Computer-generated Holograms Research Articles

Accuracy verification methodology for computer-generated hologram used for testing a 3.5-meter mirror based on an equivalent element

Accuracy verification methodology for computer-generated hologram used for testing a 3.5-meter mirror based on an equivalent element

Holographic fabrication of reconfigurable diffractive elements

The fabrication of Diffractive Optical Elements (DOEs) involves the analog patterning of material surfaces on the scale of light wavelength. This typically requires multi-step lithographic processes. Differently from the photoresists of standard lithography, thin films of amorphous azobenzene-containing polymers (azopolymers) can directly produce a structured surface using a single irradiation step with structured light. The resulting surface reliefs can be used directly as planar phase-modulating DOEs without the need for any post-exposure process. Additionally, the surface geometry and its optical functionality can be reconfigured at will. Here, we demonstrate reprogrammable and ready-to-use azopolymer diffractive gratings, lenses, and holographic projectors, produced by grayscale digital holographic patterns. By exploiting the all-optical scheme based of computer-generated holography, the diffraction behavior of the DOEs is optimized during the developing of structured surfaces. Full all-optical reconfigurability of the fabricated devices is also achieved. Our approach provides a versatile, efficient, and all-optical reversible fabrication framework for DOEs, making it a promising option to overcome the demanding, cumbersome, and irreversible fabrication processes typically involved in the realization of planar diffractive optical devices.

Extended field-of-view scanning in LiDAR systems using computer-generated holography-based spatial light modulator

A method is proposed for realizing extended field-of-view (FOV) scanning in light detection and ranging (LiDAR) systems utilizing computer-generated holography, a spatial light modulator (SLM), and an eye piece. In the proposed approach, an SLM driven by appropriately-designed computer-generated holograms (CGHs) is used to collimate the incoming rays and produce a large field of view (FOV). Five FOVs are considered for illustration purposes: 0°, -5°, -10°, -15°, and -20° For each FOV, the necessary CGH is determined by optimizing the “Binary 1″ surface in Zemax simulations and then processing the resulting phase by a modulo-2π operation. The feasibility of the proposed method is demonstrated using an experimental telescope system. The results show that the FOVs generated by the system deviate by no more than 0.566° from the target values. Moreover, the focus spot maintains a size of approximately 3 mm at two different observation planes located at 73 cm and 198 cm from the eyepiece, respectively. In other words, the system not only provides an extended FOV scanning range, but also delivers an excellent collimation performance. Overall, the results thus confirm the potential of the proposed method for large FOV scanning in practical LiDAR systems.

Fabrication of a large computer-generated hologram with high diffraction efficiency and high accuracy by scanning homogenization etching.

The diffraction efficiency, defined as the ratio of diffracted power to incident power, is one of the key working indicators for a computer-generated hologram (CGH). The CGH with high diffraction efficiency could suppress stray light and eliminate ghost images, thus improving interferometric performance in aspherical testing of low-reflectivity or large off-axis distance surfaces. However, the high-efficiency CGH is hard to precisely fabricate by traditional reactive ion etching and focusing ion beam, because it requires high etching depth with a high uniformity and sub-nanometric roughness in the glass, especially in the fabrication of a large CGH with an aperture of up to 300 mm. In this study, fabrication of the above-mentioned CGH was demonstrated via what we believe to be a new method called scanning homogenization etching (SHE), in which the ion source with a Gaussian energy distribution accurately scans the glass surface to realize homogenization etching. Different from controlling dwell time at each etching point, this paper proposes to control the scanning rate to achieve not only uniform but also quantitative depth removal in a single scan. Moreover, the depth errors in deep etching across the whole glass surface can be remarkably reduced due to homogenization effects introduced by multiple scanning etching. Finally, the target etching depth of 692.3 nm with an etching uniformity of 2.2% in the etching of a 300 mm CGH was achieved. The roughness of the etched and unetched area both have Ra values of 0.3 nm. The diffraction efficiency of working order is 39.998%, achieving 98.6% of the theoretical diffraction efficiency. In addition, the SHE is not limited by the aperture of the ion source, so it can achieve even larger diffractive optical elements with high diffraction efficiency and high accuracy.

