We propose a polarization multiplexing structure based on multilayer reflective polarized volume holographic gratings(PVGs) to improve the field of view and brightness of the augmented reality waveguide display. The multilayer structure forms the splicing of different response bandwidths by stacking PVGs with different periodic components, and realizes the expansion of the wavelength (angle) bandwidth. The polarization multiplexing structure controls the polarization of the diffracted light by controlling the rotation direction of the liquid crystal pitch in the liquid crystal material, so that both left-hand and right-hand circularly polarized light are diffracted to enhance efficiency. Based on these two structures, the wavelength bandwidth of PVG is increased by 40 nm, the angular bandwidth is increased by 10° and the diffraction efficiency is nearly doubled. In order to verify the feasibility of these two structures, we use the holographic waveguide display with OLED as the image source. The demonstrated waveguide prototype shows a complete display with a diagonal field of view of 55°. The brightness of virtual image was measured as high as 1100 cd/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with a transparency of 72% for ambient light. 

/pdf/holographic-waveguide-display-with-large-field-of-view-and-3ba4232i.pdf

Holographic Waveguide Display With Large Field of View and High Light Efficiency Based on Polarized Volume Holographic Grating

We proposed a single-layer color echelle grating combined optical waveguide structure for an augmented-reality display. In this structure, we used echelle gratings with super-wavelength periodic scale as in-coupling, relay, and out-coupling elements. The combined propagation of three light beams in the waveguide was realized by overlapping different high diffraction orders of the RGB three primary colors, and deflection of the beam direction between gratings was achieved by conical diffraction generated by the inclined grating. Using the vector diffraction theory, the structural parameters and tolerance ranges of the three types of gratings were optimized, rendering average diffraction efficiencies of the three primary colors of the in-coupling, relay, and out-coupling gratings greater than 74%, 21%, and 35%, respectively. As a result, we obtained dual-channel one-dimensional pupil dilation of the original image and a field-of-view angle of h18.9° × v36.87°.

Design of single-layer color echelle grating optical waveguide for augmented-reality display.

Multilayer waveguide structures can realize full-color AR displays of diffractive waveguides. This paper discusses the field-of-view characteristics of a waveguide system with a multilayer structure when polarization volume gratings (PVG) are applied as couplers to achieve a full-color display. The effects of the refractive index and period parameter of PVG-couplers on the field of view are investigated. In addition, a PVG waveguide sample with an optimized design is prepared. The experimental results show that the designed waveguide system can achieve a full-color AR display with a 45° diagonal field of view, which verifies the feasibility of the design and provides a potential solution for AR color waveguide display applications.

/pdf/a-study-of-the-field-of-view-performance-for-full-color-mkq8x8mb.pdf

A Study of the Field of View Performance for Full-Color Waveguide Displays Based on Polarization Volume Gratings

Polarization volume gratings (PVGs) based on polymerized cholesteric liquid crystal (CLC) films can potentially be applied as optical combiners in augmented reality (AR) smart glasses. The reflection bandwidth of PVGs is determined by the helical pitch and birefringence of the CLC, which must be tuned to match the desired color for reflection, but tuning the material properties is challenging as the materials must also be polymerizable. Here, we report a postprocessing method based on guest material penetration for the tuning of the reflection bandwidth of CLC polymer films. Solution coating of a high-birefringence guest liquid crystal on a CLC film modifies the birefringence as well as elongates the helical pitch. Penetration of a high-birefringence 5CT liquid crystal was found to cause a birefringence increase of the CLC film by approximately 18%. 

Tuning the Reflection Bandwidth of Polarization Volume Gratings by Guest Material Penetration

Waveguide displays, a highly competitive solution for augmented reality (AR), have attracted a lot of interest. A polarization-dependent binocular waveguide display using polarization volume lenses (PVLs) and polarization volume gratings (PVGs) as input and output couplers, respectively, is proposed. Light from a single image source is delivered to the left and right eyes independently according to its polarization state. Compared with traditional waveguide display systems, no additional collimation system is needed due to the deflection and collimation capabilities of PVLs. Leveraging the high efficiency, wide angular bandwidth, and polarization selectivity of liquid crystal elements, different images can be independently and accurately produced in the two eyes when the polarization of the image source is modulated. The proposed design paves the way for a compact and lightweight binocular AR near-eye display.

Single-image-source binocular waveguide display based on polarization volume gratings and lenses.

In this paper, we demonstrate a waveguide display structure which can realize a large field of view on a two-dimensional plane and a larger exit pupil size at the same time. This waveguide structure has three polarization volume gratings as its coupling elements. We use Zemax to simulate the effect of monochromatic and full-color two-dimensional exit pupil expansion and actually prepared a monochromatic waveguide with a two-dimensional exit pupil expansion structure. For the red, green, and blue light beams, it can achieve a large diffraction angle and can achieve diffraction efficiency of more than 70%. The waveguide structure shown can have an angle of view of 35° in the horizontal direction and 20° in the vertical direction, and an exit pupil of 18 mm long and 17 mm wide was achieved at the same time. As measured, the overall optical efficiency was measured as high as 118.3 cd/m2 per lumen with a transparency of 72% for ambient light.

https://www.mdpi.com/2073-4352/11/4/333/pdf

Yuning Zhang

Papers

Exit Pupil Expansion Based on Polarization Volume Grating