An astigmatic transform of a fractional-order edge dislocation

Abstract

In this work, it is theoretically and numerically demonstrated that an astigmatic transformation of a νth-order edge dislocation (shaped as a zero-intensity straight line) of a coherent light field – where ν = n+α is a real positive number, n is integer, and 0 < α < 1 is fractional – produces n optical elliptic vortices (screw dislocations) with topological charge –1, which are arranged on a straight line perpendicular to the edge dislocation and found at Tricomi function zeros. We also reveal that at a distance from the said optical vortices (OV), an extra OV with charge –1 is born on the same straight line, which departs to the periphery with α tending to zero, or gets closer to the n OVs with α tending to 1. Additionally, we find that a countable number of OVs (intensity nulls) with charge –1 are produced at the field periphery and arranged on diverging hyperbolic curves equidistant from the straight line of the n main intensity nulls. These additional OVs, which we term as “escort”, either approach the beam center, accompanying the extra “companion” OV if 0 < α < 0.5, or depart to the periphery, whereas the “companion” keeps close to the main OVs if 0.5 < α < 1. At α = 0 or α = 1, the “escort” OVs are shown to be at infinity. At fractional ν, the topological charge (TC) of the whole optical beam is theoretically shown to be infinite. Numerical simulation results are in agreement with the theoretical findings.