Phase Holograms Research Articles

Gradient of refractive index (GRIN) effect in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a photosensitive multicomponent silicate glass containing photosensitive agents such as cerium and silver. Photoinduced crystalline phase precipitation results in refractive index variations in exposed areas of PTR glass, which has been successfully used for phase hologram recording. Photosensitivity being the result of the partial absorption of the exposing radiation by cerium ions, it results in a gradient of refractive index (GRIN) along the exposure beam propagation. This GRIN is a parasitic effect that deteriorates the parameters of some types of volume Bragg gratings because of the beam deflection they produce. In this paper, the evolution of GRIN is investigated as a function of the dosage of UV exposure and the thermal treatment duration. We show that GRIN is a deterministic process that can be easily modeled and predicted using basic optics and glass science equations.

Controlling the optical fiber output beam profile by focused ion beam machining of a phase hologram on fiber tip.

A phase hologram was machined on an optical fiber tip using a focused ion beam (FIB) system so that a ring-shaped beam emerges from the fiber tip. The fiber used for this work was a commercial single-mode optical fiber patch cable for a design wavelength of 633nm with a germanosilicate core. The ring-shaped beam was chosen to ensure a simple geometry in the required phase hologram, though the Gerchberg-Saxton algorithm can be used to calculate a hologram for an arbitrary beam shape. The FIB machining took approximately 45 min at 30kV and 200pA. The radius of the resulting ring beam was 0.083m at 1m standoff, as compared to 0.1m as was initially desired. Results suggest that this imaging technique may provide a basis for a beam-shaping method with several advantages over the current commercial solutions, having permanent alignment, compactness, and mechanical robustness. However, it would appear that minimizing the speckle pattern will remain a critical challenge for this technique to become widely implemented.

Holographic generation of highly twisted electron beams.

Free electrons can possess an intrinsic orbital angular momentum, similar to those in an electron cloud, upon free-space propagation. The wave front corresponding to the electron's wave function forms a helical structure with a number of twists given by the angular speed. Beams with a high number of twists are of particular interest because they carry a high magnetic moment about the propagation axis. Among several different techniques, electron holography seems to be a promising approach to shape a conventional electron beam into a helical form with large values of angular momentum. Here, we propose and manufacture a nanofabricated phase hologram for generating a beam of this kind with an orbital angular momentum up to 200ℏ. Based on a novel technique the value of orbital angular momentum of the generated beam is measured and then compared with simulations. Our work, apart from the technological achievements, may lead to a way of generating electron beams with a high quanta of magnetic moment along the propagation direction and, thus, may be used in the study of the magnetic properties of materials and for manipulating nanoparticles.

基于计算全息和随机相位编码的三维信息的加密与显示

基于计算全息和随机相位编码的三维信息的加密与显示

Contribution of Amplitude Holographic Grating to Diffraction Efficiency of Mixed Transmission Holograms in Photorefractive Bi12TiO20 Piezocrystal

The results of experimental and theoretical studies of the readout process of the mixed transmission holograms recorded in a photorefractive Bi12TiO20 piezocrystal are presented. It is shown, that in comparison with the case of pure phase holograms, traditionally considered in photorefractive sillenite crystals, additional accounting of the amplitude component of the holographic grating leads to a qualitative and quantitative changes in the diffraction efficiency of holographic gratings recorded in this crystal, which allows to successfully interpreting the experimental results.

Characteristics of PTR Glass with Novel Modified Composition

We compare spectral and holographic characteristics of a novel material designed for manufacture of photothermorefractive (PTR) glass. The parameters of holograms recorded in classical and modified PTR glass are confronted. It is shown that unlike the classical PTR glass, the modified PTR glass does not have an additional absorption band in the blue-green spectral region. This allows one to record purely phase holograms in the visible spectral range. The amplitude of modulation of the first harmonic in the refraction factor for the modified photothermorefractive glass is twice as high as that within the classical approach.

Formation Dynamics of Transmission Holograms in Lithium Niobate Crystals Doped by Copper Through High-Temperature Diffusion

The results of experimental realization of the technology of diffusion doping of X-cut congruent lithium niobate crystals by copper and the results of studying their photorefractive and photovoltaic properties are presented. The latter was done by using the Bragg diffraction of the reading beam with a wavelength of 655 nm on the dynamic transmission volume–phase holograms recorded by laser beams having a wavelength of 532 nm. It is shown that by their photogalvanic properties the created samples, which are 1.8 mm thick, are similar to the single crystals grown by the conventional method from melt containing copper monooxide as a supplement.

