Random phase-free computer-generated hologram

Computer-generated holography with ordinary display

We propose a method of computer-generated holography (CGH) using incoherent light emitted from a mobile phone screen. In this method, we suppose a cascade of holograms in which the first hologram is a color image displayed on the mobile phone screen. The hologram cascade is synthesized by solving an inverse problem with respect to the propagation of incoherent light. We demonstrate a three-dimensional color image reproduction using a two-layered hologram cascade composed of an iPhone and a spatial light modulator.

Multiuser medical image encryption algorithm using phase-only CGH in the gyrator domain

In this paper, a multiuser medical image encryption algorithm is proposed. The proposed algorithm utilizes polar decomposition, which enables multiuser features in the proposed algorithm. A computer-generated hologram (CGH) improves the security of the proposed algorithm in the gyrator domain. The phase-only CGH-based multiuser algorithm offers advantages such as storing a large amount of information in a compact space, resistance to counterfeiting, and enhanced security. The proposed method is validated with various statistical metrics, such as information entropy, mean squared error, correlation coefficient, histogram, and mesh plots. Results confirm that the proposed algorithm is secure and robust against potential attacks, such as plaintext attacks, iterative attacks, and contamination attacks. The proposed method has a large keyspace, which makes it very difficult to be breached in real-time with existing computational power.

Comparison of double-phase hologram and binary amplitude encoding: holographic projection and vortex beam generation

Utilizing computer-generated holograms is a promising technique because these holograms can theoretically generate arbitrary waves with high light efficiency. In phase-only spatial light modulators, encoding complex amplitudes into phase-only holograms is a significant issue, and double-phase holograms have been a popular encoding technique. However, they reduce the light efficiency. In this study, our complex amplitude encoding, called binary amplitude encoding (BAE), and conventional methods including double-phase hologram, iterative algorithm, and error diffusion methods were compared in terms of the fidelity of reproduced light waves and light efficiency, considering the applications of lensless zoomable holographic projection and vortex beam generation. This study also proposes a noise reduction method for BAE holograms that is effective when the holograms have different aspect ratios. BAE is a non-iterative method, which allows holograms to be obtained more than 2 orders of magnitude faster than iterative holograms; BAE has about 3 times higher light efficiency with comparable image quality compared to double-phase holograms.

Method of calculating speckle-reduced hologram data using a convergence light wave for a computer-generated hologram

In a computer-generated hologram, random phases are required for representing object surfaces; however, speckle noise occurs in the random phases. We propose a speckle reduction method for three-dimensional virtual images in electro-holography. The method does not have random phases but instead converges the object light on the observer's viewpoint. Optical experiments demonstrated that the proposed method greatly reduced speckle noise while maintaining a calculation time comparable to that of the conventional method.

Diffraction-engineered holography: Beyond the depth representation limit of holographic displays

Holography is one of the most prominent approaches to realize true-to-life reconstructions of objects. However, owing to the limited resolution of spatial light modulators compared to static holograms, reconstructed objects exhibit various coherent properties, such as content-dependent defocus blur and interference-induced noise. The coherent properties severely distort depth perception, the core of holographic displays to realize 3D scenes beyond 2D displays. Here, we propose a hologram that imitates defocus blur of incoherent light by engineering diffracted pattern of coherent light with adopting multi-plane holography, thereby offering real world-like defocus blur and photorealistic reconstruction. The proposed hologram is synthesized by optimizing a wave field to reconstruct numerous varifocal images after propagating the corresponding focal distances where the varifocal images are rendered using a physically-based renderer. Moreover, to reduce the computational costs associated with rendering and optimizing, we also demonstrate a network-based synthetic method that requires only an RGB-D image.

Incoherent computer-generated holography

We present a method for computer-generated holography (CGH) using spatially and temporally incoherent light. The proposed method synthesizes a hologram cascade by solving an inverse problem for the propagation of incoherent light. The spatial incoherence removes speckle noise in CGH, and the temporal incoherence simplifies the optical setup, including the light source. We demonstrate two- and three-dimensional color image reproductions by a two-layer grayscale hologram cascade with a chip-on-board white light-emitting diode.

