Significance As the entrance technology of metaverse, augmented reality (AR) is highly likely to become the next generation computing platform. Near-eye display is the core foundation for the development and application of AR technology and is the direct medium for people to receive virtual information and combine reality. Near-eye display optical systems are the core component of AR, and the maturity of the display module is part of AR technology popularization. The vergence-accommodation conflict (VAC) of near-eye display systems is a key challenge restricting the large-scale AR application. Human beings obtain 80% of external information by human eye vision. To obtain the 3D display effect, near-eye display optical systems will simulate the real scenes through both eyes, displaying the independent pictures of the two eyes with certain parallax to make the brain perceive the 3D display effect. Meanwhile, the eye lens is always focused on the virtual image plane of the micro-display of the optical system, which indicates the fixed eye accommodation distance. Due to the setting of the left and right eye images with parallax, the brain will perceive the distance of the 3D image objects, and then the convergence distance of human eyes will change with the built-in image source. As a result, a mismatch is caused between the convergence distance and the accommodation distance, which is a VAC problem. When the user is in this state for a long time, visual fatigue, dizziness, and vomiting will occur. The VAC problem in the near-eye display optical system causes the display scene to deviate from people's perception in the real world, which brings bad user experience and is a major challenge to be solved for the long-term utilization and popularization of AR devices. Progress VAC is an insurmountable technical challenge in the development of AR display technology. We classify its solutions to provide references for selecting solutions suitable for different technical development needs, and review current VAC solutions, classifying them as solutions without depth information, with partial depth information, and with complete depth information (Fig. 2). In the VAC solutions without depth information, we mainly introduce Maxwellian display technology, which takes the image as a single point beam through the eye lens photocentric position and subsequently images it directly onto the retina. Thus, the limitation that the eye lens must be forced to focus can be addressed, which means the human eye lens can observe the image in different diopter states and the VAC problem can be overcome. Lin et al. implemented a MEMS-based Maxwellian display system to achieve a 33 degrees x 22 degrees display field of view with an exit pupil distance of 10 mm, converging the imaging beam into a 5 mu m spot with the best imaging quality when the spot is at the photocentric position of the human eyes. The Maxwellian technique requires the converging light to form a beam point that matches the pupil position, resulting in the natural limitation and drawback of this solution in the exit pupil range and needs to be matched with a corresponding pupil extension technique. In the VAC solutions with partial depth information, we mainly introduce multi-focal plane display technology (physical multi-focal plane and virtual multi-focal plane) and adjustable focus display technology. Cheng et al. equipped two microdisplays to implement a two-focal plane AR near-eye display module by spatial multiplexing [Fig. 6(c)]. The display solution achieves a 40 degrees display field of view, an exit pupil distance of 20 mm, and an eyebox aperture of 6 mm, enabling two different focal planes at 1.25 m and 5 m distances. In the VAC solutions with complete depth information, we introduce integrated imaging and computational holography solution. Hua et al. employed an integrated imaging unit as a stereo microdisplay image source combined with a free-form prism to form an AR near-eye display module with depth information, achieving a 33.4 degrees display field of view, an exit pupil distance of 19 mm, and an eyebox aperture of 6.5 mm [Fig. 9(a)]. Zhang et al. achieved complex amplitude wavefront reconstruction of image information by cascading amplitude holograms to provide complete depth information for AR near-eye display. Finally, the undesirable effects of VAC are eliminated to achieve a 4.8 degrees display field of view (9.4x secondary magnification) with an exit pupil distance of 10 mm. Conclusions and Prospects We present a systematic review of VAC solutions in AR near-eye display optical systems. Meanwhile, the focus is on the basic principles of the VAC problem and the technical features, implementation methods, and representative literature of current near-eye display optical solutions. The VAC problem is contrary to the physiological characteristics of human eyes in daily observation and is an inevitable difficulty for AR to enhance viewing comfort. In current VAC solutions, Maxwellian near-eye display, variable focus, and multi-focal plane near-eye display solutions have relative advantages in design and image quality.
