Portable waveguide display system with a large field of view by integrating freeform elements and volume holograms

Equalization of RGB coupling efficiencies of metasurface waveguide coupler by adjusting imaginary part of refractive index

Optical metasurfaces offer high-efficiency and flexible wavefront shaping for near-eye displays, especially in wideband waveguide couplers accommodating RGB primary colors. By leveraging the resonance characteristics of sub-wavelength periodic nanostructures, metasurfaces surpass the limitations of traditional optics that rely on multiple components and mediums. In this study, we propose adjustment of the imaginary parts of the material refractive indices as a new method to achieve balanced first-order diffraction efficiencies among RGB colors over a wide field of view (FOV) in an in-coupling metasurface waveguide coupler. Physical mechanism is investigated deeply and systematically in theory. It is found that nanostructure resonances deflect the wavefront and Poynting vector, significantly enhancing first-order diffraction efficiency, while resonance-enhanced absorption plays a crucial role in balancing the diffraction efficiency of RGB primary colors. First experimental demonstration well confirms the practical feasibility of this method and a uniform first-order diffraction efficiency of approximately 20% is achieved among RGB colors across a FOV as large as ∼30° over a single-piece glass substrate. This research provides insights into the design and mechanisms of metasurface waveguide couplers, advancing our understanding of metasurface-based RGB displays and facilitating further advancements in this field.

Automotive Augmented Reality Head-Up Displays

As the next generation of in-vehicle intelligent platforms, the augmented reality heads-up display (AR-HUD) has a huge information interaction capacity, can provide drivers with auxiliary driving information, avoid the distractions caused by the lower head during the driving process, and greatly improve driving safety. However, AR-HUD systems still face great challenges in the realization of multi-plane full-color display, and they cannot truly achieve the integration of virtual information and real road conditions. To overcome these problems, many new devices and materials have been applied to AR-HUDs, and many novel systems have been developed. This study first reviews some key metrics of HUDs, investigates the structures of various picture generation units (PGUs), and finally focuses on the development status of AR-HUDs, analyzes the advantages and disadvantages of existing technologies, and points out the future research directions for AR-HUDs.

Framework for optimizing AR waveguide in-coupler architectures

Waveguide displays have been shown to exhibit multiple interactions of light at the in-coupler diffractive surface, leading to light loss. Any losses at the in-coupler set a fundamental upper limit on the full-system efficiency. Furthermore, these losses vary spatially across the beam for each field, significantly decreasing the displayed image quality. We present a framework for alleviating the losses based on irradiance, efficiency, and MTF maps. We then derive and quantify the innate tradeoff between the in-coupling efficiency and the achievable modulation transfer function (MTF) characterizing image quality. Applying the framework, we show a new in-coupler architecture that mitigates the efficiency vs image quality tradeoff. In the example architecture, we demonstrate a computation speed that is 2,000 times faster than that of a commercial non-sequential ray tracer, enabling faster optimization and more thorough exploration of the parameter space. Results show that with this architecture, the in-coupling efficiency still meets the fundamental limit, while the MTF achieves the diffraction limit up to and including 30 cycles/deg, equivalent to 20/20 vision.

Using high-diffraction-efficiency holographic optical elements in a full-color augmented reality display system

Holographic optical elements (HOEs) play an important role in augmented reality (AR) systems. However, the fabrication of full-color HOEs is difficult and the diffraction efficiency is low. In this paper, we use the time-scheduled iterative exposure method to fabricate full-color HOEs with high diffraction efficiency. Through this method, a full-color HOE with an average diffraction efficiency of 73.4% was implemented in a single photopolymer, the highest rate yet reported. In addition, the AR system is simulated by the geometric optics method combining k-vector circle and ray tracing and structured by combining laser micro-drop and high diffraction efficiency HOEs. A good color blending effect was achieved in a full-color AR system by using the reconstruction wavelength consistent with the recording light. It can present clear holographic images in a full-color AR display system.

See-through display based on commercial photopolymer: Optimization and shrinkage effects

Nowadays augmented reality, 3D Image, mixed reality and see-through applications are very attractive technologies due to their great potential. Holographic optical elements can provide interesting solutions for injection and extraction of the image in the waveguides that are part of the see-through devices. We have developed a coupled waveguide system based on slanted transmission gratings recorded in manufactured photopolymers. In this work we optimize our schedule to a commercial photopolymer for this high demanded application. We demonstrate that high diffraction efficiencies can be obtained if we optimize the recording geometry, recording intensity and recording time for this material. In addition, we study the effects of shrinkage in our holographic system. In general shrinkage is an important drawback for holographic applications, nevertheless we demonstrate how shrinkage can help these systems open new possibilities. Lastly, we show how to significantly improve the quality of the guided image.

Metagrating meets the geometry-based efficiency limit for AR waveguide in-couplers

Recently, augmented reality (AR) displays have attracted considerable attention due to the highly immersive and realistic viewer experience they can provide. One key challenge of AR displays is the fundamental trade-off between the extent of the field-of-view (FOV) and the size of the eyebox, set by the conservation of etendue sets this trade-off. Exit-pupil expansion (EPE) is one possible solution to this problem. However, it comes at the cost of distributing light over a larger area, decreasing the overall system's brightness. In this work, we show that the geometry of the waveguide and the in-coupler sets a fundamental limit on how efficient the combiner can be for a given FOV. This limit can be used as a tool for waveguide designers to benchmark the in-coupling efficiency of their in-coupler gratings. We design a metasurface-based grating (metagrating) and a commonly used SRG as in-couplers using the derived limit to guide optimization. We then compare the diffractive efficiencies of the two types of in-couplers to the theoretical efficiency limit. For our chosen waveguide geometry, the metagrating's 28% efficiency surpasses the SRG's 20% efficiency and nearly matches the geometry-based limit of 29% due to the superior angular response control of metasurfaces compared to SRGs. This work provides new insight into the efficiency limit of waveguide-based combiners and paves a novel path toward implementing metasurfaces in efficient waveguide AR displays.

