Hologram Generation Research Articles

Improving performance of single-pass real-time holographic projection

This work describes a novel approach to time-multiplexed holographic projection on binary phase devices. Unlike other time-multiplexed algorithms where each frame is the inverse transform of independently modified target images, Single-Transform Time-Multiplexed (STTM) hologram generation produces multiple sub-frames from a single inverse transform. Uniformly spacing complex rotations on the diffraction field then allows the emulation of devices containing 2N modulation levels on binary devices by using N sub-frames. In comparison to One-Step Phase Retrieval (OSPR), STTM produces lower mean squared error for up to N=5 than the equivalent number of OSPR sub-frames with a generation time of 1N of the equivalent OSPR frame. A mathematical justification of the STTM approach is presented and a hybrid approach is introduced allowing STTM to be used in conjunction with OSPR in order to combine performance benefits.

Hardware/Software Support for Correlation Detection in Holographic Wavefront Sensors

An algorithm that automatically calculates aberrations in the holographic wavefront sensor scheme previously proposed by the authors is described, which enables measuring aberrations by means of iterative generation of holograms on a phase spatial light modulator (SLM). The practical implementation of this algorithm on the basis of the feedback of the camera and SLM is given. Using the proposed algorithm, defocusing was measured with an accuracy of λ/160.

Fast hologram generation using intermediate angular-spectrum method for high-quality compact on-axis holographic display.

In the angular-spectrum method-based computer-generated hologram, the zero-padding method is used to convert circular convolution into linear convolution. However, it will increase the calculation time and memory usage significantly. Therefore, a fast and simple method is proposed to solve the issue of the numerical convolution in the process of hologram generation by using the intermediate angular-spectrum method in this paper. Through replacing numerical Fourier transform by optical Fourier transform in the hologram generation, the calculation speed is approximately 6 times faster than that of the zero-padding method. And due to the scaling factors introduced by the Fourier lens and without the cropping operation, the reconstruction quality of the proposed method is improved significantly compared with the zero-padding method. Moreover, the optical reconstruction system is more compact than the 4-f filter system in the on-axis holographic reconstruction. Both numerical simulations and optical experiments have validated the effectiveness of the proposed method.

Improving holographic search algorithms using sorted pixel selection.

Traditional search algorithms for computer hologram generation such as Direct Search and Simulated Annealing offer some of the best hologram qualities at convergence when compared to rival approaches. Their slow generation times and high processing power requirements mean, however, that they see little use in performance critical applications. This paper presents the novel sorted pixel selection (SPS) modification for holographic search algorithms that offers mean square error reductions in the range of 14.7-19.2% for the test images used. SPS operates by substituting a weighted search selection procedure for traditional random pixel selection processes. While small, the improvements seen are observed consistently across a wide range of test cases and require limited overhead for implementation.

Foveated computer-generated hologram and its progressive update using triangular mesh scene model for near-eye displays.

We propose a computer-generated hologram technique that reduces latency caused by hologram calculations in holographic near-to-eye displays. The proposed method applies a foveated rendering technique to a triangular mesh-based computer-generated hologram to reduce the computational load while maintaining the perceived image quality. Progressive update from low resolution to high resolution is achieved with minimal computational load by overlaying a new high-resolution occluding mesh patch on a low-resolution mesh model of the scene. The reduced latency for the first hologram generation with low reconstruction resolution and its smooth update to the high-resolution reconstructions using the proposed method was verified by numerical simulations and optical experiments.

Error diffusion method with optimized weighting coefficients for binary hologram generation.

The error diffusion method can effectively reduce quality degradation by propagating thresholding errors to neighboring pixels in the conversion of a gray-scale hologram to a binary hologram. In previous works, the four weighting coefficients in error diffusion are mostly set as the Floyd-Steinberg coefficient, which was determined empirically and originally proposed for photograph processing. In this work, we point out that the Floyd-Steinberg coefficients can be suboptimal for hologram error diffusion binarization. Furthermore, the weighting coefficients are optimized for each different hologram adaptively. Compared with conventional coefficients, our optimized coefficients can better preserve the fidelity of a reconstructed image after a hologram is binarized.

Three-dimensional hologram generation method based on space-division

The layer-based method for three-dimensional (3D) hologram generation suffers from reconstruction quality decreasing. On the basis of the layer-based method, a new method based on the idea of “space-division” is proposed. The method divides the object into small pieces, named spaces; for spaces with small number of points, the point-based method is used for hologram generation, to ensure the reconstruction quality; for spaces with large number of points, the fast Fourier transform method is used to ensure the computation efficiency. An index named “quality-time ratio” Q is defined, to synthetically evaluate the influence effect of the number of portions, the number of layers and the number of points to the effect of the algorithm. The simulation results show that, the increment of the number of portions improves Q the most effectively, which proves the validity of our method. The method is further tested by computer and optical reconstructions using different objects, and it is proved that the space partitioning method can obtain reconstructed images with the same quality in about 30% fewer time.

