Analysis of surface relief grating waveguide based on finite element method

The development of Head-Mounted Displays (HMDs) for Augmented Reality (AR) has gained increasing attention due to their portability. However, traditional combiner-based HMDs are bulky, and thus limited their further application. To miniaturize the devices, diffractive waveguide devices are the best solutions. A relief grating is presented in this study. Through the Finite Element Method (FEM) rather than the traditional Rigorous Coupled-Wave Analysis (RCWA), the diffractive efficiency of relief gratings with different slant angles and different slit depths have been investigated with the wavelength around 532 nm and several incident angles. The relief grating had slits with a slant angle α, depth d, and periods a. In the FEM simulation environment, the grating was fabricated on the glass substrate surrounded by air, and the refractive indices are nglass = 1.5 and nair = 1. The diffractive efficiencies were analyzed in several slant angles with depths varying from 50 nm to 500 nm. The results showed that a deeper grating depth produced higher efficiency, and a larger incident angle resulted in stronger contributions to the corresponding diffraction order. The relief grating with different slant angles showed greater efficiency in the +1 order, while the efficiency of the −1 order decreased. The relief grating with a larger slant angle showed higher efficiency than the grating with a smaller slant angle. Overall, a larger slant angle effectively induced higher efficiency in the +1 order, and the depth can also give contributions to the diffractive efficiency enhancement.