Digital Holograms Research Articles

Experimental demonstration of single-shot chirped pulse digital holography

We experimentally demonstrated single-shot chirped pulse digital holography which can be used for measuring the sequence of ultrafast optical wavefronts in a relatively simple setup. In this method, multiple ultrafast object pulses and a chirped reference pulse are obtained from a single ultrafast pulse and these pulses are interfered to record a digital hologram and optical wavefronts at different times can be reconstructed separately from the recorded hologram. In experiment, two optical wavefronts obtained by a 35 fs pulse with the time separation of 3.7 ps were recorded and reconstructed successfully.

Reliability of the head tracking measured by Microsoft Hololens during different walking conditions

Augmented reality (AR) is a technology that expand the real environment by adding digital holograms into it. AR appears to be a promising field of development for serious games, especially for walk...

Absolute and pixel-wise measurements of vibration amplitudes using time-averaged digital holography

This paper presents scanning time-averaged digital holography, a method for measuring the amplitude distributions of harmonically oscillating objects. Each intensity image reconstructed from the time-averaged digital hologram has the form of a set of fringes that follow a zero-order Bessel function of the first kind. Modulating the phase of the light beam in the interferometer results in shifts in the bright zero-order fringes with changes in modulation depth. The developed method is based on the continuous shift in the zero-order fringe over the object surface. The magnitude of the vibration amplitude is independently evaluated for each pixel. Unlike other digital holographic vibrometry methods based on spatial phase unwrapping or fringe counting, the proposed technique can measure vibration amplitudes without the risk of fringe number confusion or 2π errors. Thus, the new method can be used to measure high-slope amplitude distributions and discontinuous/partially shaded objects. The method was experimentally verified by measuring a bending piezo actuator, and the uncertainty of the method was determined to be in the range of nanometers.

Resolution enhancement algorithm based on infrared digital holography imaging through flame

In recent years, the use of new technologies combining infrared thermal imaging and digital holographic imaging to observe the targets in the fire field has become a current research focus. In theory, flame and smoke have no effect on long-wavelength infrared digital holography, but in the real fire environment, large particles of dust from the combustion will interfere with the light path, seriously increasing the reconstruction noise of the hologram. This paper proposes a new image processing algorithm to suppress the noise of infrared digital holographic reconstruction. The algorithm uses a bilateral filter combined with the Laplacian pyramid algorithm to separate the details and energy layers of the holographic reconstructed image, filters the detail layer, and then superimposes the separated layers back into the reconstructed image by the inverse Laplacian pyramid algorithm. Therefore, the resolution of the reconstructed image is improved, and the simulation of the fire field environment proves that the algorithm has a significant effect on improving the resolution of the reconstructed image of the infrared digital hologram.

Orthographic projection images-based photon-counted integral Fourier holography.

Unlike coherent imaging techniques, light field imaging uses incoherent (white light) illumination to generate a digital hologram of three-dimensional (3D) objects in real time. Multiple projections (or elemental images) of a 3D object are captured using a microlens array attached to a digital camera. Orthographic projection images (OPIs) can be synthesized from the recorded elemental images. The synthesized intensity-based OPIs are then multiplied by the corresponding phase functions and combined to form a digital hologram (also known as an integral hologram) of a 3D object under illumination. In this study, we analyze the performance of a synthesized integral hologram under low light imaging (photon-counting) conditions. The feasibility of this technique is verified experimentally by capturing the elemental images and subsequently generating orthographic projection images and by varying photon counts to reconstruct the digital holograms.

Three-dimensional refractive index measurement of special optical fiber based on optical vortex phase-shifting digital holographic microscopy

We propose an approach for three-dimensional (3-D) refractive index (RI) measurement of special optical fiber by phase-shifting digital holographic microscopy based on optical vortex. The experimental system consists of a modified Mach–Zehnder interferometer and related lab-developed software. The phase shifts are obtained by loading different computer-generated spiral phase holograms on a liquid-crystal spatial light modulator with digital blazed grating, which is more accurate than the traditional phase-shift methods. The optical vortex generated in +1 diffraction order of the diffraction field is utilized as the reference beam. Eight digital holograms with the phase-shift increments of 0, π / 2, π, and 3π / 2 are recorded by a CCD before and after the tested optical fiber is placed in the experimental system. Then, a four-step phase-shifting algorithm and phase unwrapping algorithm are adopted to extract the phase distribution from the holograms. Based on a filtered backprojection algorithm, the 3-D RI distribution of the tested fiber is obtained at high accuracy. Experimental results demonstrate the feasibility of the proposed method for 3-D RI measurement of special optical fiber.

