Dual-focal waveguide see-through near-eye display with polarization-dependent lenses

Waveguide holography for 3D augmented reality glasses

Near-eye displays are fundamental technology in the next generation computing platforms for augmented reality and virtual reality. However, there are remaining challenges to deliver immersive and comfortable visual experiences to users, such as compact form factor, solving vergence-accommodation conflict, and achieving a high resolution with a large eyebox. Here we show a compact holographic near-eye display concept that combines the advantages of waveguide displays and holographic displays to overcome the challenges towards true 3D holographic augmented reality glasses. By modeling the coherent light interactions and propagation via the waveguide combiner, we demonstrate controlling the output wavefront using a spatial light modulator located at the input coupler side. The proposed method enables 3D holographic displays via exit-pupil expanding waveguide combiners, providing a large software-steerable eyebox. It also offers additional advantages such as resolution enhancement capability by suppressing phase discontinuities caused by pupil replication process. We build prototypes to verify the concept with experimental results and conclude the paper with discussion.

Binary geometric-phase holograms

Diffractive optics elements have exhibited many novel characteristics through various methods of employing Pancharatnam-Berry, or geometric, phase. One geometric-phase hologram (GPH) subset, consisting of a π-difference binary sampling, shows polarization-independent properties that are not present in the continuous GPH and the dynamic-phase binary analog. Here, we investigate the binary geometric-phase holograms (bin-GPHs) realized with anisotropic liquid crystal (LC) polymers. First, the optical properties of the ideal binary polarization grating are derived and simulated showing 81% cumulative first-order efficiency, polarization-independent diffraction when applying a π-switching scheme, innate odd (m = 2k + 1) diffractive orders, and variable polarization output. After, experimental results of two key bin-GPH elements, the binary polarization grating (Λ = 30μm) and binary geometric-phase lens (f/100), with π-offset regions and a 0.5μm transition pixel are presented. We found that the fabricated non-ideal bin-GPHs exhibit near-maximum theoretical polarization-insensitive diffraction efficiency and tunable polarization outputs. The simple, and scalable, fabrication of the anisotropic bin-GPH provides the potential for implementation within the next-generation near-eye displays for polarization-invariant beam-steering and waveguides.

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.

Flicker-free dual-volume augmented reality display using a pixelated interwoven integral floating technique with a geometric phase lens

A geometric phase (GP) integral floating display can provide multifocal three-dimensional (3D) augmented reality (AR) images with enhanced depth expression by switching the focal modes of the GP lens via polarization control. However, using temporal multiplexing to switch between the focal modes of GP optics causes flickering as each 3D AR image is fully presented in different frames and their temporal luminance profile becomes easily recognizable, particularly as the number of available focal modes increases. Here, we propose a novel integral floating technique to generate pixelated interwoven 3D AR images; a half of each image is spatially mixed with another and presented in both focal modes simultaneously to resolve the flickering issue. The principle was verified via experimental demonstration and optically measured data.

Tunable focal waveguide-based see-through display with negative liquid crystal lens

A see-through display based on a planar holographic waveguide with a tunable focal plane is presented. A negative liquid crystal lens is attached on the outcoupling location of the waveguide to manipulate the image distance. The continuous tunable range for the focal length is from negative infinity to -65 cm. The demonstrated prototype system provides 10.5° field-of-view (FOV) for the images not locating at infinity. The FOV for the images not locating at infinity is limited by the diameter of the liquid crystal lens. The lens function of the liquid crystal lens is polarization dependent. By controlling the polarization states of the real scene and the input information image, the liquid crystal lens keeps the see-through function for a real scene and simultaneously plays the role of a negative lens for the input information image. Compared to the see-through display system with a single focal plane, the presented system offers a more comfortable augmented reality (AR) experience.

