Directional narrowing of the diffractive pattern through a combination of spherical and spiral optical elements within a single aperture

The intensity pattern produced by the zero-order Bessel beam can be squeezed along certain directions if it interferes with the Bessel beam of a higher order. The concept of directional narrowing can be extended onto the zone plates by the division of the aperture into a set of concentric annuli; within some of the apertures the phase function of the spherical optical element is substituted or supplemented by the spiral optical element. The proposed approach is verified by the numerical simulation of the interference of Bessel beams, linear axicons, and spherical zone plates of zero order and second order. (C) 1997 Optical Society of America.