Design, stray light analysis, and fabrication of a compact head-mounted display using freeform prisms

It has been a challenge to design an optical see-through head-mounted display that is compact, lightweight, and stray-light-suppressed by using freeform optics. A new type of design based on freeform prisms is presented. The system consists of three optical elements and a micro-display. Two prisms serve as near-eye viewing optics that magnify the image displayed by the micro-display, and the other freeform lens is an auxiliary element attached to the main wedge-shaped prism to provide an undistorted see-through view of a real-world scene. The overall thickness of the optical system does not exceed 9.5 mm, and the weight is less than 9.8 g per eye. The final system is based on a 0.49-inch micro-display and features a diagonal field of view of 38°, an F/number of 1.8, with a 10 mm × 7 mm exit pupil diameter, and a 19 mm eye relief. The causes of stray light in the optical-mechanical system are investigated, and effective solutions or theoretical suppression of stray light are given. The freeform optical elements are successfully fabricated, and the system performance is carefully investigated. The results show that the performance of the optical see-through head-mounted display is adequate for practical applications.

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.

Uniformity improvement of two-dimensional surface relief grating waveguide display using particle swarm optimization

Augmented reality head-mounted displays (AR-HMDs) based on diffractive waveguides have been a challenging and rewarding research topic focusing on near-eye displays. The size of the exit pupil and uniformity of the image illuminance are two important factors that affect the display performance of the diffractive waveguide. In this paper, a novel method for optimizing high uniformity of two-dimensional (2D) diffractive waveguide is proposed. A straight-line 2D surface relief grating (SRG) waveguide with divided grating regions is designed. An illuminance uniformity evaluation model of the energy propagation process is established, and non-sequential ray tracing is utilized to optimize the diffraction efficiency of multi-regions grating to achieve illuminance uniformity distribution. Then, the uniformity distribution of the diffraction efficiency in different fields of view (FOVs) is realized by combining the particle swarm optimization (PSO) algorithm and rigorous couple wave analysis (RCWA) to optimize the grating structural parameters, which further ensures the uniformity of the exit pupil illuminance and angular illuminance. The waveguide with exit pupil expansion (EPE) has exit pupil size of 16 mm × 14 mm at an eye relief (ERF) of 20 mm, exit pupil illuminance uniformity of 91%, and angular uniformity illuminance of 64%.

Design method of a wide-angle AR display with a single-layer two-dimensional pupil expansion geometrical waveguide

Waveguide near-eye displays (NEDs) consist of a planar waveguide combiner and a coupling-in projection system. A two-dimensional geometrical waveguide (TDGW) can achieve an ultra-thin, large exit pupil diameter (XPD), wide-angle NED. The design method of a single-layer TDGW is presented and discussed in detail in this paper. A high-precision processing technology that can effectively guarantee the parallelism accuracy is also presented. A miniature coupling-in projection optics is designed with a catadioptric structure and integrated with the waveguide accordingly. Finally, a TDGW with a thickness of 1.75 mm is designed and analyzed. The results show that the stray light over the normal light is less than 0.5%, and the illuminance uniformity is well optimized. The field of view is up to 55°, and the XPD exceeds 12mm×10mm at an eye relief (ERF) of 18 mm. A proof-of-concept prototype was fabricated and demonstrated.

Simplified retinal 3D projection rendering method and system

A simplified rendering method and system for retinal 3D projection using view and depth information is proposed and demonstrated. Instead of vertex calculations, image-based techniques, including sub-image shifting, image fusion, and hole filling, combined with the depth information, are used to render the multi-view images in a display space with specific discrete depth coordinates. A set of time-division multiplexing retinal 3D projection systems with dense viewpoints is built. A near-eye display of a 3D scene with complex occlusion relationships is realized using the rendering method and system. The eye box of the retinal projection system is enlarged, and the accommodation response of the eyes is evoked at the same time, which improves the visual experience. Rendering tests are carried out using simple and complex models, which proves the effectiveness of this method. Comparative experiments prove that the proposed retinal projection method can obtain high-performance 3D images comparable to the super multi-view display method while simplifying the rendering process. Additionally, the depth of field of the experimental system can cover most of the vergence accommodation conflict sensitive range of the human eye.

Augmented reality and virtual reality displays: emerging technologies and future perspectives

With rapid advances in high-speed communication and computation, augmented reality (AR) and virtual reality (VR) are emerging as next-generation display platforms for deeper human-digital interactions. Nonetheless, to simultaneously match the exceptional performance of human vision and keep the near-eye display module compact and lightweight imposes unprecedented challenges on optical engineering. Fortunately, recent progress in holographic optical elements (HOEs) and lithography-enabled devices provide innovative ways to tackle these obstacles in AR and VR that are otherwise difficult with traditional optics. In this review, we begin with introducing the basic structures of AR and VR headsets, and then describing the operation principles of various HOEs and lithography-enabled devices. Their properties are analyzed in detail, including strong selectivity on wavelength and incident angle, and multiplexing ability of volume HOEs, polarization dependency and active switching of liquid crystal HOEs, device fabrication, and properties of micro-LEDs (light-emitting diodes), and large design freedoms of metasurfaces. Afterwards, we discuss how these devices help enhance the AR and VR performance, with detailed description and analysis of some state-of-the-art architectures. Finally, we cast a perspective on potential developments and research directions of these photonic devices for future AR and VR displays.

