Solving the key problems of head mounted display with holographic optical elements

The head mounted display (HMD) provides people with a new visual acquisition method that users can obtain the visual information provided by the external scene and the display device at the same time. However, the new display method also increases the difficulty of optical system design. The display system should be compact and lightweight, and provide sufficient field of view (FOV) in a large eye-box. The holographic optical waveguide display has the characteristics of light weight and good perspective, so it is very suitable for HMD. The holographic optical element (HOE) has a high degree of design freedom, which provides more possibilities for the optical system design. To enlarge the FOV, we use two HOEs and stack them, each HOE provides a small FOV, and the two FOVs are spliced together to form a large FOV. To extend the eye-box, we duplicate a single viewpoint several times and distribute the viewpoints evenly in space. Specifically, first, a single viewpoint of the system is copied into two viewpoints separated from each other using a multiplied HOE. Then two polarization gratings placed in parallel are used, which can convert the incident linearly polarized beam into two circularly polarized beams that separate in the opposite direction. The two polarization gratings double the two viewpoints obtained in the first step to four viewpoints. Around these viewpoints, the user can see a clear and complete image.