Design, analysis and optimization of a waveguide-type near-eye display using a pin-mirror array and a concaved reflector

Waveguides have become one of the most promising optical combiners for see-through near-eye displays due to the thickness, weight, and transmittance. In this study, we propose a waveguide-type near-eye display using a pin-mirror array and a concaved reflector with a compact outlook, optimized image uniformity and stray light. Issues have been discussed in detail, which include field of view (FOV), eye-box, resolution, depth of field (DOF), display uniformity and stray light artifacts. It can be shown that the DOF can be extended (when compared with traditional waveguide-type near-eye displays) to alleviate the vergence-accommodation conflict (VAC) problem, and the uniformity & stray light can be improved with an optimal structure. Moreover, reflective surfaces have been introduced as the input and output coupling with a compact outlook, an easy-processing structure and the achromatic performance. A prototype based on the proposed method have been successfully developed, and virtual images with an extended DOF can be shown along with the real-world.

Actuating compact wearable augmented reality devices by multifunctional artificial muscle

An artificial muscle actuator resolves practical engineering problems in compact wearable devices, which are limited to conventional actuators such as electromagnetic actuators. Abstracting the fundamental advantages of an artificial muscle actuator provides a small-scale, high-power actuating system with a sensing capability for developing varifocal augmented reality glasses and naturally fit haptic gloves. Here, we design a shape memory alloy-based lightweight and high-power artificial muscle actuator, the so-called compliant amplified shape memory alloy actuator. Despite its light weight (0.22 g), the actuator has a high power density of 1.7 kW/kg, an actuation strain of 300% under 80 g of external payload. We show how the actuator enables image depth control and an immersive tactile response in the form of augmented reality glasses and two-way communication haptic gloves whose thin form factor and high power density can hardly be achieved by conventional actuators.

Artificial muscle actuators enabled by responsive functional materials like shape memory alloys are promising candidates for compact e-wearable devices. Here, authors demonstrate augmented reality glasses and two-way communication haptic gloves capable of image depth control and immersive tactile response.

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.

Chromatic aberration correction in bi-focal augmented reality display by the multi-layer Pancharatnam-Berry phase lens

Chromatic aberration is a main obstacle for the commercial application of augmented reality displays. The current digital and optical compensation methods of reducing the chromatic aberration suffer from processing time, power consumption or complex design. Here, a simple strategy of chromatic aberration correction in bi-focal augmented reality near-eye display based on multi-layer Pancharatnam-Berry phase lens has been demonstrated and verified by experimental results. The multi-layer Pancharatnam-Berry phase lens, as a part of optical combiner, is fabricated by three liquid crystal polymer phase lenses with central wavelength in red, green, and blue, respectively. The multi-layer Pancharatnam-Berry phase lens can effectively reduce the chromatic aberration in both convex and concave mode of bi-focal augmented reality system, where the color breakup of virtual images captured in bi-focal augmented reality display is significantly alleviated. Comparing to the value of ΔK = 1.3 m^-1 in single green Pancharatnam-Berry phase lens, the multi-layer Pancharatnam-Berry phase lens system significantly reduce the ΔK to 0.45 m^-1 with reduction of 65.4%, which finally decreases the longitudinal chromatic aberration and improve the quality of images. The proposed broadband multi-layer Pancharatnam-Berry phase lens can benefit augmented reality displays and find widespread application in the near-eye displays.

Waveguide-type Maxwellian near-eye display using a pin-mirror holographic optical element array

We propose a novel, to the best of our knowledge, waveguide-type optical see-through Maxwellian near-eye display for augmented reality. A pin-mirror holographic optical element (HOE) array enables the Maxwellian view and eye-box replication. Virtual images with deep depth of field are presented by each pin-mirror HOE, alleviating the discrepancy between vergence and accommodation distance. The compact form factor is achieved by the thin waveguide and HOE couplers.

Waveguide-type see-through dual focus near-eye display with a polarization grating

Waveguide-type near-eye displays have useful properties such as compact form factor, lightweight and see-through capability. Conventional systems, however, support only a single image plane fixed at a certain distance, which may induce eye fatigue due to the vergence-accommodation conflict. In this paper, we propose a waveguide-type near-eye display with two image planes using a polarization grating. Two images with orthogonal polarizations propagate within the waveguide with different total internal reflection angles and form virtual images at different distances. The use of the polarization grating and two pairs of holographic optical elements enables dual image plane formation by a single waveguide with high transparency for the real scene. Optical experiments confirm the principle of the proposed optical system.