Design of two compact waveguide display systems utilizing metasurface gratings as couplers

We propose two approaches to design compact head mount display (HMD) systems employing metasurface gratings. In the first approach, we design and simulate a monocular optical waveguide display by applying crystalline-silicon elliptical-shaped metasurface arrays as couplers on a right trapezoid waveguide to achieve large field of view (FOV) horizontally. As such, we achieve a FOV as large as 80° that is approximately 80% higher than the FOV in traditional waveguide systems based on diffractive gratings. In the second approach, considering the polarization sensitivity feature in metasurfaces and employing the proposed structures in the first technique, we design a metasurface grating as the input coupler in a binocular HMD system. The suggested structure diffracts incident light into two opposite directions with a 53.7° deflection angle on each side. We use the finite difference time domain method to study the behavior of the proposed systems.

Optical metasurfaces for waveguide couplers with uniform efficiencies at RGB wavelengths

Optical metasurfaces hold great potential for near-eye display applications with high optical efficiency, light-weight and compactness. Taking advantage of optical resonances in subwavelength features, metasurfaces can diffract optical beams at RGB primary colors efficiently, forming superimposed virtual images over a real-world scene with merely a single glass substrate in augmented realities (AR) applications. We report for the first time a metasurface with double or triple nano-beams in each period for high angle diffraction with a uniform efficiency at RGB wavelengths. An efficiency as high as 30-40% of the first diffraction order is obtained across field of view, allowing a single piece of AR glass for light in-coupling to deliver image uniformly. The underlying physics is investigated through systematic full-vector numerical simulations. It is found that strong resonances inside the nano-beams with different sizes are the main reason for the deflection of wavefront as well as the Poynting vectors, leading to an efficient coupling of the incident light into the first-order diffraction. The resonances also manipulate the light absorption among the RGB colors for uniform efficiency. This work provides a new understanding of optimal metasurface structure for waveguide couplers using multiple nano-beams.

Nearly dispersionless multicolor metasurface beam deflector for near eye display designed by a physics-driven deep neural network

Dispersion is one of the most important issues in see-through near eye displays with waveguide technology. In particular, the nanophotonics design is challenging but demanding. In this paper, we propose a design method for a multilayer achromatic metasurface structure for near eye display application by a physics-driven generative neural network. Two in-coupling metagratings under different projector illuminations are presented and numerically verified with the absolute diffraction efficiency over 89%. A beam splitter, which provides a balance between compactness and visual comfort in a single-projector-binocular display, is also designed. Finally, we apply this method to an out-coupling metasurface with the capability of enlarging the visible region by threefold.

Holographic Recording Performance of Acrylate-Based Photopolymer under Different Preparation Conditions for Waveguide Display

This work proposes a green light-sensitive acrylate-based photopolymer. The effects of the preparation conditions for the waveguide applied volume holographic gratings (VHGs) were experimentally investigated. The optimum preparation conditions for holographic recording were revealed. After optimization, the peak of VHG diffraction efficiency reached 99%, the diffractive wavelength bandwidth increased from 13 nm to 22 nm, and the corresponding RIM was 0.06. To prove the wide application prospect of the acrylate-based photopolymer in head-mounted augmented reality (AR) displays, green monochromatic volume holographic waveguides were fabricated. The display results showed that the prototype was able to achieve a 28° diagonal FOV and possessed a system luminance of 300 cd/m².

Eye-box extended retinal projection type near-eye display with multiple independent viewpoints [Invited]

We introduce an approach to expand the eye-box in a retinal-projection-based near-eye display. The retinal projection display has the advantage of providing clear images in a wide depth range; however, it has difficulty in practical use with a narrow eye-box. Here, we propose a method to enhance the eye-box of the retinal projection display by generating multiple independent viewpoints, maintaining a wide depth of field. The method prevents images projected from multiple viewpoints from overlapping one other in the retina. As a result, our proposed system can provide a continuous image over a wide viewing angle without an eye tracker or image update. We discuss the optical design for the proposed method and verify its feasibility through simulation and experiment.

Design of a compact waveguide eyeglass with high efficiency by joining freeform surfaces and volume holographic gratings

In this paper, a compact waveguide eyeglass integrating freeform surfaces and volume holographic gratings (VHGs) is proposed for full-color display with high energy utilization. The in-coupler with four freeform surfaces collimates the light emitting from the micro image source (MIS) and couples them into the waveguide. The six-layer VHGs as an outcoupler are designed to modulate the light propagating toward the user's eye. The chromatic aberrations and aberrations are well optimized and compensated by the in-coupler. The diffraction angular bandwidth of the gratings matches the angular range of the light propagating in the waveguide. The simulation results show that our proposed eyeglass achieves a diagonal field of view (FOV) of 39.5°, the average diffraction efficiency of the outcoupler achieves 95.22%, and the diffraction uniformity is about 0.95. Because of the integrated designs and compact stable structures, the optimized display system is expected to be flexibly used in various applications.

