Digital Holograms Research Articles

Digital holographic microscopy for real-time investigation of 3D microstructural dynamics of starch-kefiran-based nanocomposite.

Investigating real-time phenomena in bio-polymers has received much attention because of their increasing demands in polymer substitution. The 3D morphometry of polymer surfaces may be very impactful in such studies. Here, digital holographic microscopy (DHM) is applied for quantitative measurement of the rare morphological changes of UV-A and UV-C exposed nanocomposites during their incubation with excess water. By reconstructing the recorded successive digital holograms, the time evolution of the swelled regions of the samples is derived. Our results clearly show that the higher water swelling of UV-A irradiated starch/kefiran/ZnO may be attributed to its higher hydrophilicity.

Magneto-Refractive Effect and Mechanism Analysis of Erbium-Ytterbium Co-Doped Silica Fiber

The magneto-refractive properties of Er/Yb co-doped silica fiber (EYDF) are important in magnetic field sensing. Here, we proposed a theoretical calculation model of the EYDF using density functional theory (DFT) and demonstrated it by an interferometric digital hologram magneto-refractive measurement system. The calculation results show that the fiber material, doped with Er/Yb atoms, has a large net spin and asymmetric spin distribution, which enables the elements to play an important role in the magnetic characteristics of the material. The experimental results show that the refractive index of the fiber decreases linearly with the increase of the magnetic field. The sensitivity of EYDF is 3.8279 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> RIU/Gs and is two orders of magnitude higher than that of single-mode fiber which is confirmed by the theoretical calculation. It demonstrated that the EYDF has advantages and potential of high sensitivity, miniaturization, and easy integration for magnetic field detection in harsh environments.

Improving the intensity-contrast image of a noisy digital hologram by convolution of Chebyshev type 2 and elliptic filters.

In this paper, a new, to the best of our knowledge, technique convolves the windowed Fourier filtering (WFF) of the Fresnel transform with the transfer functions of both Chebyshev type 2 and elliptic filters to enhance the intensity-contrast image of a noisy digital hologram. The recorded digital hologram is reconstructed by the Fresnel approach, the reconstructed intensity-contrast image is transformed by WFF, and the obtained spectrum is convolved in frequency domain with the transfer functions of Chebyshev type 2 and elliptic filters. The result of convolution is transformed by inverse WFF to produce a speckle-free image with a sharp roll-off and no ripples in both pass- and stop-bands. The experimental results with a die in the presence and absence of a rotating ground glass diffuser are shown and demonstrate that the resolution can be effectively enhanced with simple setup and procedure. The proposed technique can improve the capabilities of digital holography in three-dimensional (3D) microscopy.

Topographic Analysis of Calcite (104) Cleavage Surface Dissolution in Ethanol–Water Solutions

The interaction of organic molecules with calcite surfaces plays a key role in many geochemical, industrial and biomineralization processes, and exploring the influences of organic molecules on calcite reactions is crucial for a fundamental understanding of the reaction mechanisms. Here, we used digital hologram microscopy to explore the in situ evolution of the calcite (104) surfaces when dissolved in ethanol–water solutions, and total organic carbon analysis was applied to confirm the adsorption of ethanol by calcite. The results showed that the bulk dissolution rate of calcite decreases as the volume fraction of ethanol increases, and the topographic features of etch pits were also altered by the presence of ethanol. When exposed to too much ethanol, the etch pits’ growth was inhibited and their shapes tended to change from rhombuses in ultrapure water to triangles. Our results provide insights into the interaction between adsorbed ethanol and evolving calcite crystal, which highlights the dissolution regulation of calcite by organic molecules that could benefit a broad range of fields.

Single-Element Reflective Digital Holographic Microscopy

Digital holographic microscopy (DHM) is a well-known microscopy technique using an interferometric architecture for quantitative phase imaging (QPI) and it has been already implemented utilizing a large number of interferometers. Among them, single-element interferometers are of particular interest due to its simplicity, stability, and low cost. Here, we present an extremely simple common-path interferometric layout based on the use of a single one-dimensional diffraction grating for both illuminating the sample in reflection and generating the digital holograms. The technique, named single-element reflective digital holographic microscopy (SER-DHM), enables QPI and topography analysis of reflective/opaque objects using a single-shot operation principle. SER-DHM is experimentally validated involving different reflective samples.

