Evolution of focused Hankel waves and Bessel beams

The focusing properties of a Bessel beam are analysed and interpreted in terms of its constituent conical waves. While these waves focus to a ring image in the focal plane of the lens, the beam also exhibits an apparent focus on the axis in front of the focal plane. The physical origin of this feature is explained, and it is also shown that, under certain focusing conditions, a Bessel beam can be reconstructed beyond the focal plane. (C) 2000 Elsevier Science B.V. All rights reserved.