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A method is proposed for spatially correcting an acoustic hologram recorded on an arbitrarily oriented planar area in front of an axially symmetric ultrasound transducer. The method comprises steps for determining the direction of the transducer's axis from the measured acoustic hologram, transferring the hologram data on a plane perpendicular to this axis and centering it relative to the axis by means of a correction algorithm that executes appropriate rotations and shifts of the original hologram. The corrected hologram is used for reconstructing the structure of vibrations of the emitting surface of the transducer. A simplified correction algorithm is also considered that is applicable for transducers with narrow angular spectrum and small angles of inclination of the initial hologram to the transducer axis. The accuracy of the developed algorithms is analyzed numerically for a single focusing transducer with a uniform vibrational velocity distribution on its surface and is demonstrated experimentally for an annular 12-element array. [ABSTRACT FROM AUTHOR]

Acoustic radiation forces can remotely manipulate particles. Forces from a standing wave field align microscale particles along the nodal or anti-nodal locations of the field to form three-dimensional (3D) patterns. These patterns can be used to form 3D microstructures for tissue engineering applications. However, standing wave generation requires more than one transducer or a reflector, which is challenging to implement in vivo. Here, a method is developed and validated to manipulate microspheres using a travelling wave from a single transducer. Diffraction theory and an iterative angular spectrum approach are employed to design phase holograms to shape the acoustic field. The field replicates a standing wave and aligns polyethylene microspheres in water, which are analogous to cells in vivo, at pressure nodes. Using Gor'kov potential to calculate the radiation forces on the microspheres, axial forces are minimized, and transverse forces are maximized to create stable particle patterns. Pressure fields from the phase holograms and resulting particle aggregation patterns match predictions with a feature similarity index > 0.92, where 1 is a perfect match. The resulting radiation forces are comparable to those produced from a standing wave, which suggests opportunities for in vivo implementation of cell patterning toward tissue engineering applications. [ABSTRACT FROM AUTHOR]

The focus steering capabilities of a 1 MHz linear phased array transducer (64 rectangular elements, 14.8 × 51.2 mm aperture) intended for drug delivery applications in abdominal organs were assessed and compared with its design-stage computer model. Acoustic fields generated by the transducer and predicted by the models of an ideal array with uniformly vibrating elements and either a plane or a cylindrically focused surface were simulated using the Rayleigh integral and angular spectrum methods. The boundary conditions for the transducer were reconstructed from acoustic holography measurements performed for selected focusing configurations of the array and also synthesized from holography data measured for each of its individual elements. It was shown that the transducer field with electronic focus steering can be accurately synthesized based on the holography data of its elements, which significantly simplified acoustic field characterization. Variability of the power and directivity patterns of the array elements were analyzed. A twofold smaller range of electronic steering in the transverse direction for the transducer compared to its computer model is discussed. [ABSTRACT FROM AUTHOR]

To eliminate the diffraction effect on Terahertz (THz) imaging and improve imaging system performance, digital holography has been investigated. The size of the common THz digital hologram and the recording distance are in the same order of magnitude, which does not satisfy the Fresnel approximation conditions. Meanwhile, diffraction has a great impact on free-space propagation behavior. So the research of the influence on reconstruction performance with different reconstruction algorithms is necessary. In this paper, the numerical simulations of the recording and reconstruction process of 2.52 THz off-axis digital holography imaging have been done, and the reconstruction performances of Fresnel angular spectrum algorithm, Rayleigh-Sommerfeld convolution algorithm and angular spectrum algorithm have been compared and analyzed as the reconstruction reference wave is different. The real experimental reconstruction results have also been presented. [ABSTRACT FROM AUTHOR]

Dynamic phase-only beam shaping with a liquid crystal spatial light modulator is a powerful technique for tailoring the intensity profile or wave front of a beam. While shaping and controlling the light field is a highly researched topic, dynamic nonlinear beam shaping has hardly been explored so far. One potential reason is that generating the second harmonic is a degenerate process as it mixes two fields at the same frequency. To overcome this problem, we propose the use of type II phase matching as a control mechanism to distinguish between the two fields. Our experiments demonstrate that distributions of arbitrary intensity can be shaped in the frequency-converted field at the same quality as for linear beam shaping and with conversion efficiencies similar to without beam shaping. We envision this method as a milestone toward beam shaping beyond the physical limits of liquid crystal displays by facilitating dynamic phase-only beam shaping in the ultraviolet spectral range. [ABSTRACT FROM AUTHOR]

