大 视 场 二 维 扩 瞳 全 息 光 栅 波 导 设 计 与 加 工

Objective Augmented reality (AR) head-mounted near-eye display technology combines virtual image information with real environment, and it is the next generation of interactive display technology with potential applications in education, military, medical, consumption, and other fields. The optical waveguide scheme is a promising technology due to its thinness, compactness, and eyeglass shape, which can realize true wearability and all-day utilization. A volume holographic grating (VHG) has a good wavefront reconstruction function and can be fabricated by the laser exposure method. The process is controllable at a low cost, which is a potential AR solution. Although holographic waveguide display technology develops rapidly, its exit pupil size and field of view (FOV) are still important factors affecting the optical performance of volume holographic waveguide. VHG waveguides are difficult to achieve large FOV due to the restrictions of refractive index and angular bandwidth of holographic materials. How to ensure a large FOV display of VHG waveguide under the premise of a large exit pupil is a difficult research point, which requires a breakthrough from the design method of exit pupil expansion (EPE). Thus, we propose a design method of double VHG waveguide with two-dimensional (2D) EPE and FOV expansion. Methods To enlarge the exit pupil diameter and FOV of the VHG waveguide, we put forward a design method of double VHG waveguide with 2D-EPE and FOV expansion. The out-coupling grating of the waveguide structure is composed of double VHGs. By changing the light transmission direction in the waveguide through the structure of double VHGs, the beam is converted from one-dimensional propagation to 2D propagation for realizing the 2D-EPE of the system. We divide the incident FOV into two propagating paths, each path is responsible for half of the FOV, and the two parts of the FOV are finally spliced together to form a complete FOV. The optical principle and design method of 2D-EPE and FOV expansion are introduced in detail. Two non-coherent double grating structures are exposed on two layers of holographic material by double holographic exposure method, which is adopted to realize 2D-EPE in holographic waveguides. The proposed method not only expands the exit pupil diameter but also improves the FOV of the holographic waveguide system. Results and Discussions The VHG waveguide sample is fabricated by double holographic exposure method. The structure of the two grating positions is compact, which is conducive to glass shape modeling. A 532 nm laser is employed to illuminate HOE₁ vertically, and the diffracted light of HOE₁ is propagated in the waveguide by total reflection and then exits through the influence of HOE₂ and HOE₃ together. The exit pupil point propagates along the x and y directions, and the optical pupil point can prove the ability of the VHG waveguide to achieve 2D-EPE (Fig. 10). Digital light processing (DLP) is selected as the projection optical system to verify the display FOV of the waveguide sample. The experimental results show that the horizontal FOV of the waveguide is 48°, the vertical FOV is 27°, and the display diagonal FOV is 55° (Fig. 11). The experimental tests prove that the proposed method can realize 2D-EPE and FOV expansion. Conclusions A design method of double VHG waveguide with 2D-EPE and FOV expansion is proposed. The exit pupil diameter and display FOV of the holographic waveguide are enlarged by the combination of double VHGs. The optical propagation principle and design method of each path are introduced in detail. The volume holographic waveguide sample is fabricated by the double holographic exposure method, and the experimental test of the double VHG waveguide sample is carried out. The 2D optical pupil point propagates in the x and y directions of the exit pupil position, and the size of the exit pupil is 16 mm×13 mm. The displayed full FOV image has a horizontal FOV of 48°, a vertical FOV of 27°, and a diagonal FOV of 55°. The experimental results verify that the proposed method is beneficial to achieve 2D-EPE and FOV expansion, with a broad application prospect in AR head-mounted near-eye display.