefront structure as the vortex wavefront evolves. We report that the vortex wavefront shows a significant phase dip in the near-core region for all propagation distances, and the phase contour lines in this region are seen to spiral around the core. While the radial extent of this phase dip is seen to expand on propagation, the magnitude of the dip remains constant. Both propagating as well as evanescent components are seen to contribute to this phase dip, which we attribute to the presence of the radial component in the propagation vector near the core. The angular spectrum method as used here can be a valuable tool for probing the near-core structure of optical vortices.