A low-noise and fast optical encryption method for three-dimensional (3-D) information using the double-phase method is proposed. First, the 3-D information is encoded into a phase-only hologram (POH) by the angular-spectrum diffraction and the double-phase method. Second, the chaotic random phase mask (CRPM) is generated by the hybrid logical map and the iterative chaotic map with infinite collapses map to modulate the POH and obtain the ciphertext. As the secret key, the CRPM cannot only improve the secret key space of the scheme but also achieve the purpose of scrambling and hiding 3-D information. Third, the background noise of the decrypted image is successfully reduced by the cross-shaped filter, which is designed for the first time based on the POH spectrum distribution. The proposed scheme has successfully improved the encryption speed of the 3-D information and the quality of the reconstructed images. Numerical simulation and optical results show the effectiveness and feasibility of the proposed encryption scheme. [ABSTRACT FROM AUTHOR]

Holographic display offers the capability to generate high-quality images with a wide color gamut since it is laser-driven. However, many existing holographic display techniques fail to fully exploit this potential, primarily due to the system's imperfections. Such flaws often result in inaccurate color representation, and there is a lack of an efficient way to address this color accuracy issue. In this study, we develop a color-aware hologram optimization approach for color-accurate holographic displays. Our approach integrates both laser and camera into the hologram optimization loop, enabling dynamic optimization of the laser's output color and the acquisition of physically captured feedback. Moreover, we improve the efficiency of the color-aware optimization process for holographic video displays. We introduce a cascade optimization strategy, which leverages the redundant neighbor hologram information to accelerate the iterative process. We evaluate our method through both simulation and optical experiments, demonstrating the superiority in terms of image quality, color accuracy, and hologram optimization speed compared to previous algorithms. Our approach verifies a promising way to realize a high-fidelity image in the holographic display, which provides a new direction toward the practical holographic display. [ABSTRACT FROM AUTHOR]

Contemporary studies in polarization multiplexing are hindered by the intrinsic orthogonality constraints of polarization states, which restrict the scope of multiplexing channels and their practical applications. This research transcends these barriers by introducing an innovative nonorthogonal polarization-basis multiplexing approach. Utilizing spatially varied eigen-polarization states within metaatoms, we successfully reconstruct globally nonorthogonal channels that exhibit minimal crosstalk. This method not only facilitates the generation of free-vector holograms, achieving complete degrees-of-freedom in three nonorthogonal channels with ultra-low energy leakage, but it also significantly enhances the dimensions of the Jones matrix, expanding it to a groundbreaking 10 × 10 scale. The fusion of a controllable eigen-polarization engineering mechanism with a vectorial diffraction neural network culminates in the experimental creation of 55 intricate holographic patterns across these expanded channels. This advancement represents a profound shift in the field of polarization multiplexing, unlocking opportunities in advanced holography and quantum encryption, among other applications. Recent advances in metasurface have expanded holographic capabilities. Here, authors introduce a nonorthogonal polarization multiplexing strategy using spatially varied eigen-polarization states in metaatoms to achieve 55-channel holography, unlocking the holographic information capacity. [ABSTRACT FROM AUTHOR]

The article presents the results of experimental determination of the structure of surface vibrations of a two-dimensional phased ultrasound probe using transient acoustic holography in the frequency range from 0.1 to 10 MHz. It is shown that this method makes it possible to detect individual elements operating with parameter deviations from the nominal values, including inactive ones. A comparison is made with the results of echo pulse measurements for each element, as well as measurements of the electric capacitance of the elements. [ABSTRACT FROM AUTHOR]

Holographic displays can generate light fields by dynamically modulating the wavefront of a coherent beam of light using a spatial light modulator, promising rich virtual and augmented reality applications. However, the limited spatial resolution of existing dynamic spatial light modulators imposes a tight bound on the diffraction angle. As a result, modern holographic displays possess low étendue, which is the product of the display area and the maximum solid angle of diffracted light. The low étendue forces a sacrifice of either the field-of-view (FOV) or the display size. In this work, we lift this limitation by presenting neural étendue expanders. This new breed of optical elements, which is learned from a natural image dataset, enables higher diffraction angles for ultra-wide FOV while maintaining both a compact form factor and the fidelity of displayed contents to human viewers. With neural étendue expanders, we experimentally achieve 64 × étendue expansion of natural images in full color, expanding the FOV by an order of magnitude horizontally and vertically, with high-fidelity reconstruction quality (measured in PSNR) over 29 dB on retinal-resolution images. All holographic displays and imaging techniques are fundamentally limited by the étendue supported by existing spatial light modulators. Here, the authors report on using artificial intelligence (AI) to learn an étendue expanding element that effectively increases étendue by two orders of magnitude. [ABSTRACT FROM AUTHOR]

