Physical problems of diffraction waveguide used in large field of view

Abstract

Diffraction gratings have been widely used in waveguides. They can transmit light beams or images from the in-coupling end to the out-coupling end at predetermined positions. However, when they are applied to augmented reality and virtual reality with large field of view and color light sources, there will arise some problems such as mismatch and missing field of view, non-uniform emission, and others. Therefore, starting from these physical problems, the upper limit of the field of view for diffractive waveguide and the complete theoretical boundary formula of the field of view are derived, and on this basis, in-depth research is conducted on monochromatic waves and multicolor waves, respectively. It is concluded that the single-layer diffractive waveguide supports the theoretical upper limit of the monochromatic wave field angle of about 48° under normal high refractive index of <i>n</i> = 1.75, and supports the theoretical upper limit of the multicolor wave field angle of 26.4° for coefficient <i>q</i> = 1.3. Clearly, a larger field of view requires a higher refractive index <i>n</i> and a smaller <i>q</i> value. The boundary conditions of field integrity indicate that reducing the maximum diffraction angle of the long wave and thinning the thickness of the waveguide layer can solve the problem of missing field of view. The practical maximum diffraction angle generally does not exceed 75°, and the thickness of the waveguide layer is about 0.5 to 1.0 mm generally based on the incident field of view. Finally, a method of expanding each total internal reflection field of view into a ray tracing diagram and a distribution function of pupils to receive light energy at various angles are obtained. In this way, the optimal position of the out-coupling grating region can be achieved, and the inverse of the distribution function is used to constrain the angular distribution of the projected light or the grating efficiency, and then receiving uniform exit image at any position becomes possible. The uniformity of the monochromatic waveguide increases from 0.27 to 0.15, and the uniformity of the long wave in the single grating multicolor waveguide rises from 0.4 to 0.28. The results of these studies will undoubtedly help to solve the problem in the diffractive waveguides used in large field of view and multicolor light.