Review of computer-generated hologram algorithms for color dynamic holographic three-dimensional display

Holographic three-dimensional display is an important display technique because it can provide all depth information of a real or virtual scene without any special eyewear. In recent years, with the development of computer and optoelectronic technology, computer-generated holograms have attracted extensive attention and developed as the most promising method to realize holographic display. However, some bottlenecks still restrict the development of computer-generated holograms, such as heavy computation burden, low image quality, and the complicated system of color holographic display. To overcome these problems, numerous algorithms have been investigated with the aim of color dynamic holographic three-dimensional display. In this review, we will explain the essence of various computer-generated hologram algorithms and provide some insights for future research.

Enhancement of grayscale image display with amplitude Fourier holograms, employing a limited bandwidth phase

An alternative method is proposed to generate a modified random phase that is able to concentrate the light around a given direction, produces well-contrasted Fourier amplitude holograms, reduces the quantity and the randomness of the speckle noise in the image, and decreases the amount of data necessary for the phase definition. This modified limited bandwidth random phase uses structured random phase patterns to control the object dispersion. The resulting hologram displays an image with structured speckle noise (SSN), exhibiting similar metrics as the iterative method for hologram generation. A filtering process eliminates the SSN; the speckle contrast in the final image is reduced from 0.66 to 0.07; and the peak SNR increases from 7.21 dB to 12.62 dB. This method enhances the fine details and grayscale tone perception in the final image.

Automotive Holographic Head-Up Displays

Driver's access to information about navigation and vehicle data through in-car displays and personal devices distract the driver from safe vehicle management. The discrepancy between road safety and infotainment must be addressed to develop safely operated modern vehicles. Head-up displays (HUDs) aim to introduce a seamless uptake of visual information for the driver while securely operating a vehicle. HUDs projected on the windshield provide the driver with visual navigation and vehicle data within the comfort of the driver's personal eye box through a customizable extended display space. Windshield HUDs do not require the driver to shift the gaze away from the road to attain road information. This article presents a review of technological advances and future perspectives in holographic HUDs by analyzing the optoelectronics devices and the user experience of the driver. The review elucidates holographic displays and full augmented reality in 3D with depth perception when projecting the visual information on the road within the driver's gaze. Design factors, functionality, and the integration of personalized machine learning technologies into holographic HUDs are discussed. Application examples of the display technologies regarding road safety and security are presented. An outlook is provided to reflect on display trends and autonomous driving.

Holobricks: modular coarse integral holographic displays

Here, we propose and demonstrate a modular holographic display system that allows seamless spatial tiling of multiple coarse integral holographic (CIH) displays called "holobricks". A holobrick is a self-contained CIH module enclosing a spatial light modulator (SLM), a scanner, and periscopic coarse integral optics. Modular CIH uses a coarse pitch and small area but high-bandwidth SLM in conjunction with periscopic coarse integral optics to form the angularly tiled 3D holograms with large viewing areas and fields of view. The creation of periscopic coarse integral optics prevents the optical system from being larger than the holographic image and allows the holographic fringe pattern to fill the entire face of the holobrick. Thus, multiple holobricks can be seamlessly abutted to form a scalable spatially tiled holographic image display capable of both wide field-of-view angle and arbitrary large-size area. We demonstrate an initial prototype that seamlessly tiles two holobricks each with 1024 × 768 pixels, 40° FOV, full color, 24 fps, displaying 2D, 3D holographic stereograms, and full parallax 3D CGI Fresnel holograms.

Generation of non-iterative phase-only hologram based on a hybrid phase mask

The random phase method and quadratic phase method are most widely used in the generation of non-iterative phase holograms. However, the former leads to the reconstruction being severely disturbed by speckle noise, with serious loss of detailed information, and the latter leads to the reconstruction being contaminated with ringing artifacts. To solve these problems, we present a novel, to the best of our knowledge, method capable of generating non-iterative phase holograms, hereafter referred to as hybrid-phase-only holograms (HPOHs). Our proposal is to use a weight factor to combine the random phase and quadratic phase to generate a hybrid phase mask. The hybrid phase mask is then superimposed on the target image to obtain a complex hologram by simple Fourier transform. Followed by retaining the phase of the complex hologram, we can generate the corresponding HPOH. The effects of different weight factors on the holographic reconstructions are discussed. Numerical simulations of reconstruction quality associated with the proposed method, random phase method, and quadratic phase method are presented for comparison purposes. Optical experiments based on liquid crystal on silicon also demonstrate the validity of the method.