Enhancing computational holography with spiral phase coding

In this Letter, we propose a new, to the best of our knowledge, approach to generate computer-generated holograms (CGHs) utilizing spiral phase coding. This method can be applied to generate an array spiral phase plate that can generate array vortex spots with a high compression ratio. Moreover, the method extends its applicability to the generation of Fresnel holograms and kinoforms, resulting in edge-enhanced imaging. Theoretical analysis and experimental results demonstrate the potential of spiral phase-encoded CGHs in laser processing and image enhancement.

End-to-end compression-aware computer-generated holography.

Joint photographic experts group (JPEG) compression standard is widely adopted for digital images. However, as JPEG encoding is not designed for holograms, applying it typically leads to severe distortions in holographic projections. In this work, we overcome this problem by taking into account the influence of JPEG compression on hologram generation in an end-to-end fashion. To this end, we introduce a novel approach to merge the process of hologram generation and JPEG compression with one differentiable model, enabling joint optimization via efficient first-order solvers. Our JPEG-aware end-to-end optimized holograms show significant improvements compared to conventional holograms compressed using JPEG standard both in simulation and on experimental display prototype. Specifically, the proposed algorithm shows improvements of 4 dB in peak signal-to-noise ratio (PSNR) and 0.27 in structural similarity (SSIM) metrics, under the same compression rate. When maintained with the same reconstruction quality, our method reduces the size of compressed holograms by about 35% compared to conventional JPEG-compressed holograms. Consistent with simulations, the experimental results further demonstrate that our method is robust to JPEG compression loss. Moreover, our method generates holograms compatible with the JPEG standard, making it friendly to a wide range of commercial software and edge devices.

Calculation of Computer-Generated Hologram Over 100K Resolution Range for Realistic 3D Meta-Space Reproduction

Calculation of Computer-Generated Hologram Over 100K Resolution Range for Realistic 3D Meta-Space Reproduction

The Computer-generated Hologram Display Based on Holographic Combiner with Expanded FOV by Scanning Mirror

The Computer-generated Hologram Display Based on Holographic Combiner with Expanded FOV by Scanning Mirror

Performance Evaluation of Transmission System for Computer-Generated Holograms in Wired/Wireless Connections by Using Lossless Compression

Performance Evaluation of Transmission System for Computer-Generated Holograms in Wired/Wireless Connections by Using Lossless Compression

Depolarized Holography with Polarization-Multiplexing Metasurface

The evolution of computer-generated holography (CGH) algorithms has prompted significant improvements in the performances of holographic displays. Nonetheless, they start to encounter a limited degree of freedom in CGH optimization and physical constraints stemming from the coherent nature of holograms. To surpass the physical limitations, we consider polarization as a new degree of freedom by utilizing a novel optical platform called metasurface. Polarization-multiplexing metasurfaces enable incoherent-like behavior in holographic displays due to the mutual incoherence of orthogonal polarization states. We leverage this unique characteristic of a metasurface by integrating it into a holographic display and exploiting polarization diversity to bring an additional degree of freedom for CGH algorithms. To minimize the speckle noise while maximizing the image quality, we devise a fully differentiable optimization pipeline by taking into account the metasurface proxy model, thereby jointly optimizing spatial light modulator phase patterns and geometric parameters of metasurface nanostructures. We evaluate the metasurface-enabled depolarized holography through simulations and experiments, demonstrating its ability to reduce speckle noise and enhance image quality.

Complex RGB spatial light modulation with a dual-layer in-plane switching liquid crystal panel

Complex RGB spatial light modulators are required to produce full-color holographic displays. In particular, complex spatial light modulation, which modulates the amplitude and phase of incident light is essential for noiseless dynamic computer-generated hologram synthesis. The feasibility of full-color holographic image generation through the dual-layer in-plane switching liquid crystal plane is theoretically validated and experimentally demonstrated.

Physical unclonable functions on carbon fiber reinforced polymers using laser techniques

We have successfully fabricated physical unclonable functions (PUF)s on carbon fiber reinforced polymers (CFRP) in the form of computer-generated holograms (CGH)s using an industrial friendly UV ns laser system. The topological randomness of the fiber network and laser absorption yield laser engraved CGH that are unique. The engraved CGH serve as PUFs and this is confirmed by the exceptionally low values of probability of cloning (POC), lower than 10−7, well below the commonly accepted threshold value of 10−5 in the literature. For the PUF evaluation we have developed a novel methodology that employs digital reconstruction of images of the engraved CGH obtained from low resolution cameras with moderate magnification.

