Diffractive Waveguide Research Articles

Study of the Dispersion Compensation Double-Layer Diffractive Optical Components Based on Metasurface and Grating, and Their Application in Augmented Reality Displays

We employed a double-layer coupled diffractive optical element, based on metasurfaces and diffraction gratings, which exhibits wavefront modulation and chromatic dispersion compensation. Utilizing this double-layer coupled diffractive optical element in the optical information transmission process of a diffractive waveguide allows for the transmission of color image information using a single-layer waveguide structure. Our results demonstrate that, under the conditions of a field of view of 47° × 47°, an entrance pupil size of 2.9 × 2.9 mm2, and an exit pupil extension size of 8.9 mm, the uniformity of the brightness for each monochromatic field reached 85%, while the uniformity of color transmission efficiency exceeded 95%.

Large-size PVG-based waveguide simulation and uniformity optimization for AR-HUD

Polarization volume grating (PVG), a kind of diffractive optical element, is applied widely for augmented reality (AR) near-eye display (NED) lately. However, PVG-based AR head-up display (AR-HUD) requires a large-size exit pupil and uniform efficiency, and there is presently no systematic simulation method for this type of application. Here, we introduce a unique simulation analysis method via the large-size PVG-based waveguide technology for AR-HUD. Through the self-built particle swarm optimization (PSO) algorithm, on the waveguide structure of 290 mm × 160 mm × 3 mm, with an eyerelief distance of 600 mm, the binocular field of view uniformity reaches 35.37% at an eye box of 100 mm × 50 mm, and the monocular uniformity can reach 31.65% and 32.48% respectively. The design scheme in this paper provides guidance for the large-size diffractive waveguide display for AR-HUD.

3‐1: Invited Paper: Reality Versus Simulations in Diffractive Waveguide Combiners

Dispelix has developed an advanced design toolset capable of optimizing the image quality and performance of augmented reality near‐eye and head‐up displays based on diffractive waveguide technology. The design toolset can solve the unique challenge prevalent in diffractive optical waveguide systems by accurately simulating the operation of nano‐scale diffractive surface relief gratings in an optical waveguide with macro‐scale dimensions. This paper presents simulation results of key performance metrics and compares them to measurement data from samples manufactured by state‐of‐the‐art nanofabrication tools.

P‐47: Multi‐wavelength achromatic metacoupler for augmented reality display using adjoint method

Diffractive waveguides are a promising optical combiner technology for augmented reality (AR) as they provide the potential for the slimmest geometry and lightest weight. However, their severe chromatic aberration imposes a big challenge for full‐color display applications. In this study, we propose multi‐wavelength achromatic metacouplers to overcome the longstanding chromatic aberration issue. The metacouplers were inverse‐designed to realize a true achromatic behavior in an ultra‐thin silicon nitride surface (160 nm) with only binary nanostructures. We achieved a horizontal field of view (FOV) of 45 ° using a single waveguide with high exit‐pupil uniformity across the entire FOV range. We will report our simulation results along with our experimental validation of these. This initial study will be used to inform the future direction of the research to perfect the techniques developed.

83‐4: Invited Paper: Development of Polarization Volume Hologram Waveguide for Augmented Reality Smart Glasses

Polarization Volume Hologram (PVH) waveguide is an innovative liquid crystal‐based diffractive waveguide designed for augmented reality (AR) smart glasses, showcasing exceptional optical performance and simplified fabrication process. This presentation offers a comprehensive overview, including its working principles, advantages, performance optimization, process development, challenges and opportunities. The objective is to provide insights into PVH waveguide technology, from its inception to practical implementation.

P‐45: A simulation method for crossed‐type exit pupil based on polarization volume grating

We propose a simulation method for two‐dimensional (2D) crossed‐type exit pupil expansion based on polarization volume grating (PVG) applied in diffractive waveguide holographic display. The functions of turning and decoupling of the PVG are realized through two continuous Bragg regions, and the spatial director distribution of liquid crystal is combined with RCWA to model its diffractive characteristics and ray tracing simulation.