An Optimization Method for Hologram Generation on Multiple GPU-based Parallel Processing

An Optimization Method for Hologram Generation on Multiple GPU-based Parallel Processing

Non-iterative direct binary search algorithm for fast generation of binary holograms

The direct binary search (DBS) algorithm is an efficient method for the generation of binary holograms, but it is also an iterative method involving lengthy computation. Thus, fast non-iterative approaches are more preferred in practice even though they yield poorer results. In this paper, we propose a strategy to drastically reduce the computational time of the DBS algorithm. First, we show that convergence of the conventional DBS algorithm can be significantly improved by optimizing the order in which the pixels are examined. Then, we demonstrate the efficiency of a design based on optimization of multiple small blocks of binary pixels through parallel computation. Since each block can be optimized in parallel utilizing platforms such as those offering cloud computing services, the time to compute the final pattern is determined by the computational time for a single block. The proposed block-partition strategy involves a trade-off between the computation time and the quality of the final hologram. However, it should be noted that simply randomizing the pixel examination order during the DBS procedure reduces the computational time by 67% even without parallel computation. In summary, our proposed method facilitates easier generation of high-quality binary holograms in less time than is required by the conventional DBS.

Secure real-time generation and display of color holographic movies

In this paper we demonstrate a secure, real time color hologram generation and projection scheme. Holographic display techniques have the potential to enable the control of both the phase and amplitude of a light field. These methods have been traditionally hindered by high computational requirements. On the other hand, optical security techniques are ideal for the protection of holographic data, but have shown limited performance in actual experimental applications. Our proposal combines a double random phase encoding technique with the use of optimized random phases. In this way, we achieve fast encryption, decryption and generation of color phase only holograms for use in a projection system based on a liquid crystal on silicon spatial light modulator. We show actual experimental results confirming the effectiveness of our proposal.

Generation of Phase-Only Holograms Based on Aliasing Reuse and Application in Holographic See-Through Display System

In the point-based method, the aliasing phenomenon in the hologram plane is usually considered as a harmful phenomenon because it will degrade the reconstructed image quality. In this paper, the advantage of the aliasing phenomenon is employed to improve the reconstructed image quality of the phase-only hologram. Through analysis of the principle of this phenomenon, we proposed a method to improve the reconstructed image quality of the phase-only hologram. By controlling the strength of the aliasing, the amplitude distribution in the hologram plane can be turned into a nearly uniform distribution. It is helpful to generate an optimized phase-only hologram, which results in higher quality of the reconstructed image because the effect of lost information caused by abandon the amplitude part can be diminished. Moreover, a holographic see-through display system is designed to demonstrate the effectiveness of the proposed method.

Single SLM full-color holographic three-dimensional video display based on image and frequency-shift multiplexing

A single spatial-light-modulator (SLM) full-color holographic 3-D video display based on image and frequency-shift multiplexing (IFSM) is proposed. In the frequency-shift multiplexing (FSM), three-color holograms are multiplied with their respective phase factors for shifted-separations of their corresponding frequency-spectrums on the Fourier plane. This FSM process, however, causes three-color images to be reconstructed at the center-shifted locations depending on their multiplied phase factors. Center-shifts of those color images due to the FSM can be balanced out just by generation of three-color holograms whose centers are pre-shifted to the opposite directions to those of the image shifts with the novel-look-up-table (NLUT) based on its shift-invariance property, which is called image-shift multiplexing (ISM). These image and frequency-shifted holograms are then multiplexed into a single color-multiplexed hologram and loaded on the SLM, and from which a full-color 3-D image can be reconstructed on the optical 4-f lens system without any color dispersion just by employing a simple pinhole filter mask. Fourier-optical analysis and experiments with 3-D objects in motion confirm the feasibility of the proposed system.

Enhanced direct binary search algorithm for binary computer-generated Fresnel holograms

The direct binary search (DBS) algorithm was originally invented for the synthesis of a binary Fourier hologram, and was applied for the generation of a binary Fresnel hologram recently. DBS performs quality evaluation on every pixel. Therefore, both the quality and diffraction efficiency of the generated binary hologram are better among various algorithms of the binary hologram. However, DBS is a time-consuming algorithm and thus is impractical for the generation of high-definition computer-generated holograms. In this paper, we proposed an enhanced DBS (E-DBS) method to speed up the hologram computation. E-DBS is based on the same pixelwise evaluation strategy of DBS, but the diffraction field of a single pixel is precomputed as a lookup table. In evaluating any pixel value, only a small area in the region of interest affected by the diffraction field of single pixel is calculated. In addition, it is also found that qualified results can be obtained by using only 4% of the area of the diffraction field. As a result, the computing complexity of E-DBS can be reduced by at least 2 orders of magnitude in contrast to conventional DBS.

