Modeling and optimizing the chromatic holographic waveguide display system

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.

Subwavelength dielectric grating structures with tunable higher order resonance for achromatic augmented reality display

Color non-uniformities caused by a dispersion effect can seriously affect the image quality for a diffractive waveguide display system. In this work, we propose a subwavelength multilayered dielectric grating structure by a rigorous coupled wave analysis as a novel coupling grating, to the best of our knowledge, for waveguide-based near-eye displays to overcome the "rainbow" effect. Such a grating structure exhibits a tunable high-efficiency resonance in first-order diffraction due to resonant coupling of incident light with the grating structure. A further analysis of the resonant behaviors helps us get a clear understanding of the underlying physics for the mode excitation and resonant coupling process. The first-order resonance with a diffraction efficiency of more than 60% can be achieved with the resonant angle continuously shifted to get a large field of view. The resonant angle, diffraction efficiency, and spectral linewidth can be easily tuned by the geometrical parameters of the grating structure.

Portable autostereoscopic display based on multi-directional backlight

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

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.

Realizing the imaging simulation of reflective polarization volume gratings

Near-eye holographic waveguide display system using novel reflective polarized volume gratings (RPVG) have lately gotten a lot of interest. However, from polarization characteristics to imaging simulation, there is no systematic approach based on RPVG. Here, a full methodology for solving this problem using the rigorous coupled wave analysis (RCWA) model is presented. This self-built RCWA model is used to examine the optical behavior of RPVG. This excellent portability of the RCWA model makes it possible for RPVG as a diffractive optical element, which is integrated into the commercial optical software Zemax via a self-compiled dynamic link library (DLL), and a full-color imaging simulation of the based-RPVG waveguide display system is obtained. Our work provides an instructive imaging analysis method using the RPVG for holographic waveguide display.

Optimization of gratings in a diffractive waveguide using relative-direction-cosine diagrams

Designing diffractive waveguides for head-mounted displays requires wide-angle conical diffraction analysis of multiple gratings. In this work, diffractive waveguide design using the relative direction cosine space, which extends the direction cosine space to a relative space involving refractive indices and can describe grating diffraction through various media, is demonstrated. A diffractive waveguide was fabricated with grating periods of 382 and 270 nm, which generated a monochromatic virtual image image in green light (520 nm). The maximum field of view was measured as 39° with 0.5° deviation from the center of view.

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.