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

Enlarged Eye-Box Accommodation-Capable Augmented Reality with Hologram Replicas

Augmented reality (AR) technology has been widely applied across a variety of fields, with head-up displays (HUDs) being one of its prominent uses, offering immersive three-dimensional (3D) experiences and interaction with digital content and the real world. AR-HUDs face challenges such as limited field of view (FOV), small eye-box, bulky form factor, and absence of accommodation cue, often compromising trade-offs between these factors. Recently, optical waveguide based on pupil replication process has attracted increasing attention as an optical element for its compact form factor and exit-pupil expansion. Despite these advantages, current waveguide displays struggle to integrate visual information with real scenes because they do not produce accommodation-capable virtual content. In this paper, we introduce a lensless accommodation-capable holographic system based on a waveguide. Our system aims to expand the eye-box at the optimal viewing distance that provides the maximum FOV. We devised a formalized CGH algorithm based on bold assumption and two constraints and successfully performed numerical observation simulation. In optical experiments, accommodation-capable images with a maximum horizontal FOV of 7.0 degrees were successfully observed within an expanded eye-box of 9.18 mm at an optimal observation distance of 112 mm.

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.

Compact dual-focal augmented reality head-up display using a single picture generation unit with polarization multiplexing

The augmented reality head-up display (AR-HUD) attracts increasing attention. It features multiple focal planes to display basic and AR information, as well as a wider field of view (FOV). Using two picture generation units (PGUs) to create dual-focal AR-HUDs leads to expanded size, increased cost, and reduced reliability. Thus, we previously proposed an improved solution by dividing one PGU into two partitions that were separately imaged into two virtual images with an optical relay system. However, the resolution of the PGU was halved for either virtual image. Regarding the drawbacks, this paper proposes a dual-focal AR-HUD using one PGU and one freeform mirror. Either virtual image utilizes the full resolution of the PGU through polarization-multiplexing. By performing optical design optimization, high image quality, except for the distortion, is achieved in an eyebox of 130 by 60 mm for far (10 m, 13° by 4°) and near (2.5 m, 10° by 1°) images. Next, we propose a distortion correction method by directly inputting the distorted but clear images acquired in the design stage into the real HUD with an inversed optical path. The proposed optical architecture enables a compact system volume of 9.5 L, close to traditional single-focal HUDs. Finally, we build an AR-HUD prototype, where a polarizing reflective film and a twisted nematic liquid crystal cell achieve polarization-multiplexing. The expected image quality of the two virtual images is experimentally verified.

3D head-up display with a multiple extended depth of field based on integral imaging and holographic optical elements

We propose a 3D head-up display with a multiple extended depth of field based on integral imaging and holographic optical elements (HOEs). The system consists of two components, an integral imaging module for generating 3D target objects and a holographic combiner for off-axis diffraction and image magnification. By recording HOEs with different optical powers at different locations on the same recording material, 3D imaging at multiple depth ranges is achieved. The designed and built HUD prototype verifies the feasibility of the method. The center depth planes of the red and green 3D images are at 600 mm and 2200 mm. The theoretical depth magnifications of the red and green channels are 1.5 and 5.5, and the measured depth ranges are approximately 250 mm and 850 mm. In addition, the influencing factors and improvement methods of resolution in LED illumination or laser illumination are analyzed.

Design method of imaging optical systems using confocal flat phase elements

Imaging systems consisting of flat phase element such as diffractive optical element, holographic optical element, and metasurface have important applications in many fields. However, there is still a lack of generalized and efficient design methods of these systems, especially for systems with nonsymmetric configurations. We proposed a design method of imaging system consisting of flat phase elements based on confocal properties. The description of the generalized phase function for realizing point-to-point stigmatic imaging is derived. Given the focal length or magnification as well as the locations of the elements based on the design requirements, the phase functions can be calculated very fast and stigmatic imaging of the central field is realized. The systems can be taken as good starting points for further optimization, during which the rotationally symmetric or freeform phase terms can be added. Several design examples are demonstrated to validate the feasibility of the method. The proposed method increases design efficiency while decreasing the dependence on existing systems and skills significantly, and can be easily integrated into optical design software.