Imaging of phase objects behind scattering media is a challenging task. Intensity imaging through diffusers can be achieved based on digital holography by obtaining the complex amplitude of the diffuser in advance. As described in this paper, we experimentally demonstrate the reconstructed images of phase objects behind diffusers with different diffusion angles by digital holography. Using the complex amplitude information of the diffuser to correct the complex amplitude information of the object through the diffuser, the phase distribution of the object is obtainable behind the diffuser. Imaging of phase objects behind diffusers has been verified through experiments using a plano-convex lens and a wedge substrate as phase objects with various scattering angles. Quantitative analyses of the phase objects are performed. The lens shape can be visualized from the known refractive index. Moreover, the curvature radius can be estimated. [ABSTRACT FROM AUTHOR]

With the advantages of a large field of view, portability, and cost-effectiveness, lensless imaging has been applied widely nowadays. However, as a powerful tool for complete polarimetric characterization of microstructural and optical properties of a medium, Mueller matrix imaging has not yet been integrated in lensless imaging scheme. Here we propose a lensless inline polarization holographic system for high-speed and high-resolution Mueller matrix imaging. Liquid crystal variable retarders are introduced to realize high-speed response and avoid vibrations and positioning errors. We apply the blind deconvolution for depolarized imaging reconstruction and the back-propagation approach for polarization hologram reconstruction, respectively. The polarimetric imaging ability and resolution performance of the proposed technique are demonstrated. Furthermore, Mueller matrix images and certain quantitative polarimetric parameters of biological samples are calculated. The proposed method can be easily implemented and integrated in various lensless imaging techniques for on-chip polarimetric imaging. [ABSTRACT FROM AUTHOR]

In this study, we have proposed a digital holographic technique in which the frequency-modulated continuous-wave technique is introduced as a novel implementation of wavelength multiplexing in the time–frequency domain. In the proposed technique, the holograms are recorded with two wavelengths and the information of each hologram can be separated in the time–frequency domain by modulating the frequencies of two laser diodes at different modulation widths. Therefore, a temporal Fourier analysis is performed on each pixel of the time-series holograms whose intensity is modulated with two beat frequencies. And then, the holograms corresponding to the two wavelengths are extracted independently. Initially, a holographic system with two close wavelengths of 782.43 nm and 782.50 nm was designed to measure the surface profile of metallic gauge blocks with a known step-height of 1.16 mm in both experimental and numerical calculations. In addition, the measurement accuracy of the proposed system was investigated using both the experimental and the numerical results. Furthermore, the numerical calculation was conducted to investigate the origin of the periodic noise superimposed on the experimental results. Finally, the reduction method of the periodic noise was proposed, and the effect of the method was demonstrated using numerical calculations. [ABSTRACT FROM AUTHOR]

Methods of reconstructing holographic images of objects made of the composite based on carbon fibermodified acrylonitrile-butadiene styrene and a thin electrically conductive film are presented. Holograms are obtained in the axial scheme when the collimating lens is placed on the source axis. Results of comparison of the calculated frequency response of the model transmission coefficient with measurements on an experimental setup are presented. [ABSTRACT FROM AUTHOR]

An ideal holographic camera measures the amplitude and phase of the light field so that the focus can be numerically adjusted after the acquisition, and depth information about an imaged object can be deduced. The performance of holographic cameras based on reference-assisted holography is significantly limited owing to their vulnerability to vibration and complex optical configurations. Non-interferometric holographic cameras can resolve these issues. However, existing methods require constraints on an object or measurement of multiple-intensity images. In this paper, we present a holographic image sensor that reconstructs the complex amplitude of scattered light from a single-intensity image using reciprocal diffractive imaging. We experimentally demonstrate holographic imaging of three-dimensional diffusive objects and suggest its potential applications by imaging a variety of samples under both static and dynamic conditions. An ideal holographic camera acquires depth information about an object by measuring the light field. Here, the authors present a reference-free single-shot holographic image sensor that freely focuses the image of real-world objects via reciprocal diffractive imaging. [ABSTRACT FROM AUTHOR]

The effect of incoherence of the readout light on the level of crosstalk in wavelength-division multiplexing of reflection holograms is studied. The level of crosstalk is determined theoretically as a function of the spectral width of the readout light, the total number of multiplexed holograms, and the frequency difference between neighboring holograms. Good agreement is obtained between the theory and experiment for the case of two sinusoidal gratings. © 1999 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a holographic imaging scheme exploiting random illuminations for recording hologram and then applying numerical reconstruction and twin image removal. We use an in-line holographic geometry to record the hologram in terms of the second-order correlation and apply the numerical approach to reconstruct the recorded hologram. This strategy helps to reconstruct high-quality quantitative images in comparison to the conventional holography where the hologram is recorded in the intensity rather than the second-order intensity correlation. The twin image issue of the in-line holographic scheme is resolved by an unsupervised deep learning based method using an auto-encoder scheme. Proposed learning technique leverages the main characteristic of autoencoders to perform blind single-shot hologram reconstruction, and this does not require a dataset of samples with available ground truth for training and can reconstruct the hologram solely from the captured sample. Experimental results are presented for two objects, and a comparison of the reconstruction quality is given between the conventional inline holography and the one obtained with the proposed technique. [ABSTRACT FROM AUTHOR]

