Enhancement of imaging performance of a variable focus Fresnel zone plate based on a single, binary, phase-only SLM

It is shown that the imaging performance of a variable focus optical system based on a single, binary, phase-only spatial light modulator (SLM) can be improved in important respects by combining it with a fixed phase mask wit h either two or foul phase levels separated by pi /2. With the SLM encoded as a modified Fresnel zone plate, the images associated with one member of each of the pair of foci, +/-f, +/-f/3, +/-f/5, etc, are deflected and bloc ked by a stop enabling a single image plane to dominate the performance. An alytical expressions are derived for the peak amplitudes of the paint sprea d functions of a simple imaging system. The simulated performance of a vari able focus system is reported in which the intensity in a single selected i mage plane, scanned in the z-direction, is about an order of magnitude grea ter than other image planes and in which the scan range is approximately do ubled by switching between +/-f and -f foci. (C) 2001 Elsevier Science B.V. All rights reserved.