Abstract

Conventional holographic optical elements are limited in their utility by uncorrected field aberrations: coma for planar HOEs and astigmatism for curved elements. However, manufactured diffractive elements, produced by precision machining or photolithographic techniques, can have a designer-specified diffractive structure. This freedom, along with the choice of aperture stop location and bending of the diffractive lens substrate allows for the control of geometric aberrations. We have developed a set of design equations for a diffractive singlet which is free from third-order coma, astigmatism, and field curvature, for a specified conjugate ratio. The aberrations are controlled by the location of the diffracting zones and the placement of the aperture stop. The bending of the substrate can be used to provide a desired (non-zero) amount of spherical aberration or distortion. The aperture size is limited by the remaining spherical aberration, and a suitable tolerance can be found by applying Marechai's criterion. One application which requires a prescribed amount of distortion is a laser scanning lens. In this case, it is desired to have the lens corrected for the f-theta condition. We present an example of a flat field diffractive singlet which satisfies the f-theta condition and is suitable for scanning an 8.5-in. paper width at a resolution of 1000 dots/in.

© 1989 Optical Society of America