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.
Augmented Reality (AR) and Mixed Reality (MR) glasses stand as pivotal technological advancements in contemporary society. However, maintaining a compact and lightweight design while ensuring high-quality image viewing remains a persistent challenge. Researchers endeavor to overcome the intricate optical hurdles associated with these glasses. They suggest that waveguides incorporating two in- and out-coupling Volume Holographic Optical Elements (VHOEs) has surfaced as a promising approach, addressing these requirements and providing high see-through transmittance due to Bragg selectivity. Nonetheless, in the case of a full-color VHOE-based waveguide, the crosstalk between the RGB gratings of three primary colors within a waveguide results in the ghost images that diminish image quality. In this paper, we propose a method to eliminate ghost images and offer precise simulations aligned with experimental observations.
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.
In this paper, we will discuss about the color performance and image quality of the Volume-Holographic-Optical-Element (VHOE)-based AR devices using two different type of volume holograms: transmission or reflection-type VHOEs. The simulation results show that color distortion occurring in these two types of VHOE-based AR devices are quite different and resulting from different characteristics of Bragg diffraction from those VHOEs. The reflective VHOE-based AR devices show better overall performance in terms of color, field of view and image quality.
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.
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.
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.
A holographic optical element (HOE) simultaneously accompanied with light guiding and beam shaping
function is implemented with edge-lit holograms in this study. This holographic optical element is generated
in a polymer-dispersed- liquid-crystal (PDLC) film with 20μm thickness. In the holographic reconstruction
process of the HOE, the wavefronts emitted from the light source will propagate to the HOE and a quasi
collimation diffraction beam can be obtained from this device. We demonstrate two applications of edge-lit
HOE in this study. One demonstration is a head-mounted display (HMD) system, and the other is an
illumination device for display holograms.
A profile measurement approach using two diffractive elements to generate two fringe patterns is presented. Only one
phase measurement needed for operation. In conjunction with the endoscope, the compact design makes it possible to
inspect dynamic object inside the body cavity.
A nonvolatile reading of erasable polarization holograms in dye-doped liquid-crystal films using the same writing wavelength is presented. The recorded hologram can be easily erased with the illumination of one p-polarized wave; nevertheless, it becomes nonvolatile when reading the hologram with one s-polarized wave. We have proven the nonvolatile reading property is induced by dye adsorption on the substrate of the sample.
We recorded a hologram in a photorefractive LiNbO3:Fe crystal with a two-color recording technique in transmission geometry. The holographic recording involves a light with wavelength 633 nm for interference, and a light with wavelength 532 nm for exciting. The short wavelength light excites more charges so that the holographic recording is affected, and the storage capacity (M-number) and the sensitivity vary accordingly. We found that the optimized intensity ratio of the interference lights and the exciting light is between 50 and 60. In the optimized conditions, the M number and the sensitivity are enhanced by 43 and 35%, respectively. Because the crystal we used is a typical iron-doped LiNbO3 crystal and the short wavelength exciting light source is inexpensive now, the proposed method is easy to be achieved in most holographic systems.
We have successfully demonstrated a holographic memory in a single LiNbO3 crystal with two simultaneous
but individual readout channels. A special scheduled exposure model is derived to obtain equal diffraction
efficiency of each hologram in this memory. The simultaneous readout technique is achieved in a hybrid-multiplexed
memory using angular multiplexing and the polarization multiplexing. Polarization multiplexing
offers the mechanism of simultaneous readout for two individual orthogonally polarized images. In each
angular position of the holographic memory, these two orthogonally polarized images can be reconstructed
simultaneously and each of them can be viewed independently. After our proposed scheduled exposure,
experimental result of diffraction efficiency in each hologram becomes equal and the result is consistent with
our prediction.
In this paper, a quartz-based micro-prism array structure is newly proposed as the "parallax barrier", and such a device is
designed only for fitting the size of a 2.2 inch LCD panel, the most popular size for the TV on mobile phone. The optical
simulation software LightTools was applied to verify whether the designed structure can work or not. The parameters we
considered including the viewing angle, viewing distance, vertex angle of prism, refractive index of quartz (=1.46) and
the sub-pixel width (=66 μm). 1,000,000 rays emitted from the dual view display panel are simulated, as the result, our
designed quartz-based micro-prism array structure can successfully separate images from odd or even sub-pixels into
two different viewers and the view angle is indeed 80° as our requirement. The key to control the red, green, blue light
from different sub-pixels can be precisely guided into the same direction is ascribed to the arrangement of the vertex
angles of the relative prisms (R:47.1°, G:47.2 °, B:47.4 ° ). Three steps of lithography and the reactive ion etching can
fabricate the precise angle and size of our needs. The left and right images generated by our designed dual-view display
are quite pellucid without color difference, and very similar to our simulation.
