Search

• This paper proposes a holographic curved waveguide system for near-eye display. • An iterative algorithm is established based on angular spectrum theory to simulate the whole propagation process and analyze the behavior of light beam in the curved waveguide. • A phase compensation measures for overmodulation during beam propagation in the waveguide have been proposed. • Experiments have proven that the overmodulation can be diluted with the appropriate compensation phase over an effective distance. Generally, current type of waveguide used for near-eye displays is planar waveguide, which belongs to flat panel display. Compared to display system in three-dimensional space, flat panel display limits people's view of the 3-D world and is not conducive to the integration of virtual and reality. This paper proposes a holographic curved waveguide system for near-eye display. In order to analyze the propagation process of light beam in waveguide, an iterative algorithm is established based on angular spectrum theory to get the intensity distribution from each total internal reflection point. According to the simulation results and experimental phenomenon, we have developed phase compensation measures for overmodulation during beam propagation in the waveguide. The research shows that using curved waveguide can achieve beam transmission with less aberration after introducing an appropriate phase compensation in a distance, and it can also achieve beam transmission with lossless energy within a large radius range. Experiments have proven that the final coupling effect is basically consistent with the simulation results, this means that our proposal about curved waveguide has potential application value in near-eye display systems. [ABSTRACT FROM AUTHOR]

• Proposing a fast point-based hologram generation method using high-frequency information extraction. • Using the Gaussian filter to separate high-frequency information and low-frequency information of the 3D objects. • Improving the calculation speed by over 65%. • Combining different algorithms and leveraging their respective advantages. Computer-generated hologram based holographic 3D display is an important development direction of holographic 3D display. However, the heavy amount of calculation affects the generation speed of the holograms, which limits the further development of real-time holographic 3D display. In this paper, a fast point-based hologram generation method using high-frequency (HF) information extraction is proposed. Firstly, an image information processing method based on the Gaussian filter is proposed to separate the HF information and low-frequency (LF) information of 3D object. Then, the HF hologram is calculated by using the novel-look-up-table algorithm, while the LF hologram is calculated by the angular spectrum algorithm. Finally, the final hologram is obtained by overlaying HF hologram and LF hologram. Compared with the traditional method, the proposed method improves the generation speed of the hologram by more than 65%. Moreover, the details of 3D object can be fully reconstructed, and the quality of the reconstructed image is slightly improved with lower speckle noise. Experimental results verify the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

Microwave holography utilizes an antenna to record the hologram. In a phase shifting microwave holographic method, the antenna has to perform the 2D scanning over an area corresponding to each phase shift in reference wave. Since this methodology requires phase shifting in each scan, this makes the process lengthy and laborious. In this work, a numerical 4‐step phase shifting method is presented for generating the microwave hologram, in which a single (0° phase) hologram is recorded experimentally and the other three holograms are generated numerically by shifting the phase in reference wave. The amplitude and phase information of object is extracted using angular spectrum diffraction method. An error introduced by various components in experiments is calculated and corrected to enhance the accuracy of reconstructed phase values. The calculated phase step error for the experiment is 0.0501 rad. The quantitative and qualitative results along with the experimental phase shifting phase results are shown to envision the applications of numerical phase shifting microwave holography in imaging of metallic objects and highly scattered biological phantom. [ABSTRACT FROM AUTHOR]

Herein, we present a novel approach to the spec-radiation method (a method of acoustical holography) for determining the sound distribution on a tilted particleboard by calculating a single plane and rotation in the frequency domain. The tilted particleboard allows testing without standing waves between the transmitter and the particleboard. This eliminates the need to evaluate several parallel planes and to search for values belonging to the tilted particleboard. The numerical requirements can be optimally exploited through a combination with a flaw detectability enhancement method. The results are supported by experiments on a wooden particleboard with flaw imitations. Finally, we showed, through a comparison with the usual procedure of identifying a flaw (calculating many parallel planes and then selecting the data belonging to the tilted plane), that the calculation of the tilted plane is up to 98.5% faster and improves the detectability of flaws in a tilted particleboard. [ABSTRACT FROM AUTHOR]

For real-time evaluation of non-destructive air-coupled ultrasonic testing of wood-based materials, efficient and reliable calculation methods from ultrasonic holography are essential. Presented here is the spec-radiation method as a fast alternative to the re-radiation method. The spec-radiation method offers a more accurate and up to 88% faster evaluation than the re-radiation method for the determination of flaws in particleboards. Flaws of sub-wavelength sizes can be identified and their shape and location can be determined with this method. The spec-radiation method produces a better reproduction of the sound field than the re-radiation method, especially in the area of the measuring plane. [ABSTRACT FROM AUTHOR]

