Full-color holographic display with increased-viewing-angle [Invited]

Solution to the issue of high-order diffraction images for cylindrical computer-generated holograms

The cylindrical computer-generated hologram (CCGH), featuring a 360° viewing zone, has garnered widespread attention. However, the issue of high-order diffraction images due to pixelated structure in CCGH has not been previously reported and solved. For a cylindrical model offering a 360° viewing zone in the horizontal direction, the high-order diffraction images always overlap with the reconstruction image, leading to quality degradation. Furthermore, the 4f system is commonly used to eliminate high-order diffraction images in planar CGH, but its implementation is predictably complex for a cylindrical model. In this paper, we propose a solution to the issue of high-order diffraction images for CCGH. We derive the cylindrical diffraction formula from the outer hologram surface to the inner object surface in the spectral domain, and based on this, we subsequently analyze the effects brought by the pixel structure and propose the high-order diffraction model. Based on the proposed high-order diffraction model, we use the gradient descent method to optimize CCGH accounting for all diffraction orders simultaneously. Furthermore, we discuss the issue of circular convolution due to the periodicity of the Fast Fourier Transform (FFT) in cylindrical diffraction. The correctness of the proposed high-order diffraction model and the effectiveness of the proposed optimization method are demonstrated by numerical simulation. To our knowledge, this is the first time that the issue of high-order diffraction images in CCGH has been proposed, and we believe our solution can offer valuable guidance to practitioners in the field.

Expansion of a vertical effective viewing zone for an optical 360° holographic display

Cylindrical holography, as a promising 360° display technology, has already attracted a lot of attention. In a previous study, an optical 360° cylindrical holography has been achieved in the visible spectrum using a planar spatial light modulator (SLM) and a 45° conical mirror. Although the 360° viewing zone is successfully achieved in the horizontal direction, in the previous study, the vertical viewing zone remains as narrow as the planar holography, and its expansion is not only necessary but also potential due to the waste of vertical viewing zone in application scenarios such as tabletop and ceiling. In this paper, we propose a method of expanding the vertical effective viewing zone for optical 360° holographic display by using a conical mirror with a base angle of less than 45°. The proposed method can expand the vertical effective viewing zone by shifting the wasted vertical viewing zone into an effective vertical viewing zone from the base to the top angle direction of the conical mirror, which is up to two times theoretically. The feasibility and effectiveness of the proposed method are demonstrated by optical experiments. We believe that it would be promising in the field of augmented reality.

Holographic near-eye display system with large viewing area based on liquid crystal axicon

In this paper, a liquid crystal axicon based holographic near-eye display system with large viewing area is proposed. The viewing area of the system is extended by implementing the liquid crystal axicon. The error diffusion algorithm is used to calculate the computer-generated hologram (CGH). When incident on the liquid crystal axicon placed at the back focal plane of Fourier lens, the reconstruction light modulated by the CGH is deflected into two directions resulting in a viewing area extension. Meanwhile, to illustrate the potential of the proposed system, two-dimensional viewing area extension is demonstrated. It combines the frequency spectrum shift with the proposed system and achieves a double expansion of the horizontal viewing area and three-times expansion of the vertical viewing area. Feasibility of the proposed system is verified by optical experiments. The proposed system has potential applications in holographic augmented reality (AR) display.

Color curved hologram calculation method based on angle multiplexing

In this paper, a method of color curved hologram calculation based on angle multiplexing is proposed. The relationship between the wavelength, center angle and sampling interval of the curved holograms is analyzed for the first time by analyzing the reconstruction process of the curved holograms with different wavelengths. Based on this relationship, the color curved holograms are calculated by compensating phase to the complex amplitude distribution of the planar holograms. To eliminate the chromatic aberration, the curved holograms of different objects with the same color channel are respectively used for angle multiplexing and phase compensation, and then the color composed curved hologram is generated. Different color objects without chromatic aberration can be reconstructed by bending the composed curved hologram into different central angles. The experimental results verify the feasibility of the proposed method. Besides, the application of the proposed method in augmented reality display is also shown.

