Search Results (23)

At present, most near-eye display devices adopt flat substrates, which have problems such as limited field of view (FOV) and bulky shape, while the curved structure is expected to expand the FOV with appropriate volume. In this paper, we propose a curved holographic augmented reality (AR) near-eye display system based on holographic optical element (HOE) with the ability to expand the FOV. The system includes a display source and a HOE with curved substrate. We analyze the system by exploiting the diffraction theory between plane and curved surface, and a layered and weighted FOV optimization method using particle swarm optimization algorithm is proposed to realize the optimization of the phase of freeform HOE. Numerical and experimental results show that the proposed curved holographic near-eye display system can realize cylindrical AR display and expand the FOV of the system. It is expected to be applied to the holographic AR near-eye display in the future. Full article

The paper proposes an equivalent optical scheme of an in-line digital holographic system for particle recording and a mathematical model that establishes a one-to-one correspondence between the dimensional and spatial parameters of a digital holographic image of a particle and the imaged particle itself. The values of the model coefficients used to determine the real size and longitudinal coordinate of a particle according to its holographic image are found by calibration. The model was tested in field and laboratory conditions to calibrate a submersible digital holographic camera designed to study plankton in its habitat. It was shown that four calibration measurements are sufficient enough to determine the model coefficients, and the developed design of the submersible digital holographic camera makes it possible to perform these measurements during the recording of each hologram. In addition, this neither requires data on the refractive index of the medium with particles nor on the parameters of the optical elements of the scheme. The paper presents the results of marine experiments in the Kara Sea and the Laptev Sea, as well as in fresh water in laboratory conditions and in Lake Baikal. The error in measuring the particle size in seawater without the use of the model is 53.8%, while the error in determining their longitudinal coordinates is 79.3%. In fresh water, the same errors were 59% and 54.5%, respectively. The error in determining the position of a particle with the use of the designed mathematical model does not exceed 1.5%, and the error in determining the size is 4.8%. The model is sensitive to changes in the optical properties of the medium, so it is necessary to perform calibration in each water area, and one calibration is quite sufficient within the same water area. At the same time, the developed design of the submersible holographic camera allows, if necessary, calibration at each holographing of the medium volume with particles. Full article

Understanding the formation processes of holographic gratings in polymers as a function of material composition and processing is important for the development of new materials for holography and its associated applications. Among the processing-related factors that affect grating formation in volume holographic recording material, pre-exposure, prebaking and dark storage, as well as the associated variations in layer thickness and composition, are usually underestimated. This study highlights the influence and interaction of these factors and shows that they should not be neglected. This is of particular importance for samples with a free surface. Here, one such epoxy-based free-surface material is investigated. To determine the influence of prebaking on the holographic grating formation, as well as on the achieved refractive index contrast, angular resolved analysis of volume holographic phase gratings is applied through point-by-point scanning of the local material response. Grating characteristics are determined by comparison with simulations based on rigorous coupled wave theory. Thus, the optimal dose for prebaking can be determined, as well as the optimal exposure time, depending on the dose. The influence of dark storage on the material response is investigated over a period of 12 weeks and shows a strong dependence on the deposited energy density. Full article

In the present work, we consider an optical design of a slitless spectrograph for an existing 0.5 m-class telescope. This design concept has a number of advantages such as compact size, simplicity, and simultaneous coverage of a large field of view. A challenge with this design is correcting aberrations caused by placing a dispersing element in a converging beam. To overcome this issue, we propose to use a composite grism, which represents a combination of a prism and a volume-phase holographic grating, the latter which is split into zones with independently optimized parameters. We demonstrate two designs of such a grism. In both designs, the spectrograph operates in the range of 450–950 nm in an $F/6.8$ beam and covers a field of view of 35.6${}^{\prime }$ × 7.2${}^{\prime }$. Through advanced modeling, it is shown that a composite grism having four rectangular zones with different thickness and index modulation depth of the hologram and recorded with an auxiliary deformable mirror decreases the astigmatic elongation by a factor of 85, increases the spectral resolving power by 4.4 times, and reaches $R1389$ while increasing the average diffraction efficiency by a factor of $1.31$. If we reduce the number of zones to only two, replace the deformable mirror with two static corrector plates, and fix the hologram thickness, the corresponding performance gains still remain high: the astigmatism is reduced by a factor of 61, the spectral resolving power is up to $1.7$ times higher, reaching $R1067$, and the efficiency is increased by a factor of $1.27$. This shows that the proposed design allows the construction of a simple and compact instrument, providing high performance over the entire field of view and spectral range. Full article

Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design and fabrication of diffractive coupling elements through a holographic process that avoids the use of coupling prisms during recording and will help in the design of recording arrangements that better suit the mass production of low-cost elements, especially those designed for non-normal incidence. In this study, the recording angles needed for a range of recording wavelengths were explored for VHOE couplers designed for input angles (in air) ranging from 0° to −55°. Then, in order to validate the model, holographic recording in Bayfol HX 200 photopolymer at 532 nm was used to fabricate photopolymer VHOE couplers for 633 nm light (−45° input angle in air). Bragg curves obtained experimentally for different probe wavelengths (403 nm, 532 nm and 633 nm) confirm the recording of the desired grating structures to a precision of ±1°, and coupling is demonstrated at 633 nm with a diffraction efficiency of up to 72%. Furthermore, the model is used to identify the origins of some weaker spurious gratings observed alongside the expected ones. Full article

Continual improvements to holographic recording materials make the development of volume holographic optical elements increasingly more attainable for applications where highly efficient, lightweight diffractive optical elements can replace conventional optics. A fast-curing, water resistant photosensitive sol–gel capable of volume holographic recording has recently drawn attention for its extreme environmental and physical robustness, in particular its water/moisture and scratch resistance. However, to date, the refractive index modulation has been limited. While water-resistant properties are invaluable in the face of the weathering which many practical systems for outdoor applications will endure, high refractive index modulation is also important in order to facilitate high diffraction efficiency holograms recorded in relatively thin layers. Lower grating thickness ensures a large angular and wavelength range of operation-properties that are critical for many applications of holographic optical elements such as solar light harvesting, optical displays and illumination management. For any application where low-cost mass production is envisaged, sensitivity/writing speed is also a crucial factor. In this research, we studied the recording properties of these water-resistant photosensitive sol–gel layers at two different recording wavelengths (532 and 476 nm) and investigated methods for improving these properties. We report more than two-fold improvement of the refractive index modulation from $1.4\times {10}^{-3}$ to $3.3\times {10}^{-3}$ in layers of thickness ranging from 40–100 μm and more than an order of magnitude increase in photosensitivity/recording speed through better matching between recording wavelength and layer absorption, chemical alterations and thermal post-processing techniques. Full article

We present an array-based volume holographic optical element (vHOE) recorded as an optical combiner for novel display applications such as smart glasses. The vHOE performs multiple, complex optical functions in the form of large off-axis to on-axis wave front transformations and an extended eye box implemented in the form of two distinct vertex points with red and green chromatic functions. The holographic combiner is fabricated by our extended immersion-based wave front printing setup, which provides extensive prototyping capabilities due to independent wave front modulation and large possible off-axis recording angles, enabling vHOEs in reflection with a wide range of different recording configurations. The presented vHOE is build up as an array of sub-holograms, where each element is recorded with individual optical functions. We introduce a design and fabrication method to combine two angular and two spectral functions in the volume grating of individual sub-holograms, demonstrating complex holographic elements with four multiplexed optical functions comprised in a single layer of photopolymer film. The introduced design and fabrication process allows the precise tuning of the vHOE’s diffractive properties to achieve well-balanced diffraction efficiencies and angular distributions between individual multiplexed functions. Full article