Improved single-random-phase holographic encryption using double-phase method

An improved single-random-phase holographic encryption (SRPHE) is proposed by using double-phase method and demonstrated by a spatial light modulator (SLM). Different from the conventional SRPHE using Fresnel diffraction and the bidirectional error diffusion method, the proposed SRPHE employees the angular spectrum diffraction and double-phase method to generate a phase-only hologram, and the phase-only hologram is encrypted into ciphertext by multiplying a single random phase mask. When the ciphertext is modulated by the conjugate of the random phase mask and loaded to a SLM, the decrypted image can be achieved successfully. The improved SRPHE has two advantages of better quality of the decrypted image, and lower time-cost of hologram generation, which is compared with the conventional SRPHE. Numerical simulations and optical reconstruction results have been carried out to verify the validity of the proposed SRPHE.

Single‐Layer Bifacial Metasurface: Full‐Space Visible Light Control

AbstractOptical metasurfaces have attracted considerable attention thanks to their unprecedented ability to control the electromagnetic properties of light in subwavelength scale. These metasurface‐integrated devices not only have the merit of compact form factor but also exhibit some better performance in comparison with conventional bulk optic devices. In this work, a novel metasurface that enables full‐space light control is realized, offering an unconventional functionality that provides a new foothold for metasurface integration in optics. Multipolar interference and Pancharatnam–Berry phase are combined with each other and suggested as a physical background for the proposed device. As an example of the functionality, asymmetric beam steering and hologram generation are proposed and measured experimentally. As far as it is known, this is the first work achieving full‐space control in visible range. Thanks to its manufactural ease and unprecedented function capability, this work will offer innovative potentials to be utilized in various integrated optic devices that need miniaturization or implementation of ingenious and light‐weight optical components used in optical systems for photography, microscopy, computer vision, etc.

Gerchberg-Saxton algorithm for fast and efficient atom rearrangement in optical tweezer traps.

We demonstrate fast and efficient neutral atom rearrangements in an optical tweezer-trap array, using an enhanced hologram generation algorithm. The conventional Gerchberg-Saxton (GS) algorithm is modified to include zero-padding hologram expansion for optical tweezer sharpness, weighted iteration feedback for reduced crosstalk, and phase induction for successive phase continuity. With the new GS algorithm, we experimentally demonstrate defect-free formation of 2D atom arrays with various geometries, achieving a high loading probability of 0.98 for up to N ∼ 30 atoms. Furthermore, the hologram movie calculation speed is enhanced to cover a computational scalability up to 𝒪(103).

Аппаратно-алгоритмическое обеспечение корреляционного детектирования в голографических датчиках волнового фронта

AbstractAn algorithm that automatically calculates aberrations in the holographic wavefront sensor scheme previously proposed by the authors is described, which enables measuring aberrations by means of iterative generation of holograms on a phase spatial light modulator (SLM). The practical implementation of this algorithm on the basis of the feedback of the camera and SLM is given. Using the proposed algorithm, defocusing was measured with an accuracy of λ/160.

Laser-induced formation of holograms for generation of plasmons

Methods of fabrication of precisely adjusted surface structures are indispensable to development of new technologies. Different surface structuring techniques resulting in solitary nanobumps, random surface structures and laser induced periodic surface structures were in focus in the past. Here we consider a physical model and numerical simulation allowing understanding a high field plasmonics formation process of a smooth periodic perturbation of surface on a metal film with a period equal to the surface plasmon-polariton wavelength at a frequency of laser wave. Such surface structure is a hologram produced by thermomechanical response to interference between an incident laser wave, a reflected laser wave, and an electromagnetic field in the running plasmon-polariton wave. The following laser irradiation of the manufactured hologram generates a new plasmon-polariton wave identical to that used for formation of the hologram.

Modulating phase by metasurfaces with gated ultra-thin TiN films.

Active control over the flow of light is highly desirable because of its applicability to information processing, telecommunication, and spectroscopic imaging. In this paper, by employing the tunability of carrier density in a 1 nm titanium nitride (TiN) film, we numerically demonstrate deep phase modulation (PM) in an electrically tunable gold strip/TiN film hybrid metasurface. A 337° PM is achieved at 1.550 μm with a 3% carrier density change in the TiN film. We also demonstrate that a continuous 180° PM can be realized at 1.537 μm by applying a realistic experiment-based gate voltage bias and continuously changing the carrier density in the TiN film. The proposed design of active metasurfaces capable of deep PM near the wavelength of 1.550 μm has considerable potential in active beam steering, dynamic hologram generation, and flat photonic devices with reconfigurable functionalities.

深度学习算法及其在光学的应用

As an important branch of machine learning, deep learning has reached another climax of machine learning since its inception. Deep learning has excellent performance in many fields such as image recognition and classification, semantic segmentation, and intelligent driving and so on. At the same time, deep learning algorithms are widely used in the field of optics such as computational hologram generation and imaging, non-parameter reconstruction of digital holography, and spectral resonance curves prediction due to their abstract feature recognition and extraction characteristics, strong model building and generalization capabilities. This article detailed the basic principles of deep learning and its typical application research in image classification, super-resolution imaging, computer generated hologram and digital holography, prediction of surface plasmonics resonance curves, and structural design of metasurfaces. And future development of deep learning in the physical optical field was worth exploring.