Conventional Hologram Research Articles

Fractional Fourier-transform off-axis digital holographic imaging

A fractional Fourier-transform digital holographic imaging method with resolution enhancement features is presented. In an optical configuration, an extended fractional Fourier-transform optical setup is set in the object arm of an off-axis digital holographic recording system to record a fractional Fourier-transform hologram via the optical interference of the fractional Fourier-transform wavefront of an object wave with a reference wave. For reconstruction imaging, the reconstruction approach for fractional Fourier-transform holograms is given. In the experiment, the fractional Fourier-transform digital holograms are recorded under the different recording parameters, and their amplitude images are effectively reconstructed. The imaging results demonstrate that the reconstruction-imaging resolution of fractional-order Fourier-transform holograms is obviously enhanced compared to that of conventional image-plane holograms. The presented fractional Fourier-transform digital holographic imaging with resolution enhancement and optical configuration flexibility provides, to our knowledge, a novel way for off-axis digital holographic imaging.

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.

Enhancing efficiency of complex field encoding for amplitude-only spatial light modulator based on a neural network.

The widespread adoption of artificial neural networks for hologram synthesis can be attributed to their ability to improve image quality and reduce computational costs. In this study, we propose an alternative use of artificial neural networks to improve the optical efficiency of complex field encoding. The neural encoding significantly enhances the efficiency of amplitude-only SLMs, resulting in 2.4-fold optical efficiency enhancement with negligible image quality degradation compared to the Burch encoding method. Notably, the experimental results demonstrate that the neural encoding method has even higher image quality, providing an approximately 2.5 dB enhancement in a peak signal-to-noise ratio. The neural encoding method offers promise in mitigating a fundamental challenge of conventional amplitude-only holograms, namely its low efficiency.

Generic Vectorial Hologram Based on Simultaneous and Independent Amplitude and Phase Modulation of Orthogonal Polarization States via Single Completely Spin‐Decoupled Meta‐Particles

AbstractMetasurfaces composed of subwavelength structure arrays provide a diverse and powerful platform for generation of complicated electromagnetic (EM) fields and wavefronts manipulation and have triggered an unprecedented research upsurge in electromagnetism with the development of computer‐generated holography (CGH). Different from conventional scalar holograms only containing amplitude and phase properties, a vectorial hologram that benefits from the maneuverability of polarization of EM fields exhibits more immeasurable application potential and is simultaneously regarded as an urgent requirement. In this work, assisted by single completely spin‐decoupled meta‐particles, which perfectly combine the functions of simultaneous and independent amplitude and phase modulation of orthogonal polarization states containing left‐ and right‐handed circularly polarized (LCP&RCP) components, a vectorial hologram induced through eigenstates degeneracy with the ability of encoding arbitrary polarization is proposed in the microwave region. As the proof of concept, eight distinct holograms with specified polarizations and six‐channel polarization‐multiplexed hologram display are fabricated and characterized, further showcaing the versatility of the approach for the customization of arbitraty polarization and integration of reconstructed patterns. With the advantages of multi‐controllability and ultra‐compactness, the constructed monolayer completely spin‐decoupled paradigm for vectorial regulation may find prospective application aspects in spin detection, target perception, multi‐channel data encryption, and many other related fields.

Discrete Multi-Step Phase Hologram for High Frequency Acoustic Modulation

Abstract Acoustic holograms can recover wavefront stored acoustic field information and produce high-fidelity complex acoustic fields. Benefiting from the advantages of the huge spatial information content unmatched by traditional acoustic components, acoustic holograms pursue the realization of high-resolution complex acoustic fields and gradually tend to high-frequency ultrasound applications. However, conventional continuous phase holograms are limited by 3D printing size and the presence of unavoidable small printing errors make it difficult to achieve acoustic field reconstruction at high frequency accuracy. Here, we present an optimized discrete multi-step phase hologram. It can guarantee the reconstruction quality of image with high robustness, and properly lower the requirement of the 3D printing accuracy. Meanwhile, the concept of reconstruction similarity is proposed to refine a measure of acoustic field quality. In addition, the realized complex acoustic field at 20 MHz promotes the application of acoustic holograms in high frequencies and provides a new way to generate high-fidelity acoustic fields.

Optically transparent anisotropic holographic impedance metasurface for launching orbital angular momentum vortex wave

The visualization of electronic products is essential for the digitization and visualization of human-computer exchange in the future smart world. However, due to its large non-translucent aperture, the conventional hologram metasurface urgently needs transparency for wide scene usage. In this paper, an optically transparent holographic impedance metasurface with a low profile is investigated for generating orbital angular momentum (OAM) vortex waves in the radio-frequency domain for the first time. Indium tin oxide is applied to perform the metal functionality, which is featured by high optical transparency, and polymethyl methacrylate is proposed as the substrate material in the meta-atom design. By virtue of classic optical metasurface theory and leaky-wave principle, holographic impedance mapping is achieved, so the surface wave generated by the monopole port is effectively converted into a radiating vortex wave at 15 GHz. The prototype has been manufactured and fabricated, and the experiment results exhibit good agreements with theory and simulation analyses, showing a good prospect for OAM vortex wave launch in visualized applications.