A multiplexing scheme in volume holograms with a spherical reference wave implemented by rotating a mirror is presented. Diffraction angular selectivity of the mirror in the proposed holographic storage system is theoretically and experimentally investigated. The spherical reference waves in the presented multiplexing scheme can be replaced with plane reference waves; however, angular selectivity of the mirror with a plane reference wave is higher than that with a spherical reference wave in the system. Angular selectivity of the mirror using a spherical reference wave in the system can be increased by decreasing the distance between the mirror and the hologram. Unlike spherical reference waves, angular selectivity using a plane reference wave stays the same in this system. Although using spherical reference waves generates a lower angular selectivity, it also offers larger total accessible angular range for reference beams. Accordingly, using spherical reference waves in the proposed system will not sacrifice the storage capacity.
Volume holographic storage has received increasing attention owing to its potential high storage capacity and access rate. In the meanwhile, encrypted holographic memory using random phase encoding technique is attractive for an optical community due to growing demand for protection of information. In this paper, encryption-selectable holographic storage algorithms in LiNbO3 using angular multiplexing are proposed and demonstrated. Encryption-selectable holographic memory is an advance concept of security storage for content protection. It offers more flexibility
to encrypt the data or not optionally during the recording processes. In our system design, the function of encryption and non-encryption storage is switched by a random phase pattern and a uniform phase pattern. Based on a 90-degree geometry, the input patterns including the encryption and non-encryption storage are stored via angular multiplexing with reference plane waves at different incident angles. Image is encrypted optionally by sliding the ground glass into one of the recording waves or removing it away in each exposure. The ground glass is a key for encryption. Besides, it is also an important key available for authorized user to decrypt the encrypted information.
In this paper, we have developed the synthesized method for volume Hologram grating by layer-peeling algorithm. Layer-peeling algorithm is known as an efficient tool for synthesizing fiber Bragg grating which is one-dimension grating without absorption. By taking account for attenuation and angles of incident wave, we have modified the layer-peeling algorithm that can synthesize volume Hologram grating in lithium niobate crystal.
Two volume holographic storage schemes using random phase multiplexing technique are presented. A ground glass is spatially shifted to multiplex the signals for both storage schemes. The diffraction selectivity of the ground glass is experimental investigated and theoretically analyzed.
Multilayer storage in a shift-multiplexed holographic disk is proposed. An optical head comprising a fiber array is used for multiplayer storage in a LiNbO3 holographic disk. High longitudinal selectivity of spherical reference waves is obtained and permits the implementation of multiplayer storage in the hologram. The technique yields the efficient usage of dynamic range and promotes the storage capacity potentially.
We propose a novel method for optical security by using a randomly selected binary phase code. The phase code is recorded with holographic volume gratings, and the recording material can be regarded as an optical lock, while the original phase code acts as the key. When an arbitrary phase code is used for decryption, the decryption probability is less than 10–10 when the phase code contains only five rows. The decryption probability decreases dramatically as the row number of the code or the threshold value increases. Furthermore, the security system performs good tolerance for data loss of the key. Both the theory and experiment are demonstrated.
We propose a volume-holographic filter to perform 3D object filtering. Theoretical analysis and the corresponding experiment of the shifting tolerance of the volume-holographic filter are demonstrated. We find that the shifting selectivity is different for different shifting directions, which include laterally horizontal, laterally vertical, and longitudinal directions. The simulation results show that the shifting selectivity depends on the scale of the object and the distance between obejct and the holographic filters.
We propose a new holographic multiplexing technique in LiNbO3 with 90° geometry. The advanced concept of polarization-multiplexed holographic memory is based on photorefractive effect and photovoltaic effect. The holographic gratings are constructed in the crystal using two writing beams with isotropic and anisotropic polarizaiton recording. Even mutually orthogonal polarized waves can be used in holographic storage. The polarization multiplexing technique can be accompanied with other multiplexing method in the holographic storage, and we can increase the storage capacity doubly.