Digital holography provides access to quantitative measurement of the entire complex field, which is indispensable for the investigation of wave-matter interactions. The emerging iterative phase retrieval approach enables to solve the inverse imaging problem only from the given intensity measurements and physical constraints. However, enforcing imprecise constraints limits the reconstruction accuracy and convergence speed. Here, we propose an advanced iterative phase retrieval framework for single-shot in-line digital holography that incorporates adaptive constraints, which achieves optimized convergence behavior, high-fidelity and twin-image-free reconstruction. In conjunction with morphological operations which can extract the object structure while eliminating the irrelevant part such as artifacts and noise, adaptive constraints allow the support region to be accurately estimated and automatically updated at each iteration. Numerical reconstruction of complex-valued objects and the capability of noise immunity are investigated. The improved reconstruction performance of this approach is experimentally validated. Such flexible and versatile framework has promising applications in biomedicine, X-ray coherent diffractive imaging and wavefront sensing. [ABSTRACT FROM AUTHOR]

While recent research has shown that holographic displays can represent photorealistic 3D holograms in real time, the difficulty in acquiring high-quality real-world holograms has limited the realization of holographic streaming systems. Incoherent holographic cameras, which record holograms under daylight conditions, are suitable candidates for real-world acquisition, as they prevent the safety issues associated with the use of lasers; however, these cameras are hindered by severe noise due to the optical imperfections of such systems. In this work, we develop a deep learning-based incoherent holographic camera system that can deliver visually enhanced holograms in real time. A neural network filters the noise in the captured holograms, maintaining a complex-valued hologram format throughout the whole process. Enabled by the computational efficiency of the proposed filtering strategy, we demonstrate a holographic streaming system integrating a holographic camera and holographic display, with the aim of developing the ultimate holographic ecosystem of the future. The authors develop a deep learning-based incoherent holographic camera system in order to deliver visually enhanced holograms in real-time. The neural network filters the noise in the captured holograms, and by integrating a holographic camera and a display, they demonstrate a holographic streaming system. [ABSTRACT FROM AUTHOR]

This study evaluates a novel holographic data storage (HDS) that uses a phase-only spatial light modulator (SLM) for the multilevel complex amplitude modulation of a signal beam and the transport of intensity equation (TIE) method to detect the signal beam without interferometry, to increase the capacity of the HDS, simplifying its optical system, and improving the stability of the signal beam modulation and detection. Both the amplitude and phase of the signal beam were modulated by a computer-generated hologram displayed in a phase-only SLM, a 4-f optical system, and a pinhole placed in the Fourier plane. The complex amplitude-modulated signal beam generated by this scheme does not always perfectly match the target complex amplitude, and deviations from the amplitude and phase of the target complex amplitude may exist. It is unclear whether the TIE method, which is sensitive to the state of the beam intensity and the phase distributions to be detected (such as zero-intensity points and phase discontinuities), can accurately detect a signal beam whose complex amplitude is modulated by the modulation scheme with a phase-only SLM. Here, we demonstrate via numerical simulations and experiments that several methods of complex amplitude generation using a phase-only SLM can achieve multilevel modulation of the amplitude and phase of a signal beam and are suitable for detection by the TIE method in HDS. [ABSTRACT FROM AUTHOR]

Experimental results on frequency conversion of images by dynamic holograms and diagnostics of semiconductor materials and photorefractive and activated crystals based on the transient gratings method are presented. The possibility of visualization of infrared 3D images in real time is shown. Spectral regularities of short- (hundreds of microseconds) and long-lived (seconds) lattice recordings in photorefractive bismuth silicate crystals are established. The diffusion coefficient of excitation energy and the lifetime of excited states are measured. Techniques for compensation of induced anisotropy in active laser media and measurement of the thermal diffusivity coefficient of thin films and bulk thermoelectrics based on lead telluride are proposed. [ABSTRACT FROM AUTHOR]