Optimized Fresnel phase hologram for ringing artifacts removal in lensless holographic projection

Ringing artifacts are the main noise sources in holographic projection when the quadratic phase is introduced to suppress speckle noise. In this study, the mechanisms of ringing artifacts on reconstructed images are theoretically analyzed, which illustrates the ringing artifacts are related to the bandwidth properties of the reconstructed wave field. Based on the frequency analysis, a band-limited iterative algorithm is proposed to optimize the phase hologram in the Fresnel domain. The proposed method can effectively suppress the ringing artifacts as well as the speckle noise of the Fresnel hologram by optimizing the phase distribution with bandwidth constraint. Numerical simulations and optical experiments have been performed to validate the proposed method for providing quality reconstructions in lensless holographic projection.

Dual-task convolutional neural network based on the combination of the U-Net and a diffraction propagation model for phase hologram design with suppressed speckle noise

In this paper, a dual-task convolutional neural network based on the combination of the U-Net and a diffraction propagation model is proposed for the design of phase holograms to suppress speckle noise of the reconstructed images. By introducing a Fresnel transmission layer, based on angular spectrum diffraction theory, as the diffraction propagation model and incorporating it into U-Net as the output layer, the proposed neural network model can describe the actual physical process of holographic imaging, and the distributions of both the light amplitude and phase can be generated. Afterwards, by respectively using the Pearson correlation coefficient (PCC) as the loss function to modulate the distribution of the amplitude, and a proposed target-weighted standard deviation (TWSD) as the loss function to limit the randomness and arbitrariness of the reconstructed phase distribution, the dual tasks of the amplitude reconstruction and phase smoothing are jointly solved, and thus the phase hologram that can produce high quality image without speckle is obtained. Both simulations and optical experiments are carried out to confirm the feasibility and effectiveness of the proposed method. Furthermore, the depth of field (DOF) of the image using the proposed method is much larger than that of using the traditional Gerchberg-Saxton (GS) algorithm due to the smoothness of the reconstructed phase distribution, which is also verified in the experiments. This study provides a new phase hologram design approach and shows the potential of neural networks in the field of the holographic imaging and more.

Holographic and speckle encryption using deep learning

Vulnerability analysis of optical encryption schemes using deep learning (DL) has recently become of interest to many researchers. However, very few works have paid attention to the design of optical encryption systems using DL. Here we report on the combination of the holographic method and DL technique for optical encryption, wherein a secret image is encrypted into a synthetic phase computer-generated hologram (CGH) by using a hybrid non-iterative procedure. In order to increase the level of security, the use of the steganographic technique is considered in our proposed method. A cover image can be directly diffracted by the synthetic CGH and be observed visually. The speckle pattern diffracted by the CGH, which is decrypted from the synthetic CGH, is the only input to a pre-trained network model. We experimentally build and test the encryption system. A dense convolutional neural network (DenseNet) was trained to estimate the relationship between the secret images and noise-like diffraction patterns that were recorded optically. The results demonstrate that the network can quickly output the primary secret images with high visual quality as expected, which is impossible to achieve with traditional decryption algorithms.

Speckleless color dynamic three-dimensional holographic display based on complex amplitude modulation

In this paper, we propose a method to implement a speckleless color dynamic three-dimensional holographic display by modulating amplitude and phase distribution simultaneously. Computer-generated holograms are calculated with an initial uniform phase, and the speckle noise of reconstructed images is suppressed effectively. We perform both numerical simulations and optical experiments to demonstrate the effectiveness of the proposed method. The numerical simulations show that the proposed method can achieve speckleless reconstruction and the optical experiments provide a good color dynamic display effect. It is expected that the proposed method could be widely applied to realize high-quality color dynamic holographic displays in the future.

Fast generation of 360-degree cylindrical photorealistic hologram using ray-optics based methods

Due to the large pixel pitch and limited size of spatial light modulator (SLM), the field of view (FOV) of current holographic display is greatly restricted. Cylindrical holography can effectively overcome the constraints of FOV. However, the existent algorithms of cylindrical hologram are all based on the wave-optics based approach. In this paper, to the best of our knowledge, we adopt the ray-optics based approach in the generation of cylindrical computer generated hologram (CCGH) for the first time. Information of parallax images captured from three-dimensional (3D) objects using a curved camera array is recorded into a cylindrical hologram. Two different recording specific algorithms are proposed, one is based on the Fast Fourier Transform (FFT) method, and another is based on the pinhole-type integral imaging (PII) method. The simulation results confirm that our proposed methods are able to realize a fast generation of the cylindrical photorealistic hologram.

