Patent ID: 7099084

Claim:
A diffractive modulating element for modulating and diffracting an incident wave, comprising a) a first plate assembly, the first plate assembly comprising a first zone plate with a set of alternate zones being reflective and the remaining zones being transmissive to the incident wave, wherein the zone plate being a Fresnel zone plate consisting of a series of concentric circular zones with the width of the zones not equal to each other and the radii Rn of the zones determined by the zone plate equation; b) a second plate assembly, the second plate assembly comprising a second reflective zone plate complementary to said first zone plate, the second plate assembly being positioned substantially parallel to said first plate assembly, and the second plate assembly receiving the incident wave after the first plate assembly, wherein the zone plate being a Fresnel zone plate consisting of a series of concentric circular zones with the width of the zones not equal to each other and the radii Rn of the zones determined by the zone plate equation; c) displacing means for changing the relative distance between the first and second plate assemblies in a direction normal to the assemblies; whereby the displacing means changes the relative distance between the first and second zone plates, and between a first configuration wherein the relative distance is m/2 times the wavelength of the incident wave, m is an integer number, the first and second zone plates act to reflect the incident wave as a plane mirror, and a second configuration wherein the relative distance is m/2+¼ times the wavelength of the incident wave, the first and second zone plates act to diffract the incident wave into a series of focal points along the axis of the zone plates as a phase zone plate.