Photopolymerisable glasses are holographic recording materials which provide good recording capability, improved dimensional stability, and negligible shrinkage. Recently, a novel photopolymerisable hybrid sol-gel (PHSG) for holographic recording of volume gratings has been reported. The PHSG has significantly improved gelation time and high water resistance, both of which make it an attractive material for mass production of holographic optical elements (HOEs) with no sensitivity to ambient humidity. In order to achieve full control over the performance of the material and further improve its properties, a study of grating formation under holographic patterning is essential. This paper reports characterisation of the grating recording in PHSG. The approach is based on the analysis of grating parameters during exposure and post-recording dark processes. The obtained results suggest that photopolymerisation of the methacrylate groups is the main contributor to the creation of refractive index modulation during exposure. During the dark process, the enhancement of the refractive index modulation is observed, probably due to further polycondensation. The observations made facilitate controlled and predictable diffraction efficiency of gratings recorded on the PHSG, thereby furthering the prospect of the development of HOEs with customisable specification. Full article

The formation of volume holograms in photosensitive polymers is a complex process under the influence of many interacting factors: material composition and processing, exposure conditions, and pre-exposure affect the development and final characteristics of holographic gratings. In order to better understand the interplay of these influencing factors, the detailed investigations of holographic recording in a new organic material are performed and the results are presented here. The material response and performance of an epoxy-based free surface material designed for volume holography are investigated. For this purpose, time-resolved investigation of volume holographic grating growth is performed on the one hand. Spatially resolved analysis of volume holographic phase gratings by point-by-point scanning of the local material response to the Gaussian intensity distribution of the recording beams is carried out on the other hand. Thus, the influence of pre-exposure on the temporal grating formation, as well as on the final obtained refractive index contrast, was determined. The various effects observed can be explained by the consumption of photosensitive compounds and prior crosslinking in the course of pre-exposure. Rather unexpected effects are that, on the one hand, pre-exposed gratings emerge with ever more complete null diffraction at the transition point and, on the other hand, a stabilizing effect of some degree of pre-exposure on regions exposed with low intensity was identified. Full article

Functionalised holograms are important for applications utilising smart diffractive optical elements for light redirection, shaping and in the development of sensors/indicators. This paper reports on holographic recording in novel magnetic nanocomposites and the observed temperature change in dry layers and liquid samples exposed to alternating magnetic field (AMF). The nanocomposite consists of N-isopropylacrylamide (NIPA)-based polymer doped with magnetic nanoparticles (MNPs), and local heating is achieved through magnetic induction. Here, volume transmission holographic gratings (VTHGs) are recorded with up to 24% diffraction efficiency (DE) in the dry layers of magnetic nanocomposites. The dry layers and liquid samples are then exposed to AMF. Efficient heating was observed in the liquid samples doped with Fe₃O₄ MNPs of 20 nm average size where the temperature increased from 27 °C to 64 °C after 300 s exposure to 111 mT AMF. The temperature increase in the dry layers doped with the same nanoparticles after exposure to 4.4 mT AMF was observed to be 6 °C. No temperature change was observed in the undoped layers. Additionally, we have successfully recorded Denisyuk holograms in the magnetic nanocomposite materials. The results reveal that the magnetic nanocomposite layers are suitable for recording holograms and need further optimisation in developing holographic indicators for mapping AMFs. Full article

Global warming is a very topical issue, therefore the search for new renewable energy sources is considered of fundamental importance. Among these, solar energy offers great possibilities considering that the amount of sunlight hitting the Earth ‘s surface in an hour and a half is enough to meet the world’s electricity consumption for a complete year. Generally, solar concentrators are used to collect the solar radiation and to concentrate it at a single focal point. These devices consist in a set of mirrors or mechanical structures to reduce the area of a photovoltaic cell, which is typically very expensive. Volume transmission phase holographic optical elements could be opportunely designed and realized to obtain a simple, lightweight, compact and inexpensive planar solar concentrator. With the aim of bringing scientific attention to this still developing topic, in this work we critically report a complete investigation on a new photopolymeric material obtained by sol-gel reactions used as possible recording material for volume holographic solar concentrators; as a proof of concept, both terrestrial and extreme environments, such as space, are considered as potential applications. Full article

