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31 December 2003 Design and optimization considerations of multi-focus phase-only diffractive elements
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Most commonly used Fourier-domain optical diffractive structures are focused in infinity. For many applications, however, structures with a focal plane in finite distance are needed (s.c. Fresnel-domain structures). Furthermore, in special cases, structures with simultaneous multiple focuses at different distances are of concern. As it turned out, design and optimization algorithms for Fourier-domain structures can be effectively used in such cases, only after some modifications. In this contribution, design procedures based on iterative approaches such as iterative Fourier transform algorithm (IFTA) and direct binary search (DBS), applied to designing such structures, are presented. Furthermore, the elements with tilted focal plane have been of interest due to their potential applications, and design algorithms for such elements have been developed. A comparison of computer simulated and experimental reconstructions is also discussed. The IFTA and DBS algorithms have been analyzed, improved and efficiently implemented to achieve high performance for a general class of Fresnel-domain multifocal diffractive structures. Convergence of the algorithms has been studied with respect to various important design parameters. In comparison with Fourier-domain structures, also several new quality parameters for Fresnel-domain element quality evaluation have been identified, and their importance has been demonstrated. The designed structures have been fabricated using two different technologies, i.e. realization of designed structures (1) on a nematic liquid crystal spatial light modulator, (2) using e-beam lithography. Several designed testing elements have been fabricated with different design and fabrication parameters chosen in e-beam photoresist, and their performance have been evaluated, achieving a good qualitative agreement with computer simulations.
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Marek Skeren, Ivan Richter, and Pavel Fiala "Design and optimization considerations of multi-focus phase-only diffractive elements", Proc. SPIE 5182, Wave-Optical Systems Engineering II, (31 December 2003);

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