1×N wavelength selective adaptive optical power splitter for wavelength-division-multiplexed passive optical networks

Abstract In this paper, a 1× N wavelength selective adaptive optical power splitter (WS-AOPS) suitable for wavelength-division-multiplexed passive optical network (WDM-PON) systems is proposed and experimentally demonstrated based on the use of an Opto-VLSI processor. The input signals with different wavelengths can be split and coupled into N output ports with variable splitting ratios. A proof-of-principle 1×2 WS-AOPS structure driven by optimized multicasting phase holograms uploaded onto the Opto-VLSI processor is developed, demonstrating an arbitrary splitting ratio for each input wavelength channel over C-band.

Generation of phase-only holograms by laser ablation of nanoparticulate ITO layers

Phase holograms offer great potential e.g. for laser beam shaping and imaging applications. However, the generation of these structures typically requires multi-step processes which tend to be time-consuming and expensive, especially if high-quality structures are required. In this work we demonstrate a flexible and inexpensive method which allows for the production of binary phase-only holograms by laser ablation of ITO nanoparticle layers. Since these layers are ablated free of residues, accurately defined phase shifts can be achieved. While arbitrarily shaped structures can be generated, the ITO layer thickness can be adjusted in order to freely tune the phase shift for the desired wavelength. In the diffraction patterns generated by the holograms we observed an excellent zero order suppression.

Generation of phase-only Fresnel hologram based on down-sampling.

We present a novel non-iterative method for generating phase-only Fresnel holograms. The intensity image of the source object scene is first down-sampled with uniform grid-cross lattices. A Fresnel hologram is then generated from the intensity and the depth information of the sampled object points. Subsequently, only the phase component of the hologram is preserved, resulting in a pure phase hologram that we call the sampled-phase-only hologram (SPOH). Experimental evaluation reveals that the numerical, as well as the optical reconstructed images of the proposed phase-only hologram derived with our method are of high visual quality. Moreover, the reconstructed optical image is brighter, and less affected by phase noise contamination on the hologram as compared with those generated with existing error-diffusion approaches.

Holographic multi-focus 3D two-photon polymerization with real-time calculated holograms.

Two-photon polymerization enables the fabrication of micron sized structures with submicron resolution. Spatial light modulators (SLM) have already been used to create multiple polymerizing foci in the photoresist by holographic beam shaping, thus enabling the parallel fabrication of multiple microstructures. Here we demonstrate the parallel two-photon polymerization of single 3D microstructures by multiple holographically translated foci. Multiple foci were created by phase holograms, which were calculated real-time on an NVIDIA CUDA GPU, and displayed on an electronically addressed SLM. A 3D demonstrational structure was designed that is built up from a nested set of dodecahedron frames of decreasing size. Each individual microstructure was fabricated with the parallel and coordinated motion of 5 holographic foci. The reproducibility and the high uniformity of features of the microstructures were verified by scanning electron microscopy.

Highly efficient volume hologram multiplexing in thick dye-doped jelly-like gelatin.

Dye-doped jelly-like gelatin is a thick-layer self-developing photosensitive medium that allows single and multiplexed volume phase holograms to be successfully recorded using pulsed laser radiation. In this Letter, we present a method for multiplexed recording of volume holograms in a dye-doped jelly-like gelatin, which provides significant increase in their diffraction efficiency. The method is based on the recovery of the photobleached dye molecule concentration in the hologram recording zone of gel, thanks to molecule diffusion from other unexposed gel areas. As an example, an optical recording of a multiplexed hologram consisting of three superimposed Bragg gratings with mean values of the diffraction efficiency and angular selectivity of ∼75% and ∼21', respectively, is demonstrated by using the proposed method.

Extended focus imaging in digital holographic microscopy: a review

The microscope is one of the most useful tools for exploring and measuring the microscopic world. However, it has some restrictions in its applications because the microscope’s depth of field (DOF) is not sufficient for obtaining a single image with the necessary magnification in which the whole longitudinal object volume is in focus. Currently, the answer to this issue is the extended focused image. Techniques proposed over the years to overcome the limited DOF constraint of the holographic systems and to obtain a completely in-focus image are discussed. We divide them in two macro categories: the first one involves methods used to reconstruct three-dimensional generic objects (including techniques inherited from traditional microscopy, such as the sectioning and merging approach, or multiplane imaging), while the second area involves methods for objects recorded on a tilted plane with respect to hologram one (including not only the use of reconstruction techniques and rotation matrices, but also the introduction of a numerical cubic phase plate or hologram deformations). The aim is to compare these methods and to show how they work under the same conditions, proposing different applications for each.