Two-step acceleration calculation method to generate curved holograms using the intermediate plane in a three-dimensional holographic display

Nowadays, curved computer-generated holograms are widely applied to increase the field of view. However, heavy computational load restricts the development of curved computer-generated holograms. In this paper, we propose a two-step acceleration calculation method to generate curved holograms by using the intermediate plane, which is placed between the object and a curved computer-generated hologram. The first step is the calculation of the intermediate plane by an improved accurate highly compressed lookup-table method. In the second step, we execute diffraction calculation from the intermediate plane to obtain a curved computer-generated hologram. Numerical simulations and optical experiments are performed to demonstrate that the proposed method is an efficient method for reducing calculation time. Additionally, the increase of field of view using a curved hologram is also numerically demonstrated. It is expected that our method can be combined with a curved display screen to realize three-dimensional holographic displays in the future.

Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography

We propose a full-color see-through three-dimensional (3D) display method based on volume holography. This method is based on real object interference, avoiding the device limitation of spatial light modulator (SLM). The volume holography has a slim and compact structure, which realizes 3D display through one single layer of photopolymer. We analyzed the recording mechanism of volume holographic gratings, diffraction characteristics, and influencing factors of refractive index modulation through Kogelnik’s coupled-wave theory and the monomer diffusion model of photopolymer. We built a multiplexing full-color reflective volume holographic recording optical system and conducted simultaneous exposure experiment. Under the illumination of white light, full-color 3D image can be reconstructed. Experimental results show that the average diffraction efficiency is about 53%, and the grating fringe pitch is less than 0.3 μm. The reconstructed image of volume holography has high diffraction efficiency, high resolution, strong stereo perception, and large observing angle, which provides a technical reference for augmented reality.

Method of curved composite hologram generation with suppressed speckle noise

In this paper, a curved composite hologram (CCH) generation method with suppressed speckle noise is proposed. The recorded 3D object is considered as many layers with different depths and the planar hologram (PH) for each layer is generated accordingly. Then the PH is transformed to curved hologram (CH). The CH of the recorded 3D object can be generated by superposing the CHs for all the layers. Also, the linear phase factor is superposed to the CH of the object. For different objects, the bending angle and linear phase factor of the hologram are different. The CCH is generated by superimposing the CHs of different objects. Finally, the CCH is encoded by using the error diffusion method and optimized by superposing of the digital lens. When the CCH is reproduced from different angles, the reconstructed images with suppressed speckle noise can be displayed. The experimental results verify the feasibility of the proposed method.

Holographic display method with a large field of view based on a holographic functional screen

In this paper, we propose a method to increase the field of view (FOV) in a holographic display. Different from the traditional method, a large-sized computer-generated hologram (CGH) is generated, and a holographic function screen is used in the proposed method. The CGH is formed by superposition of interference fringes. The diffraction boundary angle of the interferogram is set to be equal to the maximum diffraction angle of the reconstructed light. In the holographic reconstruction, three spatial light modulators (SLMs) arranged side by side in a linear configuration are used to load the CGH. The holographic functional screen is used for eliminating the seams between the SLMs and further enlarging the diffraction light. With the proposed method, the reconstructed light after each image point is expanded, so that the FOV can be increased effectively. Experimental results prove the feasibility of the proposed method.

Holographic display system with adjustable viewing angle based on multi-focus optofluidic lens

In this paper, a holographic display system with adjustable viewing angle is proposed. The system consists of a collimated beam, a spatial light modulator (SLM), a multi-focus optofluidic (MFO) lens and an aperture. The MFO lens with high focal power is produced and it consists of two substrates, one multilayer substrate and two chambers. When the liquids are pulled in/out from the channels, the curvature of the liquid-liquid interface changes due to the surface tension and adsorption between the liquids and the multilayer substrate. The relationship between the parameters of the MFO lens and the holographic display viewing angle is revealed for the first time. Based on the theoretical analysis, the mechanisms of the high focal power and mechanical stability of the proposed MFO lens are also clarified. The experiments show that the focal power of the proposed MFO lens can be varied from -20 D (m^-1) to 4 D (m^-1), respectively. By using the MFO lens, the viewing angle of the holographic display system can be adjusted without moving any components mechanically. Meanwhile the setup of the system is greatly simplified. The experimental results verify the feasibility of the system, and it is expected to bring new ideas to the holographic display with large viewing angle.