Deep Learning-Based Framework for Fast and Accurate Acoustic Hologram Generation.

Acoustic holography has been gaining attention for various applications, such as noncontact particle manipulation, noninvasive neuromodulation, and medical imaging. However, only a few studies on how to generate acoustic holograms have been conducted, and even conventional acoustic hologram algorithms show limited performance in the fast and accurate generation of acoustic holograms, thus hindering the development of novel applications. We here propose a deep learning-based framework to achieve fast and accurate acoustic hologram generation. The framework has an autoencoder-like architecture; thus, the unsupervised training is realized without any ground truth. For the framework, we demonstrate a newly developed hologram generator network, the holographic ultrasound generation network (HU-Net), which is suitable for unsupervised learning of hologram generation, and a novel loss function that is devised for energy-efficient holograms. Furthermore, for considering various hologram devices (i.e., ultrasound transducers), we propose a physical constraint (PC) layer. Simulation and experimental studies were carried out for two different hologram devices, such as a 3-D printed lens, attached to a single element transducer, and a 2-D ultrasound array. The proposed framework was compared with the iterative angular spectrum approach (IASA) and the state-of-the-art (SOTA) iterative optimization method, Diff-PAT. In the simulation study, our framework showed a few hundred times faster generation speed, along with comparable or even better reconstruction quality, than those of IASA and Diff-PAT. In the experimental study, the framework was validated with 3-D printed lenses fabricated based on different methods, and the physical effect of the lenses on the reconstruction quality was discussed. The outcomes of the proposed framework in various cases (i.e., hologram generator networks, loss functions, and hologram devices) suggest that our framework may become a very useful alternative tool for other existing acoustic hologram applications, and it can expand novel medical applications.

Phase-only hologram generated by a convolutional neural network trained using low-frequency mixed noise.

A phase-only hologram generated through the convolution neutral network (CNN) which is trained by the low-frequency mixed noise (LFMN) is proposed. Compared with CNN based computer-generated holograms, the proposed training dataset named LFMN includes different kinds of noise images after low-frequency processing. This dataset was used to replace the real images used in the conventional hologram to train CNN in a simple and flexible approach. The results revealed that the proposed method could generate a hologram of 2160 × 3840 pixels at a speed of 0.094 s/frame on the DIV2K valid dataset, and the average peak signal-to-noise ratio of the reconstruction was approximately 29.2 dB. The results of optical experiments validated the theoretical prediction. The reconstructed images obtained using the proposed method exhibited higher quality than those obtained using the conventional methods. Furthermore, the proposed method considerably mitigated artifacts of the reconstructed images.

Parametric hologram optimization for enhanced underwater wireless optical communication.

The performance of the underwater optical communication (UWOC) systems was primarily limited by the low optical transmission efficiency due to the beam divergence and water interference. It has been proved in our previous works that holographic beam shaping can effectively increase the optical transmission efficiency and therefore the communication distances and speed. The conventional hologram optimisation method treated each pixel as an independent variable, leading to a large search space and a slow process. In this work, we proposed to use a small set of parameters to describe the beam shaping holograms that were able to limit the beam divergence and compensate for the wavefront distortion. This significantly reduced the number of variables to be optimised and enabled the optimisation to be more efficient and effective. In a proof-of-concept experiment based on the off-the-shelf components, the proposed method was able to generate the optimal hologram within 20 iterations while achieving a tenfold increase in the optical transmission efficiency for a 30 m link at 100 Mbps.

Reconstructing subwavelength resolution terahertz holographic images.

Computer-generated holography typically generates terahertz (THz) holographic images with a pixel size larger than wavelength. We propose a multi-foci metalens model to reconstruct THz holographic images with subwavelength resolution. The designed devices are realized based on dielectric metasurfaces consisting of silicon micropillars with spatially variant orientations. By exploiting quasi-continuous profile of focal points as the pixels of a holographic image, a metalens can reconstruct a high-resolution target image on its focal plane. The effects of size and pitch of each sub-diffraction focal point on imaging quality and pixel resolution are discussed. The intensity distribution at each focal point indicates that the reconstructed images have subwavelength resolution. In comparison with conventional hologram designs, this design method can be used to reconstruct THz holographic images with subwavelength resolution, which have potential applications in THz communication, information security and anti-counterfeiting.