We propose a new single-shot digital holography that measures the complex amplitude high precisely using a single image sensor. In this technology, a signal beam passes through a diamond-shaped low-pass filter, and a reference beam passes through a chessboard pattern phase-shift plate, where the phase difference between adjacent pixels is π/2. The signal and reference beams interfere, and the interference fringe is captured by a single image sensor. In a computer, an image is acquired by extracting the phase-dependent component from the interference fringe. The real and imaginary parts were obtained separately from the inside and outside of the diamond shape in the Fourier plane of the acquired image, respectively. Subsequently, the real and imaginary parts were combined to reconstruct the complex amplitude. This method can obtain the signal light in the diamond-shaped band of the Fourier plane, where the information of the signal light is concentrated generally, so this method can measure high precisely. Numerical analysis showed that the signal-to-noise ratio of the intensity and phase of this method were more than 4.8 and 2.8 dB higher, respectively, than two-step parallel phase-shift digital holography, and more than 7.3 and 4.8 dB higher, respectively, than Fourier fringe analysis, an off-axis digital holography technique. Moreover, the range of the reference intensity that maintains high-precision measurements was clarified, and the reconstruction accuracy against noise in the interference fringe or reference beam intensity was investigated. [ABSTRACT FROM AUTHOR]

Off-axis digital holography in the terahertz waves allows three-dimensional visualizations of optically opaque volumetric objects in a single exposure and with angular separation of the zero-order component. Forming the tilted reference beam usually requires a set of mirrors, which can disturb the polarization, causing the loss of contrast in fringes at the detector. Here we present a mirror-less approach with 3D-printed transmissive diffractive optical elements, providing unaltered polarization and successful holographic measurements at 0.1 THz without any pre- or postprocessing of interference fringes. [ABSTRACT FROM AUTHOR]

Phase imaging microscopy under Gabor regime has been recently reported as an extremely simple, low cost and compact way to update a standard bright-field microscope with coherent sensing capabilities. By inserting coherent illumination in the microscope embodiment and producing a small defocus distance of the sample at the input plane, the digital sensor records an in-line Gabor hologram of the target sample, which is then numerically post-processed to finally achieve the sample's quantitative phase information. However, the retrieved phase distribution is affected by the two well-known drawbacks when dealing with Gabor's regime, that is, coherent noise and twin image disturbances. Here, we present a single-shot technique based on wavelength multiplexing for mitigating these two effects. A multi-illumination laser source (including 3 diode lasers) illuminates the sample and a color digital sensor (conventional RGB color camera) is used to record the wavelength-multiplexed Gabor hologram in a single exposure. The technique is completed by presenting a novel algorithm based on a modified Gerchberg–Saxton kernel to finally retrieve an enhanced quantitative phase image of the sample, enhanced in terms of coherent noise removal and twin image minimization. Experimental validations are performed in a regular Olympus BX-60 upright microscope using a 20X 0.46NA objective lens and considering static (resolution test targets) and dynamic (living spermatozoa) phase samples. [ABSTRACT FROM AUTHOR]

By leveraging frequency-domain diagonal extension (FDDE) imaging in terahertz (THz) in-line digital holography, the resolution of THz digital holography would be dramatically improved. The holography system in this work which utilizes a CO2-pumped 2.52 THz (118.83 μm) continuous-wave laser as the source and a pyroelectric-array camera with a pixel pitch of 100 μm as the detector could attain a resolution of 150 μm (~1.26 λ) near to the resolution limit of the system. FDDE imaging is a simple and effective method for resolution enhancement, where no new detector or other equipment is employed. Samples are imaged twice from different angles and then the high-frequency components of images are stitched to improve the resolution. Different from the traditional FDDE, weight-FDDE (w-FDDE) which can reduce the visibility distinction between different orientations of the synthetic picture by adding weight factors to produce a superior synthetic image is proposed in this work. [ABSTRACT FROM AUTHOR]

We perform shock tube experiments with high-speed digital holography to investigate Richtmyer–Meshkov instabilities on an air-sulfur hexafluoride gaseous interface and understand the formation of windward surface waves in catastrophic hydrometeor aerobreakup. The catastrophic aerobreakup phenomenon is described by an extremely abrupt disintegration of the drop mass upon impulsive acceleration, the existence of which is still a matter of debate in the scientific community as little relevant evidence is available for support and the shock strengths needed are typically very high (Mach numbers ≥ 6), posing safety concerns, experimental complexities and higher operation costs. A gaseous interface created with a bubble film herein requires a much lower shock strength (Mach 1.2) to form similar windward instabilities providing an opportunity to obtain high resolution data in much simpler and safer laboratory tests. In addition, the use of digital holography allows the captured wavefront to be reconstructed at different depths and determine the three-dimensional location and shape of the bubble interface with greater precision than otherwise afforded by two-dimensional imaging. Our results reveal an exponential growth for RM instabilities on the gaseous bubble interface immediately after shock passage that shows excellent agreement with the linearized Rayliegh–Taylor theory and closely resembles the windward unstable waves observed by past studies in the catastrophic hydrometeor aerobreakup. The growth rates can be varied by modulating the pre-shock interfacial perturbation and provide a possible pathway in future to control catastrophic aerobreakup times that can be of importance to many hypersonic flight related problems due to adverse weather conditions. [ABSTRACT FROM AUTHOR]