Full color holographic display system based on intensity matching of reconstructed image

In this paper, a full color holographic display system based on the intensity matching of the reconstructed image is proposed. The system consists of three color collimated beams, three spatial light modulators (SLMs), three beam splitters, three lenses, three irises, a prism and a receiving screen. The three SLMs are used to load three color holograms, respectively. In order to eliminate the undesirable light in the reconstructed images and adjust the light intensities, the irises which contain two functions of both light intensity attenuation and aperture variation are cleverly produced. Finally, by using the prism, three color images can be coincident on the receiving screen. Experimental results verify the feasibility of the proposed system.

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.

Holographic Three-Dimensional Virtual Reality and Augmented Reality Display Based on 4K-Spatial Light Modulators

In this paper, we propose a holographic three-dimensional (3D) head-mounted display based on 4K-spatial light modulators (SLMs). This work is to overcome the limitation of stereoscopic 3D virtual reality and augmented reality head-mounted display. We build and compare two systems using 2K and 4K SLMs with pixel pitches 8.1 μm and 3.74 μm, respectively. One is a monocular system for each eye, and the other is a binocular system using two tiled SLMs for two eyes. The viewing angle of the holographic head-mounted 3D display is enlarged from 3.8 ∘ to 16.4 ∘ by SLM tiling, which demonstrates potential applications of true 3D displays in virtual reality and augmented reality.

Progress in virtual reality and augmented reality based on holographic display

The past, present, and future industry prospects of virtual reality (VR) and augmented reality (AR) are presented. The future of VR/AR technology based on holographic display is predicted by analogy with the VR/AR based on binocular vision display and light field display. The investigations on holographic display that can be used in VR/AR are reviewed. The breakthroughs of holographic display are promising in VR/AR with high resolution. The challenges faced by VR/AR based on holographic display are analyzed.

Pursuing High Quality Phase-Only Liquid Crystal on Silicon (LCoS) Devices

Fine pixel size and high-resolution liquid crystal on silicon (LCoS) backplanes have been developed by various companies and research groups since 1973. The development of LCoS is not only beneficial for full high definition displays but also to spatial light modulation. The high-quality and well-calibrated panels can project computer generated hologram (CGH) designs faithfully for phase-only holography, which can be widely utilized in 2D/3D holographic video projectors and components for optical telecommunications. As a result, we start by summarizing the current status of high-resolution panels, followed by addressing issues related to the driving frequency (i.e., liquid crystal response time and hardware interface). LCoS panel qualities were evaluated based on the following four characteristics: phase linearity control, phase precision, phase stability, and phase accuracy.

Binocular holographic three-dimensional display using a single spatial light modulator and a grating

In this paper, a binocular holographic three-dimensional (3D) display system combining a single spatial light modulator (SLM) and a grating is proposed and implemented. A synthetic phase-only hologram of the left and right 3D perspective images of an object is calculated by the layer-based Fresnel diffraction method according to the depth information, and uploaded onto the SLM for holographic 3D reconstruction with correct depth cues. The grating is designed and fabricated to guide the reconstructed left and right 3D perspective images to the corresponding eyes. Optical experiments demonstrate that the proposed system can successfully present binocular holographic 3D images with both the accommodation effect and binocular parallax, which enables observation free of the accommodation-vergence conflict and visual fatigue problem.