Matrix optics representation and imaging analysis of a light-field near-eye display

Integral-imaging-based (InI-based) light-field near-eye display (LF-NED) is an effective way to relieve vergence-accommodation conflict (VAC) in applications of virtual reality (VR) and augmented reality (AR). Lenslet arrays are often used as spatial light modulator (SLM) in such systems. However, the conflict between refocusing on a virtual object point from the light-field image (LF image) and focusing on the image plane of the lenslets leads to degradation of the viewing effect. Thus, the light field (LF) cannot be accurately restored. In this study, we introduce matrix optics and build a parameterized model of a lenslet-array-based LF-NED with general applicability, based on which the imaging process is derived, and the performance of the system is analyzed. A lenslet-array-based LF-NED optical model is embodied in LightTools to verify the theoretical model. The simulations prove that the model we propose and the conclusions about it are consistent with the simulation results. Thus, the model can be used as the theoretical basis for evaluating the primary performance of an InI-based LF-NED system.

Design of an ultra-thin, wide-angle, stray-light-free near-eye display with a dual-layer geometrical waveguide

The field of view (FOV) of a geometrical waveguide display is limited by the total internal reflection (TIR) condition (related with the index of glass) and the stray light generated inside the waveguide. A novel concept of an ultra-thin, wide-angle, stray-light-free, optical see-through near-eye display (NED) with a dual-layer geometrical waveguide is proposed in this paper. In the dual-layer waveguide, the two waveguides have different structures and are responsible for two different FOVs which are spliced together to form the entire FOV. The stray light of the dual-layer waveguide is analyzed and an optimized structure to suppress the stray light is designed. An optimized coupling-in structure is designed and a progressive optimization method is proposed for optimizing the illuminance uniformity of the entire FOV across the exit pupil. A dual-layer waveguide with a total thickness of 3.0 mm and stray light of less than 1% is designed. The FOV is 62° in the pupil-expanding direction, and the diameter of the exit pupil (EPD) is 10 mm at an eye relief (ER) of 18 mm. A compact projection optic is designed and finally is integrated with the dual-layer waveguide.

Full-color see-through near-eye holographic display with 80° field of view and an expanded eye-box

A full-color see-through near-eye holographic display is proposed with 80° field of view (FOV) and an expanded eye-box. The system is based on a holographic optical element (HOE) to achieve a large FOV while the image light is focused at the entrance to human pupil and the image of entire field enters human eye. As we know, one of the major limitations of the large FOV holographic display system is the small eye-box that needs to be expanded. We design a double layer diffraction structure for HOE to realize eye-box expansion. The HOE consists of two non-uniform volume holographic gratings and a transparent substrate. Two fabricated holographic gratings are attached to front and back surfaces of the substrate to multiplex image light and achieve eye-box expansion. Simultaneously, the HOE is also manufactured for RGB colors to realize full-color display. The experiment results show that our proposed display system develops 80° round FOV and an enlarged eye-box of 7.5 mm (H) ×5 mm (V) at the same time. The dynamic display ability is also tested in the experiments. The proposed system provides a new solution for the practical application of augmented reality display.

Augmented Reality and Virtual Reality Displays: Perspectives and Challenges

As one of the most promising candidates for next-generation mobile platform, augmented reality (AR) and virtual reality (VR) have potential to revolutionize the ways we perceive and interact with various digital information. In the meantime, recent advances in display and optical technologies, together with the rapidly developing digital processers, offer new development directions to advancing the near-eye display systems further. In this perspective paper, we start by analyzing the optical requirements in near-eye displays poised by the human visual system and then compare it against the specifications of state-of-the-art devices, which reasonably shows the main challenges in near-eye displays at the present stage. Afterward, potential solutions to address these challenges in both AR and VR displays are presented case by case, including the most recent optical research and development, which are already or have the potential to be industrialized for extended reality displays.

Extended-viewing-angle waveguide near-eye display with a polarization-dependent steering combiner

A waveguide-based near-eye display (WNED) with an extended viewing angle using a polarization-dependent steering combiner (PDSC) is proposed. The novel eyepiece-combiner is composed of polarization gratings and polarization optics attached to the outcoupler part of the waveguide, which can control the output beam path depending on the polarization state. The viewing angle limited by the grating properties can be extended up to twice. Also, an ultrathinness of about 1.4 mm is suitable for the WNED. The demonstrated prototype system achieves a horizontal field of view of 33.2°, which is 2 times wider than the conventional structure (without the PDSC). The proposed configuration can resolve the viewing angle issue for the WNED.

Effect of surface roughness of optical waveguide on imaging quality and a formula of RSE tolerance and incident angle

The optical waveguide is a lightweight and portable scheme for augmented reality near-eye display devices. However, the surface roughness of the waveguide affects its imaging performance, which has not been studied. In this work, we investigate the light scattering caused by the root-mean-square roughness of the waveguide surface and present two methods to numerically analyze the modulation transfer function (MTF) of the display system. Here, we consider the effects of different surface roughness, incident angle, and incident wavelength on the scattering distribution when other conditions are constant. For a simplified optical waveguide display system, the MTF degradation and the variation of the tolerance is calculated. And when the MTF (@ 40 cycles/mm) is required to be 0.3 and the incident angles of the total reflection surface are 45°, 55°, 65° and 75°, the random surface error (RSE) tolerances are 0.207λ₀, 0.255λ₀, 0.347λ₀ and 0.566λ₀ (λ₀=0.5461µm), respectively. We find a formula descripting the relationship between RSE tolerance and incident angle. If the RSE tolerance exceeds the value of the formula at an angle, the imaging quality of the system will drop significantly. The formula can predict tolerances and incident angles and provide basic tool for imaging quality analysis and manufacturing for optical waveguide AR/VR display systems.