Uniform and efficient beam shaping for high-energy lasers

Phase-only beam shaping with liquid crystal on silicon spatial light modulators (SLM) allows modulating the wavefront dynamically and generating arbitrary intensity patterns with high efficiency. Since this method cannot take control of all degrees of freedom, a speckle pattern appears and drastically impairs the outcome. There are several methods to overcome this issue including algorithms which directly control phase and amplitude, but they suffer from low efficiency. Methods using two SLMs yield excellent results but they are usually limited in the applicable energy due to damage to the SLM's backplane. We present a method which makes use of two SLMs and simultaneously gives way for high-energy laser applications. The algorithm and setup are designed to keep the fluence on the SLMs low by distributing the light over a large area. This provides stability against misalignment and facilitates experimental feasibility while keeping high efficiency.

Hybrid holographic Maxwellian near-eye display based on spherical wave and plane wave reconstruction for augmented reality display

The holographic Maxwellian display is a promising technique for augmented reality (AR) display because it solves the vergence-accommodation conflict while presenting a high-resolution display. However, conventional holographic Maxwellian display has the inherent trade-off between depth of field (DOF) and image quality. In this paper, two types of holographic Maxwellian displays, the spherical wave type and the plane wave type, are proposed and analyzed. The spherical wavefront and the plane wavefront are produced by a spatial light modulator (SLM) for Maxwellian display. Due to the focusing properties of different wavefronts, the two types of display have complementary DOF ranges. A hybrid approach combining the spherical wavefront and plane wavefront is proposed for a large DOF with high image quality. An optical experiment with AR display is demonstrated to verify the proposed method.

Frequency-based optimized random phase for computer-generated holographic display

Random phases with all frequency components lead to excessive diffusions of object waves, resulting in loss of detail in holographic reconstructions. In this study, the effects of random phases with various frequencies on holographic reconstruction results are evaluated. The optimized maximal value of the random phases is analyzed. Utilizing the evaluation results, we propose a frequency-based optimized random phase that reduces the unfavorable effect of the insufficient dynamic range of computer-generated holograms and prevents excessive diffusions by traditional random phases. Utilizing the optimized random phase, which improves the reconstruction quality significantly, we can commendably reconstruct both contours and details.

Analysis of numerical diffraction calculation methods: from the perspective of phase space optics and the sampling theorem

Diffraction calculations are widely used in applications that require numerical simulation of optical wave propagation. Different numerical diffraction calculation methods have their own transform and sampling properties. In this study, we provide a unified analysis where five popular fast diffraction calculation methods are analyzed from the perspective of phase space optics and the sampling theorem: single fast Fourier transform-based Fresnel transform, Fresnel transfer function approach, Fresnel impulse response approach, angular spectrum method, and Rayleigh-Sommerfeld convolution. The evolutions of an input signal's space-bandwidth product (SBP) during wave propagation are illustrated with the help of a phase space diagram (PSD) and an ABCD matrix. It is demonstrated that all of the above methods cannot make full use of the SBP of the input signal after diffraction; and some transform properties have been ignored. Each method has its own restrictions and applicable range. The reason why different methods have different applicable ranges is explained with physical models. After comprehensively studying and comparing the effect on the SBP and sampling properties of these methods, suggestions are given for choosing the proper method for different applications and overcoming the restrictions of corresponding methods. The PSD and ABCD matrix are used to illustrate the properties of these methods intuitively. Numerical results are presented to verify the analysis, and potential ways to develop new diffraction calculation methods are also discussed.

Non-iterative phase hologram generation with optimized phase modulation

A non-iterative algorithm is proposed to generate phase holograms with optimized phase modulation. A quadratic initial phase with continuous distributed spectrum is utilized to iteratively optimize the phase modulation in the reconstruction plane, which can be used as an optimized phase distribution for arbitrary target images. The phase hologram can be calculated directly according to the modulated wave field distribution in the reconstruction plane. Fast generation of the phase holograms can be achieved by this non-iterative implementation, and the avoidance of the random phase modulation helps to suppress the speckle noise. Numerical and optical experiments have demonstrated that the proposed method can efficiently generate phase holograms with quality reconstructions.

