Fresnel zone plate with multiple layers of delay zones

TLDR: The Fresnel zone plate as mentioned in this paper is composed of a base plate (20), and a plurality of layers or orders (21, 22, etc.) of transparent phase delay zones arranged on the base plate.

Abstract: The present Fresnel zone plate is comprised of a base plate (20), and a plurality of layers or orders (21, 22, etc.) of transparent phase delay zones arranged on the base plate. The zones of each succeeding order are narrower than those of the previous order, and are thinner to provide half the delay of the previous order. The radii of the zone boundaries are defined by the boundary formula Sj(n) = ∑(bμn)/2j-1, where j = integers representing order number, n = boundary number, b = focal length, and μ = wavelength. The innermost zone boundary is boundary number 1. The phase delays introduced by the delay zones are defined by the delay formula (0.5j+k)μ, where j = integers representing order number, k = is preferably 0 but may be any integer, and μ = wavelength. For example, with 3 orders of delay zones, the first order provides a 1/2μ delay, the second order provides a 1/4μ delay, and the third order provides an 1/8μ delay. Since each succeeding order is narrower than the previous order, a combination of 7 distinct delays are provided, from 1/8μ to 7/8μ, in 1/8μ increments. The delay increments are thus fine enough to correct most of the phase differences in the waves that passed through the zone plate to produce 97.5 % of the amplitude and 95 % of the intensity of an ideal lens. Alternative embodiments include a concave zone plate (35-37), a zone plate with linear delay zones (40-42), a reflective zone plate (80-84), and a flat prism (85-87).