Volume holographic printing using unconventional angular multiplexing for three-dimensional display

Holographic Grating Enhancement Induced by a Dual-Photo-Initiator System in PMMA Substrate Polymers

Polymer systems induced by the reaction between monomers and photo-initiators play a crucial role in the formation of volume-phase gratings. In this paper, we fabricated a dual-photo-initiator photopolymer by doping EY (Eosin Yellow) molecules into a TI (Titanocene, Irgacure 784@BASF) dispersed PMMA (poly-[methyl methacrylate]) substrate system, with the aim of promoting the diffusion and polymerization processes in volume holographic storage. The two-wave interference system is adopted to record a permanent grating structure in our materials. The temporal diffraction variations of photopolymerization (during the interference exposure) and dark diffusion (after the interference exposure) processes have been investigated and analyzed. Aiming to analyze the influence of EY doping ratios on holographic performances, some key parameters were examined in the experiment. We first measured the temporal evolution of diffraction efficiency, then an exponential fitting was adopted to obtain the response time. Finally, the angular selectivity was evaluated by the Bragg condition after holographic recording. Also, the temporal evolution of each component is described by the nonlocal polymerization-driven diffusion model with a dual-photo-initiator composition, theoretically. Furthermore, we experimentally achieved the holographic grating enhancement in both the dark diffusion and photopolymerization processes by doping appropriate EY concentrations, respectively. This work provides a foundation for the acceptability of TI&EY/PMMA polymers in further holographic storage research.

Depth of field and resolution-enhanced integral imaging display system

Depth of field (DOF) and resolution are mutually restricted in integral imaging (II) display. To overcome the trade-offs, we propose an II display system that simultaneously enhances the DOF and resolution. The system consists of a transmissive mirror device (TMD), a semi-transparent mirror (STM), and two II display units. Each II display unit consists of a 4K display screen and a micro-lens array (MLA). Benefiting from the parallel placement of the TMD and the STM, two central depth planes are reconstructed, which effectively enhances the DOF. Meanwhile, the resolution in the overlapping DOF region is increased to two times due to the interpolation of the light field information from two II display units. The impact of the distance between the two II display units and the TMD on the 3D image quality is analyzed. In geometric optics, a distance between the II two display units and the TMD is optimized to eliminate ghost images. In wave optics, a distance is optimized to eliminate 3D pixel gaps by exploiting the diffraction effect of the TMD. Both the geometric and wave optics are considered simultaneously to obtain a high-quality 3D image without ghost images and 3D pixel gaps. A DOF and resolution-enhanced II display system is developed, and the experimental results verify its feasibility.

System to eliminate the graininess of an integral imaging 3D display by using a transmissive mirror device

We propose a system to eliminate the graininess of an integral imaging 3D display by using a transmissive mirror device (TMD). The proposed system consists of a 2D display, a micro-lens array (MLA), and a TMD. The TMD comprises square apertures with mirror-reflective inner wall. The light rays pass through the square aperture to form a diffraction spot, and the diffraction light intensity has a Sinc-function distribution. Therefore, the TMD can be used as an optical low-pass filter. In a certain imaging range, the mainlobe of the Sinc-function distribution is almost unchanged. The TMD has the property of a volumetric optical low-pass filter. It can interpolate the interval between discrete 3D pixels. Therefore, the TMD can be used to eliminate the graininess. The resolution of the 3D image is improved by 2.12 times. The experimental results verify the feasibility of the proposed system.

Locally controllable 2D/3D mixed display and image generation method

In this paper, a locally controllable two-dimensional (2D)/ three-dimensional (3D) mixed display system and corresponding image generation method are proposed. The proposed system is mainly composed of a collimating backlight module (CBM) and a light control module (LCM). The CBM provides collimating polarized light. The LCM modulates a part of the collimating polarized light to form point light sources for 3D display and the other part to form scattered light sources for 2D display. The 2D and 3D display states can be locally controlled by using a pixelated mask loaded on a polarization switching layer. In addition, a corresponding image generation method is proposed. According to observer's demand, the parallax image is divided into target image area and residual image area by using deep learning matting algorithm, and a 2D/3D mixed light field image with full parallax 3D target image and high-resolution 2D residual image is generated. We developed a prototype based on the proposed locally controllable 2D/3D mixed display structure and generated two sets of 2D/3D mixed light field image with different target objects and residual objects from the same parallax images. The experimental results demonstrated the effectiveness of our proposed system and the corresponding image generation method. High-resolution 2D image and full parallax 3D image were displayed and locally switched in the experimental system.

Common-path angular-multiplexing holographic data storage based on computer-generated holography

An unconventional angular-multiplexed recording technique is proposed for holographic data storage based on a computer-generated hologram (CGH) technique. While general angular-multiplexed recording techniques require a Mach-Zehnder interferometer to record data pages as volume holograms, the proposed method records ones with a common-path configuration with the help of a CGH technique, which prevents the optical setup from being bulky. In the proposed method, the CGH reconstructs signal and reference beams simultaneously, and these beams interfere in a recording medium. By changing the diffraction angle of the reference beam from the CGH, angular multiplexing is accomplished with a common-path optical setup without additional optical elements. Multiplexed recording of four data pages is demonstrated in a proof-of-principle experiment, which indicates the feasibility of the proposed method.

