Comparative analysis of off-axis digital hologram binarization by error diffusion

The digital micromirror device DMD is widely used in visible light projection, special-purpose spatial light modulation, and infrared scene simulation, due to its high resolution, uniformity and energy concentration. In some applications that require high frame rate scene image display, it is necessary to ensure that the DMD displays high gray level images at a high frame rate. However, the display frame rate of the pulse width modulation (PWM) method is limited by the minimum time required for DMD loading data, unable to achieve high frame rate display. Although the DMD binary display mode can meet the requirements of high frame rate display, it cannot meet the requirements of high gray level. The paper proposes an image display method that uses light source and DMD to synchronize modulation. Decompose the high-order gray image into bit planes according to the gray threshold, and the DMD displays each bit plane in binary mode under the trigger of an sync pulse. The intensity of the illumination laser source is modulated by an acousto-optic modulator to match the bit plane and the radiated laser power. This method makes use of the function of high frame rate display in DMD binary mode cooperate with light intensity modulation of illumination laser source, and realizes high frame rate and high dynamic range image display. With this method, the maximum frame rate of 8-bit gray level image with 1920 ×1080 resolution can reach 2KHz. The experimental system has realized 200Hz frame rate display of 8-bit gray level image with 1920 ×1080 resolution.

This paper presents our implementation of a multi-view autostereoscopic 3D display system by using time-multiplexing approach, digital micro-mirror devices (DMD), and the optical Wedgereg. The whole system consists of three major parts: high frame rate DMD projector, an active shutter, and an optical Wedgereg. This research investigates and develops the approaches to interface and control DMDs at very high bit rates and ultra-high frame rates to drive a novel flicker-free 3D flat panel device, with a wide viewing area and without need of special viewing glasses or head-tracking system. It needs to design a digital interface circuit capable of receiving video image data in a conventional format and translating it into a form appropriate for display on a DMD light modulator. This approach differs from existing ones in that it facilitates the modulation of the DMD to achieve much high frame rates, though with a trade-off in greyscale resolution. The outcomes of this research will have the great potential for commercialising the autostereoscopic display systems

High-speed optical reconstruction of 3D-scenes can be achieved using digital holography with binary digital micromirror devices (DMD) or a ferroelectric spatial light modulator (fSLM). There are many algorithms for binarizing digital holograms. The most common are methods based on global and local thresholding and error diffusion techniques. In addition, hologram binarization is used in optical encryption, data compression, beam shaping, 3D-displays, nanofabrication, materials characterization, etc. This paper proposes an adaptive binarization method based on a combination of local threshold processing, hologram division into blocks, and error diffusion procedure (the LDE method). The method is applied for binarization of optically recorded and computer-generated digital holograms of flat objects and three-dimensional scenes. The quality of reconstructed images was compared with different methods of error diffusion and thresholding. Image reconstruction quality was up to 22% higher by various metrics than that one for standard binarization methods. The optical hologram reconstruction using DMD confirms the results of the numerical simulations.

A digital micro-mirror device is one of the most frequently used spatial light modulators for holographic three-dimensional displays due to its fast refresh rate. The modulation by the digital micro-mirror device is, however, limited to the binary amplitude modulation, and it degrades the reconstruction image quality. In this paper, we propose a novel binary hologram encoding technique which applies the error diffusion algorithm considering the carrier wave of the hologram. The error diffusion weights designed for the hologram carrier wave suppress the binarization noise around the carrier wave where the most signal energy is concentrated, which enhances the reconstruction quality. The combination with the time-multiplexing enables speckless enhanced-quality three-dimensional reconstruction with shallow depth of focus. The proposed technique is verified by simulations and optical experiments.

As a portable device, such as PDA and cell phone, a small size build in virtual input device is more convenient for complex input demand. A few years ago, a creative idea called 'virtual keyboard' is announced, but up to now there's still no mass production method for this idea. In this paper we'll show the whole procedure of making a virtual keyboard. First of all is the HOE (Holographic Optical Element) design of keyboard image which yields a fan angle about 30 degrees, and then use the electron forming method to copy this pattern in high precision. And finally we can product this element by inject molding. With an adaptive lens design we can get a well correct keyboard image in distortion and a wilder fan angle about 70 degrees. With a batter alignment of HOE pattern lithography, we're sure to get higher diffraction efficiency.

We present a holographic multi-path (HMP) approach to extend the eye-box of the off-axis reflective holographic optical element (HOE) combiners for micro-OLED augmented reality (AR) near-eye displays (NEDs). The compact form factor design requirement highly constrains the eye-box and the field of view (FOV) in the off-axis reflection HOE AR combiners. In this Letter, we show that multiplexing multiple independent optical paths into a HOE effectively extends the eye-box. In the proposed multi-path multiplexing approach, separate partial images of a micro-OLED are routed along the multiplexed beam paths and combined to generate a complete AR image. The experiment verifies that the multi-path multiplexing achieves a twofold increase in the eye-box compared to previous results, enhancing the user experience for AR applications.