Structured light reconstruction by Computer-Generated hologram in defect state

The generation of structured light from Computer-Generated Hologram (CGH) using spatial light modulators (SLM) has a wide range of applications, but there is currently a lack of research on localized defects in SLM devices. Considering that the local damage of SLM can lead to hologram defect, its result can be equivalent to the process of normal hologram generated structured beam with its near field being blocked. In this paper, CGH loaded onto digital micromirror device (DMD) is used to generate structured beams, and two cases of local defect states at the center and edge of the DMD are discussed. The results show that the reconstructed light field obtained by central defect is quite similar to the original light field, while the reconstructed light field obtained by edge defect is a much similar to the lower case of the original light field. The article further demonstrates the reconstructed results in coherent superpositions of Hermite-Gaussian modes using DMD-generated structured beams based on the single mode regime. This study expands the application scenarios for the generation of structured light beams with SLM.

Broadband multifunctional metasurfaces enabling polarization multiplexed focused vortex array generation

Metasurfaces are ultrathin artificially engineered structures exquisitely designed to tailor the wavefront, polarization, and amplitude of incident light, offering a versatile platform for implementing compact and integrated photonics devices. Metasurface-based structured light generation has gained considerable attention in the scientific community for its various exotic applications, including optical communication, particle manipulation, and high-resolution lithography. State-of-the-art design techniques include polarization multiplexing, multiple vortices beam generation, and vortex arrays. However, there needs to be more techniques for generating polarization switchable vortex arrays, despite the apparent advantages in significant application fields, necessitating a simple design technique. Here, we present a straightforward design technique utilizing computer-generated holography (CGH) and photonic spin Hall effect (PHSE) to realize a metadevice platform capable of generating broadband multiple optical vortex arrays with switchable polarization output. Our approach involves designing phase masks for the vortex arrays using CGH, which are then encoded onto the metasurface. By leveraging PSHE, the designed metasurfaces produce the desired optical response depending on the input polarization state. Using the proposed design technique as proof of concept, different all-dielectric metasurfaces capable of generating optical vortex arrays of different dimensions are demonstrated. The results verify the effectiveness of the proposed methodology. The design has potential applications in data communications, storage, quantum computation, nanoscale physical manipulation, and microscopy.

Mixed compensation for the testing of large convex aspheres

Since the testing of convex mirrors often requires a compensator with a larger aperture than the mirror under test, this is undoubtedly a trade-off not worth pursuing. Moreover, for surfaces with higher levels of asphericity, it further escalates the challenges in testing. In this study, high-accuracy testing of large convex aspheric surfaces with rotary symmetry was achieved via a method that combines sub-aperture stitching and Computer-Generated Holograms (CGH) compensation testing. Applying the methodology outlined in this study, practical testing was conducted on a large-aperture convex aspheric mirror with a diameter of 538 mm. The outcomes demonstrated that the center testing data and the outer ring testing data were stitched by the stitching algorithm to obtain the surface error of the full-aperture. The results were compared to those of Luphoscan and compensation testing, with a residual Root Mean Square (RMS) less than λ/20 (λ = 632.8 nm). Finally, the error analysis of the entire testing process showed that the accuracy was better than λ/50. The results demonstrate that the method can achieve highly accurate test of large convex aspheric surfaces.

Real-time 4K computer-generated hologram based on encoding conventional neural network with learned layered phase

Learning-based computer-generated hologram (CGH) demonstrates great potential for real-time high-quality holographic displays. However, real-time 4K CGH generation for 3D scenes remains a challenge due to the computational burden. Here, a variant conventional neural network (CNN) is presented for CGH encoding with learned layered initial phases for layered CGH generation. Specifically, the CNN predicts the CGH based on the input complex amplitude on the CGH plane, and the learned initial phases act as a universal phase for any target images at the target depth layer. These phases are generated during the training process of the coding CNN to further optimize the quality. The CNN is trained to learn encoding 3D CGH by randomly selecting the depth layer in the training process, and contains only 938 parameters. The generation time for a 2D 4K CGH is 18 ms, and is increased by 12 ms for each layer in a layered 3D scene. The average Peak Signal to Noise Ratio (PSNR) of each layer is above 30dB in the depth range from 160 to 210 mm. Experiments verify that our method can achieve real-time layered 4K CGH generation.

