Pixel-level phase filters for off-axis shifting of sinc envelope in holographic projection.

Off-axis projection is a common practice for reconstructions of Fourier holograms displayed on liquid crystal on silicon (LCoS) spatial light modulators (SLMs), as it spatially separates the image from the undiffracted light. Binary gratings encoded within the holograms enable maximum angular separation. However, as a result, two mirror images of equal intensities are present in the reconstruction. To introduce asymmetry to the intensity distribution and suppress one of those images, we propose a phase mask with a subpixel pattern. Presented results prove the potential of in-built SLM phase-mask layers for optimizing efficiency of the off-axis holographic projection.

Vortex beam as a positioning tool.

Remote positioning by precise measurements of lateral displacements of laser beams at large distances is inevitably disturbed by the influence of atmospheric turbulences. Here we propose the use of optical vortices, which exhibit lower transversal variations at an experimentally validated range of 100 meters. We show the higher precision of the localization of vortex points as compared with standard centroid-based assessment of Gaussian beams. Numerical simulations and experimental measurements show further improvements by averaging of the positions of up to four secondary vortices forming a stable constellation when higher values of the topological charges are used.

Accelerating hologram generation using oriented-separable convolution and wavefront recording planes.

Recently, holographic displays have gained attention owing to their natural presentation of three-dimensional (3D) images; however, the enormous amount of computation has hindered their applicability. This study proposes an oriented-separable convolution accelerated using the wavefront-recording plane (WRP) method and recurrence formulas. We discuss the orientation of 3D objects that affects computational efficiency, which is overcome by reconsidering the orientation, and the suitability of the proposed method for hardware implementations.

Real-valued layer-based hologram calculation.

Layer-based hologram calculations generate holograms from RGB and depth images by repeating diffraction calculations using complex Fourier transforms (FTs). Holograms generated as such are suitable for near-eye display and can be easily reconstructed with good image quality, but they are computationally expensive because of multiple complex-valued operations, including complex FTs. In this study, we propose an acceleration method for layer-based hologram calculations by reducing time-consuming complex-valued operations using the real-valued FT and Hartley transform as real linear transformations. Real linear transformations transform real input data to real output data; thus, the proposed method generates amplitude holograms. Thus, we also propose a technique to convert holograms generated by real linear transformations into phase-only holograms using the half-zone plate process and digitalized single-sideband method while maintaining the calculation acceleration. The proposed method can speed up hologram calculations by a factor of around three while maintaining the same image quality as the conventional method.

Impact of Surfactant and Calcium Sulfate Type on Air-Entraining Effectiveness in Concrete.

The paper presents the evaluation of the influence of calcium sulfate on the air void microstructure in concrete and its action mechanism depending on the character of the air-entraining agent. Gypsum dehydration has been previously proven to negatively influence the air void structure of air-entrained concrete. Ettringite, nucleating from tricalcium aluminate and calcium sulfate, influences the adsorption and mode of action of anionic-based polycarboxylate ether admixtures. The authors suspected the admixture's air-entraining mechanism was also affected by these characteristics. Gypsum dehydration was confirmed to influence the air void structure. In the case of the anionic surfactant, the content of air bubbles smaller than 300 µm was lower compared to cement with gypsum and hemihydrate. On the other hand, the content of air voids with a diameter up to 60 µm, which are the most favorable, was higher. The results obtained led to the conclusion that the mechanism of air entrainment was twofold, and in most cases occurred through the lowering of surface tension and/or through the adsorption of surfactant on cement grains. The adsorptive mechanism was proved to be more effective in terms of the total air content and the structure of the air void system. The results and conclusions of the study provide guidelines to determine the proper surfactant type to reduce the risk of improper air entrainment of concrete, and emphasize the importance of gypsum dehydration of cement in the process of air entrainment.

Suppression of spurious image duplicates in Fourier holograms by pixel apodization of a spatial light modulator.

Holographic projectors and near-eye displays are a promising technology with truly three-dimensional, natural viewing and excellent energetic efficiency. Spatial light modulators with periodic pixel matrices cause image duplicates, which distract the viewer and waste energy of the playback beam. We present the engineering of the far field intensity envelope, which suppresses higher-order image duplicates in the simplest possible optical setup by physically changing the shape of modulator pixels with attached apodizing masks. Numerical and experimental results show the limited number of perceived duplicates and better uniformity in off-axis projections for the price of compromised energetic efficiency due to amplitude masks.