Independent Multichannel Wavefront Modulation for Angle Multiplexed Meta‐Holograms

AbstractMeta‐holograms have been investigated widely because of their capabilities that are not feasible in conventional holograms, such as full‐color, broadband, and bifacial image generation performance. In particular, a high degree of freedom in designing meta‐atoms provides opportunities to multiplex holograms by using light properties, such as angle, polarization, and wavelength. However, further development of techniques for meta‐holograms recording multiple wavefronts is still necessary to expand the amount of optical information and improve the security level of holographic encryption. In this study, detour phase holograms are combined with the spatial multiplexing method to record four phase profiles in a single metasurface device. Independent wavefront modulation, which can process four channels as a bijective function, is realized by engineering the scattering properties of the meta‐atom. Based on the concept, meta‐holograms are demonstrated experimentally, independently generating four different images depending on the incident angles with high fidelity. Because of the expanded capacity and independence, the proposed device provides a potential platform for optical devices in various fields, such as holography, multifunctional flat optics, and off‐axis optical devices.

Acoustic Hole-Hologram for Ultrasonic Focusing With High Sensitivity

Acoustic hologram enable the capability in the acoustic pattern control and drive many applications of ultrasound. However, hologram has not been applied to ultrasound imaging because the medical application of acoustic holograms is limited owing to the sound reflection and scattering at the acoustic holographic surface and its internal attenuation. In this study, we propose an acoustic holographic computing method and present a disc-shaped hologram with a central hole comparing with a conventional hologram using the iterative angular spectrum approach. The results show that the transducer with hole hologram achieves the required focusing effect with the full width at half maximum about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\lambda $ </tex-math></inline-formula> , high sensitivity increased by 92.7%, and a low two-way insertion loss with −15.8 dB compared to conventional hologram. Pulse-echo and ultrasonic imaging are achieved by a transducer with two holograms which shows new applications of acoustic hologram in ultrasound.

Realization of amplitude-type hologram based on nanostructured metasurfaces

Conventional Amplitude-type holograms are generated by recording the interference fringes on a photosensitive material. In this paper, taking advantage of the characteristics of metasurfaces that can continuously control the amplitude of electromagnetic waves, a silver-nanopolarizer-based metasurface is proposed to realize amplitude-type holograph. An off-axis meta-hologram is designed and fabricated with e-beam lithography. And experimental results show that it can project a holographic image in the far field with high-fidelity, indicating that this amplitude hologram has the ability of continuous amplitude modulation and is safe and stable, so it has application spaces in holography, optical holography, holographic encryption, anti-counterfeiting, etc.

Deep neural network for multi-depth hologram generation and its training strategy.

We present a deep neural network for generating a multi-depth hologram and its training strategy. The proposed network takes multiple images of different depths as inputs and calculates the complex hologram as an output, which reconstructs each input image at the corresponding depth. We design a structure of the proposed network and develop the dataset compositing method to train the network effectively. The dataset consists of multiple input intensity profiles and their propagated holograms. Rather than simply training random speckle images and their propagated holograms, we generate the training dataset by adjusting the density of the random dots or combining basic shapes to the dataset such as a circle. The proposed dataset composition method improves the quality of reconstructed images by the holograms generated by the network, called deep learning holograms (DLHs). To verify the proposed method, we numerically and optically reconstruct the DLHs. The results confirmed that the DLHs can reconstruct clear images at multiple depths similar to conventional multi-depth computer-generated holograms. To evaluate the performance of the DLH quantitatively, we compute the peak signal-to-noise ratio of the reconstructed images and analyze the reconstructed intensity patterns with various methods.

Off-axis digital holographic multiplexing for rapid wavefront acquisition and processing

Off-axis holographic multiplexing involves capturing several complex wavefronts, each encoded into off-axis holograms with different interference fringe orientations, simultaneously, with a single camera acquisition. Thus, the multiplexed off-axis hologram can capture several wavefronts at once, where each one encodes different information from the sample, using the same number of pixels typically required for acquiring a single conventional off-axis hologram encoding only one sample wavefront. This gives rise to many possible applications, with focus on acquisition of dynamic samples, with hundreds of scientific papers already published in the last decade. These include field-of-view multiplexing, depth-of-field multiplexing, angular perspective multiplexing for tomographic phase microscopy for 3-D refractive index imaging, multiple wavelength multiplexing for multiwavelength phase unwrapping or for spectroscopy, performing super-resolution holographic imaging with synthetic aperture with simultaneous acquisition, holographic imaging of ultrafast events by encoding different temporal events into the parallel channels using laser pulses, measuring the Jones matrix and the birefringence of the sample from a single multiplexed hologram, and measuring several fluorescent microscopy channels and quantitative phase profiles together, among others. Each of the multiplexing techniques opens new perspectives for applying holography to efficiently measure challenging biological and metrological samples. Furthermore, even if the multiplexing is done digitally, off-axis holographic multiplexing is useful for rapid processing of the wavefront, for holographic compression, and for visualization purposes. Although each of these applications typically requires a different optical system or processing, they all share the same theoretical background. We therefore review the theory, various optical systems, applications, and perspectives of the field of off-axis holographic multiplexing, with the goal of stimulating its further development.