Modeling and optimizing the chromatic holographic waveguide display system

A ray-tracing model is developed based on coupled wave theory for a volume holographic grating, which is the most important element of the holographic waveguide display but not accessibly integrated in current optical design software. The model fully and faithfully represents the angular selectivity, wavelength selectivity, polarization, and other properties for the in-coupling, out-coupling, and expansion gratings. It is especially important that the model is compatible with the current optical design software. In this paper, combining with other mature optical simulation functions of Zemax, integrated models are built for typical holographic waveguide display configurations, including image source, collimation element, gratings, waveguide plates, and approximate eye. It could provide the retina image at different viewing positions, based on which the main performance characteristics of a holographic waveguide display, such as field of view, color uniformity, eye box, and light efficiency, could be easily derived. Consequently, it provides a valuable guiding approach for the design and optimization of holographic waveguide displays.

Holographic waveguide HUD with in-line pupil expansion and 2D FOV expansion

A method of head-up display is presented that uses an in-line surface relief grating attached to a waveguide propagation head-up display to achieve a large field of view without the need for large-projection optics. Horizontal pupil expansion is achieved using an extraction hologram that is multiple times the size of the injection hologram and is recorded with modulated diffraction efficiency. Vertical pupil expansion is achieved by coupling the surface relief grating to the waveguide surface between the injection and extraction holograms. The grating replicates the beam along the propagation direction, which allows for a larger field of view at the extraction. Using this technique, both a Zemax OpticStudio computer model and a physical system demonstrator achieve a field of view of 16^∘×14.25^∘.

Switchable pupil expansion propagation using orthogonal superposition varied-line-spacing H-PDLC gratings in a holographic waveguide system

To solve the problem of small field of view in traditional holographic waveguides, this paper proposes a waveguide-coupling technique using orthogonal superposition varied-line-spacing (VLS) gratings. These gratings expand the x and y directions and orthogonally superimpose to achieve transmission in a waveguide and enlarge the pupil field of view. At the optical coupling input end and the coupling output end of a waveguide, one-dimensional VLS gratings are used to realize horizontal expansion of the image. Then a vertical VLS grating is orthogonally superimposed at the exit end to realize vertical expansion of the image. Finally, the waveguide transmission and expansion of the image are completed. In the experiment, holographic polymer dispersed liquid crystal one-dimensional VLS gratings in the x and y directions are fabricated and coupled with a waveguide. An image source with a diameter of 0.5 cm is waveguide-transferred and coupled out, and then passed through the vertical grating. Amplification is performed to obtain an expanded image of a diameter of 2.28 cm. In this study, the diffraction characteristics of the grating used to realize pupil expansion in the holographic waveguide system are analyzed and simulated. It is calculated that the diffraction efficiency of the VLS gratings can reach 80% or more in the 532 nm band. Additionally, the characteristics of an electronically controlled switch are studied. Experimental results show that the method can be used for expanding the field of view and can be applied to waveguide systems for image transmission and expansion.

A holographic waveguide based eye tracking device

We demonstrated the feasibility of using holographic waveguide for eye tracking. A custom-built holographic waveguide, a 20 mm × 60 mm × 3 mm flat glass substrate with integrated in- and out-couplers, was used for the prototype development. The in- and out-couplers, photopolymer films with holographic fringes, induced total internal reflection in the glass substrate. Diffractive optical elements were integrated into the in-coupler to serve as an optical collimator. The waveguide captured images of the anterior segment of the eye right in front of it and guided the images to a processing unit distant from the eye. The vector connecting the pupil center (PC) and the corneal reflex (CR) of the eye was used to compute eye position in the socket. A 3D printed model eye, which has a similar corneal curvature of human eye and laser pointer tube holder at the tail for simulation of eye gaze on a screen, was used for prototype validation. The benchtop prototype demonstrated a linear relationship between the angular eye position and the PC/CR vector over a range of 60 horizontal degrees and 40 vertical degrees. This prototype eye tracker has a tracking accuracy of 0.72 degree and tracking precision of 0.50 degree over the whole tracking range. These results confirmed that the holographic waveguide technology could be a feasible platform for developing a wearable eye tracker. Further development can lead to a compact, see-through eye tracker, which allows continuous monitoring of eye movement during real life tasks, and thus benefits diagnosis of oculomotor disorders.

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

A waveguide near-eye display (NED) with a dual-focal plane using a polarization-dependent lens device is proposed. The novel optical device is composed of a geometric phase holographic lens, a wave plate, and a circular polarizer, which is operating as a concave lens or a see-through optical window, depending on the polarization state of the input beam. Such property and ultra-thinness of about 1.5 mm can be applied to a combiner-eyepiece lens for augmented reality. This optical device attached to the waveguide provides two depth planes with polarization multiplexing. We have demonstrated that our proof-of-concept system has image planes at infinity and 20 diopters. The devised system can be expected to offer a better immersive experience, compared to a NED system with a single-focal plane.

