Arago-Poisson diffraction spot observed in the shadow area of an axicon lens

Abstract

In this paper we examined both theoretically and experimentally the diffraction field along the axis of propagation of the zero order Bessel distribution produced by an axicon. An axicon is typically used to obtain quasi non-diffracting beams. We found that an Arago-Poisson diffraction spot, normally only observed behind an opaque circular obstacle, can also be observed in the shadow area of an axicon lens. Both theoretical and experimental analysis indicate that the intensity distribution of the diffraction spot follows a zero order Bessel function in the paraxial region. The diffraction spot diameter increases linearly with the propagation distance in the shadow area while its energy decreases with propagation distance. The illuminated region and Arago-Poisson diffraction spot never overlap because the diameters of either increase linearly with propagation distance. Therefore, in theory, the Arago-Poisson diffraction spot could propagate to infinity. Our observation may be useful to produce an accurate straight line reference in commercial alignment systems.