Recent Advances in Photoalignment Liquid Crystal Polarization Gratings and Their Applications

Liquid crystal (LC) circular polarization gratings (PGs), also known as Pancharatnam–Berry (PB) phase deflectors, are diffractive waveplates with linearly changed optical anisotropy axes. Due to the high diffraction efficiency, polarization selectivity character, and simple fabrication process, photoalignment LC PGs have been widely studied and developed especially in polarization management and beam split. In this review paper, we analyze the physical principles, show the exposure methods and fabrication process, and present relevant promising applications in photonics and imaging optics.

Design of a near-eye display measurement system using an anthropomorphic vision imaging method

We developed a new near-eye display measurement system using anthropomorphic vision imaging to measure the key parameters of near-eye displays, including field-of-view (FOV), angular resolution, eye box, and virtual image depth. The characteristics of the human eye, such as pupil position, pupil size variation, accommodation function, and the high resolution of the fovea, are imitated by the proposed measurement system. A FOV scanning structure, together with a non-vignetting image-telecentric lens system, captures the virtual image from the near-eye display by imitating human eye function. As a proof-of-concept, a prototype device was used to obtain large-range, high-resolution measurements for key parameters of near-eye displays.

Interaction between sampled rays' defocusing and number on accommodative response in integral imaging near-eye light field displays

In an integral imaging near-eye light field display using a microlens array, a point on a reconstructed depth plane (RDP) is reconstructed by sampled rays. Previous studies respectively suggested the accommodative response may shift from the RDP under two circumstances: (i) the RDP is away from the central depth plane (CDP) to introduce defocusing in sampled rays; (ii) the sampled ray number is too low. However, sampled rays' defocusing and number may interact, and the interaction's influence on the accommodative response has been little revealed. Therefore, this study adopts a proven imaging model providing retinal images to analyze the accommodative response. As a result, when the RDP and the CDP coincide, the accommodative response matches the RDP. When the RDP deviates from the CDP, defocusing is introduced in sampled rays, causing the accommodative response to shift from the RDP towards the CDP. For example, in a system with a CDP of 4 diopters (D) and 45 sampled rays, when the RDP is at 3, 2, 1, and 0 D, the accommodative response shifts to 3.25, 2.75, 2, and 1.75 D, respectively. With fewer rays, the accommodative response tends to further shift to the CDP. Eventually, with fewer than five rays, the eye accommodates to the CDP and loses the 3D display capacity. Moreover, under different RDPs, the ray number influences differently, and vice versa. An x-y polynomial equation containing three interactive terms is finally provided to reveal the interaction between RDP position and ray number. In comparison, in a pinhole-based system with no CDP, the accommodative response always matches the RDP when the sampled ray number is greater than five.

Compensation of color breaking in bi-focal depth-switchable integral floating augmented reality display with a geometrical phase lens

A bi-focal integral floating system using a geometrical phase (GP) lens can provide switchable integrated spaces with enhanced three-dimensional (3D) augmented reality (AR) depth expression. However, due to the chromatic aberration properties of the GP lens implemented for the switchable depth-floating 3D images, the floated 3D AR images with the red/green/blue (R/G/B) colors are formed at different depth locations with different magnification effects, which causes color breaking. In this paper, we propose a novel technique to resolve the color breaking problem by integrating the R/G/B elemental images with compensated depths and sizes along with experiments to demonstrate the improved results. When we evaluated the color differences of the floated 3D AR images based on CIEDE2000, the experimental results of the depth-switchable integral floating 3D AR images showed that the color accuracies were greatly improved after applying a pre-compensation scheme to the R/G/B sub-images in both concave and convex lens operation modes of the bi-focal switching GP floating lens.

Foveated display system based on a doublet geometric phase lens

We propose a new concept of a foveated display with a single display module. A multi-resolution and wide field of view (FOV) can be simultaneously achieved using only a single display, based on temporal polarization-multiplexing. The polarization-dependent lens set functions as an optical window or beam expander system depending on the polarization state, which can provide two operating modes: fovea mode for a high-resolution and peripheral mode for a wide viewing angle. By superimposing two-mode images, the proposed system supports a foveated and wide FOV image without an ultra-high-resolution display. We demonstrate the feasibility of the proposed configuration through the proof-of-concept system.