Collective matrix of spatial light modulators for increased resolution in holographic image projection.

We present a method to increase the resolution of holographically projected images by the use of a collective matrix of two phase-only spatial light modulators. As a result of fine alignment and common coherent illumination of the modulators, a synthetic aperture is formed with a doubled number of active pixels. In this paper we present theoretical analysis and numerical simulations which stand in good agreement for different distances between the apertures of modulators. We have achieved experimentally controlled field interference from both modulators on the projection screen, exhibiting increased resolution in one spatial direction and residual effects of the dead space between modulators. Then we have proved numerically possibility to extend our approach on bigger synthetic apertures consisted of more than two modulators.

Graphene-based plastic absorber for total sub-terahertz radiation shielding.

Increasing the requirements on telecommunications systems such as the need for higher data rates and connectivity via the Internet of things results in continuously increasing amounts of electromagnetic radiation in ever-higher telecommunications bands (up to terahertz). This can generate unwanted electromagnetic radiation that can affect the operation of electronic devices and human health. Here, we demonstrate that nonconductive and lightweight, graphene-based composites can shield more than 99.99% of the electromagnetic energy in the sub-THz range mainly via absorption. This contrasts with state-of-the-art electromagnetic radiation shielding materials that simply redirect the energy of the radiation from a protected area via conduction-based reflection mechanisms. This shifts the problem of electromagnetic pollution from one place to another. We have demonstrated that the proposed composites can be fabricated by industrial compatible methods and are characterized by specific shielding efficiency values that exceed 30 dB cm3 g-1, which is more than those for typical metals used today. Therefore these materials might help to solve the problem of electromagnetic environmental pollution.

Terahertz 3D printed diffractive lens matrices for field-effect transistor detector focal plane arrays.

We present the concept, the fabrication processes and the experimental results for materials and optics that can be used for terahertz field-effect transistor detector focal plane arrays. More specifically, we propose 3D printed arrays of a new type - diffractive multi-zone lenses of which the performance is superior to that of previously used mono-zone diffractive or refractive elements and evaluate them with GaN/AlGaN field-effect transistor terahertz detectors. Experiments performed in the 300-GHz atmospheric window show that the lens arrays offer both a good efficiency and good uniformity, and may improve the signal-to-noise ratio of the terahertz field-effect transistor detectors by more than one order of magnitude. In practice, we tested 3 × 12 lens linear arrays with printed circuit board THz detector arrays used in postal security scanners and observed significant signal-to-noise improvements. Our results clearly show that the proposed technology provides a way to produce cost-effective, reproducible, flat optics for large-size field-effect transistor THz-detector focal plane arrays.

Assessment of imaging with extended depth-of-field by means of the light sword lens in terms of visual acuity scale.

We present outcomes of an imaging experiment using the refractive light sword lens (LSL) as a contact lens in an optical system that serves as a simplified model of the presbyopic eye. The results show that the LSL produces significant improvements in visual acuity of the simplified presbyopic eye model over a wide range of defocus. Therefore, this element can be an interesting alternative for the multifocal contact and intraocular lenses currently used in ophthalmology. The second part of the article discusses possible modifications of the LSL profile in order to render it more suitable for fabrication and ophthalmological applications.

Fiber-based real-time color digital in-line holography.

An extremely simple setup for real-time color digital holography using single-mode fibers as light guides and a directional coupler as a beam-splitting device is presented. With the directional coupler we have two object beams and one residual crosstalk used as a reference beam. This facilitates the adjustment and improves robustness. With the use of graphics processing units, real-time hologram reconstruction was possible. Due to adaptation of the optical setup and scaling, zero-order and complex image influence is highly reduced.

Evaluation of the shadow effect in terahertz kinoform gratings.

The experimental and numerical evaluation of the shadow effect in kinoform diffractive gratings for the terahertz (THz) range is given. This effect limits the diffractive efficiency of dense gratings, which are the base of the elements suited for convenient beam focusing and imaging in THz. The observed effect of redirecting most of the incident energy into stray -1st diffractive order is observed and discussed. The presented results show the great significance of the shadow effect in selected kinoform gratings and prove the utility of the used methodology of numerical simulations.

Diffuserless holographic projection working on twin spatial light modulators.