Large field-of-view lensless holographic dynamic projection system with uniform illumination and U-net acceleration

Optical lithography with its 193nm technology is pushed to reach and shift its limits even further. There is strong demand on innovations in illumination part of exposure tools. Current illumination systems consisting of diffractive and refractive optical elements offer numerous benefits such as optimized laser beam shape with high homogeneity and high numerical aperture enabling high efficiency. LIMO's unique production technology is capable to manufacture free form surfaces on monolithic arrays larger than 250mm with high precision and reproducibility. Different kinds of intensity distributions with best uniformities or customized profiles have been achieved by using LIMO's refractive optical elements. Recently LIMO pushed the limits of this lens production technology and was able to manufacture first diffractive optical elements (DOE) based on continuous relief's profile. Beside for the illumination devices in lithography, DOEs find wide use in optical devices for other technological applications, such as optical communications and data processing. Up to now DOE designs follow the principle of phase diffraction gratings. Its diffraction structure with a periodic phase profile performs a superposition of beams with predefined energy ratios. Due to the application for high precise laser-beam shaping and beam splitting in optical technologies and optical fiber networks, number of grating orders is increased up to some tens or even hundreds. Classic lithographic technologies lead to quantized (step-like) profiles of diffractive micro-reliefs, which causes a decrease of beam splitter's diffractive efficiency. The newest development of LIMO's microlens fabrication technology allows us to make a step from free programmable microlens profiles to diffractive optical elements with high efficiency. Our first results of this approach are demonstrated in this paper. Diffractive beam splitters are presented. A special mathematical method is used to design diffractive optical elements with continuous surface profiles. Comparison between theoretical simulations and experimental results shows very good correlation.

Near-eye displays (NEDs) for augmented and virtual reality (AR/VR) are spotlighted because they have a possibility to provide much more immersive experiences never possible before. With the virtue of recent progress in sensors, optics, and computer science, several commercial products are already available, and the consumer market is expanding rapidly. However, there are several challenging issues for AR and VR NEDs to become closer to our lives. Here, we will explore these issues and important topics for AR and VR, and introduce some of the ideas to overcome them: diffractive optical elements (DOEs), retinal projection displays, and 3D display with focus cues. First, unlike VR with a simple optical system, AR that needs to merge an artificial image with an outer scene requires additional optics. The diffractive elements have the merit of being thin and transparent, suitable for the image combiner. Among them, holographic optical elements (HOEs) have great potential as they can record the desired volume grating from the simple lens to the complex wavefront using light interference. Second, in order to wear the NEDs for a long time, it must deal with the visual fatigue as well as the form factor. Retinal projection display can effectively prevent the vergence-accommodation conflict problem even with a simple optical design. In the retinal projection display, the light rays from the display are adjusted to converge into a small point using a lens. It ensures a wide depth range in which the images are clearly visible. Furthermore, it is possible to provide observers with accurate focus cues for the alleviation of visual fatigue via multi-layer displays and holographic displays. Recently, we conceived tomographic NED that can reproduce dense focal planes. We confirm that this system provides quasi-continuous focus cues, semi-original contrast, and considerable depth of field. The experimental results of our prototypes are explained. We also explain the recent activities of using deep learning in holographic NED system.

A holographic optical element (HOE) made by photopolymer can be used as an image combining element. Use of HOE enables higher transparency and compact configuration with light and thin form factor. However, unlike other conventional optical elements such as partial mirrors or concave lenses, full color implementation involves some issues to consider since HOE has high selectivity that only reacts to predefined wavelength or angle of incidence light. Also, a shrinkage of photopolymer during the curing process causes grating detuning effect that modulates the characteristic of HOE. In this study, we present a methodology for implementing a full color HOE using a wavelength multiplexing. Especially, pre-compensation recording method is presented that can calibrate wavelength mismatch and shrinkage effect. With the proposed methods, simulation results and experimental result are also provided.

Optical microsensors for fluid flow diagnostics

Recent advances in compression molding of glass optical elements for mass production offer the potential of extending this technology to elements with micro and nano scale features. In this research, glass diffractive optical elements (DOEs) with lateral features in the order of 10 µm and vertical height of 330 nm were fabricated using a fused silica glass mold and a special low Tg (glass transition temperature) glass material K-PG325. Molded DOEs were studied using an atomic force microscope (AFM) and scanning electron microscope (SEM) to evaluate the glass molding process capability. Optical testing of the molded DOEs was a further demonstration of the effectiveness of the molding process for high volume micro and diffractive optical component fabrication. The combination of two high-precision, high-volume processes, i.e., semiconductor batch process for optical mold making and glass molding for DOE replication, is an effective alternative manufacturing method for high-quality, low-cost optical components. The reported experiment is a detailed illustration of the glass molding process capability. With further process optimization a robust manufacturing process can be developed for mass production of diffractive and micro glass optical elements.

In recent years surface-relief Diffractive Optical Elements (DOEs), a specialized class of diffractive optical elements, have revolutionized design of optical systems and instruments. Surface-relief diffractive optical components improve performance of the existing optical systems and increase systems compactness and reliability. Surface-relief DOEs can be fabricated as multilevel binary optical elements, or as continuous surface-relief optical elements. A surface-relief DOE is being designed such that incident light interacts with the topography of the surface-relief microstructure including the size and shape characteristics of surface profile features such as indentations and projections as well as their dimensional relationship, to perform a desired optical function. Binary optical elements are being made by the same microlithographic and ion etching techniques used to produce VLSI circuits. Computer-generated continuous surface-relief DOE is recorded by direct laser-beam or e-beam writing in photoresist. Continuous surface-relief DOE can be also recorded in photoresist by interferometric (holographic) technique. After development recorded in photoresist surface-relief microstructures are being ion-beam etched into quartz or other inorganic optical substrates.

Fabrication of optical transmission elements in an SiN x membrane

Laguerre-Gaussian (LG) beams are used in many research fields, including microscopy, laser cavity modes, and optical tweezing. We developed a holographic method to generate pure LG modes (amplitude and phase) with a binary amplitude-only digital micromirror device (DMD) as an alternative to the commonly used phase-only spatial light modulator. The advantages of such a DMD include very high frame rates, low cost, and high damage thresholds. We have shown that the propagating shaped beams are self-similar and their phase fronts are of helical shape as demanded. We estimate the purity of the resultant beams to be above 94%.