Evaluation of quadratic phase hologram calculation algorithms in the Fourier regime

The display of phase-only holograms with a spatial light modulator (SLM) has many applications due to its potential for dynamic three-dimensional projection of arbitrary patterns. We describe an innovative modification of the quadratic phase method for hologram calculation that uses error diffusion for initialization of an iterative phase retrieval algorithm. We compare the performance of our algorithm to other variations of hologram calculation approaches that use the quadratic phase method in the Fourier regime. Parameter variation is conducted for finding the differences and limits of the methods. Experiments with an SLM show the validity of the simulations.

Simple and effective calculation method for computer-generated hologram based on non-uniform sampling using look-up-table

Heavy computational complexity and imprecise reconstruction of objects are crucial problems in computer-generated holograms. In this paper, we propose a non-uniform sampling based on novel compressed look up table method to generate holograms. The method consists of two steps: in the first step, the non-uniform basic modulation factors are precalculated and stored in look-up-table. Secondly, fringe patterns for other points are obtained by simply shifting and multiplying the pre-calculated non-uniform basic modulation factors, and the final computer-generated hologram is obtained by adding them all together. The proposed method eliminates the redundant information properly and modulates the reconstructed images precisely. Numerical simulation results show proposed method reduces the memory usage, speeds up computation time and the quality of reconstructed images do not degrade evidently compared with uniform sampling method. Optical experiments results are in good agreement with numerical simulation results. The proposed method is simple, effective and could be applied in the holographic field in the future.

Computational holographic Maxwellian near-eye display with an expanded eyebox

The Maxwellian near-eye displays have attracted growing interest in various applications. By using a confined pupil, a Maxwellian display presents an all-in-focus image to the viewer where the image formed on the retina is independent of the optical power of the eye. Despite being a promising technique, current Maxwellian near-eye displays suffer from various limitations such as a small eyebox, a bulky setup and a high cost. To overcome these drawbacks, we present a holographic Maxwellian near-eye display based on computational imaging. By encoding a complex wavefront into amplitude-only signals, we can readily display the computed histogram on a widely-accessible device such as a liquid-crystal or digital light processing display, creating an all-in-focus virtual image augmented on the real-world objects. Additionally, to expand the eyebox, we multiplex the hologram with multiple off-axis plane waves, duplicating the pupils into an array. The resultant method features a compact form factor because it requires only one active electronic component, lending credence to its wearable applications.

Reducing the memory usage of computer-generated hologram calculation using accurate high-compressed look-up-table method in color 3D holographic display

In this paper, we propose an accurate high-compressed look-up-table method that uses less memory to generate the hologram. In precomputation, we separate the longitudinal modulation factors and only calculate the basic horizontal and vertical factors. Therefore, we obtain other horizontal and vertical modulation factors of object points by simply shifting the basic horizontal and vertical modulation factors while computing holograms. We perform numerical simulations and optical experiments to verify the proposed method. Numerical simulation results show that the proposed method has the least memory usage, the fastest computation time and no distortion. The optical experimental results are in accord with the numerical simulation results. The proposed method is simple and effective to calculate computer-generated holograms for color dynamic holographic display with high speed, less memory usage and high accuracy that could be applied in the holographic field in the future.

Low-noise high-efficiency double-phase hologram by multiplying a weight factor

This Letter describes a method of generating a phase-only hologram, which improves the lack of diffraction efficiency and peripheral noise phenomenon, which is a problem of the existing double-phase hologram (DPH) method. The proposed weight factor DPH method can be obtained by a simple matrix operation, so there is no need for additional computing power. The optimal weight factor value depends on the image size, so we have found the optimal value for each image size. The numerical simulation and optical experiments were conducted to compare the images generated using the proposed weigh factor DPH method, the existing DPH method, and the random phase method. The peak signal-to-noise ratio of the images obtained through the proposed weight factor is improved by 14.6% compared to the existing DPH.