Computational see-through screen camera based on a holographic waveguide device

This study proposes a novel computational imaging system that integrates a see-through screen (STS) with volume holographic optical elements (vHOEs) and a digital camera unit. Because of the unique features of the vHOE, the STS can function as a holographic waveguide device (HWD) and enable the camera to capture the frontal image when the user gazes at the screen. This system not only provides an innovative solution to a high-quality video communication system by realizing eye-contact but also contributes to other visual applications due to its refined structure. However, there is a dilemma in the proposed imaging system: for a wider field of view, a larger vHOE is necessary. If the size of the vHOE is larger, the light rays from the same object point are diffracted at different Bragg conditions and reflect a different number of times, which causes blurring of the captured image. The system imaging process is analyzed by ray tracing, and a digital image reconstruction method was employed to obtain a clear picture in this study. Optical experiments confirmed the effectiveness of the proposed HWD-STS camera.

Design of off-axis reflective imaging systems based on freeform holographic elements

Holographic optical element (HOE) can be used in many areas in optics due to its characteristics of thin structure, flexible wavefront reconstruction/control ability and angular/wavelength selectivity. In this paper, we propose a design method of off-axis reflective imaging systems based on freeform HOEs, which are fabricated by freeform wavefronts. The freeform HOEs offer many degrees of design freedom and can correct the aberrations in nonsymmetric imaging systems. The initial imaging system with freeform HOEs is generated using a point-by-point design approach, and is used for the preliminary design of the imaging system and the freeform recording system of each HOE. Then a joint optimization is conducted for all the systems, simultaneously considering the imaging performance, the diffraction efficiency, the system constraints and fabrication to get the final design results. To validate the feasibility and effectiveness of the proposed method, an off-axis reflective head-up display system with good performance based on freeform HOEs is designed and fabricated. Detailed procedures of the design and development process of the prototype are demonstrated.

Automotive Holographic Head-Up Displays

Driver's access to information about navigation and vehicle data through in-car displays and personal devices distract the driver from safe vehicle management. The discrepancy between road safety and infotainment must be addressed to develop safely operated modern vehicles. Head-up displays (HUDs) aim to introduce a seamless uptake of visual information for the driver while securely operating a vehicle. HUDs projected on the windshield provide the driver with visual navigation and vehicle data within the comfort of the driver's personal eye box through a customizable extended display space. Windshield HUDs do not require the driver to shift the gaze away from the road to attain road information. This article presents a review of technological advances and future perspectives in holographic HUDs by analyzing the optoelectronics devices and the user experience of the driver. The review elucidates holographic displays and full augmented reality in 3D with depth perception when projecting the visual information on the road within the driver's gaze. Design factors, functionality, and the integration of personalized machine learning technologies into holographic HUDs are discussed. Application examples of the display technologies regarding road safety and security are presented. An outlook is provided to reflect on display trends and autonomous driving.

Holographic curved waveguide combiner for HUD/AR with 1-D pupil expansion

We are presenting the optical ray tracing as well as an experimental prototype of a curved waveguide combiner with pupil expansion for augmented reality (AR) and mixed reality (MR) glasses. The curved waveguide combiner takes advantage of holographic optical elements both for injection and extraction of the image to correct the aberrations introduced during the propagation of light inside the waveguide. The holographic curved combiner presented has a cylindrical outer radius of curvature of 171.45 mm with a field of view of 13° (H) × 16° (V) at a viewing distance of 1 cm with a 5 × horizontal 1 dimension pupil expansion for an eyebox of 6.2 mm × 42.7 mm.

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.

Examining aberrations due to depth of field in holographic pupil replication waveguide systems

Pupil expansion using waveguide propagation and pupil replication has been a popular method of developing head-up displays and near-to-eye displays. This paper examines one of the limits of pupil replication, which involves projecting images at a finite distance through a single waveguide by holographic optical elements and seeing the image doubling artifact. A Zemax model and a demonstrator were developed to determine the cause of image doubling. A relationship between the designed outcoupled image distance of a waveguide, pupil size, optical path length, and angle of image doubling is established. In waveguide pupil replication, the internally propagating light should be close to collimated to mitigate image doubling. We also provide a solution to project the image at different distances, which is an important factor for some applications, such as automotive head-up display and the seamless integration of augmented reality information with the natural environment.