We proposed an aerial projection 3D display based on integral imaging. It is composed of a projector, a lens-array holographic optical element (HOE), and two parabolic mirrors. The lens-array HOE is a diffraction grating and is made by the volume holography technique. The lens-array HOE can be produced on a thin glass plate, and it has the optical properties of a lens array when the Bragg condition is satisfied. When the display beams of the element image array (EIA) are projected on the lens-array HOE, 3D images can be reconstructed. The two parabolic mirrors can project 3D images into the air. The Bragg-unmatched light simply passes through the lens-array HOE. Therefore, the aerial projection 3D images appear to be imaged in the air without any medium. In the experiment, a BenQ projector was used for the projection of 3D images, with a resolution of 1600 × 1200. The diameter and the height of each parabolic mirror are 150 mm and 25 mm, respectively. The inner diameter of the parabolic mirror is 40 mm. The 3D images were projected in the air, and the experimental results prove the correctness of our display system. Full article

An optical see-through two-dimensional (2D)/three-dimensional (3D) compatible display using variable-focus lens and multiplexed holographic optical elements (MHOE) is presented. It mainly consists of a MHOE, a variable-focus lens and a projection display device. The customized MHOE, by using the angular multiplexing technology of volumetric holographic grating, records the scattering wavefront and spherical wavefront array required for 2D/3D compatible display. In particular, we proposed a feasible method to switch the 2D and 3D display modes by using a variable-focus lens in the reconstruction process. The proposed system solves the problem of bulky volume, and makes the MHOE more efficient to use. Based on the requirements of 2D and 3D displays, we calculated the liquid pumping volume of the variable-focus lens under two kinds of diopters. Full article

Diffraction waveguides are widely used in augmented reality devices as information display systems for the introduction of data into the human visual field in order to supplement information about the world around us. This paper formulates the principles of radiation conversion in diffraction waveguides made of photo-thermo-refractive glass on the basis of multiplexed volume holographic gratings, and the advantages and disadvantages of high spectral-angular selectivity are analyzed. In the optical scheme, each of the superimposed volume holographic gratings in the diffraction waveguide forms a corresponding part of the composite angular field of view of the augmented reality device. A proposed mathematical model based on angular multiplexing made it possible to synthesize the diffraction optical element for a new type of diffraction waveguide made from photo-thermo-refractive glass and to create a prototype with an angular resolution of at least 3.0 ± 0.5′, with a brightness change in the image of less than 20% and with a composite angular field of view of 32°. Full article

Diffractive optical elements (DOEs) have been in development for many years and are an exciting technology with the capability to re-direct light, using diffraction rather than refraction. Holographic Optical Elements (HOEs) are a subset of diffractive optical elements for which the photonic structure is created holographically, i.e., by recording a specific interference pattern in a suitable, photosensitive optical material. Volume HOEs are of particular interest for some applications because of their very high diffraction efficiency and single diffracted order; however, high dispersion and angular wavelength selectivity still present significant challenges. This paper explores a method for producing a compound DOE useful for situations where elements designed for two separate target wavelengths can be advantageously combined to achieve a highly efficient HOE with reduced dispersion. A photopolymer material consisting of two independently sensitized laminated layers is prepared and used in sequential holographic recording at two different wavelengths. The photonic structures recorded are investigated through examination of their diffraction peaks and comparison with the structure predicted by modeling. Finally, the device is illuminated with an expanded diverging beam at both target wavelengths and with white light, and a strong diffracted beam is observed. Full article