All-optical diffractive neural networked terahertz hologram.

Holography has garnered an explosion of interest in tremendous applications, owing to its capability of storing amplitude and phase of light and reconstructing the full-wave information of targets. Spatial light modulators, metalenses, metasurfaces, and other devices have been explored to achieve holographic images. However, the required phase distributions for conventional holograms are generally calculated using the Gerchberg-Saxton algorithm, and the iteration is time-consuming without Fourier transform or other acceleration techniques. Few studies on designing holograms using artificial intelligence methods have been conducted. In this Letter, a three-dimensional (3D)-printed hologram for terahertz (THz) imaging based on a diffractive neural network (DNN) is proposed. Target imaging letters "THZ" with uniform field amplitude are assigned to a predefined imaging surface. Quantified phase profiles are primarily obtained by training the DNN with the target image and input field pattern. The entire training process takes only 60 s. Consequently, the hologram, that is, a two-dimensional array of dielectric posts with variational heights that store phase information, is fabricated using a 3D printer. The full-wave simulation and experimental results demonstrate the capability of the proposed hologram to achieve high-quality imaging in the THz regime. The proposed lens and design strategy may open new possibilities in display, optical-data storage, and optical encryption.

Lensfree super-resolved imaging based on adaptive Wiener filter and guided phase retrieval algorithm

We present a data-efficient framework to achieve a super-resolved reconstruction in lensfree on-chip microscopy. Instead of using conventional 3D-stack holograms, our method accomplishes a 5 × reduction of data size without the loss of the imaging resolution, in which merely 30 raw holograms are recorded. The experiment of the resolution target demonstrates a half-pitch lateral resolution of 780 nm across a large field-of-view (FOV) of 28.6 mm2. For the view of H&E stained intestine pathology slide, our method shows an equivalent resolution but a 20-fold expansion of FOV in comparison with a 10 × commercial wide-field microscope. This work will be a useful tool for the miniaturization of whole slide pathological microscopy.

Design of a reconfigurable broadband greyscale multiplexed metasurface hologram.

Holograms are a promising state-of-the-art technique that are able to reproduce fully three-dimensional images. However, most elementary holograms only recover a nonadjustable image restricted by a certain amplitude and phase. Recently, the concept of the reconfigurable metasurface has come into sight. The reconfigurable metasurfaces can be tuned dynamically through a physics control signal from outside. Here we design and realize a series of novel, to the best of our knowledge, reconfigurable metasurfaces. Using these devices, an advanced metasurface hologram effect could be easier to achieve. Moreover, the reconfigurable property of such tunable metasurface enables independent control of advanced multiplexed channel functionalities to exist in comparison with the conventional static metasurface holograms. Our method creates unprecedented display and virtual effects, and it opens a novel way to a more flexible methodology of dynamic control of electromagnetic waves.

Formation of Dynamic and Binocular Three-Dimensional Images in Protective Holograms with Zero Diffraction Order

Variants of implementing trackograms, one of the types of zero-order holograms, for the formation of protective features for non-expert verification of their authenticity are considered. The three-dimensional image in trackograms is formed by glares on the walls of toroidal surfaces-tracks that represent relief stripes with the recording material. Variations in the shape of tracks, their length, and the number of sections of different tracks at one point in the image can provide security features that are convenient for non-expert identification of holograms. Track sizes and depths are compatible with the technology of conventional embossed holograms. Photos of holograms forming images in white light are shown.

Computer-generated photorealistic hologram using ray-wavefront conversion based on the additive compressive light field approach

The conventional computer-generated hologram reconstructing photorealistic three-dimensional (3D) images based on ray-wavefront conversion has the disadvantage of spatio-angular resolution trade-off. In this Letter, we propose for the first time, to the best of our knowledge, a computer-generated photorealistic hologram without spatio-angular resolution trade-off based on the additive compressive light field (CLF) approach. The original light field is compressed into multiple layer images through numerical optimization based on the additive light field principle. Then, by independently calculating the wave propagation from each layer image to the hologram plane and adding them together, a CLF hologram is generated. Since the CLF information is presented through a holographic method, the advantage of high resolution in CLF is preserved while the limitation of the number of physically stacked layers (such as liquid crystal displays) is removed, leading to higher quality, larger depth of field, and higher brightness compared with a conventional CLF display. The proposed method is verified with a photorealistic optical experiment.