Phase-only Hologram Generation Research Articles

Two-constraint-free dual-domain optimised random phase-only hologram

To overcome the main limitations of the optimised random phase (ORAP) non-iterative hologram generation method, which include the support constraints (position, size, and shape) and the fixed window size of the space domain, we propose a two-constraint-free dual-domain optimised random phase (TCF-DD-ORAP) method for phase-only hologram generation. Firstly, the dual-domain optimised random phase (DD-ORAP) is generated in the dual domain (the space or frequency domain), both of which can be used to non-iteratively generate holograms for different target images. Next, the support constraints of the target image are relaxed in the space domain (position, size, and shape), and the generated DD-ORAP can then be used to generate holograms for the target images with arbitrary support in the image space. Finally, a crop operation is applied to target images with different window sizes, and the TCF-DD-ORAP corresponding to the target window size can be cropped for CGH generation. The TCF-DD-ORAP method proposed in this paper overcomes the dual limitations of fixed support and window size that arise in the original ORAP method of CGH generation, and can therefore significantly reduce the computational cost and enhance the flexibility of CGH generation. Numerical and optical experiments validate the effectiveness and superiority of our proposed TCF-DD-ORAP method.

Comparative Study of Phase-Only Hologram Generation Algorithms Based on Iteration

Comparative Study of Phase-Only Hologram Generation Algorithms Based on Iteration

Optimized phase-only hologram generation for high-quality holographic display.

In holographic displays, the Gerchberg-Saxton (GS) method has been widely used to generate the phase-only hologram (POH). However, as the constraint strategy of the GS method cannot optimize the POH well enough, the display quality is degraded. Various methods have been proposed to solve this problem, such as introducing the dummy area, using an initial quadratic phase, and modifying the amplitude constraint strategy. This paper proposes a simple and effective iterative method to optimize POH for the high-quality reconstructed image, which is a combination of the above three methods. In the proposed method, the target image is padded with zeros, and the padded image is divided into the target and non-target region. The initial phase in the target region is reset to the quadratic phase, and the optimized parameters α and β are introduced to modify the amplitude constraint strategy. In the iterative process, this strategy is applied to the target region. Numerical and optical experiments were conducted to verify the effectiveness of the proposed method. The results show that the speckle noise is effectively suppressed, and the quality of the reconstructed image is improved.

Generation of phase-only holograms with high-diffraction-order reconstruction by a U-Net-based neural network: A phase grating perspective

Implicit periodic structure in phase-only holograms will result in many diffraction orders in the diffraction field. We analyzed the diffraction pattern from a phase gratings point of view and proved that the diffraction orders were jointly influenced by the phase factor, the single-beam diffraction factor, and the multibeam interference factor. According to the analysis, we proposed the high-diffraction-order angular spectrum method (HDO-ASM) for the numerical reconstruction of high diffraction orders. Different from the conventional methods of removing high diffraction orders, we chose to reconstruct target images in high diffraction orders with HDO-ASM and a U-Net-based neural network. Finally, the 4 K phase-only holograms with high-diffraction-order reconstruction were generated in 0.09s and had a mean reconstruction quality of 34.3 dB (PSNR) in the DIV2K valid dataset. Theoretical and experimental results demonstrated that there are few speckle noises and fringes in the reconstructed images of holograms generated by the proposed method.

Diffraction model-informed neural network for unsupervised layer-based computer-generated holography.

Learning-based computer-generated holography (CGH) has shown remarkable promise to enable real-time holographic displays. Supervised CGH requires creating a large-scale dataset with target images and corresponding holograms. We propose a diffraction model-informed neural network framework (self-holo) for 3D phase-only hologram generation. Due to the angular spectrum propagation being incorporated into the neural network, the self-holo can be trained in an unsupervised manner without the need of a labeled dataset. Utilizing the various representations of a 3D object and randomly reconstructing the hologram to one layer of a 3D object keeps the complexity of the self-holo independent of the number of depth layers. The self-holo takes amplitude and depth map images as input and synthesizes a 3D hologram or a 2D hologram. We demonstrate 3D reconstructions with a good 3D effect and the generalizability of self-holo in numerical and optical experiments.