Thermal Properties of Bayfol® HX200 Photopolymer

Bayfol^® HX200 photopolymer is a holographic recording material used in a variety of applications such as a holographic combiner for a heads-up display and augmented reality, dispersive grating for spectrometers, and notch filters for Raman spectroscopy. For these systems, the thermal properties of the holographic material are extremely important to consider since temperature can affect the diffraction efficiency of the hologram as well as its spectral bandwidth and diffraction angle. These thermal variations are a consequence of the distance and geometry change of the diffraction Bragg planes recorded inside the material. Because temperatures can vary by a large margin in industrial applications (e.g., automotive industry standards require withstanding temperature up to 125°C), it is also essential to know at which temperature the material starts to be affected by permanent damage if the temperature is raised too high. Using thermogravimetric analysis, as well as spectral measurement on samples with and without hologram, we measured that the Bayfol^® HX200 material does not suffer from any permanent thermal degradation below 160°C. From that point, a further increase in temperature induces a decrease in transmission throughout the entire visible region of the spectrum, leading to a reduced transmission for an original 82% down to 27% (including Fresnel reflection). We measured the refractive index change over the temperature range from 24°C to 100°C. Linear interpolation give a slope 4.5×10-4K-1 for unexposed film, with the extrapolated refractive index at 0°C equal to n0=1.51. This refractive index change decreases to 3×10-4K-1 when the material is fully cured with UV light, with a 0°C refractive index equal to n0=1.495. Spectral properties of a reflection hologram recorded at 532 nm was measured from 23°C to 171°C. A consistent 10 nm spectral shift increase was observed for the diffraction peak wavelength when the temperature reaches 171°C. From these spectral measurements, we calculated a coefficient of thermal expansion (CTE) of 384×10-6K-1 by using the coupled wave theory in order to determine the increase of the Bragg plane spacing with temperature.

See-through holographic display with randomly distributed partial computer generated holograms

Holographic displays have the feature to show images out of the plane of the device itself, which is especially favored for augmented reality (AR) applications where the images need to be merged with the real world. In existing cases of AR holographic display, a combiner is used to converge the light path of the display image and surrounding scene toward the viewer's eye. In this paper, the idea of combining the holographic device and the combiner has been proposed, resulting in a see-through holographic display. In order to maintain the see-through quality of the device, the concept of partial hologram has been introduced, which means only a part of the area on the device has the holographic fringe pattern while leaving the rest fully transparent. Experiment and theoretical investigation shows that an evenly yet randomly distributed partial hologram provides the best holographic image quality assuming a fixed percentage of the holographic area on the device. A passive computer generated hologram (CGH) with two phase levels has been designed and fabricated for the verification. With partial hologram sharing 25% of the whole area, the CGH exhibits 90.9% of total transmission and 72.2% of parallel transmission. The demonstration shows a high see-through quality while providing a clear holographic image.

Design and fabrication of an off-axis four-mirror system for head-up displays

Head-up displays (HUDs), as one kind of the augmented reality (AR) applications, enable users to enhance the situational awareness. In this paper, we propose a design of a HUD employing the freeform off-axis four-mirror construction, in which the eyebox and eye relief as large as 120mm×80mm and 400 mm, respectively, are derived. In the part of system design, the biocular parallax is evaluated and corrected. An optimization strategy in forward ray-tracing mode is presented to directly govern the biocular parallax, and the convergence, divergence, and dipvergence are finally constrained within 3.5 mrad, 1.5 mrad, and 2.0 mrad, respectively. An overall design procedure of a HUD, including initial configuration, structure constraints, optimization method, and parallax correction, is illustrated in detail. After analyzing the tolerance, a proof-of-concept prototype with the field of 24^∘×15^∘ is developed to demonstrate the image quality.

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^∘.