Generating double focal spots by focusing a radially polarized double-ring-shaped beam with an annular classical axicon

Abstract

Focusing properties of a radially polarized double-ring-shaped beam with an annular classical axicon are numerically investigated based on vector Debye theory. Double focal spots and a flat-topped beam can be generated by choosing appropriate values of the pupil to the beam ratio β, numerical aperture, and annular obstruction. The distance between the twin spots, their depth of focus and the depth of the flat-topped beam are considerably affected by numerical aperture and annular obstruction. These focal shapes may find applications in laser printing, multifocal microscopy, and material processing.