Quantitative Phase Imaging Using Deep Learning-Based Holographic Microscope

Digital holographic microscopy enables the measurement of the quantitative light field information and the visualization of transparent specimens. It can be implemented for complex amplitude imaging and thus for the investigation of biological samples including tissues, dry mass, membrane fluctuation, etc. Currently, deep learning technologies are developing rapidly and have already been applied to various important tasks in the coherent imaging. In this paper, an optimized structural convolution neural network PhaseNet is proposed for the reconstruction of digital holograms, and a deep learning-based holographic microscope using above neural network is implemented for quantitative phase imaging. Living mouse osteoblastic cells are quantitatively measured to demonstrate the capability and applicability of the system.

Real-time heart rate monitoring system for cardiotoxicity assessment of Daphnia magna using high-speed digital holographic microscopy

Machine vision techniques for monitoring heart rates in aquatic bioassays have been applied to cardiotoxicity assessment. However, the requisite large data sizes and long calculation times make long-term observations of heart rates difficult. In this study, we developed a real-time heart rate monitoring system for individual Daphnia magna in a water chamber mounter that immobilizes their movement in 100mL media. Heart rates are calculated from real-time, time-resolved relative phase information from digital holograms acquired with a 200 fps camera and parallel computation using a graphics processing unit. With this system, we monitored the real-time changes in the heart rates of individual D. magna specimens exposed to H2O2 as a positive control for reactive oxygen species (ROS) levels in an aquatic environment for 10h, a period long enough to observe dynamic heart rate responses to the mounting process and exposure and to establish heart rate trends. An additional group analysis was conducted to compare to conventional cardiotoxicity assessment, with results of both assessments showing that the heart rates reduced according to ROS level by H2O2 exposure concentration. Notably, the results of our real-time dynamic heart rate monitoring in aquatic conditions indicated that establishing a relaxation heart rate before measurements could improve the accuracy of toxicity assessment. It is believed that the system developed in this study, achieving the simultaneous measurement, analysis, and display of reconstructed results, will find wide application in other aquatic bioassays.

Angular spectrum matching for digital holographic microscopy under extremely low light conditions.

Digital holographic microscopy (DHM) is a future three-dimensional (3D) microscopy due to its high-resolution and high-precision 3D images. Thus, it is getting attention in bioinformatics, semiconductor defect detection, etc. However, some limitations still exist. Especially, high-speed holographic imaging requires high-power lasers, which are difficult to image on highly absorbent or light-sensitive samples. To overcome these issues, we propose a new, to the best of our knowledge, digital hologram recovery algorithm called angular spectrum matching (ASM), which achieves hologram imitation to recover holograms in digital holography at low light intensities. The hologram used for the background phase comparison is recorded without objects; thus, no power limitation is required. The ASM utilizes this background hologram to recover dark holograms. We present experimental results showing improved DHM numerical reconstructions and recovered holograms under extremely low light conditions.

Digital holographic method to study stress formation in photopolymer during live recording of holograms

Stress formation during the recording and bleaching of photopolymer holograms results in distortion of the internal diffractive structure and causes many undesirable effects. This is especially important in the case of holographic solar concentrators and similar holographic optical elements that are supposed to meet stringent specifications. We report a new digital holographic method to measure and study stress formation in photopolymers during hologram recording. A glass substrate, with one of its sides diffusely reflecting, is effectively applied to facilitate the recording of good digital holograms captured using a CMOS camera. A green-sensitive photopolymer is used for the study. Digital holograms recorded in successive time intervals of exposure were overlaid and numerically reconstructed. High-contrast stress fringes were generated, and the preliminary results are presented.

Holographic tomography: techniques and biomedical applications [Invited

Holographic tomography (HT) is an advanced label-free optical microscopic imaging method used for biological studies. HT uses digital holographic microscopy to record the complex amplitudes of a biological sample as digital holograms and then numerically reconstruct the sample's refractive index (RI) distribution in three dimensions. The RI values are a key parameter for label-free bio-examination, which correlate with metabolic activities and spatiotemporal distribution of biophysical parameters of cells and their internal organelles, tissues, and small-scale biological objects. This article provides insight on this rapidly growing HT field of research and its applications in biology. We present a review summary of the HT principle and highlight recent technical advancement in HT and its applications.