Simulation of gradient period polarization volume gratings for augmented reality displays

Augmented reality (AR) displays are gaining attention as next-generation intelligent display technologies. Diffractive waveguide technologies are progressively becoming the AR display industry's preferred option. Gradient period polarization volume holographic gratings (PVGs), which are considered to have the potential to expand the field of view (FOV) of waveguide display systems due to their wide bandwidth diffraction characteristics, have been proposed as coupling elements for diffraction waveguide systems in recent years. Here, what we believe to be a novel modeling method for gradient period PVGs is proposed by incorporating grating stacking and scattering analysis utilizing rigorous coupled-wave analysis (RCWA) theory. The diffraction efficiency and polarization response were extensively explored using this simulation model. In addition, a dual-layer full-color diffractive waveguide imaging simulation using proposed gradient period PVGs is accomplished in Zemax software using a self-compiled dynamic link library (DLL), achieving a 53° diagonal FOV at a 16:9 aspect ratio. This work furthers the development of PVGs by providing unique ideas for the field of view design of AR display.

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.

Angular uniformity improvement of diffractive waveguide display based on region geometry optimization.

Augmented reality (AR) near-eye displays have significantly progressed due to advances in nanostructure fabrication. However, for diffractive waveguide AR displays requiring exit pupil expansion, the angular uniformity of each exit pupil position still needs to improve. In this paper, an angular uniformity improvement method based on region geometry optimization is proposed. This optimization method essentially introduces the interaction number of the light with the grating as one of the variables to manipulate the energy distribution. This distribution is obtained by the rigorous coupled wave analysis (RCWA) method and ray tracing process and is further optimized by a multi-objective genetic algorithm. A model is built, and the feasibility of the proposed method is verified. The diffractive waveguide system has a 10m m×10m m exit pupil size at the eye relief of 25mm and a field of view (FOV) of 21∘×12∘. After the optimization, the overall optical efficiency of the central field and the angular uniformity at the center exit pupil position increased from 0.9% and 66% to 3.1% and 80%, respectively.

Tandem neural network-assisted inverse design of highly efficient diffractive slanted waveguide grating.

Virtual reality devices featuring diffractive grating components have emerged as hotspots in the field of near-to-eye displays. The core aim of our work is to streamline the intricacies involved in devising the highly efficient slanted waveguide grating using the deep-learning-driven inverse design technique. We propose and establish a tandem neural network (TNN) comprising a generative flow-based invertible neural network and a fully connected neural network. The proposed TNN can automatically optimize the coupling efficiencies of the proposed grating at multi-wavelengths, including red, green, and blue beams at incident angles in the range of 0°-15°. The efficiency indicators manifest in the peak transmittance, average transmittance, and illuminance uniformity, reaching approximately 100%, 92%, and 98%, respectively. Additionally, the structural parameters of the grating can be deduced inversely based on the indicators within a short duration of hundreds of milliseconds to seconds using the TNN. The implementation of the inverse-engineered grating is anticipated to serve as a paradigm for simplifying and expediting the development of diverse types of waveguide gratings.

Theoretical efficiency limit of diffractive input couplers in augmented reality waveguides.

Considerable efforts have been devoted to augmented reality (AR) displays to enable the immersive user experience in the wearable glasses form factor. Transparent waveguide combiners offer a compact solution to guide light from the microdisplay to the front of eyes while maintaining the see-through optical path to view the real world simultaneously. To deliver a realistic virtual image with low power consumption, the waveguide combiners need to have high efficiency and good image quality. One important limiting factor for the efficiency of diffractive waveguide combiners is the out-coupling problem in the input couplers, where the guided light interacts with the input gratings again and get partially out-coupled. In this study, we introduce a theoretical model to deterministically find the upper bound of the input efficiency of a uniform input grating, constrained only by Lorentz reciprocity and energy conservation. Our model considers the polarization management at the input coupler and can work for arbitrary input polarization state ensemble. Our model also provides the corresponding characteristics of the input coupler, such as the grating diffraction efficiencies and the Jones matrix of the polarization management components, to achieve the optimal input efficiency. Equipped with this theoretical model, we investigate how the upper bound of input efficiency varies with geometric parameters including the waveguide thickness, the projector pupil size, and the projector pupil relief distance. Our study shines light on the fundamental efficiency limit of input couplers in diffractive waveguide combiners and highlights the benefits of polarization control in improving the input efficiency.