Geometrical-lightguide-based head-mounted lightfield displays using polymer-dispersed liquid-crystal films

Integrating the promising waveguide or lightguide optical combiners to head-mounted lightfield display (LF-HMD) systems offers a great opportunity to achieve both a compact optical see-through capability required for augmented or mixed reality applications and true 3D scene with correct focus cues required for mitigating the well-known vergence-accommodation conflict. Due to the non-sequential ray propagation nature of these flat combiners and the ray construction nature of a lightfield display engine, however, adapting these two technologies to each other confronts several significant challenges. In this paper, we explore the feasibility of combining an integral-imaging-based lightfield display engine with a geometrical lightguide based on microstructure mirror arrays. The image artifacts and the key challenges in a lightguide-based LF-HMD system are systematically analyzed and are further quantified via a non-sequential ray tracing simulation. We further propose to utilize polymer-dispersed liquid-crystal (PDLC) films to address the inherent problems associated with a lightguide combiner such as increasing the viewing density and improving the image coupling uniformity. We finally demonstrate, to our best knowledge, the first lightguide-based LF-HMD system that takes the advantages of both the compact form factor of a lightguide combiner and the true 3D virtual image rendering capability of a lightfield display.

Development of an ultra-compact optical combiner for augmented reality using geometric phase lenses

We present an ultra-compact optical combiner using a waveguide and geometric phase lenses (GPL) for augmented reality displays. By sandwiching the output coupler of a planar waveguide between two flat, thin GPLs, we create two optical sub-systems with different optical powers for displaying the virtual objects and transmitting the ambient light rays, respectively. We implemented our method in a scanning-based Maxwellian display and demonstrated the augmentation of an all-in-focus Maxwellian-view image with real-world objects within a 15° field of view. Our device is light (50 g) and thin (4 mm), making it well suited for wearable applications.

Finite-depth and vari-focal head-mounted displays based on geometrical lightguides

Existing waveguides and lightguides in optical see-through augmented reality (AR) displays usually guide collimated light, which results in a fixed image depth at optical infinity. In this paper, we explore the feasibility of integrating a lightguide with a varifocal optics engine to provide correct focus cues and solve the vergence-accommodation conflict in lightguide-based AR displays. The image performance and the cause of artifacts in a lightguide-based AR display with a varifocal optics engine are systematically analyzed. A non-sequential ray tracing method was developed to simulate the retinal image and quantify the effects of image focal depth on the image performance and artifacts for a vari-focal display engine of different depths. A prototype with varying image depths from 0 to 3 diopters was built and the experimental results validate the proposed system. A digital correction method is also proposed to correct the primary image artifact caused by the physical structure of the lightguide.

Retinal projection type lightguide-based near-eye display with switchable viewpoints

We present a retinal-projection-based near-eye display with switchable multiple viewpoints by polarization-multiplexing. Active switching of viewpoints is provided by the polarization grating, multiplexed holographic optical elements and polarization-dependent eyepiece lens that can generate one of the dual-divided focus groups according to the pupil position. The lightguide-combined optical devices have a potential to enable a wide field of view (FOV) and short eye relief with compact form factor. Our proposed system can support a pupil movement with an extended eyebox and mitigate image problem caused by duplicated viewpoints. We discuss the optical design for guiding system and demonstrate that proof-of-concept system provides all-in-focus images with 37 degrees FOV and 16 mm eyebox in horizontal direction.

Computational holographic Maxwellian near-eye display with an expanded eyebox

The Maxwellian near-eye displays have attracted growing interest in various applications. By using a confined pupil, a Maxwellian display presents an all-in-focus image to the viewer where the image formed on the retina is independent of the optical power of the eye. Despite being a promising technique, current Maxwellian near-eye displays suffer from various limitations such as a small eyebox, a bulky setup and a high cost. To overcome these drawbacks, we present a holographic Maxwellian near-eye display based on computational imaging. By encoding a complex wavefront into amplitude-only signals, we can readily display the computed histogram on a widely-accessible device such as a liquid-crystal or digital light processing display, creating an all-in-focus virtual image augmented on the real-world objects. Additionally, to expand the eyebox, we multiplex the hologram with multiple off-axis plane waves, duplicating the pupils into an array. The resultant method features a compact form factor because it requires only one active electronic component, lending credence to its wearable applications.