Fresnel Zoneplates

A fresnel zoneplate is diffractive optic which acts like a circular grating. The period of the zoneplate decreases with radial distance from the center.  Similar to how a grating diffracts light into multiple orders, a Fresnel zoneplate creates multiple focuses. If an order sorting aperture is used to select a single order, then the zoneplate acts similarly to a refractive lens.