Compact pupil-expansion AR-HUD based on surface-relief grating.

Augmented reality head-up display (AR-HUD) using diffractive waveguide is a challenging research field. It can drastically reduce the system volume compared with AR-HUD based on freeform mirror. However, one of the remaining challenges that affects the performance of the diffractive waveguide is to expand the eye-box while maintaining the illuminance uniformity. In this paper, a one-dimensional pupil expansion diffractive optical waveguide system for AR-HUD is presented. The optimization of grating parameters is based on scalar diffraction theory and rigorous coupled wave analysis (RCWA). Then, the illuminance uniformity is optimized through non-sequential ray tracing. We simulate and construct a waveguide-based AR-HUD. The presented AR-HUD realized an exit pupil size of 80 mm × 15 mm and a field of view of 10° × 5° at the wavelength of 532 nm.

Design of diffractive waveguide collimating lens with high optical efficiency in edge field

Design of diffractive waveguide collimating lens with high optical efficiency in edge field

Design of diffractive waveguide collimating lens with high optical efficiency in edge field

Design of diffractive waveguide collimating lens with high optical efficiency in edge field

Study on Two-Dimensional Exit Pupil Expansion for Diffractive Waveguide Based on Holographic Volume Grating

Diffraction gratings are becoming a preferred option for waveguide head-mounted in–out coupling devices due to their flexible optical properties and small size and light weight. At present, diffraction waveguide coupling devices for AR head-mounted displays (HMD) have difficulties such as a long development cycle and complicated processing. In this paper, we first establish a set of two-dimensional (2D) grating ray tracing models, based on which we determine the initial architecture of the dual-region two-dimensional exit pupil expansion (2D-EPE) AR-HMD holographic waveguide diffraction system. Second, we propose a honeycomb coupled grating array and optimize the optical energy utilization and brightness uniformity of the holographic waveguide and use a custom dynamic linked library (DLL) function to implement the ray tracing of the 2D grating and add a probabilistic splitting function to the DLL, which reduces the single simulation time from 11.853 min to 1.77 min. We also propose a holographic lithography device composed of holographic optical elements (HOEs) and a method for preparing HOEs. Finally, in order to obtain the diffraction efficiency preoptimized by the above DLL for the uniformity of the exit pupil brightness and light energy utilization, we inverse design with the preparation process parameters as the optimization variables and develop the adaptable electromagnetic calculation program Holo-RCWA. Using Holo-RCWA with nondominated sorting genetic algorithm II (NSGA-II), we inverse design to obtain the process parameters satisfying the diffraction efficiency distribution, and the optimization time of the whole system is reduced from 2–3 days to 10 h. This work is of great significance for AR/VR applications.

Gradient polarization volume grating with wide angular bandwidth for augmented reality.

Angular bandwidth, which is critical to field-of-view, plays important role in diffractive optical waveguide augmented reality display. However, design and fabrication of large angular bandwidth is still a challenge. Herein, we demonstrate a liquid crystal reflective gradient polarization volume grating with three-dimensional gradient periodic structure for waveguide near-eye display. Two-beam polarization interference with special designed periodic gradient photomask are applied to chiral-dopant reactive mesogens doped with ultraviolet dye for generating gradient three-dimensional configuration of liquid crystals, resulting in gradient polarization volume grating with extended angle bandwidth of 61° while keeping 80% diffraction efficiency, with peak efficiency near 100%. The proposed gradient polarization volume grating provides an effective method to broaden the angular bandwidth in waveguide for wide field-of-view augmented reality display.