See-through display combined with holographic display and Maxwellian display using switchable holographic optical element based on liquid lens

We report a switchable holographic optical element based on a liquid lens for a see-through display. For the switchable holographic optical element, we recorded two optical components in the holographic film in two steps. A numerical simulation was also done to define the recording and reconstruction conditions. After the recording process, the entire system was changed from 4f optics to Maxwellian optics by changing wavefront of the reference wave using a liquid lens. The diffraction efficiency was 0.46 for a single element recording and around 0.14 for a double element recording. The holographic display and the Maxwellian display were successfully switched without any crosstalk. The field of view and eye box of the holographic display were 1° and 4.36 mm, respectively, and the field of view and the eye box of the Maxwellian display were 3.8° and 23.2 um, respectively. In the proposed system, spatial frequency filtering by the liquid lens and image shape distortion seriously affected the hologram image. However, we successfully verified the feasibility of our proposed switchable holographic optical element using a liquid lens.

Time multiplexing technique of holographic view and Maxwellian view using a liquid lens in the optical see-through head mounted display

We report a liquid lens based optical see-through head mounted display that can simultaneously display both a maxwellian view and a hologram. Holograms are reconstructed by an angular spectrum layer based synthesis method. A hologram and Maxwellian view are simultaneously displayed by focusing the liquid lens from 0 D to 20 D with 60 Hz. The hologram is reconstructed at a position 1.5 m from the eye, and it is confirmed that the Maxwellian view is clear, even if the focus of the eye changes from 50 cm to 1.7 m. In the proposed system, the liquid lens acts as a low-pass filter. Since the PSNR is about 23 dB in the currently used 10 mm diameter liquid lens, the image quality is not adequate. However, we successfully verify the feasibility of our proposed system. In addition, if a large diameter liquid lens of 30 mm or more is applied, excellent image quality of 30 dB or more can be realized.

Computer-generated hologram generation method to increase the field of view of the reconstructed image

In this paper, a computer-generated hologram (CGH) generation method is proposed to increase the field of view (FOV) in the holographic display. The CGH is generated through accumulating interference patterns of all object points. The size of each interference pattern is equal to the sum of the size of the recorded object and the spatial light modulator (SLM), so the size of the interference pattern is increased. The position of the interference pattern is related to that of the corresponding recorded object point. In the reconstruction process, three SLMs in a planar configuration are used to load the CGH. The seams between the SLMs are eliminated by beam splitters. Meanwhile, the boundaries of the diffraction light of all interference patterns are parallel with each other. Compared with the holographic display method using one SLM, the experiments demonstrate that the proposed method can reduce the demand for viewing distance and expand the FOV obviously in the horizontal direction, especially for the large-sized recorded object.

Multi-region phase calibration of liquid crystal SLM for holographic display

The liquid crystal spatial light modulator (SLM) is able to provide flexible wavefront control, whereas the initial phase and its response distortions will heavily influence the modulation accuracy. The currently existing calibration methods are tedious and time consuming. A novel multi-region calibration method for minimizing those distortions is proposed. The entire panel is divided into several local regions based on the similarity of phase response characteristic. The nonlinear phase response and static phase distortion of each local region are calibrated in the iterative division procedure. The calibration method is theoretically analyzed and experimentally verified. For the Jasper 4 K SLM panel, when five local regions are built, the root mean error of linear phase shifts is reduced to 0.1 rad and the compensation accuracy of the static phase distortion reaches 0.24 wavelength. The calibrated SLM is applied for the color holographic display and the results show that the reconstructed image quality is improved significantly. The proposed method is simpler and faster because of the reasonable regional division and lower calibration complexity. It could be used for the calibration of various phase only or complex modulators with high space bandwidth product in the future.

Digital Holography and 3D Imaging: introduction to the joint feature issue in Applied Optics and Journal of the Optical Society of America B

The OSA Topical Meeting on Digital Holography and 3D Imaging (DH) was held 25-28 July 2016 in Heidelberg, Germany, as part of the Imaging Congress. Feature issues based on the DH meeting series have been released by Applied Optics (AO) since 2007. This year, AO and the Journal of the Optical Society of America B (JOSA B) jointly decided to have one such feature issue in each journal. This feature issue includes 31 papers in AO and 11 in JOSA B, and covers a large range of topics, reflecting the rapidly expanding techniques and applications of digital holography and 3D imaging. The upcoming DH meeting (DH 2017) will be held from 29 May to 1 June in Jeju Island, South Korea.