An improved efficient projection of holographic images is presented. It uses two phase spatial light modulators (SLMs) with two iteratively optimized Fresnel holograms displayed simultaneously--each for one modulator. The phase distribution on the second modulator is taking into account the light distribution coming from the first one. A pixelated structure of the modulator and fluctuations of liquid-crystal molecules cause a zero-order peak that was separated in experiment. Use of two SLMs gives clear and containing almost no speckles images. Thanks to the compensation of phase distribution from the first modulator, we can abandon diffusers in the iterative process and that is why we can control both amplitude and phase distribution in the image plane independently.

Simple holographic projection in color.

Extremely simplified image projection technique based on optical fibers and a single Spatial Light Modulator is presented. Images are formed by addressing the modulator with especially iterated Fourier holograms, precisely aligned on the projection screen using phase factors of lenses and gratings. Focusing is done electronically with no moving parts. Color operation is done by spatial side-by-side division of the area of the modulator. Experimental results are given, showing good image quality and excellent resistance to obstructions in the light path. Speckles are suppressed by micro-movements of the screen and by time-averaging of a number of holograms into the final image.

Diffractive paper lens for terahertz optics.

Passive terahertz (THz) setups require optical elements with large diameters for optimal harvesting of weak signals. High f-number implies sophisticated aspheric designs to ensure optimal resolution and good energetic efficiency. Trial and error testing of such optics is expensive and numerical modeling is time consuming; hence, we propose extremely cheap diffractive lenses for THz made of regular paper. They are easy to manufacture even with large diameters, and the optical function can be easily customized, which can be used for initial experimental testing of THz setups. Characterization of the proposed diffractive lenses with time-domain spectroscopy is presented and discussed.

Highly efficient broadband double-sided Fresnel lens for THz range.

Modern passive THz setups require effective optical elements with a large numerical aperture. Here we propose a new type of the optical element for THz applications, which is a broadband double-sided Fresnel-like lens with an optimized thickness. The optimization is performed to obtain a very low attenuation, low material cost, and small weight in the element media. It also provides achromatic properties for the assumed wavelength range. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.

Efficient image projection by Fourier electroholography.

An improved efficient projection of color images is presented. It uses a phase spatial light modulator with three iteratively optimized Fourier holograms displayed simultaneously--each for one primary color. This spatial division instead of time division provides stable images. A pixelated structure of the modulator and fluctuations of liquid crystal molecules cause a zeroth-order peak, eliminated by additional wavelength-dependent phase factors shifting it before the image plane, where it is blocked with a matched filter. Speckles are suppressed by time integration of variable speckle patterns generated by additional randomizations of an initial phase and minor changes of the signal.

Off-axis metallic diffractive lens for terahertz beams.

A diffractive optical element for off-axis focusing of terahertz radiation is presented. It was designed in a nonparaxial regime and manufactured in a metal slab by laser cutting of curved stripes. The optical function of the structure includes focusing and deflecting the illuminating beam of a chosen frequency in a particular place. Therefore, the element acts as both a spatial and a spectral filter; hence it is especially suitable for separating the terahertz signal from a broadband thermal load in passive detection devices. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.

Color image projection based on Fourier holograms.

A method of color image projection is experimentally validated. It assumes a simultaneous illumination of a spatial light modulator (SLM) with three laser beams converging in a common point on a projection screen. The beams are masked with amplitude filters so that each one illuminates one third of the area of the SLM. A Fourier hologram of a chosen color component of an input image is calculated, and its phase pattern is addressed on a corresponding part of the SLM area. A full-color flat image is formed on the screen as a result of color mixing. Additional techniques of image optimization are applied: time-integral speckle averaging and an off-axis shift of a zero-order peak. Static and animated experimental results of such a color holographic projection with a good image quality are presented.

Experimental evaluation of a full-color compact lensless holographic display.

An iterative phase retrieval method for a lensless color holographic display using a single light modulator is experimentally validated. The technique involves iterative calculation of a three-plane synthetic hologram which is displayed on a SLM simultaneously lit with three laser beams providing an RGB illumination. Static and animated two-dimensional flicker-free full color images are reconstructed at a fixed position and captured using a high resolution CMOS sensor. The image finesse, color fidelity, contrast ratio and influence of speckles are evaluated and compared with other techniques of holographic color image encoding. The results indicate the technique superior in a case of full-color real-life pictures which are correctly displayed by this ultra-compact and simple projection setup.