Holographic Three-Dimensional Virtual Reality and Augmented Reality Display Based on 4K-Spatial Light Modulators

In this paper, we propose a holographic three-dimensional (3D) head-mounted display based on 4K-spatial light modulators (SLMs). This work is to overcome the limitation of stereoscopic 3D virtual reality and augmented reality head-mounted display. We build and compare two systems using 2K and 4K SLMs with pixel pitches 8.1 μm and 3.74 μm, respectively. One is a monocular system for each eye, and the other is a binocular system using two tiled SLMs for two eyes. The viewing angle of the holographic head-mounted 3D display is enlarged from 3.8 ∘ to 16.4 ∘ by SLM tiling, which demonstrates potential applications of true 3D displays in virtual reality and augmented reality.

Methods of optimizing and evaluating geometrical lightguides with microstructure mirrors for augmented reality displays

Waveguide or lightguide technology has been widely used in the state-of-the-art, see-through, near-eye displays to reduce system weight and form factor. Although a few of the current products use a geometrical lightguide as an optical combiner, its design and performance assessment methods have been barely discussed. In this paper, by taking into account the factors affecting retinal image quality, we presented novel methods for quantifying and evaluating the optical performances and artifacts of geometrical lightguides based on microstructure mirror arrays, and proposed new merit functions and a novel process for systematic optimization of such lightguides. A lightguide design example implementing the evaluation and optimization methods are demonstrated, and the resulted lightguide is then further utilized as a combiner for the design of a lightweight, glasses-like, see-through, near-eye display.

Holographic waveguide head-up display with 2-D pupil expansion and longitudinal image magnification

Head-up displays (HUDs) are used in aircraft to overlay relevant flight information on the vehicle's externals for pilots to view with continued focus on the far field. In these systems, the field of view (FOV) is traditionally limited by the size of the projection optics. Though classical HUD systems take a significant amount of space in the flight deck, they have become a necessity in avionic transportation. Our research aims to reduce the size of the HUD footprint while offering a wide FOV projected in the far field with an expanded pupil. This has been accomplished by coupling the image-bearing light into a waveguide under total internal reflection conditions, redirecting that light in the orthogonal direction, and then outcoupling the light toward the pilot. Each step was achieved using holographic optical elements. The injection hologram has optical power to obtain longitudinal magnification, whereas the redirection hologram expands the pupil in one dimension and the extraction hologram expands the pupil in a second dimension. Varying diffraction efficiency along the direction of the light propagation ensures even image intensity throughout the expanded pupil. We used ray tracing optical simulations to optimize the design of the system and present a fully operational demonstrator of the HUD. This HUD produces an image with a FOV of 24°×12.6° at a viewing distance of 4.5 in. (114 mm) from the waveguide, with infinite longitudinal magnification and 1.9× by 1.6× horizontal and vertical pupil expansion, respectively.

On-axis near-eye display system based on directional scattering holographic waveguide and curved goggle

The contradiction between the field of view (FOV), luminance uniformity (LU) and weight has always restricted the development of augmented reality display systems. An on-axis near-eye display (NED) system based on directional scattering holographic waveguide (DSHW) and curved goggle is proposed in order to realize a large FOV with high LU, light weight, and conformal design capability. The DSHW which consists of a linear volume holographic grating and holographic diffuser is used to deliver the virtual image and construct a transparent directional emission display screen with high LU. The curved goggle is used to project the image on the display screen into human eye and form a large FOV, with a suitable exit pupil diameter (EPD) and eye relief distance (ERF) and while keeping the external scene visible. Our proposed NED achieved an FOV of 44° horizontal (H) × 12° vertical (V) easily, which is almost consistent with the theoretical design. The EPD is 6 mm, ERF is 18.6 mm, and LU is about 88.09% at full viewing angle. The system is lightweight and flexible, which can be further applied in the next-generation, integrated protection-display helmet system through conformal optical design.

Integrated holographic waveguide display system with a common optical path for visible and infrared light

We propose an integrated holographic waveguide display system. An infrared volume holographic grating (IVHG) and a visible light grating are recorded on the same waveguide to achieve the purpose of a common light path for system miniaturization. Simulated and experimental results verify the feasibility of this method. The coupling efficiencies of the infrared module for eye tracking and the visible light module for augmented reality (AR) display are 40% and 45%. The holographic waveguide has a weight of only 4.3 grams. It is believed that this technique is a good way to achieve a light and thin eye tracking near-eye display.

Efficient coupling to a waveguide by combined gratings in a holographic waveguide display system

Gratings are widely used as coupling parts in a waveguide display system for achieving a much lighter and more compact system, but their diffraction efficiency needs to be improved. Two combined gratings for integrating a subwavelength binary grating and volume holographic grating (VHG) are applied as incoupler and outcoupler of a holographic waveguide display system. Two basic design rules are put forward to guarantee the maximum diffraction energy guided into the waveguide and finally coupled out to enter into the user's eyes: one is the grating vector matching rule, the other is the refractive index matching rule on the interface of the binary grating and the VHG. The finite element method is used to simulate the couple-in parts and the whole waveguide display system. The combined grating with the metal binary grating is different from that with a dielectric binary grating for achieving higher diffraction efficiency and an additional second peak in the diffraction efficiency curve varied with the relative position between the binary grating and the VHG. The simulation results indicate that a VHG+Ag combined grating can obtain much higher diffraction efficiency compared to gold, aluminum, and other dielectric materials. In addition, several factors such as the Bragg wavelength, the index modulation of VHG, binary grating thickness, and the filling factor of the binary grating are discussed for the VHG+Ag combined grating. Moreover, a higher diffraction efficiency in the holographic waveguide system can be obtained by using VHG+Ag-VHG+Ag combined gratings as the incoupler and outcoupler.

