Design, production and performance of circular fresnel zone plates.

This paper describes the design and the spark-erosion production technique for circular Fresnel zone plates with focal lengths between 3 m and 40 m when illuminated by a HeNe laser emitting light at 633 nm. Their image pattern is theoretically investigated by modifying Lommel's original approach to the problem in order to determine the effects of a decentered beam, varying zone numbers and a range of gaussian illumination parameters. The equations derived are evaluated and their results graphically illustrated. The theoretical predictions are experimentally verified for three typical zone plates. The results are discussed with particular regard to their bearing upon the use of zone plates as the optical reference elements in an alignment system at present in an advanced stage of development at the U.K. National Physical Laboratory.