Three-dimensional computer-generated holography based on the hybrid iterative angular spectrum algorithm.

This paper proposes a novel three-dimensional hologram calculation method based on the angular spectrum approach, with the aim of reducing the noise generated during the hologram reconstruction process. The proposed algorithm divides the spatial domain into multiple layers and employs the angular spectrum method to propagate the image between these layers, thus avoiding the paraxial approximation. To enhance the quality of the hologram, an error iteration algorithm is utilized to alleviate the occurrence of aliasing errors when directly superimposing holograms. Moreover, constraint factors are introduced between different layers within the same region to effectively utilize spatial resources for multi-image reconstruction, thereby mitigating the noise caused by inter-layer crosstalk. The feasibility of the proposed method is demonstrated through numerical simulations and optical experiments, highlighting its potential applicability to a wide range of three-dimensional reconstruction algorithms.

Performance analysis of versatile video coding for encoding phase-only hologram videos.

In recent decades, holographic technology has made significant progress with the development of novel hologram generation methods and three-dimensional rendering devices. Nevertheless, the substantial size of holographic data presents a significant challenge to its practical applications and thus necessitates the implementation of an efficient coding solution. In this study, we evaluate the efficiency of various coding tools within the state-of-the-art video coding standard, Versatile Video Coding, for encoding video of computer-generated phase-only hologram. Specifically, we examine the coding performance of transform/in-loop filter/screen-content coding tools. Through extensive encoding experiments and various statistical analyses, we investigated the limitations of existing standard codecs that do not account for the unique signal characteristics of phase-only holograms (POHs). The effects of coding artifacts on the visual quality of numerical reconstructions rendered from compressed POHs are also analyzed in detail. These comprehensive performance evaluations will provide valuable insights for developing efficient coding strategies for POH videos.

An Interactive Holographic Multimedia Technology and Its Application in the Preservation and Dissemination of Intangible Cultural Heritage

Digital technology offers numerous advantages, such as preserving the authenticity, replicating reality, and facilitating dissemination. It enables the preservation of intangible cultural heritage (ICH) in its original form and allows for the creation of comprehensive graphic, audio, and visual databases. Among these technologies, holographic technology holds promise for protecting ICH and promoting its dissemination. This paper focuses on interactive holographic technology and presents the design and implementation of a dynamic holographic display system that combines digital hologram (DH) and computer-generated hologram (CGH) to showcase 3D images consisting of both virtual and real objects. Real-time loading of DH into a spatial light modulator enables the optical reproduction of real objects, while the loading of two CGHs into other spatial light modulators facilitates the optical reproduction of virtual objects. Computational holography allows for the addition of virtual information, such as coordinate text, and the fusion of the three reconstructed images in space, resulting in an augmented reality experience and enhanced 3D display of real objects. An experimental setup employing three liquid crystal on silicon (LCOS) devices confirms the validity of the proposed method. Compared to other techniques, this approach demonstrates improved image signal-to-noise ratio, reduced alignment errors, and wider coverage of light traversal for laser 3D reconstruction images. The holographic technology presented in this paper enables the fusion display of real and virtual scenes and real-time two-way interaction between the audience and virtual images. This research holds significant practical value in promoting the effective dissemination and protection of ICH.

Improving the imbalance of the light intensity of 3D wire-frame projection with electro-holography by superimposing a phase error.

The CG-line method is an algorithm for generating computer-generated holograms (CGHs), a digitally recording medium for three-dimensional images in electro-holography. Since the CG-line method is specialized for projecting three-dimensional wireframe objects, it can calculate CGH with a very low computational load. However, the reconstructed image of the conventional CG-line method suffers from unintended light imbalance depending on the object shape, which disturbs the understandability of the projecting image. Therefore, we propose a method for reducing light imbalance by imposing phase error that controls light according to the line shape. Consequently, we reduced light imbalance by maintaining the high computational speed.