Low-light phase imaging using in-line digital holography and the transport of intensity equation

In one of our recent studies, we have shown that a large defocusing distance in the transport of intensity equation (TIE) is required to retrieve the phase information in low-light conditions (OSA Continuum 3 (2020) 236). It has been found that image misalignment issues come into the picture due to translation of the camera for large distances, which creates artifacts in phase recovery. The issue of misalignment can be overcome if, instead of the object, we use the digital hologram of the object for the required defocused intensities and then apply the TIE. In this paper, we demonstrate low-light phase imaging by combining digital holography with the TIE. We reconstruct the required multiple intensity distributions from the captured in-line digital hologram. To implement the low-level light illumination, a variable neutral density filter has been used while recording the in-line digital hologram. Simulation and experimental results with different objects (United States air-force chart and glue drop) are presented. To simulate the low-light conditions, a Poisson distribution based photon-counting imaging technique has been applied.

Noncollinear degenerate phase modulation in samples with inhomogeneous optical nonlinear properties [Invited

Digital inline pump-probe holography can be applied to estimate parameters of samples' optical nonlinear properties. Here we propose a mathematical model to describe noncollinear degenerate phase modulation in samples with inhomogeneities of nonlinear refractive index over all three dimensions, namely, two-layered samples and samples with local impurities. The impact of sample parameters in the considered configurations is analyzed. We show that analysis of inline digital holograms obtained by time-resolved inline digital holography can be successfully used for rapid detection and characterization of various types of nonlinear refractive index inhomogeneities.

Analysis on viewing angle of holographic image reconstructed from a digital Fourier hologram in a holographic display

We analyze the viewing angle of a holographic image reconstructed from the digital Fourier hologram with an enhanced numerical aperture (NA). The viewing angle of the reconstructed image depends on the NA of the digital hologram that is determined by a focal length of Fourier lens and hologram aperture size, which is irrespective of a location of imaging plane. The enhanced-NA digital hologram reconstructs the image with an angle larger than a diffraction angle of a hologram pixel. We also characterize the aliasing effect for this type of digital Fourier hologram, and find that the alias-free region exists even at a high numerical aperture. Numerical simulation and optical experiments are conducted to verify this interpretation of the viewing angle of holographic images.

Generating arbitrary arrays of circular Airy Gaussian vortex beams with a single digital hologram

Circular Airy Gaussian vortex (CAGV) beams have gained great popularity in areas of research such as optical tweezers and optical communications due to their fascinating properties, such as auto-focusing and self-healing. The propagation dynamics of these beams is dictated by their topological charge and launch angle. For example, larger topological charges and positive launch angles enhance the maximum intensity in the focal plane while simultaneously shortening the focal length of autofocusing. Crucially, while the generation of single CAGV beams has been widely reported, the simultaneous generation of multiple CAGV beams, has remained challenging. Here, we put forward a novel technique that enables the simultaneous generation of multiple CAGV beams with independent topological charges or initial launch angles from a single digital hologram encoded on a spatial light modulator (SLM). Our technique enables the independent manipulation of each CAGV beam, their topological charge and launch angle, at refresh rates limited only by the SLM (60 Hz). This technique paves the way for the simultaneous manipulation of microparticles in three dimensions and provides with an alternative way to realize optical communications with multiple spatial modes of light.

Compression of complex-valued hologram data using line index coding and image compression standards

Human Visual System (HVS) records 3D information of a scene while photography of a scene can record only 2D information. A hologram can record the 3D information of a scene as that of HVS and reproduce the same. Hologram recording in digital form requires large amount of memory and opens a window to challenging area of Hologram data compression techniques. The hologram records the depth information of a scene in the phase of complex data. Complex data represented in both rectangular and polar form are exploited to produce Line Index Coding (LIC) process. Before Line Index Coding, normalization of hologram is done to produce normalized hologram. After normalization Line Index Coding corresponding to complex representation is done for selected step size. After Line Index Coding the JPEG or JPEG2000 compression scheme is used to provide more compression. Analysis of the results through subjective and visual quality methods show that the combination of line index coding and JPEG coding produce significant compression when compared to the result obtained through LIC alone. This paper gives a scope for Digital Hologram Watermarking Systems, speeding up of transmission of 3D scenes, storage requirements in surveillance, multimedia and biomedical applications.

