In this presentation, we will delve into the evolution of volume holographic optical elements (VHOEs), discussing their development alongside calculation models such as the VOHIL model. We will unveil a novel visualization scheme rooted in this model, aimed at elucidating Bragg diffraction within intricate volume holograms. Lastly, we will showcase the latest advancements in the application of VHOEs, particularly in the realm of lightguide-based Augmented Reality (AR) and Mixed Reality (MR) glasses.
The CGH technique is crucial for MR displays, resolving the VAC issue with three-dimensional image generation. However, drawbacks include excessive device volume and speckle noise from coherent light. The lightguides with VHOE couplers and LED light source are employed to address these issues. In this study, we employed an LED as the light source to reduce the speckle noise. The volume holographic optical elements (VHOEs) as the in-coupler and out-coupler are designed as bandpass filters. The VHOEs filtered the CGH display's effective wavelength to inhibit image degradation caused by dispersion. Furthermore, the aberration caused by the lightguide was analyzed and compensated in this study. The design method, simulation results, and experimental results are discussed in this work.
We proposed a Maxwellian-view display based on Volume Holographic Optical Element (VHOE) and light-guide for see-through Head-Mounted Display (HMD) system. The diffractive light-guide is advantageous because of compact construction that leads to smaller and lighter devices. Furthermore, the advantage of VHOEs lies in their potential to achieve reducing energy loss and increasing battery endurance, owing to the higher diffraction efficiency. The Maxwellian-view display is unique because the image quality is almost not affected by the observer's focus distance or the diopter of their pupils. The proposed system utilizes a VHOE with linear grating as the in-coupling device and a VHOE with convex lens function as the out-coupling. In order to achieve a high Field of View (FOV), a prefabricated holographic lens with a high Numerical Aperture (NA) was utilized to record the out-coupling. The proposed device achieved the diagonal FOV as 50°. In this study, the detailed fabrication method of the holographic light-guide based on VOHEs was presented. Furthermore, the design method in order to improve image quality was also proposed. The optical simulation for determining image quality and optimizing was achieved based on the ray tracing method. In this case, astigmatism aberration caused by the diffractive light-guide degraded the image quality. Therefore, a cylindrical lens is necessary if the compensation of astigmatism is desired.
In this study, a near-eye display system is presented based on a dual-layer holographic light-guide structure. Each holographic light-guide is generated with wavelength multiplexing for color display and is designed to offer half FOV for the whole system. The full FOV of the system can reach 50°. The holographic light-guide utilized Volume Holographic Optical Elements (VHOEs) with linear grating function as the in-coupler and out-coupler. For a light-guide display, the full-color information light must propagate in the light-guide within Total Internal Reflection (TIR) condition. Therefore, it is imperative that the device must employ the gratings with distinct periods for directing red, green, and blue information light. Then the information light with different colors must propagate in different light-guides, in order to prevent crosstalk and ghost noise induced by Raman-Nath gratings, such as surface relief grating, with different periods. In comparison, the VHOEs can record the gratings with distinct periods on a single material with low crosstalk because of their strong wavelength selectivity. In the wavelength multiplexing process, each photopolymer material recorded three gratings for red, green, and blue images. Each grating primarily affects the information light of its corresponding color and guides the information light with different colors into the same light-guide within the TIR condition. In our case, a single waveguide provided the color images with about 25 degrees horizontal FOV. The diffraction efficiency of the primary information light is much larger than crosstalk noise.
The development of Head-Mounted Displays (HMDs) for Augmented Reality (AR) has gained increasing attention due to their portability. However, traditional combiner-based HMDs are bulky, and thus limited their further application. To miniaturize the devices, diffractive waveguide devices are the best solutions. A relief grating is presented in this study. Through the Finite Element Method (FEM) rather than the traditional Rigorous Coupled-Wave Analysis (RCWA), the diffractive efficiency of relief gratings with different slant angles and different slit depths have been investigated with the wavelength around 532 nm and several incident angles. The relief grating had slits with a slant angle α, depth d, and periods a. In the FEM simulation environment, the grating was fabricated on the glass substrate surrounded by air, and the refractive indices are nglass = 1.5 and nair = 1. The diffractive efficiencies were analyzed in several slant angles with depths varying from 50 nm to 500 nm. The results showed that a deeper grating depth produced higher efficiency, and a larger incident angle resulted in stronger contributions to the corresponding diffraction order. The relief grating with different slant angles showed greater efficiency in the +1 order, while the efficiency of the −1 order decreased. The relief grating with a larger slant angle showed higher efficiency than the grating with a smaller slant angle. Overall, a larger slant angle effectively induced higher efficiency in the +1 order, and the depth can also give contributions to the diffractive efficiency enhancement.
A polymer-dispersed liquid crystal (PDLC) device is fabricated to serve as a diffuser for random phase encoding of reference beams in holographic storage system. By tuning the applied voltage on the PDLC device, three individual random phase reference waves which are orthogonal to each other can be generated. Therefore, holographic multiplexing scheme can be performed using voltage modulation on the PDLC diffuser. In addition, shift multiplexing technique still can be performed in the storage system. When compared with conventional shift multiplexing, using electrically tunable PDLC diffuser offers additional multiplexing mechanism in the presented technique. Using the proposed PDLC diffuser, the multiplexing numbers can be enhanced three times owing to voltage modulation. A holographic storage system implemented using shift multiplexing and voltage modulation on PDLC diffuser is demonstrated.
A three-dimensional/two-dimensional (3-D/2-D) convertible display was proposed. The display system offers 2-D images via the traditional display technology and offers 3-D images via the computer-generated hologram (CGH) technology. In this system, 2-D and 3-D images are generated from the same amplitude-modulated spatial light modulator (SLM). In the 2-D display mode, the panel displays an amplitude-modulated image and the 2-D image is projected on a polarization-dependent diffuser (PD) screen. In the 3-D display mode, the SLM panel displays CGHs to generate 3-D images. The zero-order diffraction of the amplitude-type CGH will be blocked by a spatial filter (SF), which is designed by a polarizer. Furthermore, a liquid crystal element was employed to modulate the polarization state of the reconstructed light to let the 3-D image quality not to be reduced by the PD. Finally, the proposed system can offer 2-D and 3-D images both clearly. The system suppresses the zero-order diffraction noise of the amplitude-type CGH system successfully.
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