Reliable phase-only spatial light modulators (SLMs) are in demand for accurate phase modulation. However, the nonlinear optical response of liquid crystals and the limited manufacturing process can lead to the spatial nonuniformity of the phase modulation of the SLM. The transfer from the grayscale to the modulated phase can be different from the lookup table (LUT) shown in the SLM manual. The SLM should be measured for calibration. We propose a calibration method based on digital holography to calibrate the spatial nonuniformity of phase modulation of the SLM. Using a self-generated grating, the SLM involved system is converted to the calibration system based on the principle of digital holography. The in-situ strategy for low cost and efficient calibration was demonstrated with optical experiments using a 4K (3840 × 2160 pixels) phase-only SLM. The spatial nonuniformity was calibrated to decrease by more than 75% using only a beam splitter and an imaging sensor.
The 5G network has the advantage of ultra-high transmission bandwidth, which will accelerate the developments of new three-dimensional (3D) communication systems. Computer-generated holographic display is an ideal solution for 3D communication because it could provide realistic 3D effect. Currently, only the amplitude of the wavefront, rather than the phase, is recorded by the traditional image capture system, which could not provide 3D information for the 3D computer-generated holographic display. Based on the light-field camera, a capture system employed in the computergenerated holographic display is developed. It could calculate position of every object point, and render the point-cloud model of the target 3D object. The layer-based angular-spectrum method is also employed in this capture system to calculate the computer-generated holograms (CGHs) of the 3D object. Simulations and experiments have verified that the proposed capture system is able to utilize the transmission bandwidth of 5G network. The proposed system realizes quasi real-time output of large size CGHs. The reconstruction quality of these CGHs is preferable. The proposed capture system for 3D holographic communication would have a broad application prospect through further optimizations and developments.
Phase-only holograms could be displayed with a single phase-only SLM free from the zero-order diffraction and the twin image. Addition of random phase to the object light in computer-generated holograms (CGHs) can widely diffuse the object light and to avoid its concentration on the CGH. But it causes speckle noise in the reconstructed image. Speckle reducing methods could be classified as iterative and non-iterative methods. Non-iterative methods are fast and effective, such as random displaced phase distribution (RDPD) and error diffusion (ED). The drawbacks are degradation of the reconstruction at different spatial frequencies.
Point spread function (PSF) works as the impulse response of optical reconstruction system. Its Fourier transform, optical transfer function (OTF), gives a set of coefficients for plane waves of various spatial frequencies and orientations. The evaluation of OTF could quickly determine which spatial frequency components are passed or attenuated for the CGH display. The non-iterative methods for speckle noise reduction on reconstructed images in spatial frequency domain is analyzed.
The relationship between the transmission distance and the number of Fresnel zone plate rings is evaluated. The influence of different transmission distance and the number of Fresnel zone-plate rings are analyzed. Their parameters of the best reconstructed image are obtained. A point light source is taken as an example, a phase-only spatial light modulator with the resolution of 3840×2160 pixels and the pixel interval of 3.74 μm is used for experimental verification. The numerical simulation and optical experiment results show that the optimal reconstructed image has a transmission distance of 200 mm. Meanwhile, the optimal Fresnel zone ring number is 8 under the same conditions. This study provides the optimal parameters for the spatial light modulators with different size.
Metasurface is used to manipulate the optical field recently. In holography, the complex amplitude computer generated hologram can improve the quality of the reconstructed image. However, the current devices limit the application of complex amplitude modulation. Several works have been done for complex amplitude modulation by metasurface. In this work, a novel metasurface structure has been proposed to realize complex amplitude modulation. This kind of metasurface can modulate arbitrary complex amplitude. Furthermore, it has a thinner thickness, making it easier to fabricate.
Holographic data storage is expected to realize capacity of terabytes as well as fast data-transfer rate of Gbits/sec on a disk format. This kind of performance will be a potential solution for next-generation storage of big data in diversities of applications such as cloud servers and ultra-high definition systems. In this work, we analyze and optimize the axial response of the holographic data storage, aiming at maximizing storage density while suppressing the inter-page crosstalk on the reconstructed data pages. The optical model of the three dimensional hologram recorded in the medium is presented. Based on the reconstructed data page model, inter-layer crosstalk is also analyzed by use of orthogonal reference patterns. The signal to noise ratio and bit error rate of the reconstructed data page are improved. Experiments are conducted to obtain the axial shift selectivity curves and verify the predicted storage density and capacity of a holographic optical disk.
Partially coherent light source has been used in holographic display due to less speckle noise and lower cost. Different from laser, it has a low temporal and spatial coherence. The reconstructed image would be blurred by the illumination properties such as size, wavelength bandwidth and divergence angle range of partially coherent light source. However, due to the limitation of the pupil diameter and the human eye’s sensitive wavelength, the blur of the reconstructed image cannot be recognized within a confined limit. The mathematical model of diffraction intensity distribution for holographic display is derived. The relationship between the illumination properties of partially coherent light source and the reconstruction results is simulated. The results suggest a criterion for the maximum size, wavelength bandwidth and divergence angle range.
Stereo depth is the most important factor for the 3D experience when viewing an autostereoscopic display. In this paper, we investigate the influence of viewing distance and viewing angle on stereo depth. First, we build the ideal stereo depth model based on the physiological limitation. Second, we establish a wave aberration model based on diffraction theory. The simulation and experimental results agree with the theoretical analyses. The model is of significant importance for giving a guidance on display system designing.
Gold nanorod has generated great research interests due to its tunable surface plasmon resonance (SPR). The mechanism of the SPR effect on the enhancements of optical performance for the volume holographic polymer is investigated. The resonance wavelength is dependent on the aspect ratio of the nanorod. Theoretical model for the localized surface plasmon resonance effect are developed and simulated for the interactions between the photopolymer components and nanorods in the gold nanorod doped volume holographic photopolymer. The experimental evaluation of the material suggests a novel candidate for potential applications in high-density optical data storage and high-resolution holographic display.