О возможности исследования доли неоднородностей оптических нелинейных сред с помощью осевой цифровой голографии с временным разрешением

An approach to determining the fraction of local inhomogeneities nonlinear refractive index n2 in a transparent medium is considered. Data recording was performed using inline digital holography with femtosecond time resolution, where a set of probe beam inline digital holograms in the Fresnel diffraction zone to be recorded at various time delays between the pump and probe pulses. Digital holograms contain diffraction patterns that have information about the induced degenerate non-collinear phase modulation formed in the presence of nonlinear refractive index inhomogeneities. The developed algorithm is based on the calculation of the diffraction patterns Fourier spectrum and analysis of the intensity at zero spatial frequency. The proposed method has been validated in numerical modeling. An approach to determining the fraction of local inhomogeneities nonlinear refractive-index coefficient n2 in a transparent medium is considered. Data recording was performed using time-resolved inline digital holography, where a set of inline digital holograms in the Fresnel diffraction zone to be recorded in a probe beam at various time delays between the pump and probe pulses. Digital holograms contain diffraction patterns that have information about the induced degenerate noncollinear phase modulation formed in the presence of nonlinear refractive index inhomogeneities. The developed algorithm is based on the calculation of the diffraction patterns Fourier spectrum and analysis of the intensity at zero spatial frequency. The proposed method has been validated in numerical simulation.

Determination of the Diffusion Coefficient of Urea Solution Using Double Exposure Digital Holographic Interferometry (DEDHI) to Study Plant Growth-=SUP=-*-=/SUP=-

In this paper, we have used the double exposure digital holographic interferometry (DEDHI) technique to determine the diffusion coefficient of urea in pure distilled water at 27oC (room temperature) and it is used further to predict urea movement in soil by considering the concentration of urea solution and diffusion coefficient to study early plant growth. We have recorded several consecutive digital holograms of diffusing solutions of urea and pure distilled water at different times instantaneously on the CCD chip and processed them with H-Digital numerical reconstruction software on the computer. It is observed that as the concentration of urea solution increases from 0 to 4 M, diffusion coefficient 'D' of urea slightly increases from 12.2 to 13.8 × 10^-6 cm^2/s, which considerably affects the growth of the plants.

Watermarking for Digital Hologram by a Deep Neural Network and its Training Considering the Hologram Data Characteristics

Watermarking for Digital Hologram by a Deep Neural Network and its Training Considering the Hologram Data Characteristics

Probing the limits of orbital angular momentum generation and detection with spatial light modulators

Spatial light modulators (SLMs) are popular tools for generating structured light fields and have fostered numerous applications in optics and photonics. Here, we explore the limits of what fields these devices are capable of generating and detecting in the context of so-called vortex beams carrying orbital angular momentum (OAM). Our main contributions are to quantify (theoretically and experimentally) how the pixelation of the SLM screen affects the quality of the generated vortex mode and to offer useful heuristics on how to optimise the performance of the displayed digital hologram. In so doing, we successfully generate and detect a very high order optical vortex mode with topological charge , the highest achieved to date using SLMs. Since the OAM degree of freedom is frequently touted as offering a potentially unbounded state space, we hope that this work will inspire researchers to make more use of higher-order vortex modes.

Innovative Technology Applications in Science Education: Digital Holography

The use of technology in education gained importance in the 21st century and the use of innovative technologies in education-technology integration came to the agenda. This study examines students’ attitudes, thoughts, suggestions and opinions about digital hologram, which is one of the innovative technologies. The method used in the study was sequential explanatory approach, which is a mixed research method in which quantitative and qualitative methods are used together. The study was carried out with 418 students from six different public secondary schools affiliated to the Provincial Directorate of National Education in the Aegean Region. During the application process of the study, biodiversity, cells and divisions, DNA and genetic code, systems in our body topics, which are included in the science curriculum of the 5th, 6th, 7th and 8th grades were supported with digital holograms in a five-week period. Digital hologram attitude scale (DHAS) and digital hologram reflection form (DHRF) developed by researchers were used to collect data. Independent samples t-test and one-way analysis of variance (ANOVA) were used in the analysis of the data obtained from DHAS, whereas content analysis was used in the analysis of the data obtained from DHRF. As a result of the research, students' attitudes towards digital hologram were found to be positive. In addition, students' attitudes towards digital hologram did not differ significantly according to gender, grade, and school, but they differed according to science course academic achievement score in favor of students with a score between 85-100. Moreover, students evaluated the digital hologram as a useful tool that can contribute to academic learning and can be used in science class in the topics such as solar system and planets, space research, and living creatures. Based on these results, it is recommended to use the digital hologram at all grade levels and for the topics covered by science curriculum.