Polarization information is useful for various applications such as remote sensing and biomedical imaging. In general, polarization information is obtained using a polarization camera; however, the measured data are two-dimensional (2D) images, which limits its application. In this paper, three-dimensional (3D) polarization imaging by use of optical scanning holography (OSH) is proposed. Polarization-dependent digital holograms are recorded by changing directly the transmission axis of a polarizer in front of a photo detector. In the proof-of-principle experiment, partially Stokes parameters are calculated from the obtained holograms and are compared with the results obtained by a polarization camera. Experimental results show that the proposed method can measure a polarization object although there is the object behind a scattering medium, which opens the door of alternative imaging fields. [ABSTRACT FROM AUTHOR]

Despite recent significant progress in real-time, large-area computer-generated holography, its memory requirements and computational loads will be hard to tackle for several decades to come with the current paradigm based on a priori calculations and bit-plane writing to a spatial light modulator. Here we experimentally demonstrate a holistic approach to serial computation and repeatable writing of computer-generated dynamic holograms without Fourier transform, using minimal amounts of computer memory. We use the ultrafast opto-magnetic recording of holographic patterns in a ferrimagnetic film with femtosecond laser pulses, driven by the on-the-fly hardware computation of a single holographic point. The intensity-threshold nature of the magnetic medium allows sub-diffraction-limited, point-by-point toggling of arbitrarily localized magnetic spots on the sample, according to the proposed circular detour-phase encoding, providing complex modulation and symmetrical suppression of upper diffractive orders and conjugated terms in holographically reconstructed 3-D images. Holography recreates both the amplitude and wave front of a three dimensional object, meaning that the observer perceives the image in the nearly same way as they would the true object. Creating such holographic images is challenging computationally, and requires extremely fast display update. Here, the authors combine a fast memoryless computation algorithm with the ultra-rapid writing based on all-optical switching of a ferrimagnetic film. [ABSTRACT FROM AUTHOR]

This roadmap article focuses on spatially incoherent digital holography (IDH). Representative IDH methods such as optical scanning holography (OSH), Fresnel incoherent correlation holography (FINCH), coded aperture correlation holography (COACH), IDH with a Fresnel zone aperture, and IDH with an interferometer along with a state-of-the-art optical device are introduced as modern IDH methods. We describe these IDH techniques with applications of three-dimensional (3D) imagers, 3D thermography, and 3D microscopy. [ABSTRACT FROM AUTHOR]

Fiber-based micro-endoscopes are a critically important tool for minimally-invasive deep-tissue imaging. However, current micro-endoscopes cannot perform three-dimensional imaging through dynamically-bent fibers without the use of bulky optical elements such as lenses and scanners at the distal end, increasing the footprint and tissue-damage. Great efforts have been invested in developing approaches that avoid distal bulky optical elements. However, the fundamental barrier of dynamic optical wavefront-distortions in propagation through flexible fibers limits current approaches to nearly-static or non-flexible fibers. Here, we present an approach that allows holographic, bend-insensitive, coherence-gated, micro-endoscopic imaging using commercially available multi-core fibers (MCFs). We achieve this by adding a partially-reflecting mirror to the distal fiber-tip, allowing to perform low-coherence full-field phase-shifting holography. We demonstrate widefield diffraction-limited reflection imaging of amplitude and phase targets through dynamically bent fibers at video-rate. Our approach holds potential for label-free investigations of dynamic samples. Fiber-based micro-endoscopes are minimally invasive, but 3D imaging is limited by the need for bulky optical elements or rigid fibers. Here, the authors demonstrate holographic imaging through flexible and dynamically bent multi-core fibers by adding partially reflecting mirror to the distal fiber-tip. [ABSTRACT FROM AUTHOR]

