Abstract

A new, simple possibility for producing multilevel phase diffraction gratings is proposed. The method utilizes properties of a diffraction field of two identical cross type Ronchi diffraction gratings. The gratings are spaced by a self-imaging distance, and a plane of observation is located at 1/4 of that distance behind the second grating, where an image of the resulting diffraction structure is formed. The whole system is illuminated by a plane, spatially coherent wave. By controlling mutual, lateral displacement between both diffraction gratings, it is possible to accomplish formation of rectangular structures of variable dimensions within unitary cells of the diffraction grating on the output. Thus, by placing a plate covered with a photoactive substance in the observation plane and assuming a linear relation between the thickness of a photographic emulsion and the amount of absorbed light, it is possible to generate a two-dimensional, multilevel phase grating. Experiments have shown that it is possible to assure formation of rectangular structures as small as 5 μm × 5 μm of acceptable quality, thus being sufficient to produce a 10-level phase grating with a density of 20 lines/mm. The method proposed is better suited for gratings of relatively low spatial frequencies (1-20 lines/mm) and many phase levels.

© 1992 Optical Society of America