When topological derivatives met regularized Gauss-Newton iterations in holographic 3D imaging

We propose an automatic algorithm for 3D inverse electromagnetic scattering based on the combination of topological derivatives and regularized Gauss-Newton iterations. The algorithm is adapted to decoding digital holograms. A hologram is a two-dimensional light interference pattern that encodes information about three-dimensional shapes and their optical properties. The formation of the hologram is modeled using Maxwell theory for light scattering by particles. We then seek shapes optimizing error functionals which measure the deviation from the recorded holograms. Their topological derivatives provide initial guesses of the objects. Next, we correct these predictions by regularized Gauss-Newton techniques devised to solve the inverse holography problem. In contrast to standard Gauss-Newton methods, in our implementation the number of objects can be automatically updated during the iterative procedure by new topological derivative computations. We show that the combined use of topological derivative based optimization and iteratively regularized Gauss-Newton methods produces fast and accurate descriptions of the geometry of objects formed by multiple components with nanoscale resolution, even for a small number of detectors and non convex components aligned in the incidence direction.

Digital hologram watermarking by embedding Fresnel-diffracted watermark data

Since holograms are very costly content, security against holograms is very important. Hologram watermarking has been researched for a long time as a security solution for the digital hologram distributed on the network as the hologram is digitized. We deal with a digital watermarking for digital hologram contents as ownership protection tool. Since holograms have different optical characteristics from natural images, a watermarking technique using these characteristics should be needed. We therefore selected Fresnel diffraction for this property. Both of the host holograms and the watermarks are diffracted more than once for each. The results are refracted to concentrate the diffracted data into the localized regions of the transformed plane so that the region occupied one region among them. From this process, we change and select the region of the watermark. We experimented our scheme with various test images for various attacks on data-manipulating attack and geometric attack. It is apparently recognizable for eye inspection. From the results, we expect the proposed watermarking scheme to be an appropriate solution for protecting the ownership of holograms.

High-precision phase extraction of biological tissues based on lateral shear with zig-zag scanning in off-axis digital holography

We propose a method combining off-axis digital holography (DH) with one-dimensional (1D) lateral shear based on Zig-zag scanning to extract high-precision phase distribution of biological tissues. Firstly, object wavefront is reconstructed using the numerical reconstruction algorithm from digital hologram. Secondly, Zig-zag scanning is introduced to reorder the object wavefront from matrix to 1D vector, allowing the retrieved phase value to change smoothly without value saltation. Then, 1D lateral shear process is performed to acquire the shear vector, from which the phase difference vector is extracted by arctangent calculation. Given the small amount of shear, the phase difference vector is extracted without unwrapping process. Afterwards, the phase vector is obtained through numerical accumulation. Finally, the real phase distribution matrix is retrieved by inverse Zig-zag scanning. The proposed method has high precision in phase extraction of biological tissues. The simulated and experimental results demonstrate the feasibility and precision superiority of the proposed method.

Generation of aerosol-particle light-scattering patterns from digital holograms.

The similarity between the light-scattering pattern of a particle in the near-forward direction and diffraction from the particle's silhouette is investigated. Images of irregularly shaped free-flowing aerosol particles are obtained from digital hologram measurements, which are then binarized to yield a silhouette. Application of Huygens's principle to the silhouette generates an approximate scattering pattern, which when compared to the true measured pattern shows good agreement for particles much larger than the wavelength of light.

Method of correcting stitching errors in reconstructing a synthetic-aperture digital hologram with seams

ABSTRACTWe propose a correction method based on the shift theorem of the Fourier transform for the non-integer pixel stitching error in a synthetic-aperture digital hologram with seams. After being Fourier transformed, the sub-holograms stemming from the dislocated recording planes are multiplied by a phase factor and then superposed onto in-place sub-holograms. Finally, the entire hologram is inverse Fourier transformed for reconstruction. This method is applied to simulated particle detection. The results show that the correlation coefficient (CC) between the reconstructed image and the simulated target image is 1.5 times, by comparison with an image that does not involve the phase-factor correction, and particles at the seams are clearly visible. In addition, the effect of noise on the method is small. These verify the feasibility of the method.

Focus prediction in digital holographic microscopy using deep convolutional neural networks.

Deep artificial neural network learning is an emerging tool in image analysis. We demonstrate its potential in the field of digital holographic microscopy by addressing the challenging problem of determining the in-focus reconstruction depth of Madin-Darby canine kidney cell clusters encoded in digital holograms. A deep convolutional neural network learns the in-focus depths from half a million hologram amplitude images. The trained network correctly determines the in-focus depth of new holograms with high probability, without performing numerical propagation. This paper reports on extensions to preliminary work published earlier as one of the first applications of deep learning in the field of digital holographic microscopy.