Holographic Grating Enhancement of TI/PMMA Polymers in the Dark Diffusion Process

The dark diffusion enhancement process (DDEP) caused by photopolymerization during the pre-exposure of TI/PMMA (titanocene dispersed methyl methacrylate matrix) polymers was theoretically analyzed and experimentally investigated, revealing the holographic grating enhancement of TI/PMMA polymers in the post-exposure process without additional operations. The diffusion of photo-initiators and photoproducts dominated the grating enhancement process after exposure. We adopted two pre-exposure methods, long-time (second level) and short-time (millisecond level) laser exposure, at 532 nm, to investigate the DDEP during the post-exposure process. A five-fold enhancement in grating strength was achieved in consecutive long-time pre-exposures, while a two-fold grating development was examined after short-time exposure. Additionally, the exposure durations and repetition rates influenced the grating increment of the DDEP. This study provided a basis for the feasibility of holographic application in TI/PMMA photopolymers via the dark diffusion effect.

Design of diffractive optical element projector for a pseudorandom dot array by an improved encoding method

Here we achieved the structured light patterns of a pseudorandom dot array by a single diffractive optical element. The dot array can be applied to achieve three-dimensional imaging. First, the pseudorandom dot array was generated by the proposed improved encoding methods, which are an improved formula-method-based encoding algorithm and an improved enumeration-method-based encoding algorithm. Second, diffractive optical elements were designed as dot projectors to generate pseudorandom dots by the Gerchberg-Saxton algorithm. Pseudorandom dot arrays with different sizes were generated to validate the proposed encoding methods. A pseudorandom dot array with a maximal size of 713×449 was experimentally achieved. By analyzing the intensity distribution of the projecting pattern, the projected dots have a unique window of 7×7, and the dot array is distortion free. The proposed encoding methods, optimization algorithm, and applied fabrication technology have potential applications in three-dimensional imaging, three-dimensional sensing, shape measurement, and deformation measurement with high decoding speed.

High-resolution augmented reality 3D display with use of a lenticular lens array holographic optical element

An augmented reality (AR) three-dimensional (3D) display based on one-dimensional integral imaging (1DII), by using a lenticular lens array holographic optical element (LLA-HOE), is proposed. The 3D image of the 1DII has higher vertical resolution compared with the image of conventional integral imaging whose resolution is sharply reduced for providing quasi-continuous viewpoints in both the horizontal and vertical directions. The proposed 3D display consists of a projector and an LLA-HOE and is compact. As an image combiner, the LLA-HOE can diffract Bragg-matched light rays that have the same wavelength and incident angle as the original reference wave; it can also function as a lenticular lens array to reconstruct a 3D image but transmit other light rays emitted from the surroundings. In the experiment, an LLA-HOE of 80  mm×80  mm size is recorded, and a combination of a high-resolution 3D virtual image and a real 3D object is presented by the proposed AR 3D display.

Multiplexed holographic non-axial-scanning slit confocal fluorescence microscopy

A non-axial-scanning multi-plane microscopic system incorporating multiplexed volume holographic gratings and slit array detection to simultaneously acquire optically sectioned images from different depths is presented. The proposed microscopic system is configured such that multiplexed volume holographic gratings are utilized to selectively produce axial focal points in two or more planes inside the sample, and then to use confocal slit apertures to simultaneously image these multiple planes onto corresponding detection areas of a CCD. We describe the design, implementation, and experimental data demonstrating this microscopic system's ability to obtain optically sectioned multi-plane images of fluorescently labeled standard micro-spheres and tissue samples without scanning in axial directions.

Computer-generated hologram generation method to increase the field of view of the reconstructed image

In this paper, a computer-generated hologram (CGH) generation method is proposed to increase the field of view (FOV) in the holographic display. The CGH is generated through accumulating interference patterns of all object points. The size of each interference pattern is equal to the sum of the size of the recorded object and the spatial light modulator (SLM), so the size of the interference pattern is increased. The position of the interference pattern is related to that of the corresponding recorded object point. In the reconstruction process, three SLMs in a planar configuration are used to load the CGH. The seams between the SLMs are eliminated by beam splitters. Meanwhile, the boundaries of the diffraction light of all interference patterns are parallel with each other. Compared with the holographic display method using one SLM, the experiments demonstrate that the proposed method can reduce the demand for viewing distance and expand the FOV obviously in the horizontal direction, especially for the large-sized recorded object.

Method of single-step full parallax synthetic holographic stereogram printing based on effective perspective images' segmentation and mosaicking

With the principle of ray-tracing and the reversibility of light propagation, a new method of single-step full parallax synthetic holographic stereogram printing based on effective perspective images' segmentation and mosaicking (EPISM) is proposed. The perspective images of the scene are first sampled by a virtual camera and the exposing images, which are called synthetic effective perspective images, are achieved using the algorithm of effective perspective images' segmentation and mosaicking according to the propagation law of light and the viewing frustum effect of human eyes. The hogels are exposed using the synthetic effective perspective images in sequence to form the whole holographic stereogram. The influence of modeling parameters on the reconstructed images are also analyzed, and experimental results have demonstrated that the full parallax holographic stereogram printing with the proposed method could provide good reconstructed images by single-step printing. Moreover, detailed experiments with different holographic element sizes, different scene reconstructed distances, and different imaging planes are also analyzed and implemented.