Design of a 2 diopter holographic progressive lens

In this contribution, we investigate the use of holographic optical elements (HOEs) as progressive addition lenses (PALs). We design HOEs with high diffraction efficiency (DE) using the Fourier Modal Method (FMM) and optimize an optical system comprising two of these HOEs to fulfill the optical function of a 2 diopter (dpt) PAL. The resulting design is a holographic PAL (hPAL) exhibiting high DE and limited angular color error (CE) with a distribution of spherical power and astigmatism equivalent to its refractive counterpart. To our knowledge, our contribution is the first complete design of an hPAL. While our HOE design method is shown for PALs here, it has the potential to improve other applications of HOEs as well.

Liquid-crystal-based polarization volume grating applied for full-color waveguide displays

In this Letter, we demonstrate polarization volume grating (PVG)-based couplers for a double-layer waveguide display to realize a full-color near-eye display. The polarized interference exposure with photo-alignment methods was employed to generate a birefringent spiral configuration with two-dimensional periodicity in a chiral-dopant reactive mesogen material. Such a structure presents a unique highly efficient single-order Bragg diffraction with polarized selectivity. The prepared PVG couplers exhibited over 80% diffraction efficiency with large diffraction angles at spectra of blue (457 nm), green (532 nm), and red (630 nm). The demonstrated waveguide prototype showed a full-color display with a diagonal field of view of around 35°. The overall optical efficiency was measured as high as 118.3  cd/m² per lumen with a transparency of 72% for ambient light.

Narrowband wavelength selective waveguide for see-through glasses

See-through glasses are a new type of portable mobile device; however, most existing designs cannot balance the requirements of field of view (FOV), optical efficiency, and system volume at the same time. Here we propose a design for ultra-compact see-through glasses by using a waveguide with embedded narrowband minus filters as the optical combiner. The optical combiner demonstrates wavelength selectivity and optional narrowband(s) and is based on inorganic materials. Compelling advantages of our approach are high light efficiency, stray ray suppression, full-color capacity, good stability, and low cost. A detailed calculation model is provided for analysis of the optical properties of the waveguide, and a proof-of-concept prototype is demonstrated that can convey a horizontal FOV of 31.4°. The FOV can be further enlarged by parameter adjustments.

Design method of nonsymmetric imaging systems consisting of multiple flat phase elements

Imaging systems consisting of flat phase elements can realize the same functions and applications of conventional geometric optical systems, as well as the ones using aspherical or freeform optics, but can achieve more compactness, lighter-weight and easier-alignment. In addition, it is easy to integrate multiple phase elements into a single flat element. Here we propose a novel design method and realize the design of off-axis nonsymmetric imaging systems consisting of multiple flat phase elements. Compared with other traditional design methods of phase elements, the whole design process starts from an initial system using simple true geometric planes. The phase profiles or functions are generated point-by-point directly based on the given system specifications and configuration. In comparison with other direct or point-by-point design methods of flat phase elements, the rays of multiple fields and pupil positions are employed in the design framework. Closed-form phase functions of multiple flat elements are designed quickly and effectively by connecting and integrating the real three-dimensional space and the phase function space. This method can be taken as a fast phase retrieval method to some degree. To demonstrate the feasibility of the proposed design method, we present a high-performance compact system as design example. The design method and framework depicted in this paper can be applied in many areas, such as virtual reality (VR) and augmented reality (AR), miniature cameras, high-performance telescopy, microscopy, and illumination design.

Astigmatism and deformation correction for a holographic head-mounted display with a wedge-shaped holographic waveguide

In this study, a head-mounted display (HMD) system based on a wedge-shaped holographic waveguide that can present holographic virtual images with tunable distance is achieved. The compact computer-generated-hologram system using a spatial light modulator was employed to offer the dynamic image, where the probe beam for the hologram reconstruction is a convergent wave, and the DC term of the diffraction wave can be blocked by a barrier. The wedge-shaped holographic waveguide element was used as the combiner of the HMD system to generate a compact structure. A wedge with a polished surface was designed for in-coupling the image into the waveguide, and a reflection-type holographic optical element (HOE) was used for out-coupling the image from the waveguide. The astigmatism aberration and deformation of the diffraction images at various distances are analyzed and then are compensated. Finally, the virtual image can be obtained without aberration with experimental verification.

High-performance integral-imaging-based light field augmented reality display using freeform optics

A new integral-imaging-based light field augmented-reality display is proposed and implemented for the first time, to our best knowledge, to achieve a wide see-through view and high image quality over a large depth range. By using custom-designed freeform optics and incorporating a tunable lens and an aperture array, we demonstrated a compact design of a light field head-mounted-display that offers a true 3D display view of 30° by 18°, maintains a spatial resolution of 3 arc minutes across a depth range of over 3 diopters, and provides a see-through field of view of 65° by 40°.

