This paper presents a breakthrough in waveguide-based augmented reality (AR) displays by introducing an anomalous polarization conversion phenomenon in polarization volume gratings (PVGs). The study addresses the long-standing challenge of low in-coupling efficiency and poor image uniformity in waveguide-based AR displays. The key innovation lies in the discovery that PVGs can function as waveplates when the incident light direction does not satisfy the Bragg condition, enabling efficient polarization conversion. This property allows for a significant improvement in both in-coupling efficiency and uniformity without compromising eyebox continuity. The research demonstrates that by leveraging this polarization conversion, the in-coupling efficiency and uniformity of a 50° field of view (FoV) waveguide-based AR display can be improved by 2 times and 2.3 times, respectively, compared to conventional couplers. The results are supported by both experimental measurements and simulations. The study also shows that by incorporating an additional polarization compensation film at the in-coupler, nearly all light can be effectively coupled into the waveguides. The paper discusses the underlying mechanisms of light loss in waveguide-based AR displays, including absorption in high-index waveguide substrates, pupil expansion limitations, and multiple interactions at the in-coupler. These factors contribute to the low efficiency and poor uniformity in current waveguide displays. The proposed solution using PVGs effectively circumvents these limitations by utilizing the unique polarization conversion properties of PVGs. The study also explores the potential of this polarization conversion phenomenon in various waveguide display designs, including full-color displays and multi-layer PVGs. The results indicate that the polarization conversion phenomenon can significantly enhance the in-coupling efficiency and uniformity in waveguide-based AR displays, paving the way for more efficient and higher-quality AR systems. The findings highlight the potential of PVGs as a superior in-coupler compared to conventional couplers, offering a promising solution for the future of waveguide-based AR displays.This paper presents a breakthrough in waveguide-based augmented reality (AR) displays by introducing an anomalous polarization conversion phenomenon in polarization volume gratings (PVGs). The study addresses the long-standing challenge of low in-coupling efficiency and poor image uniformity in waveguide-based AR displays. The key innovation lies in the discovery that PVGs can function as waveplates when the incident light direction does not satisfy the Bragg condition, enabling efficient polarization conversion. This property allows for a significant improvement in both in-coupling efficiency and uniformity without compromising eyebox continuity. The research demonstrates that by leveraging this polarization conversion, the in-coupling efficiency and uniformity of a 50° field of view (FoV) waveguide-based AR display can be improved by 2 times and 2.3 times, respectively, compared to conventional couplers. The results are supported by both experimental measurements and simulations. The study also shows that by incorporating an additional polarization compensation film at the in-coupler, nearly all light can be effectively coupled into the waveguides. The paper discusses the underlying mechanisms of light loss in waveguide-based AR displays, including absorption in high-index waveguide substrates, pupil expansion limitations, and multiple interactions at the in-coupler. These factors contribute to the low efficiency and poor uniformity in current waveguide displays. The proposed solution using PVGs effectively circumvents these limitations by utilizing the unique polarization conversion properties of PVGs. The study also explores the potential of this polarization conversion phenomenon in various waveguide display designs, including full-color displays and multi-layer PVGs. The results indicate that the polarization conversion phenomenon can significantly enhance the in-coupling efficiency and uniformity in waveguide-based AR displays, paving the way for more efficient and higher-quality AR systems. The findings highlight the potential of PVGs as a superior in-coupler compared to conventional couplers, offering a promising solution for the future of waveguide-based AR displays.