Continuous Depth Control of Phase-Only Hologram With Depth Embedding Block

Digital holography is a promising candidate for advanced display, although several obstacles remain, such as the problem of heavy time consumption in the generation of phase-only holograms. Recently, deep-learning-based methods have achieved the real-time generation of holograms while maintaining high image quality. However, the holograms created with deep neural networks can reproduce images only at a specific distance because their target depth is fixed in the training process. This paper suggested and demonstrated a deep neural network that can continuously control the depth of the phase-only hologram. The network takes a target depth and an input image and generates a phase-only hologram. We added a depth embedding block that moves the hologram latent vector depending on the target depth. Thus, we can change the location of the image plane without retraining. The numerical and optical experiments show that the network understands the relationship between the depth and the appearance of the phase-only hologram. As a result, phase-only holograms generated with the proposed network can reconstruct images with around 25-dB PSNR.

Alternative constraints for improved multiplane hologram generation.

In this work, we introduce a modified hologram plane constraint to improve the accuracy of the global Gerchberg-Saxton (GGS) algorithm used for multiplane phase-only hologram generation. This constraint consists of a modified phase factor that depends on the amplitude of the field in the hologram plane. We demonstrate that this constraint produces an increase in the mean correlation coefficient between the reconstructed planes from a multiplane hologram and the corresponding amplitude targets for each plane. Furthermore, this constraint can be applied together with a mixed constraint in the reconstruction planes, leading to a more uniform and controllable reproduction of a target intensity distribution. To confirm the validity of our proposal, we show numerical and experimental results for multiplane holograms with six discrete planes, using both high and low contrast targets. For the experimental results, we implement a holographic projection scheme based on a phase-only spatial light modulator.

High-speed computer-generated holography using an autoencoder-based deep neural network.

Learning-based computer-generated holography (CGH) provides a rapid hologram generation approach for holographic displays. Supervised training requires a large-scale dataset with target images and corresponding holograms. We propose an autoencoder-based neural network (holoencoder) for phase-only hologram generation. Physical diffraction propagation was incorporated into the autoencoder's decoding part. The holoencoder can automatically learn the latent encodings of phase-only holograms in an unsupervised manner. The proposed holoencoder was able to generate high-fidelity 4K resolution holograms in 0.15 s. The reconstruction results validate the good generalizability of the holoencoder, and the experiments show fewer speckles in the reconstructed image compared with the existing CGH algorithms.

Generation and experimental reconstruction of optimized Fresnel random phase-only holograms

In this paper, we demonstrate a method for random phase optimization in the Fresnel domain that enables the fast generation of phase-only holograms (POHs). Our proposal consists of applying the iterative Fresnel transform algorithm (IFrTA) to optimize a random phase with the parameters of a given holographic projection system. Then, the resulting optimized Fresnel random phase is used for the generation of POHs of several objects. We compare the performance of our proposed method to the direct generation of holograms using IFrTA, and quantitatively study the quality of the reconstructed object varying the generation parameters. Our results show significantly improved computation speed and minor loss of accuracy compared with the direct application of the IFrTA. We present both numerical and optical reconstructions obtained with a holographic projection scheme based on a phase-only spatial light modulator to verify the validity and effectiveness of our proposal.

Adaptive weighted Gerchberg-Saxton algorithm for generation of phase-only hologram with artifacts suppression.

In the conventional weighted Gerchberg-Saxton (GS) algorithm, the feedback is used to accelerate the convergence. However, it will lead to the iteration divergence. To solve this issue, an adaptive weighted GS algorithm is proposed in this paper. By replacing the conventional feedback with our designed feedback, the convergence can be ensured in the proposed method. Compared with the traditional GS iteration method, the proposed method improves the peak signal-noise ratio of the reconstructed image with 4.8 dB on average. Moreover, an approximate quadratic phase is proposed to suppress the artifacts in optical reconstruction. Therefore, a high-quality image can be reconstructed without the artifacts in our designed Argument Reality device. Both numerical simulations and optical experiments have validated the effectiveness of the proposed method.