Image quality improvement of random phase-free holograms by addressing the cause of ringing artifacts

Holographic projectors utilize holography techniques, although there are several barriers to realizing holographic projections. One such challenge is the deterioration of the hologram image quality caused by speckle noise and ringing artifacts. Several methods designed to reduce the speckle noise and ringing artifacts have been proposed. However, these methods require multiple diffraction calculations and a significant amount of computational time. In this paper, we reveal that ringing artifacts are due to object light being recorded on the edge of the hologram and that the high-frequency component of the original image leaks outside of the recording area of the hologram when the random phase-free method is used. Therefore, this study proposes an object light centering method that prevents object light from being recorded on the edge of the hologram and prevents the high-frequency component of the original image from leaking outside the recording area of the hologram, which removes the ringing artifact and extends the random phase-free method to an off-axis hologram.

Full image reconstruction with reduced speckle noise, from a partially illuminated Fresnel hologram, using a structured random phase

In computer generated holograms, a random phase is added to spread out the object wave, but this introduces a strong speckle noise in the reconstructed image. Spreading out the object wave helps to record a large object in a smaller hologram. We propose a random repeated and displaced phase, which reduces the speckle noise in the reconstructed image and spreads out the object information in a limited area. The phase is independent of the object, and it could be calculated by sections reducing the hologram computing time. We present simulated and experimental results that prove the improvement in quality of the reconstructed image and the whole object reconstruction, using only a small part of the hologram.

Fast method for ringing artifacts reduction in random phase-free kinoforms

In holographic projection, ringing artifacts and degradation appear when the speckle noise problem is solved based on the random phase-free method. In this paper, we present a fast method to suppress the ringing artifacts in random phase-free kinoforms. We first reduce the distance between the hologram plane and the focal plane, and keep the focal length of the convergence light unchanged in the random phase-free method. Next, the complex amplitude is modulated using a single spatial light modulator. Consequently, the ringing artifacts and speckle noise reduction in the reconstructed image can be achieved. At the same time, the computing speed can be increased with our proposed method. Numerical simulations and optical experiments have validated the feasibility of the proposed method.

Generalized single-sideband three-dimensional computer-generated holography

The conjugate image and zero-order beam have considerable influences on the optical reconstructions in computer-generated holographic display systems based on the amplitude spatial light modulators. We propose a generalized single-sideband method for suppressing the unwanted terms in computer-generated holography. Computer-generated holograms (CGHs) are calculated based on frequency filtering of the object wave, which redistributes the diffraction wave in spatial frequency domain for spectrum filtering during optical reconstruction. Numerical simulations and optical experiments demonstrate that the proposed method is generally effective for different kinds of CGH algorithms to reconstruct quality three-dimensional scenes that are free of conjugate image and zero-order beam.

Enhanced see-through near-eye display using time-division multiplexing of a Maxwellian-view and holographic display

In this paper, we suggest the improved integration of a holographic display and a Maxwellian-view display using time-division multiplexing and describe an image rendering process for the proposed system. In general, the holographic displays have a resolution limit when used to represent a virtual 3D scene. In the proposed system, the holographic display processed relatively few layers of the virtual 3D scene, while the remaining objects were processed with a Maxwellian-view display to which was applied a Gaussian smoothing filter. Hence, we obtained the retaining holographic image quality, expanding the field of view, and reducing the computation time of the proposed system. The holographic display of the proposed system had an image size of 28 mm × 28 mm with a field of view of 1.02° and a 10.8 mm eye box. The Maxwellian-view display had an image size of 230 mm × 230 mm with a field of view of 22.6 ° and a 0.9 mm eye box diameter. Each display was integrated in time-division multiplexing of 40 Hz, and the proposed system was successfully verified.

Formulas of partially spatial coherent light and design algorithm for computer-generated holograms

Formulas of partially spatial coherent light are derived and its corresponding design algorithm is proposed for generating computer-generated holograms based on partially spatial coherent light. The partially coherent light is divided into two terms: spatially absolute coherent part and incoherent part. The former is propagated by angular spectrum method, while the latter is based on the optical transfer function. The related formulas are derived where the coherent function (degree of coherence) is related. A modified iterative algorithm is further developed for optimizing the phase distributions. Numerical simulations and optical experiments are both performed to verify the proposed algorithm. The results obtained by the proposed method and the traditional method are compared, and it is clear that the speckle contrasts in optical experiments are improved more than 46%, and the image quality is obviously improved. This method could also provide new applications for three-dimensional holographic display, beam shaping, and other wave-front modulation techniques.