Holography is one of the most prominent approaches to realize true-to-life reconstructions of objects. However, owing to the limited resolution of spatial light modulators compared to static holograms, reconstructed objects exhibit various coherent properties, such as content-dependent defocus blur and interference-induced noise. The coherent properties severely distort depth perception, the core of holographic displays to realize 3D scenes beyond 2D displays. Here, we propose a hologram that imitates defocus blur of incoherent light by engineering diffracted pattern of coherent light with adopting multi-plane holography, thereby offering real world-like defocus blur and photorealistic reconstruction. The proposed hologram is synthesized by optimizing a wave field to reconstruct numerous varifocal images after propagating the corresponding focal distances where the varifocal images are rendered using a physically-based renderer. Moreover, to reduce the computational costs associated with rendering and optimizing, we also demonstrate a network-based synthetic method that requires only an RGB-D image. Improving the image depth perception of holograms while maintaining high image quality is a current challenge. Here the authors propose an efficient solution relying on a multi-plane hologram technique that reconstruct different blurred images and sharply focused images depending on a propagation distance. [ABSTRACT FROM AUTHOR]

Background: The state-of-the-art holographic displacement measurements can only measure one dimensional displacement with one digital holography (DH) setup. To measure 3D displacements of curved diffuse objects, both 3D shape and displacement measurements are needed which requires additional optical systems and results in complex systems. Objective: In this work, we propose a digital image correlation (DIC)-assisted DH method for 3D shape measurement and 3D displacement measurement of curved diffuse objects with one DH setup. Methods: The proposed approach is based on the combination of two popular optical techniques, DH and DIC. Specfically, 3D shape is measured by tilting the illumination angle of the object wave of DH. Then the in-plane displacement component is measured with the assistance of 2D-DIC from the intensity images of DH which are laser speckle patterns in essence. At last, the out-of-plane displacement component is determined by the displacement along sensitivity vector which is measured from the phase images of DH and a displacement compensation quantity due to 3D shape and the measured in-plane displacements. Results: Hence, with the assistance of 3D shape and 2D-DIC method, 3D displacements of curved diffuse objects are measured. The metrological performance of the proposed scheme is verified by real experiments. Conclusions: The results demonstrate that the proposed DIC-assisted DH method for 3D shape measurement and 3D displacement measurement of curved diffuse objects with high accuracy. [ABSTRACT FROM AUTHOR]

Ultrathin lensless fibre endoscopes offer minimally invasive investigation, but they mostly operate as a rigid type due to the need for prior calibration of a fibre probe. Furthermore, most implementations work in fluorescence mode rather than label-free imaging mode, making them unsuitable for general medical diagnosis. Herein, we report a fully flexible ultrathin fibre endoscope taking 3D holographic images of unstained tissues with 0.85-μm spatial resolution. Using a bare fibre bundle as thin as 200-μm diameter, we design a lensless Fourier holographic imaging configuration to selectively detect weak reflections from biological tissues, a critical step for label-free endoscopic reflectance imaging. A unique algorithm is developed for calibration-free holographic image reconstruction, allowing us to image through a narrow and curved passage regardless of fibre bending. We demonstrate endoscopic reflectance imaging of unstained rat intestine tissues that are completely invisible to conventional endoscopes. The proposed endoscope will expedite a more accurate and earlier diagnosis than before with minimal complications. Lensless fibre endoscopes are minimally invasive, but are often rigid and require calibration and fluorescence labelling. Here, the authors present a flexible endoscope based on a bare fibre bundle and a lensless Fourier holographic imaging configuration to detect weak reflections from unstained biological tissues. [ABSTRACT FROM AUTHOR]

The twin image problem, well known in Gabor holography, greatly obstructs the output of high-quality holograms. Previous solutions include numerical and instrumental means to eliminate or mitigate the issue. The proposed method demonstrates the feasibility of using the sagitta angle in a spherical reference beam as an intrinsic reference angle within in-line holography. Along with a miniaturized version of the object, to allow for a wide range of object sizes, from millimeters to meters, with a small variation within the original optical system. The inherent reference angle allows for a separation of the twin images in the reconstruction of a Gabor hologram, while maintaining the system on axis. Under lens-less Fourier criteria, the peripheral information of the visual field interferes with a spherical wave to generate an interference pattern that results in a hologram with separated images. [ABSTRACT FROM AUTHOR]

Optical knots and links have attracted great attention because of their exotic topological characteristics. Recent investigations have shown that the information encoding based on optical knots could possess robust features against external perturbations. However, as a superior coding scheme, it is also necessary to achieve a high capacity, which is hard to be fulfilled by existing knot-carriers owing to the limit number of associated topological invariants. Thus, how to realize the knot-based information coding with a high capacity is a key problem to be solved. Here, we create a type of nested vortex knot, and show that it can be used to fulfill the robust information coding with a high capacity assisted by a large number of intrinsic topological invariants. In experiments, we design and fabricate metasurface holograms to generate light fields sustaining different kinds of nested vortex links. Furthermore, we verify the feasibility of the high-capacity coding scheme based on those topological optical knots. Our work opens another way to realize the robust and high-capacity optical coding, which may have useful impacts on the field of information transfer and storage. Robust and high capacity optical coding will be at the base of future developments of information transfer and storage. Here the authors develop a topological all-optical coding scheme, which possesses good stability and a high capacity, using nested vortex knots and links. [ABSTRACT FROM AUTHOR]

