Marek Skeren

Beam steering experiment with two cascaded ferroelectric liquid-crystal spatial light modulators

The design, construction, and evaluation of a laser beam steerer that uses two binary ferroelectric liquid-crystal (FLC) spatial light modulators (SLMs) operated in conjunction are presented. The system is characterized by having few components and is in principle lossless. Experimentally, a throughput of approximately 20% was achieved. The simple system design was achieved because of the high tilt angle FLC material used in the SLMs, which were specifically designed and manufactured for this study. By coherently imaging the first SLM onto the second SLM, pixel by pixel, we obtained an effective four-level phase structure with a phase step of 90 degrees. An appropriate alignment procedure is presented. The beam steering performance of the system is reported and analyzed.

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.

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.

Enhancement of Wavelength Selectivity of Color Holograms Based on Surface Plasmons

Setup for enhancement of wavelength selectivity of transmission holographic filters based on surface plasmon is presented. Volume grating is used on the input side of the device for modifying the dispersive properties of the element.

Synthetic Image Holograms

This chapter is dedicated to synthetic image holograms the elements which can create a reconstruction of a 3D object for observation with the human eye. Holography as a technique of image recording and reconstruction has been extensively developed from sixties of the twentieth century. During this time there have been various attempts to synthesize holograms artificially without the presence of the real object in the classical recording setup. Different approaches have been used, several trying to synthesize the three-dimensional object from two-dimensional views using the classical recording setup, the others trying to calculate the microstructure of the hologram completely in a computer. Today, we can divide synthetic holography into twomajor streams, the first containing the methods for creating the image for observation by human eye and the second consisting of approaches for designing the synthetic diffractive structures for general wavefront generation. The former techniques can exploit various imperfections of human vision and omit several parameters of the optical wave. The latter techniques are usually based on the direct calculation of the microstructure and they try to create the reconstruction in its full complexity. Only the first group of synthetic image holograms will be analyzed in this chapter. The synthetic approach to hologram creation can have several advantages, but also noticeable disadvantages. The most important advantages are connected with flexibility in modifying the recorded object. First, the object need not to exist in reality in a form of a physical model. For most synthetic approaches, it is fully sufficient to have a 3D computer model for preparing the recording data. Also for real physically existing objects it could be tricky to perform the recording process in a classical setup. For example, various outdoor scenes such as buildings and others could not be included in the laboratory setups. Generally, the scaling possibility is very limited in classical holography, so the recorded object (or its model) must be of final size. On the other hand, it is easy to scale the computer model of an object. The next problem is in various corrections of color properties, surface textures, and general fine tuning of the recorded object. While such operations are very simple in the case of computer models, they could bring insoluble problems for real physical models. The stability of the object is also very important. It is crucial to highly stabilize the object for recording in classical holographic recording setup (when exposing with a continuous-wave laser), whereas in a computer stability is not a problem. This can apply also for holograms of living objects or dynamic scenes, where it is easy to take snaphots using photographic techniques, but holographic exposure is almost impossible. Finally, according to the recording technology chosen, other parameters of the synthetic hologram can be highly superior to those of classical holograms (e.g. fidelity of color mixing, contrast of the image, etc.). 10

Real-time Generation of Synthetic Diffractive Structures Using GPU

Formerly, the applications of the holographic optical tweezers were limited by the speed of the algorithm used for design of the diffractive structures. Nowadays algorithm can run on the GPU, which enables new design possibilities.

Compact Optical Tweezers Based on SLM for Real-Time Optical Trapping and Manipulation

We report a compact holographic optical tweezers based on an LCoS SLM. Optical traps are generated by diffraction of light on the Fresnel-type hologram generated by a fast parallel algorithm that enables real-time 3-D manipulation.