In this work, a compact indirect microwave holographic setup is proposed and developed by utilizing the low cost, specifically designed directive antennae as transmitter and receiver. Microwave holograms are recorded while scanning the 2D plane using (x‐y) motorized translation stage. The recorded holograms are then reconstructed numerically employing the angular spectrum diffraction phenomenon. The quality of the reconstructed amplitude image is further enhanced through the deep neural network, in order to combat with the low resolution of reconstructed images. The quantitative and qualitative results exploit the possibility of developing the miniaturized indirect microwave holographic system for near field applications. [ABSTRACT FROM AUTHOR]

• Envelope phase method is presented to generate parallel Bessel arrays with high SNR. • The calculation method of the incomplete axicon phase holograms is deduced. • Three different methods for generating Bessel arrays are proposed. • The number, length and axial intensity distribution of the Bessel arrays can be adjusted by the programmable SLM. Bessel beam arrays have important applications in ultra-laser processing, but most of its implementation methods have the disadvantages of complex process, high cost or poor effect. In this paper, a Bessel beam array generation method is proposed based on envelope phase holograms. Applying the programmable characteristics of spatial light modulator (SLM), the parallel Bessel beam array with adjustable spacing, quantity and intensity is simulated. The diffraction pattern of the Bessel array is discussed based on the angular spectrum diffraction theory. The phase expression of the envelope superposition morphology is deduced based on the analysis of the transmission function of the axicon. Three different methods are introduced and the quality of the arrays generated by these methods are compared. Simulation and experimental results show that the diffraction pattern of Bessel beam array generated by the envelope phase holograms can keep the Bessel beams as parallel with high signal noise ratio. The research of this paper will provide a theoretical basis for the development and application of the parallel Bessel beam array. [ABSTRACT FROM AUTHOR]

Abstract: Coupled wave equations that consider two-dimensional distribution and phases of gratings are derived by using weak grating and slow varying approximation. The result of angular selectivity on rand binary data page shows that each minimum is nonzero and each maximum that is not the middle maximum is a little bit smaller than that on plane wave volume hologram. Angular multiplexing is also modeled using the same method, and the middle page is read out in two different angular spacings. The relationship about PSNR versus angular spacing at different multiplexing pages is analyzed to reflect crosstalk between pages. All simulative results show this method is effective for volume holograms simulation. [Copyright &y& Elsevier]

We used digital interference holography (DIH) for in vitro imaging of human optic nerve head and retina. Samples of peripheral retina, macula, and optic nerve head from two formaldehyde-preserved human eyes were dissected and mounted onto slides. Holograms were captured by a monochrome CCD camera (Sony XC-ST50, with 780 × 640 pixels and pixel size of ∼9 µm). Light source was a solid-state pumped dye laser with tunable wavelength range of 560-605 nm. Using about 50 wavelengths in this band, holograms were obtained and numerically reconstructed using custom software based on NI LabView. Tomographic images were produced by superposition of holograms. Holograms of all tissue samples were obtained with a signal-to-noise ratio of approximately 50 dB. Optic nerve head characteristics (shape, diameter, cup depth, and cup width) were quantified with a few micron resolution (4.06-4.8 µm). Multiple layers were distinguishable in cross-sectional images of the macula. To our knowledge, this is the first report of DIH use to image human macular and optic nerve tissue. DIH has the potential to become a useful tool for researchers and clinicians in the diagnosis and treatment of many ocular diseases, including glaucoma and a variety of macular diseases. [ABSTRACT FROM AUTHOR]

Surface reconstruction for micro-samples with large discontinuities using digital holography is a challenge. To overcome this problem, multi-incidence digital holographic profilometry (MIDHP) has been proposed. MIDHP relies on the numerical generation of the longitudinal scanning function (LSF) for reconstructing the topography of the sample with large depth and high axial resolution. Nevertheless, the method is unable to reconstruct surfaces with large gradients due to the need of: (i) high precision focusing that manual adjustment cannot fulfill and (ii) preserving the functionality of the LSF that requires capturing and processing many digital holograms. In this work, we propose a novel MIDHP method to solve these limitations. First, an autofocusing algorithm based on the comparison of shapes obtained by the LSF and the thin tilted element approximation is proposed. It is proven that this autofocusing algorithm is capable to deliver in-focus plane localization with submicron resolution. Second, we propose that wavefield summation for the generation of the LSF is carried out in Fourier space. It is shown that this scheme enables a significant reduction of arithmetic operations and can minimize the number of Fourier transforms needed. Hence, a fast generation of the LSF is possible without compromising its accuracy. The functionality of MIDHP for measuring surfaces with large gradients is supported by numerical and experimental results. [ABSTRACT FROM AUTHOR]