Bragg polarization gratings for wide angular bandwidth and high efficiency at steep deflection angles

Optical films and surfaces using geometric phase are increasingly demonstrating unique and sometimes enhanced performance compared to traditional elements employing propagation phase. Here, we report on a diffraction grating with wider angular bandwidth and significantly higher average first-order efficiency than the nearest prior art of metasurfaces, volume holographic gratings, and surface-relief gratings configured to achieve a steep deflection angle. More specifically, we demonstrate a liquid crystal (LC) polymer Bragg polarization grating (PG) with large angular bandwidth and high efficiency in transmission-mode for 532 nm wavelength and 400 nm period. Angular bandwidth was significantly increased by arranging two slanted grating layers within the same monolithic film. First, we studied the optical properties with simulation and identified a structure with 48° angular bandwidth and 70% average first-order efficiency. Second, we fabricated a sample using a photo-aligned chiral nematic LC, where the two grating slants were controlled by the chiral dopants. We measured 40° angular bandwidth, 76% average efficiency, and 96% peak efficiency. Strong input polarization sensitivity (300:1 contrast) and spectral bandwidth (200 nm) mostly matched prior PGs. This approach is especially advantageous for augmented-reality systems and nonmechanical beam steering.

Holographic waveguide heads-up display for longitudinal image magnification and pupil expansion

The field of view of traditional heads-up display systems is limited by the size of the projection optics. Our research is focused on overcoming this limitation by coupling image-bearing light into a waveguide using holographic elements, propagating the light through that waveguide, and extracting the light several times with additional holographic optical elements. With this configuration, we demonstrated both longitudinal magnification and pupil expansion of the heads-up display. We created a ray-trace model of the optical system to optimize the component parameters and implemented the solution in a prototype that demonstrates the merit of our approach. Longitudinal magnification is achieved by encoding optical power into the hologram injecting the light into the waveguide, while pupil expansion is obtained by expanding the size of the hologram extracting the light from the waveguide element. To ensure uniform intensity of the image, the diffraction efficiency of the extracting hologram is modulated according to the position. Our design has a 12°×8° field of view at a viewing distance of 10 in. (250 mm), with infinite longitudinal magnification and a 1.7× lateral pupil expansion.

Highly efficient waveguide display with space-variant volume holographic gratings

We propose a highly efficient waveguide display based on space-variant volume holographic gratings (SVVHGs). The local period and slant angle of the SVVHG vary along the tangential direction, enabling variant incident angles to satisfy the Bragg condition of the local gratings. As a result, we enlarge the field of view (FOV) without using the conventional multiplexing scheme, while achieving high efficiency and large FOV at the same time. We experimentally record the SVVHGs on Bayfol HX200 films. We demonstrate that the proposed display can achieve 31.9% system efficiency for a broadband light source and 52.3% for a coherent light source, 20° FOV, and high brightness uniformity, making it a promising candidate for widespread applications in the augmented reality (AR) industry.

Design of a uniform-illumination binocular waveguide display with diffraction gratings and freeform optics

Uniform illuminance over the expanded exit pupil is an important requirement for waveguide display systems with a wide field of view (FOV). To address this issue, we develop a monochromatic binocular waveguide display in this paper. Two surface-relief diffraction gratings are designed as in-couplers and out-couplers. The parameters of the gratings are optimized to achieve uniform diffraction efficiency distributions over a broad angular range. The grating couplers enable the system to realize a diagonal FOV of 40°. A freeform surface prism is designed as the projection optics. The diameters of the two exit pupils are 12 mm in the expanding direction at an eye relief of 19 mm.

See-through near-eye displays enabling vision correction

We propose a see-through near-eye display, which is dedicated to the visually impaired users who suffer from refractive anomalies. Our solution is characterized by a pair of corrective lenses coated with multiplexed volume holograms. Its key performance including diffraction efficiency, field of view, modulation transfer function, and distortion has been studied.

Design of a large field-of-view see-through near to eye display with two geometrical waveguides

A novel waveguide near to eye display (WGNED), with new in-coupling and propagation subsystems, is proposed for the first time, to our knowledge, to enlarge the vertical field-of-view (FOV) and the vertical size of the eye box. Two waveguides are stacked-one is for in-coupling and the other for out-coupling. A freeform prism is used to correct the aberrations. These components are combined to form the WGNED. We have simulated such a system; as a result, we show that it achieves a FOV of 30°horizontal (H)×60°vertical (V) and an eye box of about 15  mm (H)×12  mm (V). The modulation transfer function of the system is larger than 0.3 at 33  lp/mm and the distortion is smaller than 5%.

Polarization volume grating with high efficiency and large diffraction angle

We propose a polarization volume grating (PVG), which exhibits nearly 100% diffraction efficiency and large diffraction angle. Both reflective and transmissive PVGs can be configured depending on application preference. Such a PVG is polarization-sensitive so that it can split an incident unpolarized beam into two well-separated yet polarized beams. These outstanding features make PVG a strong candidate for photonic and display applications. To investigate and optimize the diffraction properties, we build a rigorous simulation model based on finite element method. To illustrate its potential applications, we propose a simple 2D/3D wearable display using a planar waveguide comprising of two reflective PVGs.

Monocular 3D see-through head-mounted display via complex amplitude modulation

The complex amplitude modulation (CAM) technique is applied to the design of the monocular three-dimensional see-through head-mounted display (3D-STHMD) for the first time. Two amplitude holograms are obtained by analytically dividing the wavefront of the 3D object to the real and the imaginary distributions, and then double amplitude-only spatial light modulators (A-SLMs) are employed to reconstruct the 3D images in real-time. Since the CAM technique can inherently present true 3D images to the human eye, the designed CAM-STHMD system avoids the accommodation-convergence conflict of the conventional stereoscopic see-through displays. The optical experiments further demonstrated that the proposed system has continuous and wide depth cues, which enables the observer free of eye fatigue problem. The dynamic display ability is also tested in the experiments and the results showed the possibility of true 3D interactive display.