In this paper, we present a novel method the étendue expansion of near-eye holographic displays through the generation of a sparse eye-box. Conventional holographic near-eye displays have suffered from narrow field of view or narrow eye-box due to the limited étendue supported by a spatial light modulator. We focus on the fact that these displays typically form a dense eye-box, which could be an excessive investment of the limited étendue. By rearranging the eye-box in a sparse manner, the practical étendue can be extended. With a properly designed sparse eye-box shape, it can provide the 3D holographic images and ensure continuous light entrance to the pupil. To create a sparse eye-box, we utilize a lens array as an eyepiece lens. We optimize the spatial light modulator's phase profile for the proposed system and analyze the impact of the use of the lens array eyepiece on the holographic image quality. In particular, we focus on the lens array specification of the lenslet pitch and the focal length, deriving feasible specifications based on our analysis. We experimentally demonstrate a near eye see-through display using the proposed system and verify the étendue expansion.

We propose a new thin and flat virtual reality (VR) display design using a Fresnel lenslet array, a Fresnel lens, and a polarization-based optical folding technique. The proposed optical system has a wide field of view (FOV) of 102°x102°, a wide eye-box of 8.8 mm, and an ergonomic eye-relief of 20 mm. Simultaneously, only 3.3 mm of physical distance is required between the display panel and the lens, so that the integrated VR display can have a compact form factor like sunglasses. Moreover, since all lenslet of the lenslet array is designed to operate under on-axis condition with low aberration, the discontinuous pupil swim distortion between the lenslets is hardly observed. In addition, all on-axis lenslets can be designed identically, reducing production cost, and even off-the-shelf Fresnel optics can be used. In this paper, we introduce how we design system parameters and analyze system performance. Finally, we demonstrate two prototypes and experimentally verify that the proposed VR display system has the expected performance while having a glasses-like form factor.

Holographic optical elements (HOEs) have a wide range of applications, including their emerging use in virtual and augmented reality displays, but their design and fabrication have remained largely limited to configurations using simple wavefronts. In this paper, we present a pipeline for the design, optimization, and fabrication of complex, customized HOEs that enhances their imaging performance and enables new applications. In particular, we propose an optimization method for grating vector fields that accounts for the unique selectivity properties of HOEs. We further show how our pipeline can be applied to two distinct HOE fabrication methods. The first uses a pair of freeform refractive elements to manufacture HOEs with high optical quality and precision. The second uses a holographic printer with two wavefront-modulating arms, enabling rapid prototyping. We propose a unified wavefront decomposition framework suitable for both fabrication approaches. To demonstrate the versatility of these methods, we fabricate and characterize a series of specialized HOEs, including an aspheric lens, a head-up display lens, a lens array, and, for the first time, a full-color caustic projection element.

In this paper, we present a concept for speckle reduction using optical superposition instead of temporal superposition. The proposed speckle reduction method does not contain any mechanical movement nor time-multiplexing technique. Therefore, the system is stable and free from the frame rate degradation or additional computation load. In addition, the angle diversity method is adopted with the proposed method to intensify the speckle reduction. In order to apply the angle diversity method, the required minimum angle difference is analyzed theoretically. To realize the proposed speckle reduction concept, the system is implemented with practical optical components. The system includes an echelon, which is a stair-shaped glass and a static diffuser. The echelon is employed to decrease the spatial coherence of the incident beam. The static diffuser imposes the band-limited random phases on each of the split beams that have passed through the echelon. Also, the static diffuser is used to combine mutually incoherent split beams into a single speckle-reduced beam. The proposed speckle-reduced light source is applied to the holographic display to demonstrate the speckle reduction effect. Experimental results show that the speckle contrast of the reconstructed image is reduced by 55%.

Holographic displays have great potential to realize mixed reality by modulating the wavefront of light in a fundamental manner. As a computational display, holographic displays offer a large degree of freedom, such as focus cue generation and vision correction. However, the limited bandwidth of spatial light modulator imposes an inherent trade-off relationship between the field of view and eye-box size. Thus, we demonstrate the first practical eye-box expansion method for a holographic near-eye display. Instead of providing an intrinsic large exit-pupil, we shift the optical system's exit-pupil to cover the expanded eye-box area with pupil-tracking. For compact implementation, a pupil-shifting holographic optical element (PSHOE) is proposed that can reduce the form factor for exit-pupil shifting. A thorough analysis of the design parameters and display performance are provided. In particular, we provide a comprehensive analysis of the incorporation of the holographic optical element into a holographic display system. The influence of holographic optical elements on the intrinsic exit-pupil and pupil switching is revealed by numerical simulation and Wigner distribution function analysis.

We introduce an augmented reality near-eye display dubbed "Retinal 3D." Key features of the proposed display system are as follows: Focus cues are provided by generating the pupil-tracked light field that can be directly projected onto the retina. Generated focus cues are valid over a large depth range since laser beams are shaped for a large depth of field (DOF). Pupil-tracked light field generation significantly reduces the needed information/computation load. Also, it provides "dynamic eye-box" which can be a break-through that overcome the drawbacks of retinal projection-type displays. For implementation, we utilized a holographic optical element (HOE) as an image combiner, which allowed high transparency with a thin structure. Compared with current augmented reality displays, the proposed system shows competitive performances of a large field of view (FOV), high transparency, high contrast, high resolution, as well as focus cues in a large depth range. Two prototypes are presented along with experimental results and assessments. Analysis on the DOF of light rays and validity of focus cue generation are presented as well. Combination of pupil tracking and advanced near-eye display technique opens new possibilities of the future augmented reality.