Image steganography based on digital holography and saliency map

A gray-level intensity image can be employed as a host image for hiding a watermark image to protect information security. Past research works demonstrate that a gray-level hidden image can be embedded into the host image by a digital phase-only holography method. However, the fidelity of retrieved watermark image from the host image is not very satisfactory and the host image quality is degraded due to the insertion of external data bits, especially when observers focus on the saliency regions in the host image. To address this problem, we propose a steganography method based on digital holography and the saliency map of the host image. First, we calculate the hidden capacity (number of bits to be replaced) for each host image pixel based on the weighted sum of pixel intensity and saliency value. Next, a multilevel phase-only digital hologram of the watermark image will be calculated by the Gerchberg–Saxton method under the constraint of the hidden capacity of host image. Finally, we embedded the multilevel phase-only digital hologram into the host image by replacing a corresponding number of bits in each pixel. In this way, the host image can preserve good image fidelity for its saliency regions even if we hide a large amount of digital hologram data into the host image. The experimental results show that the quality of retrieved watermark image from the host image and the quality of saliency regions in the watermarked host image, in our proposed scheme, are superior to the state-of-the-art works reported.

Digital holography data compression

Digital holography processing is a research topic related to the development of novel visual immersive applications. The huge amount of information conveyed by a digital hologram and the different properties of digital holographic data with respect to conventional photographic data require a comprehension of the performance and limitations of current image and video standard techniques. This paper proposes an architecture for objective evaluation of the performances of state-the-art compression techniques applied to digital holographic data.

Localizing and Characterizing Colloidal Particles Scattering Using Lens-free Holographic Microscopy**Supported by the National Key R&D Program of China under Grant No 2016YFA0202102, the Fundamental Research Funds for the Central Universities under Grant No 14380078, and the Scientific Research Foundation of Graduate School of Nanjing University under Grant No 2017CL02.

Lens-free holographic microscopy could achieve both improved resolution and field of view (FOV), which has huge potential applications in biomedicine, fluid mechanics and soft matter physics. Unfortunately, due to the limited sensor pixel size, target objects could not be located to a satisfactory level. Recent studies have shown that electromagnetic scattering can be fitted to digital holograms to obtain the 3D positions of isolated colloidal spheres with nanometer precision and millisecond temporal resolution. Here, we describe a lens-free holographic imaging technique that fits multi-sphere superposition scattering to digital holograms to obtain in situ particle position and model parameters: size and refractive index of colloidal spheres. We show that the proposed method can be utilized to analyze the location and character of colloidal particles under large FOV with high density.

The effect of martensite volume fraction on topography of dual phase steels

Digital holographic microscopy in reflection mode provides 3D visualization of metallographic surfaces within an adjustable field of view, a lateral resolution close to diffraction limit, and a sub-wavelength axial accuracy. In this paper, we utilize reflective digital holographic microscopy (rDHM) to evaluate the mean height between ferrite and martensite phases in dual phase steel (DPS) samples. The data extracted from processing of several digital holograms show that the mean height difference between the phases is reduced for the samples of higher martensite volume fraction. rDHM has the potential to be used for the multi-phase samples that are difficult to be analyzed by conventional metallographic methods.

Topological charge measurement of vortex beams by phase-shifting digital hologram technology.

We propose a direct measurement method that is applicable to both integral and fractional vortex beams. In this approach, the phase distribution of the vortex beam is visualized via the phase-shifting digital holography technique. The least square method is initiatively employed to improve the measurement precision. The maximal error of the experimental results is below 4.8%.

Method for increasing the view field of THz holograms

In this paper we demonstrate the method of widefield amplitude-phase object images reconstruction from the spatially-limited broadband terahertz wavefield spectral components distributions in the Fresnel diffraction zone, based on the wave propagation equations iterative solution algorithm. Using of this method allows one to overpass the spatial resolution limit of an object image, reconstructed from the digital pulse terahertz hologram in low-frequency components of the THz radiation, used in the research. In its turn the created software allows to simulate the process of holograms registration in case of limited detector aperture and to reconstruct the full images of an object.

The usability of discrete representation of holograms

In this paper, we analyze the existing methods of digital representation of images and fields, based on calculating digital holograms of various objects. We studied discrete Fourier, cosine and Fresnel transform are compared, advantages and disadvantages of each. Cases of the use of each transformationare suggested. We estimated the quality of reconstructed images using objective metrics such as signal-to-noise ratio and root-mean-square error. The computational complexity of the considered discrete transformations is analyzed.

Comparative appraisal of global and local thresholding methods for binarisation of off-axis digital holograms

Binary digital and computer-generated holograms are used in a number of digital micromirror device (DMD) applications, including holographic displays, media characterisation, optical encryption, and others. Binarisation is one of the most simple and efficient methods of hologram compression. In this paper, 12 local and 18 global thresholding techniques of different groups were analysed and compared based on attribute similarity, clustering, entropy, histogram shape, and local adaptive factors. Optically recorded off-axis digital holograms of different objects were binarised using these methods. The amplitude quality of the obtained reconstructed images was compared using PSNR and SSIM values. The clustering-based methods achieved the highest quality. The results of binarisation by global and local methods were comparable on average.