The spot of Arago: New relevance for an old phenomenon

Abstract

The ‘‘spot of Arago’’ has been a controversial topic since its inception in 1818 when Poisson predicted its existence in an attempt to discredit Fresnel’s wave theory of light. Arago performed the experiment and found the surprising prediction was true, thus putting Fresnel’s theory on a firm technical foundation. In recent years, the spot of Arago, which exists as a bright spot at the center of the geometrical shadow of a circular obstruction, has caused substantial grief in various high-energy laser applications and has come to be considered more of a nuisance than a curiosity. This paper suggests that the size and shape of the spot of Arago is characteristic of the wave-front aberrations of the incident beam and can therefore be used to advantage as a beam sample for wave-front analysis of annular beams. The implementation of this wave-front sampling scheme would eliminate the requirement for a special beam-sampling optical component and thus reduce to a minimum the deleterious effects upon the beam frequently accompanying the use of such components. Both experimental and numerical results are presented along with a discussion of the capabilities and limitations of this particular beam sample for diagnostic purposes.