Pavel Fiala

Reflection Holographic Portraits

This paper presents a method of producing highly efficient reflection holographic portraits. The procedure has two stages, facilitating a maximum optimization of the entire process with respect both to the diffraction efficiency of the final product and to safety when the human eye is exposed to a pulsed laser beam. The procedure makes it possible to produce any number of reflection copies, with increased diffraction efficiency, from a single original.

Analysis of binary diffraction gratings: comparison of different approaches

Diffraction characteristics of periodic binary diffractive structures are studied using three different theoretical approaches. Namely rigorous coupled-wave analysis (RCWA), Kogelniks two-wave method, and the scalar method of transmittance have been applied to study diffraction characteristics and have been compared, and their applicability has been determined. For comparison purposes, a binary surface-relief grating made from an isotropic non-absorbing substrate in the planar diffraction regime, and a transverse electric polarized incident wave, are considered in this paper. Additionally, by calculating the diffraction efficiency of such gratings using the RCWA method, ith special respect to grating profile depth and period, it was possible to describe and explain the complex behaviour of diffraction efficiency, that is to find regions with typical diffraction regimes, of all gratings of a chosen kind, in this case given by the binary grating profile. Moreover, it has been shown ho the classical scalar transmittance method for analysis of thin diffraction gratings can be modified by an estimation of the propagation process through the grating depth in order to describe the volume phase synchronism dependence on the relative grating period.

Synthetic diffractive elements for security applications realized on an enhanced integral dot-matrix system

One of the important fields of application of synthetic diffractive structures is optical document security. Several methods of security enhancement of diffractive elements for security applications are presented, namely, high carrier-frequency cryptograms and noise-covered elements are introduced. Structures are designed with respect to the fabrication on special enhanced integral dot-matrix system.

Design of binary phase-only diffractive optical elements for laser beam shaping

This contribution concentrates on a study and comparison of non- iterative design methods (error diffusion methods) based on a hardclip approach with iterative methods, namely iterative Fourier transform algorithm (IFTA), in designing binary phase-only diffractive optical elements (BPDOEs) for laser beam shaping. Two error diffusion methods as a non-iterative methods were considered, implemented and used for designing BPDOEs: the classical algorithm of error diffusion (ED) and the signal window minimum average error algorithm (SWMAE). Also, different schemes and approaches of IFTA algorithm were analyzed, implemented, and compared. The methods together with the simple hardclip method were analyzed for both diffusive and fan-out type objects: the signal-to-noise ratio and the diffraction efficiency dependencies on both the spatial-bandwidth product and on the signal-window position were obtained, enabling to determine the optimum design parameters and constraints within each method. As for the IFTA method, the role and a proper shape of the scale factor were analyzed, and a new way of characterization of the algorithm convergence was introduced, using the spectrum representation. The practical realization using e-beam lithography technology shows a good qualitative agreement with designed types of elements.

Design and optimization considerations of multi-focus phase-only diffractive elements

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.

Behavior And Properties Of Real Holographic Recording Materials

The paper discusses actual knowledge and problems of the behaviour of real holographic recording materials with a gelatine host, especially silver halide emulsions. The properties of materials are studied on reflection diffraction gratings, where diffraction efficiency, optical noise. spectral or angular selectivity, transfer linearity of image modulation, e.t.c., are the investigated characteristics. The theoretical model of real holographic gratings, including the problems of shrinkage and deformation of grating and the nonuniformity of grating modulation, is formulated. The theory and the experiments are carried out both for holograms formed by dielectric particles and for holograms formed by colloidal silver particles. In conclusion the fundamental relations among the final diffraction properties of holograms and certain factors of production and processing of recording materials are indicated.

Iterative Fourier transform algorithm: different approaches to diffractive optical element design

This contribution focuses on the study and comparison of different design approaches for designing phase-only diffractive optical elements (PDOEs) for different possible applications in laser beam shaping. Especially, new results and approaches, concerning the iterative Fourier transform algorithm, are analyzed, implemented, and compared. Namely, various approaches within the iterative Fourier transform algorithm (IFTA) are analyzed for the case of phase-only diffractive optical elements with quantizied phase levels (either binary or multilevel structures). First, the general scheme of the IFTA iterative approach with partial quantization is briefly presented and discussed. Then, the special assortment of the general IFTA scheme is given with respect to quantization constraint strategies. Based on such a special classification, the three practically interesting approaches are chosen, further-analyzed, and compared to eachother. The performance of these algorithms is compared in detail in terms of the signal-to-noise ratio characteristic developments with respect to the numberof iterations, for various input diffusive-type objects chose. Also, the performance is documented on the complex spectra developments for typical computer reconstruction results. The advantages and drawbacks of all approaches are discussed, and a brief guide on the choice of a particular approach for typical design tasks is given. Finally, the two ways of amplitude elimination within the design procedure are considered, namely the direct elimination and partial elimination of the amplitude of the complex hologram function.

Laser beam shaping by binary and multilevel phase-only diffractive optical elements

This contribution focuses on the study and comparison of different design methods for designing phase-only diffractive optical elements (PDOEs) for different possible applications in laser beam shaping. Both binary and multilevel phase-only diffractive optical elements are considered, theoretically studied, and compared, using diffusive types of input objects. Especially, new results and approaches, concerning the iterative Fourier transform algorithm, are analyzed, implemented, and compared (the scale factor and the algorithm convergence characterization through the spectrum representation). The two important output design parameters, i.e. the signal-to-noise ratio and the diffraction efficiency characteristics are presented by means of evaluating their dependencies on, e.g. the number of iterations, the number of phase levels, and the signal-window position). Next, we concentrate on the design of Fresnel domain PDOEs, generalizing the classical IFTA scheme (both with a single and double focus) types of PDOEs. The application of the appropriate scale factors for each particular case is presented, and typical design results for both cases are shown. For practical realization, e-beam lithography technology is used, showing a good qualitative agreement with theoretical designs.

Photopolymer recording material with AgBr nanoparticles for optical holography

A new self-developing recording material with silver bromide nanoparticles and photopolymerization system has been prepared and tested as a recording medium for optical holography. Through the method of the real-time measurement of a diffraction grating growth, we have shown that an efficient volume phase grating is formed during a holographic exposure. The refractive index modulation of the formed grating is several times higher than in the case of standard acrylamide-based photopolymers with a similar composition. The material response on different exposure parameters has been measured and obtained results have been discussed. The transfer of nanoparticles within the recording layer, which is the main cause of the grating growth, has been experimentally verified through the direct observation of the grating micro-structure with the scanning electron microscope.

Influence of the diffractive structure parameters on the observation of a synthetic stereogram

A detailed view into the problematics of diffraction on the structure of synthetic stereogram of dot-matrix type is presented. The diffractive structure, composed of elementary diffractive gratings, is described, as well as the process of diffraction. The distribution of the diffracted light is very important because the diffracted wave in fact provides the observation of the stereogram, and is thus crucial for its spatial perception. The relations between a behavior of the diffracted wave and parameters of the stereogram structure are calculated. The presented results help to find the optimal balance between the conception of a designer and the technical possibilities for synthetic stereography.