Pre-compensation method for optimizing recording process of holographic optical element lenses with spherical wave reconstruction

Compensated DOE in a VHG-based waveguide display to improve uniformity

Augmented reality head-mounted displays (AR-HMDs) utilizing diffractive waveguides have emerged as a popular research focus. However, the illuminance uniformity over the fields of view (FOV) is often unsatisfactory in volume holographic grating (VHG) based waveguide displays. This paper proposes a high uniformity AR waveguide display system. Firstly, the angular uniformity of the VHG-based waveguide displays is analyzed. Subsequently, diffractive optical elements (DOEs) are seamlessly integrated onto the outer coupling surface of the waveguide substrate to improve the angular uniformity through phase compensation. To design the DOE phase, the multi-objective stochastic gradient descent (MO-SGD) algorithm is proposed. A single DOE is used to compensating various images form the image source. A hybrid loss, which includes the learned perceptual image patch similarity (LPIPS) metric, is applied to enhance the algorithm performance. Simulation results show that the proposed method effectively suppresses illumination degradation at the edge FOV in exit pupil images of the waveguide display system. In the results, the peak signal-to-noise ratio (PSNR) is improved by 5.54 dB. Optical experiments validate the effectiveness of the proposed method. The measured nonuniformity (NU) against FOVs is improved by 53.05% from 0.3749 to 0.1760.

Effect of shrinkage in photopolymer film on the information transmitted through the holographic waveguide for near eye displays

Due to shrinkage in photopolymer materials, the angle of the reconstruction beam in holographic optical elements (HOEs) does not match with the Bragg condition, resulting in a decreased amount of light in the desired direction or loss of transmitted information to rematch the Bragg condition. Thus, to ensure final display features it is imperative to precompensate the shrinkage effect. We derived simplified expressions for precompensation in recording geometries of required HOEs in holographic waveguide-based Maxwellian near eye displays. An acceptable range of detuning from the Bragg angle is also analyzed. The experimentally measured 4.95% shrinkage in photopolymer film for 0° and 45° recording angles of beams was precompensated using -0.86∘ and 43.7° recording angles. Theoretical results are validated through simulation and experiments.

Super multi-view near-eye display with a lightguide combiner

We propose a lightguide-type super multi-view near-eye display that uses a digital micromirror device and a LED array. The proposed method presents three-dimensional images with a natural monocular depth cue using a compact combiner optics which consists of a thin lightguide and holographic optical elements (HOEs). Feasibility of the proposed method is verified by optical experiments which demonstrate monocular three-dimensional image presentation over a wide depth range. We also analyze the degradation of the image quality stemming from the spectral spread of the HOEs and show its reduction by a pre-compensation exploiting an adaptive moment estimation (Adam) optimizer.

Analysis on image quality of a holographic lens with a non-converging signal wave for compact near-eye displays

We analyze an image quality of a holographic lens (HL) in order to implement compact near-eye displays using a flat-panel-type micro-display panel. The proposed method utilizes a non-converging signal wave in a fabrication process of the HL, so that it provides affordable eye-box size with minimizing the aberration due to rays in the off-Bragg condition. For analyzing and optimizing the HL based on the non-converging signal wave, we introduce a comprehensive analysis model for an assessment of the image quality in the HL. The analysis model, inspired from the conventional lens design strategy for near-eye displays, evaluates the focal spot quality for incident rays forming each pixel with considering the on- and off-Bragg diffraction. The theoretical analysis is validated by simulation results using a volume hologram model in Zemax OpticStudio. As experimental verifications, we realize a prototype system using photopolymer-based HLs in a green color with the high transmittance of 89.3%. The image quality of the HLs is analyzed, which coincides well with the proposed analysis and assessment metric. By building a compact experimental setup employing the HL and a micro-organic light emitting diode display, we present see-through images with 8.0 mm of eye-box with reduced aberrations.

Uniformity improvement of a reconstructed-holographic image in a near-eye display system using off-axis HOE

When a near-eye display (NED) device reproduces an image at a location close to the eye, the virtual image is implemented at a large angle. The uniformity of the image is unbalanced due to the change in diffraction efficiency by the hologram recording angle and angular selectivity. This study proposes a method for implementing an optimal uniform image by analyzing the diffraction efficiency and the reconstructed image was analyzed using angular selectivity generated while reproducing the source point of the diffused image as an intermediate element by holographic optical element (HOE). This research provides practical results for displaying high diffraction efficiency and immersive holographic images in the NED system with HOE as uniformed intermediate elements.

