Abstract

Diffractive elements such as digital holograms¹ and Dammann gratings² can be used to generate an array of diffraction orders with specified intensity. The part of the diffraction pattern that contains these orders is called the signal window. The elements are designed using scalar diffraction theory; i.e., geometrical optics is used to predict the transmitted or reflected wave-front, and the wave propagation outside the element is calculated by paraxial approximations. For scalar diffraction theory to be valid the features of the element must be large compared to the wavelength. Correspondingly, the grating periods are large and the diffraction angles are small; the elements operate in the paraxial domain. The maximum diffraction angle within the signal window can be estimated via the knowledge of the number of features necessary to realize a specified optical function and the minimum feature size; 5° results as an upper limit. A more flexible choice of the diffraction angles may broaden the scope of possible applications of diffractive elements. Fortunately, the limit is not a general one within the frame of diffractive optics.

© 1994 IEEE