Optical imprinting subwavelength-period liquid crystal polarization gratings with dual-twist templates.

In this Letter, we report a dual-twist template imprinting method to fabricate subwavelength-period liquid crystal polarization gratings (LCPGs). In other words, the period of the template must be reduced to 800 nm-2 µm, or even smaller. To overcome the inherent problem that the diffraction efficiency shrinks as the period decreases, the dual-twist templates were optimized by rigorous coupled-wave analysis (RCWA). With the help of the rotating Jones matrix to measure the twist angle and thickness of the LC film, the optimized templates were fabricated eventually, and the diffraction efficiencies were up to 95%. Therefore, subwavelength-period LCPGs with a period of 400-800 nm were imprinted experimentally. Our proposed dual-twist template provides the possibility for fast, low-cost, and mass fabrication of large-angle deflectors and diffractive optical waveguides for near-eye displays.

Off-Axis Polarization Volume Lens for Diffractive Waveguide

In augmented reality diffractive waveguide technology, the light field needs to be collimated before being transmitted into the diffractive waveguide. Conventional schemes usually require additional collimating optics to collimate the light from the micro-image source and guide it into the waveguide in-coupling elements. In order to meet the needs of head-mounted devices and further miniaturize the equipment, this paper proposes a waveguide device that combines collimation and coupling by using a reflective polarization volume lens (PVL). A related model is also established and simulated to calculate the diffraction and transmission characteristics of the PVL element, and is then improved to fit the experiment. The diffraction lens studied in this paper has high diffraction efficiency with a large off-axis angle, which can fold the optical path and reduce considerably the volume of the optical system when applied to the waveguide system.

Method for Compensating Aberrations of a Virtual Image Formed by an Augmented Reality Display Based on a Cylindrical Diffractive Waveguide

To date, planar waveguides are under development for augmented reality systems with waveguide combiners. The next step of their development is the transition to curved waveguides, which could make the combiners more ergonomic. In the present work, a method has been developed that makes it possible to minimize aberrations of a virtual image during its in-coupling to and out-coupling from a cylindrical waveguide. The method is based on the use of in-coupling and out-coupling diffractive optical elements (DOE) with a variable grating period, which provide virtual image pre-aberration when radiation is in-coupled into the waveguide and compensation of the waveguide curvature at the out-coupling. Analytical laws are derived for the period variation of the in-coupling and out-coupling DOEs for an arbitrary curvature of a cylindrical waveguide. These dependences were optimized to minimize virtual image aberrations when using a radiation source with finite dimensions. Samples of cylindrical concentric PMMA waveguides with a curvature radius of 150 mm and in-coupling/out-coupling holographic optical elements (HOEs) have been created. The transmission of test monochrome virtual images through these waveguides without doubling and breaks in the field of view has been experimentally demonstrated.

High-efficiency and compact two-dimensional exit pupil expansion design for diffractive waveguide based on polarization volume grating.

We propose a two-dimensional exit pupil expansion (2D-EPE) design of a diffractive waveguide (DW) based on polarization volume grating (PVG). The designed waveguide structure and pupil expansion principle are introduced in this paper. The light propagation behavior and available field of view (FoV) of the proposed waveguide are investigated by simulations. In addition, the waveguide sample based on the proposed design is prepared, and an imaging system based on a monochromatic MicroLED projector is built for AR imaging experiments. The experimental results show that the prepared waveguide system can achieve a clear AR display with a diagonal FoV of 30° and obtain an exit pupil magnification of nearly 20 times compared to the entrance pupil size. The optical imaging efficiency was measured to be 3.85%, and the backward light leakage rate was as low as 8.7%. This work further enhances the feasibility and practicality of the PVG-waveguide technology and provides a promising candidate for AR-DW applications.