An optical security holographic memory encrypted by a random phase key and decrypted by a holographic key (a holographic optical element; HOE) is presented and demonstrated. The security hologram is based on protecting the access to the memory from the unauthorized users by encoding the reference waves. The holographic decryption key is fabricated by storing the wavefront information of the set of reference beams. Each addressed reference beam is stored in the HOE through holographic multiplexing technique. The decryption process is achieved by using the reconstruction reference beam to access the secure data. This security hologram system is suitable for practical application since the decryption key can be controlled copied. As to our best knowledge, this is the first experimental impementation to replace a random phase mask key.
Three all-optical systems, including a fiber displacement sensing system, an angular sensing system and a 2D-to-2D pattern transfer system based on holography interconnect are presented. Photorefractive crystals, the volume hologram recorder serves as the database and signal processor in the systems. All the processes include detection of the external signal, signal transfer, signal processing and the display of results are performed by optical means. Keywords: Volume hologram, Holographic interconnection, Random phase encoding, Angular multiplexing.
Optical interconnections based on random phase encoding using a ground glass in a volume hologram have been presented. In the system, a crystal plays the role of interconnector, which translates Chinese words into English words. The stimulation results and experimental results of the translation system are demonstrated.
We have studied the characteristic of the angular multiplexing with spherical reference wave in volume holographic storage. Through the theoretical analysis and the corresponding experimental observation, the angular sensitivity is almost independent of the distances between the point source and the hologram. Accordingly, an all- optical angular sensing system have been proposed and demonstrated.
We investigate the tolerance to data loss of a double random phase encoding encryption system. The signal-to-noise ratio of the decrypted image is investigated when a part of the encrypted image is lost. The SNR can be enhanced by reducing the ratio of the sampling number of the image to the total sampling number when electronic implementation is performed. In optical implementation, the SNR can be increased by using a phase mask with higher spatial frequency. Both the theory and computer simulations are demonstrated.
An all-optical fiber sensing system with interconnection in volume holograms through random phase encoding has been demonstrated. The random phase is generated from the speckles of a multi-mode fiber. The experimental and stimulation results are also presented.
Three dimensional shifting sensitivity of volume holograms based on random phase encoding using a ground glass has been theoretically analyzed. There are different shifting tolerances in different shifting directions, which include laterally horizontal, laterally vertical, and longitudinal directions. The shifting sensitivity depends on the diameter of the illumination region on the random phase plate, the thickness of the hologram and the distance between them. We apply the theoretical calculation to a degeneracy condition, a point object, and theoretically analyze the shifting tolerance of shifting multiplexing in holographic storage and holographic confocal microscope.
In this report, we demonstrate and discuss a holographic storage system, where two holographic multiplexing methods are included: one is angular multiplexing and the other one is random phase multiplexing through a ground glass. The latter is responsible to the optical encryption when the stored pattern needs to be encrypted. We study the tolerance to the displacement of the ground glass in three-dimension. We find that it is a function of both the illumination area of the ground glass and the thickness of the volume hologram.
Double random phase encoding technique is a valuable and effective method for optical image encryption. However, a precise alignment is required when optical setup is performed. In this paper, we investigate the shift tolerance property of the technique. The theory of the robustness to data loss of the encrypted image is proposed. According to the theory, we propose a simple and novel method to improve the shift tolerance of the decrypting phase mask. Both theory and computer simulation are presented.
An optical security system using a random selected binary phase code is proposed. A random binary phase encoded light is incident on a holographic recording material and writes gratings in the material with a reference plane wave. The recording material can be regarded as an optical lock while the phase code acts as the key. As the pixel number is 5 X 20, the decryption probability is less than 10-10 when arbitrary phase code is used for decryption. The decryption probability decreases dramatically as the pixel number increases. Furthermore, the security system performs good tolerance to data loss of the phase code.
We demonstrate an encryption-selectable holographic multiplexing storage in LiNbO3. The storage of multiple holograms is based on angular multiplexing and random phase encoding. Phase conjugates of the reference beams are used to read out those stored patterns. The sensitivity of the displacement of the ground glass is analyzed theoretically.
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