Deep-learning-generated holography

We present a method for computer-generated holography based on deep learning. The inverse process of light propagation is regressed with a number of computationally generated speckle data sets. This method enables noniterative calculation of computer-generated holograms (CGHs). The proposed method was experimentally verified with a phase-only CGH.

Holographic near-eye display system based on double-convergence light Gerchberg-Saxton algorithm

In this paper, a method is proposed to implement noises reduced three-dimensional (3D) holographic near-eye display by phase-only computer-generated hologram (CGH). The CGH is calculated from a double-convergence light Gerchberg-Saxton (GS) algorithm, in which the phases of two virtual convergence lights are introduced into GS algorithm simultaneously. The first phase of convergence light is a replacement of random phase as the iterative initial value and the second phase of convergence light will modulate the phase distribution calculated by GS algorithm. Both simulations and experiments are carried out to verify the feasibility of the proposed method. The results indicate that this method can effectively reduce the noises in the reconstruction. Field of view (FOV) of the reconstructed image reaches 40 degrees and experimental light path in the 4-f system is shortened. As for 3D experiments, the results demonstrate that the proposed algorithm can present 3D images with 180cm zooming range and continuous depth cues. This method may provide a promising solution in future 3D augmented reality (AR) realization.

Non-iterative phase-only Fourier hologram generation with high image quality

We report a novel and non-iterative method for the generation of phase-only Fourier hologram for image projection. Briefly, target image is first added with a special quadratic phase and then padded with zeros. A complex Fourier hologram is generated via the simple fast Fourier transform. Subsequently, the error diffusion algorithm is applied to convert the complex hologram into a phase-only hologram. The numerical, as well as the optical reconstructed images with the proposed method are of higher visual quality and contain less speckle noise compared to the original random phase method, which add the random phase to the target image and then preserve the phase component of the complex hologram. The influences of quadratic phase and zero-padding on the image quality are also discussed in detail.

Speeding up image quality improvement in random phase-free holograms using ringing artifact characteristics

A holographic projector utilizes holography techniques. However, there are several barriers to realizing holographic projections. One is deterioration of hologram image quality caused by speckle noise and ringing artifacts. The combination of the random phase-free method and the Gerchberg-Saxton (GS) algorithm has improved the image quality of holograms. However, the GS algorithm requires significant computation time. We propose faster methods for image quality improvement of random phase-free holograms using the characteristics of ringing artifacts.

Speckle reduced lensless holographic projection from phase-only computer-generated hologram

This paper presents a method for the implementation of speckle reduced lensless holographic projection based on phase-only computer-generated hologram (CGH). The CGH is calculated from the image by double-step Fresnel diffraction. A virtual convergence light is imposed to the image to ensure the focusing of its wavefront to the virtual plane, which is established between the image and the hologram plane. The speckle noise is reduced due to the reconstruction of the complex amplitude of the image via a lensless optical filtering system. Both simulation and optical experiments are carried out to confirm the feasibility of the proposed method. Furthermore, the size of the projected image can reach to the maximum diffraction bandwidth of the spatial light modulator (SLM) at a given distance. The method is effective for improving the image quality as well as the image size at the same time in compact lensless holographic projection system.

Acceleration of the calculation speed of computer-generated holograms using the sparsity of the holographic fringe pattern for a 3D object

In computer-generated hologram (CGH) calculations, a diffraction pattern needs to be calculated from all points of a 3-D object, which requires a heavy computational cost. In this paper, we propose a novel fast computer-generated hologram calculation method using sparse fast Fourier transform. The proposed method consists of two steps. First, the sparse dominant signals of CGHs are measured by calculating a wavefront on a virtual plane between the object and the CGH plane. Second, the wavefront on CGH plane is calculated by using the measured sparsity with sparse Fresnel diffraction. Experimental results proved that the proposed method is much faster than existing works while it preserving the visual quality.

High-accuracy method for holographic image projection with suppressed speckle noise

Iterative Fourier transform algorithms are widely used for creating holograms in holographic image projection. However, the reconstructed image always suffers from the speckle noise severely due to the uncontrolled phase distribution of the image. In this paper, a new iterative method is proposed to eliminate the speckle noise. In the iteration, the amplitude and phase in the signal window in the output plane are constrained to the desired distribution and a special object-dependent quadratic phase distribution, respectively. Since the phase of the reconstructed image is assigned artificially, the speckle noise came from the destructive interference between the sampling points with random and erratic phase distribution can be eliminated. To verify the method, simulations and experiments are performed. And the result shows that high quality, low noise images can be achieved.

