2D photonic-crystal-type surface relief grating for light with an azimuth angle

With the development of virtual reality, the projection design of the head-mounted display (HMD) requires improvement as well. The surface relief grating (SRG) is one of the popular diffraction elements to achieve light guiding through the waveguide. Using the SRG as the coupler, incoming information light can be coupled into the waveguide by the 1-order diffraction. To increase possible design options for the HMD, SRG with complex geometry has been designed to fit more diffraction conditions. In this paper, a 2D SRG is proposed to form by periodically arranging slanted glass pillars on the surface of a glass waveguide, just like a photonic crystal. The structure can suppress all the other diffraction orders except the 0-order and the [1, 0] order. The best diffraction efficiency of the [1, 0] order can be obtained when the incident light is incident with not only an incident angle but also an azimuth angle. The geometric parameters of the pillars are investigated and optimized with the diffraction efficiency by the finite element method, and the relationship between the incident angles and the diffraction efficiency will also be discussed. The proposed structure has the potential in both the in-coupler and the out-coupler, respectively, for integrating light coming from different azimuth angles or for outputting light to different azimuth angles.