Angular-Response Balancing in 4H-SiC Diffractive Waveguides Using Oppositely Oriented Slanted Gratings

High-index 4H-silicon carbide (4H-SiC) provides a larger waveguide k-space than conventional glass and is therefore attractive for wide field-of-view (FOV) diffractive augmented reality waveguides. However, the angular response of a slanted grating is asymmetric. For one grating-vector direction, the diffraction efficiency remains high over a wide positive-angle range but decreases rapidly at negative angles, leading to nonuniform image brightness over a broad field. Reversing the grating vector reverses this response. The field is therefore divided at 0°, with the positive and negative FOV channels assigned to two mirror-related slanted-grating domains with opposite grating vectors. The grating parameters for the red (648 nm), green (548 nm), and blue (448 nm) wavelengths were optimized separately using particle swarm optimization. Their angular diffraction responses were calculated by rigorous coupled-wave analysis and then introduced into Zemax for system-level ray tracing. At 548 nm, the optimized 4H-SiC, n = 1.8 glass, and n = 2.0 glass gratings provided effective angular bandwidths of approximately 88°, 31°, and 37°, respectively. Under the same 4H-SiC material and system conditions, the +G/−G arrangement reduced the root-mean-square (RMS) irradiance nonuniformity from approximately 40.3% to 32.8% and increased the effective fill factor from 40.8% to 61.5% compared with the single-+G configuration. In a separate system-level comparison, the optimized n = 1.8 glass and 4H-SiC models yield RMS irradiance nonuniformities of 44.0% and 32.8% and effective fill factors of 46.1% and 61.5%, respectively. These results connect the broad angular response of the optimized 4H-SiC grating with the system-level balancing produced by opposite grating-vector assignment. The optimized grating parameters and detector-plane response maps also provide a computational basis for subsequent fabrication and experimental validation of the proposed 4H-SiC waveguide design.