Holographic characteristics of a modified photothermorefractive glass

The spectral and holographic characteristics of classical and modified photothermorefractive glass are compared. It shows that modified photothermorefractive glass has no additional absorption bands in the blue–green region, as is the case in classical photothermorefractive glass. This makes it possible to record purely phase holograms in modified photothermorefractive glass. The refractive-index increment is a factor of 2 greater in modified photothermorefractive glass than in classical photothermorefractive glass.

Tailoring 3-Component Photoinitiating Systems for Use as Efficient Photopolymerizable Holographic Material

To enhance the efficiency of photopolymerizable systems as holographic recording materials, the use of 3-component photoinitiating systems (PIS) was explored. In order to get more insight into the hologram formation, gratings' recording curves were compared to those of monomer conversion obtained by RTFTIR spectroscopy. This work outlines the differences between the photoinitiating systems. A holographic 3-component recording material giving rise to thick phase holograms with both high diffraction yield and high rate of formation is highlighted.

Note: laser beam scanning using a ferroelectric liquid crystal spatial light modulator.

In this work we describe laser beam scanning using a ferroelectric liquid crystal spatial light modulator. Commercially available ferroelectric liquid crystal spatial light modulators are capable of displaying 85 colored images in 1 s using a time dithering technique. Each colored image, in fact, comprises 24 single bit (black and white) images displayed sequentially. We have used each single bit image to write a binary phase hologram. For a collimated laser beam incident on the hologram, one of the diffracted beams can be made to travel along a user defined direction. We have constructed a beam scanner employing the above arrangement and demonstrated its use to scan a single laser beam in a laser scanning optical sectioning microscope setup.

Hybrid coding method of multiple orbital angular momentum states based on the inherent orthogonality

We put forward a hybrid coding method of multiple orbital angular momentum (OAM) states based on the inherent orthogonality. Theoretical analysis shows this new coding method can tremendously improve the coding efficiency compared to the conventional way, and the OAM mode can be extended to all the transverse orthographic modes such as Bessel beams or Laguerre-Gauss modes. We also present a typical scheme to achieve the encoding of hybrid OAM states based on a concentric ring resonator array. We further suggest a digital device and a phase hologram pattern to decode the hybrid OAM states.

Holographic Image Projection on Tilted Planes by Phase-Only Computer Generated Hologram Using Fractional Fourier Transformation

We propose a new method to compute the phase-only computer-generated hologram (CGH) that can modulate an image on a tilted plane. We used the fractional Fourier transformation to compute light propagation between the hologram plane and the tilted plane. The phase-only hologram was obtained by an iterative algorithm. We proposed a method to calculate the inverse optical propagation from the hologram plane to a tilted plane by coordinate rotation and a simple interpolation in the Fourier domain. The experimental results show that the projected image on the tilted plane can be reconstructed by a phase-only hologram generated by our iterative algorithm. We also show that the quality of the projected image on the tilted plane is better than the image projected by the phase hologram generated by traditional method.

Highly efficient electron vortex beams generated by nanofabricated phase holograms

We propose an improved type of holographic-plate suitable for the shaping of electron beams. The plate is fabricated by a focused ion beam on a silicon nitride membrane and introduces a controllable phase shift to the electron wavefunction. We adopted the optimal blazed-profile design for the phase hologram, which results in the generation of highly efficient (25%) electron vortex beams. This approach paves the route towards applications in nano-scale imaging and materials science.

Shortening method for optical reconstruction distance in digital holographic display with phase hologram

We present a method to solve a distance issue. In digital holography, the reconstruction distance is different from the original one because of the difference of the pixel size between an imaging device and a display device. In general, the distance is larger because the pixel size of the display is larger than that of the imaging device. This makes it hard to recognize perception of the stereoscopic effect when holographic reconstructed images are used for the stereopsis system. Typically, a numerical propagation method and a spherical phase addition are used to shorten the distance. However, each method is not shortened enough. To clear the criterion, the limitation of each method is verified. By combining two methods, the reconstruction distance is shortened from 4440 to 547 mm. In addition, it is successfully shown that the proposed combining method is useful for the stereopsis by visual perception evaluation.