Off-axis varifocal augmented reality near-eye display with holographic astigmatism compensation

Volume holographic grating (VHG)-based off-axis optical combiners supersede conventional macro-optics with thin flat optics. However, VHG-based optics suffers from holographic astigmatism, leading to astigmatism aberration. This issue becomes markedly more severe in varifocal optics implemented under the off-Bragg conditions. Nevertheless, holographic astigmatism can be alleviated by the asymmetric grating vector in VHGs. We propose the off-axis varifocal-based AR-NED with transmission- and reflection-type holographic lenses. These holographic lenses are designed to compensate for holographic astigmatism by counteracting the asymmetric grating vector. This scheme is analyzed by the k-vector diagram and lens equation of both holographic lenses. The proposed off-axis varifocal optics is validated by the AR demonstration using the focus tunable lens. Ultimately, across a depth range of 0 to 3.3 diopters, the experimental results maintain a modulation transfer function of 0.5 around 9 lp/mm and an average laser speckle contrast of 6.3%, without significant degradation in image quality during dynamic focal plane shifts, and with improved astigmatism correction.