Portable autostereoscopic display based on multi-directional backlight

A multi-directional backlight autostereoscopic display system with high resolution, low crosstalk, and motion parallax is developed in this paper. The proposed multi-directional backlight system is based on the Bragg mismatched reconstruction of volume holographic optical element (VHOE), and includes a set of light sources which are uniformly arrayed along one direction. Each light source produces its corresponding directional lighting to follow the human eye position detected by an eye tracker. Two scenarios are presented to build the multi-directional backlight system. The prism-type backlight system which guides the incident beam with a prism is relatively simple and easy to implement. The waveguide-type one which employs a transflective film to expand the incident light beam within the waveguide and modulate the intensity of the incident beam, is relatively thin and is applicable to large-area display. Two prototypes are built and the effectiveness of the proposed autostereoscopic display system is verified by the experimental results.

Projection-type see-through near-to-eye display with a passively enlarged eye-box by combining a holographic lens and diffuser

We propose a projection-type see-through near-to-eye display by combining two holographic optical elements (HOEs), a holographic lens with the on-axis projection configuration and a holographic diffuser. The proposed method provides an enlarged eye-box by virtue of diffusing properties of an HOE diffuser (HOED) without any temporal multiplexing methods. In this paper, a thorough analysis on imaging characteristics of an HOE lens (HOEL) according to the projection configuration is provided, so that we optimize the recording geometry of the HOEL with the passively enlarged eye-box. The theoretical analysis is validated by simulation results using a volume hologram model in OpticStudio. As experimental verifications, we realize a prototype of the proposed method using the photopolymer-based HOEs in a single color. The fabricated HOEL and HOED show high transmittance of 84.9% and 62.2%, respectively. By using the fabricated HOED with a diffusing angle over 20 ° and an angular selectivity of 8.7 °, the prototype successfully provides see-through images with the eye-box larger than 5 mm in width.

Design of a high-resolution holographic waveguide eye-tracking system operating in near-infrared with conventional optical elements

The eye-tracking system plays an essential role in the augmented reality (AR) eyewear. Waveguide volume holographic optical elements (HOE) that can be made with high efficiency, thin form, and lightweight are well-suited for this application. Traditional holographic lenses formed with spherical wavefronts at visible wavelengths and used for near-infrared (NIR) eye-tracking systems suffer from significant image aberrations, image tilt, and ghost images. This work describes a complete holographic lens design methodology that systematically addresses these issues and details the specifications of conventional optical elements that provide the optimized wavefronts for the hologram construction beams. The resulting waveguide HOE NIR eye-tracking system has an image resolution of ∼10 lp/mm when the designed holographic lens is deposited on a waveguide with a refractive index of 1.8 and thickness of 0.5 mm.

Crosstalk Reduction in Voxels for a See-Through Holographic Waveguide by Using Integral Imaging with Compensated Elemental Images

The representation of three-dimensional volumetric pixels, voxels, is an important issue for the near-to-eye displays (NEDs) to solve the vergence-accommodation conflict problem. Although the holographic waveguides using holographic optical element (HOE) couplers are promising technologies for NEDs with the ultra-thin structure and high transparency, most of them have presented only a single and fixed depth plane. In this paper, we analyze the imaging characteristics of holographic waveguides, particularly to represent the arbitrary voxels and investigate the voxel duplication problem arising from the non-collimated light from the voxels. In order to prevent the image crosstalk arising from the voxel duplication, we propose an adjustment method for the emission angle profile of voxels by using the integral imaging technique. In the proposed method, the sub-regions of elemental images, which correspond to the duplicated voxels, are masked in order to optimize the emission angle of integrated voxels. In the experimental verification, a see-through integral imaging system, based on the organic light-emitting diode display and a holographic waveguide with the thickness of 5 mm, was constructed. The fabricated HOE in the waveguide showed high diffraction efficiency of 72.8 %, 76.6%, and 72.5 % for 460 nm, 532 nm, and 640 nm lasers, respectively. By applying the masked elemental images, the proposed method resulted in a reduced crosstalk in the observed voxels by 2.35 times. The full-color experimental results of see-through holographic waveguide with integral imaging are provided, whereby the observed 3D images are presented clearly without the ghost images due to the voxel duplication problem.

Focus issue introduction: 3D image acquisition and display: technology, perception, and applications

This feature issue of Optics Express is organized in conjunction with the 2020 OSA conference on 3D image acquisition and display: technology, perception and applications which was held virtually in Vancouver from 22 to 26, June 2020 as part of the imaging and sensing congress 2020. This feature issue presents 29 articles based on the topics and scope of the 3D conference. This review provides a summary of these articles.