US Patent 10038840 Diffractive optical element using crossed grating for pupil expansion

In an optical display system that includes a waveguide with multiple diffractive optical elements (DOEs), an in-coupling DOE couples light into the waveguide, an intermediate DOE provides exit pupil expansion in a first direction, and an out-coupling DOE provides pupil expansion in a second direction and couples light out of the waveguide. The intermediate DOE is configured with a crossed grating which is a three-dimensional microstructure that is periodic in two directions. The crossed grating provides multiple optical paths to a given point in the DOE in which differences in optical path lengths are larger than the coherence length. The crossed grating in the intermediate DOE may provide increased display uniformity in the optical display system by reducing the “banding” resulting from optical interference that is manifested as dark stripes in the display. The crossed grating can further enable the optical display system to be more tolerant to manufacturing variations.