Noniterative multiplane holographic projection

In this paper, we introduce a mixed complex and phase-only constraint for noniterative computer generation of phase-only holograms from multiplane intensity distributions. We are able to reproduce three-dimensional intensity distributions with the same number of planes achieved with the Gerchberg-Saxton (GS) algorithm; at the same time, we maintain the fast computation time of a noniterative method. In this way, we enable the possibility of multiplane light field control in dynamic applications. We show numerical results for three- and eight-plane holograms, for different interplane distances-using either the same or different amplitude constraints in each plane. In all of these tests, our method results in a comparable or better reconstruction quality than the GS algorithm, while achieving a significant decrease in computing time. Finally, we experimentally demonstrate the capability of our proposal to achieve multiplane holographic projection.

Optimized random phase tiles for non-iterative hologram generation.

In this work, we introduce a technique for fast, high-quality, non-iterative generation of phase-only holograms from both 2D and 3D scenes. In this technique, we generate an optimized random phase tile which behaves like a small diffuser, spreading the amplitude of a section of the scene throughout the hologram plane. Each section of the scene is multiplied by this tile and then propagated to the hologram plane by means of the Fresnel transform. The contribution from each tile is added, resulting in a phase-only hologram of the scene. The optimized random phase tiles can be generated for any distance between the hologram plane and the object using an iterative Fresnel algorithm. Afterwards, this tile can be used to generate holograms from any number of objects without the need for further iterative algorithms. These holograms present increased quality after reconstruction compared to similar non-iterative hologram generation techniques. Both numerical and optical experiments are carried out, demonstrating the effectiveness of our proposal.

Iterative Phase-Only Hologram Generation Based on the Perceived Image Quality

Image quality metrics are a critical element in the iterative Fourier transform algorithms (IFTAs) for the computer generation of phase-only holograms. Conventional image quality metrics such as root-mean-squared error (RMSE) are sensitive to image content and unable to reflect the perceived image quality accurately. This would have a significant impact on the calculation speed and the quality of the generated hologram. In this work, the structure similarity (SSIM) was proposed as an image quality metric in IFTAs due to its ability to predict the perceived image quality in the presence of the white Gaussian noise and its independence on the image content. This would enable IFTAs to terminate when further iterations would no longer lead to improvement in the perceived image quality. As a result, up to 75% of unnecessary iterations were eliminated by the use of SSIM as the image quality metric.

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.

Comparing noniterative methods for phase-only Fresnel holograms, with speckle reduction, using a low-resolution spatial-light modulator in mostly phase mode

We analyze 10 cases of noniterative methods for speckle noise reduction in phase-only hologram generation. The analyzed methods are bidirectional error diffusion, random phase-free, double-phase, random displaced phase distribution (RDPD), and complex field encoding (CFE). The first three methods have been used before in the Fresnel hologram display, whereas the last two methods have been used in the area of holographic memory recording and generation of optical fields, respectively. We study the last two methods as alternatives to the generation of a Fresnel hologram without random phase (RP) and compare their results with the first three methods. We found that the RDPD also suffers a high degradation of low frequencies, so we introduce a modification for reducing the degradation effect. This method allows control of the spread angle of the object wave propagation. We also found that the CFE achieves a precise reconstruction of the object in different planes without the addition of any RP, with a very high signal-to-noise-ratio (SNR). We compare the results of 10 cases and discuss the advantages of each one. We include the peak signal-to-noise-ratio and SNR measurements of each case. We present simulated and experimental results.

Generation of phase-only holograms by laser ablation of nanoparticulate ITO layers

Phase holograms offer great potential e.g. for laser beam shaping and imaging applications. However, the generation of these structures typically requires multi-step processes which tend to be time-consuming and expensive, especially if high-quality structures are required. In this work we demonstrate a flexible and inexpensive method which allows for the production of binary phase-only holograms by laser ablation of ITO nanoparticle layers. Since these layers are ablated free of residues, accurately defined phase shifts can be achieved. While arbitrarily shaped structures can be generated, the ITO layer thickness can be adjusted in order to freely tune the phase shift for the desired wavelength. In the diffraction patterns generated by the holograms we observed an excellent zero order suppression.

Phase-only hologram generation based on integral imaging and its enhancement in depth resolution

We introduce a phase-only hologram generation method based on an integral imaging, and propose an enhancement method in representable depth interval. The computational integral imaging reconstruction method is modified based on optical °ow to obtain depth-slice images for the focused objects only. A phase-only hologram for multiple plane images is generated using the iterative Fresnel transform algorithm. In addition, a division method in hologram plane is proposed for enhancement in the representable minimum depth interval.