## 1 Introduction  
Virtual reality (VR) and augmented reality (AR) have been hot topics recently, and 3D technology is an important part of VR technology [1]. VR is widely used in various fields, such as architecture, gaming, and education, where it has a promising future in the development of VR. Compared to holographic displays, conventional optics present several disadvantages for near-to-eye 3D products for VR and AR [2]. Although 3D techniques are popular in current VR and AR applications, most 3D head-mounted displays (HMD) are based on stereoscopic 3D display technology, i.e., left and right eyes get two images with binocular parallax. The human brain can combine these two images into a 3D image. However, these images in the viewing angle of the stereoscopic 3D display are separate two-dimensional (2D) images, which are different from the continuous wavefront of 3D objects or 3D digital models. When images were displayed outside the range of accommodation, visual fatigue was induced [3; 4; 5; 6]. The stereoscopic 3D display is not suitable for everyone to watch for a long time, because not everyone's pupil distance is within the applicable range. Therefore, 3D techniques should be improved to overcome the drawback in VR and AR. It is a conventional 3D technology in VR head-mounted displays, as shown in Figure 1.  
To overcome the limitation of the stereoscopic 3D technique, some researchers have studied some other 3D techniques in HMD. [PERSON] et al. used an LED light source instead of lasers as the reading light source in the holographic HMD and produced a nice effect [7]. [PERSON] et al. did some work on astigmatism aberration compensation of the holographic HMD, and their system can present holographic 3D images in a see-through fashion clearly in a wide depth range [8]. [PERSON] et al. presented a compact waveguide display system integrating free-form elements and volume holograms; this design makes the system more compact because of the waveguide [9]. [PERSON] et al. used a transparent volume hologram for a holographic projection HMD and filmed the holograms to show that the see-through holographic projection HMD system can present three-dimensional images clearly [10]. [PERSON] et al. designed real-time tomographic holography for AR, which used a large-aperture holographic optical element (HOE) and obtained good results, using a large-aperture HOE to project an off-axis, viewpoint-independent virtual image 1 m away [11]. [PERSON] [PERSON] et al. used an active shutter for HMD application to get a more precise system [12]. [PERSON] et al. used photopolymer for a full-color waveguide-type HMD and obtained a compact system, and it provided wide angular selectivity and can fabricate high-quality full-color HOEs [13]. [PERSON] et al. used a mirror-lens to propose a see-through AR display and achieve an optimized optical recording condition of the mirror-lens HOE [14]. [PERSON] et al. used an index-matched anisotropic crystal lens to propose a holographic see-through near-eye display, and the system showed the possibility of a holographic display with a large field of view [15]. [PERSON] et al. proposed a holographic near-eye display system based on a double-convergence light algorithm and provided a promising solution in future 3D AR realization [16]. [PERSON] et al. proposed a near-eye foveated holographic display [17]. Among these 3D techniques, holography is thought to be a good candidate for true 3D VR, because digital 3D holography based on spatial light modulators (SLMs) can provide the holographic 3D property of wavefront reconstruction and realize real-time video rate dynamic display [18; 19; 20; 21]. In this paper, we present a holographic 3D HMD and study its 3D properties, such as viewing angle and depth of field dependent on the pixel size of SLMs. We believe that with the development of SLMs and digital information technology, holographic 3D VR and AR can be applied in the future.