Beam interaction and targeted optimization methods for AR waveguide design

Waveguide combiners for augmented reality are a popular architecture because they can remain compact while delivering a large etendue to the user. Waveguides propagate light via total internal reflection, passing light from the display engine to the user through the substrate itself. While the operating principles are straightforward, the design is much more complex, with thousands of possible paths for light inside the glass. We introduce a method for simulating light propagation through the waveguide based on beam interactions with surfaces rather than non-sequential ray tracing. This method calculates spatial efficiency one hundred times faster than non-sequential ray tracing. It can also calculate how the waveguide affects image quality at any eyebox position using clipped beams to obtain the modulation transfer function. We also introduce a targeted optimization approach, making optimization faster and more effective. This approach localizes the optimization for each field to only those parts of the waveguide that contribute to the eyebox, mitigating losses of light that are unnecessarily out-coupled elsewhere. Using this method, we optimize an L-shaped waveguide and a crossed-grating waveguide using target diffraction efficiency values as variables and efficiency, uniformity, and image quality as optimization goals. In this ideal case, we quadruple the L-shaped waveguide's efficiency and double the crossed-grating waveguide's efficiency compared to similar designs in other work.