Broadband metasurface holograms: toward complete phase and amplitude engineering

As a revolutionary three-dimensional imaging technique, holography has attracted wide attention for its ability to photographically record a light field. However, traditional phase-only or amplitude-only modulation holograms have limited image quality and resolution to reappear both amplitude and phase information required of the objects. Recent advances in metasurfaces have shown tremendous opportunities for using a planar design of artificial meta-atoms to shape the wave front of light by optimal control of both its phase and amplitude. Inspired by the concept of designer metasurfaces, we demonstrate a novel amplitude-phase modulation hologram with simultaneous five-level amplitude modulation and eight-level phase modulation. Such a design approach seeks to turn the perceived disadvantages of the traditional phase or amplitude holograms, and thus enable enhanced performance in resolution, homogeneity of amplitude distribution, precision, and signal-to-noise ratio. In particular, the unique holographic approach exhibits broadband characteristics. The method introduced here delivers more degrees of freedom, and allows for encoding highly complex information into designer metasurfaces, thus having the potential to drive next-generation technological breakthroughs in holography.

Color computer-generated hologram generation using the random phase-free method and color space conversion

We propose two calculation methods of generating color computer-generated holograms (CGHs) with the random phase-free method and color space conversion in order to improve the image quality and accelerate the calculation. The random phase-free method improves the image quality in monochrome CGH, but it is not performed in color CGH. We first aimed to improve the image quality of color CGH using the random phase-free method and then to accelerate the color CGH generation with a combination of the random phase-free method and color space conversion method, which accelerates the color CGH calculation due to down-sampling of the color components converted by color space conversion. To overcome the problem of image quality degradation that occurs due to the down-sampling of random phases, the combination of the random phase-free method and color space conversion method improves the quality of reconstructed images and accelerates the color CGH calculation. We demonstrated the effectiveness of the proposed method in simulation, and in this paper discuss its application to lensless zoomable holographic projection.

Non-iterative phase hologram computation for low speckle holographic image projection

Phase-only spatial light modulators (SLMs) are widely used in holographic display applications, including holographic image projection (HIP). Most phase computer generated hologram (CGH) calculation algorithms have an iterative structure with a high computational load, and also are prone to speckle noise, as a result of the random phase terms applied on the desired images to mitigate the encoding noise. In this paper, we present a non-iterative algorithm, where simple Discrete Fourier Transform (DFT) relations are exploited to compute phase CGHs that exactly control half of the desired image samples (those on even - or odd - indexed rows - or columns) via a single Fast Fourier Transform (FFT) and trivial arithmetic operations. The encoding noise appearing on the uncontrolled half of the image samples is reduced by the application of structured, non-random initial phase terms so that speckle noise is also kept low. High quality reconstructions are obtained under temporal averaging of several SLM frames. Interlaced video within half of the addressable image area is readily deliverable without frame rate division. Our algorithm provides about 6X and 20X reduction in computational cost compared to IFTA and FIDOC algorithms, respectively. Simulations and experiments verify that the algorithm constitutes a promising option for real-time computation of phase CGHs.

Holographic multi-projection using the random phase-free method

We demonstrated holographic multi-projection using the random phase-free method and the iterative method. Holographic multi-projection is a method of projecting multiple different images focused on different screens at the same time. The random phase-free method succeeded in improving the image quality. By applying the iterative method to the random phase-free method, the image quality was improved further. The results of our numerical reconstruction and optical experiments confirm that the proposed method improves the image quality. The peak signal-to-noise ratios of the reconstructed images using the proposed method and the conventional method are 30.66 and 13.61 dB, respectively.

Random phase-free kinoform for large objects

We propose a random phase-free kinoform for large objects. When not using the random phase in kinoform calculation, the reconstructed images from the kinoform are heavy degraded, like edge-only preserved images. In addition, the kinoform cannot record an entire object that exceeds the kinoform size because the object light does not widely spread. In order to avoid this degradation and to widely spread the object light, the random phase is applied to the kinoform calculation; however, the reconstructed image is contaminated by speckle noise. In this paper, we overcome this problem by using our random phase-free method and error diffusion method.