Holographic display is considered as the most promising three-dimensional (3D) display due to its unique feature of reconstructing arbitrary wavefronts. However, the limited étendue, which hinders the immersive experience of observers, remains a major unresolved issue in holographic display technique. In this paper, we propose a novel approach to tweak the constraints of étendue by expanding the energy envelope in holographic display via mutually coherent multi-illumination. The proposed concept contains both a light source design for generating a mutually coherent multi-directional wave and a computer-generated hologram optimization framework for providing high-resolution 3D holograms. To verify the proposed approach, a benchtop prototype of a holographic near-eye display providing an intrinsic large exit-pupil is implemented. The experimental results clearly show that the exit-pupil is effectively expanded by four times and an appropriate viewpoint image is reconstructed according to the view position. [ABSTRACT FROM AUTHOR]

The coherence holography offers an unconventional way to reconstruct the hologram where an incoherent light illumination is used for reconstruction purposes, and object encoded into the hologram is reconstructed as the distribution of the complex coherence function. Measurement of the coherence function usually requires an interferometric setup and array detectors. This paper presents an entirely new idea of reconstruction of the complex coherence function in the coherence holography without an interferometric setup. This is realized by structured pattern projections on the incoherent source structure and implementing measurement of the cross-covariance of the intensities by a single-pixel detector. This technique, named structured transmittance illumination coherence holography (STICH), helps to reconstruct the complex coherence from the intensity measurement in a single-pixel detector without an interferometric setup and also keeps advantages of the intensity correlations. A simple experimental setup is presented as a first step to realize the technique, and results based on the computer modeling of the experimental setup are presented to show validation of the idea. [ABSTRACT FROM AUTHOR]

Holography is a promising approach to implement the three-dimensional (3D) projection beyond the present two-dimensional technology. True 3D holography requires abilities of arbitrary 3D volume projection with high-axial resolution and independent control of all 3D voxels. However, it has been challenging to implement the true 3D holography with high-reconstruction quality due to the speckle. Here, we propose the practical solution to realize speckle-free, high-contrast, true 3D holography by combining random-phase, temporal multiplexing, binary holography, and binary optimization. We adopt the random phase for the true 3D implementation to achieve the maximum axial resolution with fully independent control of the 3D voxels. We develop the high-performance binary hologram optimization framework to minimize the binary quantization noise, which provides accurate and high-contrast reconstructions for 2D as well as 3D cases. Utilizing the fast operation of binary modulation, the full-color high-framerate holographic video projection is realized while the speckle noise of random phase is overcome by temporal multiplexing. Our high-quality true 3D holography is experimentally verified by projecting multiple arbitrary dense images simultaneously. The proposed method can be adopted in various applications of holography, where we show additional demonstration that realistic true 3D hologram in VR and AR near-eye displays. The realization will open a new path towards the next generation of holography. [ABSTRACT FROM AUTHOR]

Accurate depiction of waves in temporal and spatial is essential to the investigation of interactions between physical objects and waves. Digital holography (DH) can perform quantitative analysis of wave–matter interactions. Full detector-bandwidth reconstruction can be realized based on in-line DH. But the overlapping of twin images strongly prevents quantitative analysis. For off-axis DH, the object wave and the detector bandwidth need to satisfy certain conditions to perform reconstruction accurately. Here, we present a reliable approach involving a coupled configuration for combining two in-line holograms and one off-axis hologram, using a rapidly converging iterative procedure based on two-plane coupled phase retrieval (TwPCPR) method. It realizes a fast-convergence holographic calculation method. High-resolution and full-field reconstruction by exploiting the full bandwidth are demonstrated for complex-amplitude reconstruction. Off-axis optimization phase provides an effective initial guess to avoid stagnation and minimize the required measurements of multi-plane phase retrieval. The proposed strategy works well for more extended samples without any prior assumptions of the objects including support, non-negative, sparse constraints, etc. It helps to enhance and empower applications in wavefront sensing, computational microscopy and biological tissue analysis. [ABSTRACT FROM AUTHOR]

The fundamentals of the theory of 3D hologram, which was developed by the author many years ago and has been applied in many fields of holography and nonlinear optics, are briefly described. In December 2010, the author reported the mode theory in Readings in the Memory of Academician D.S. Rozhdestvensky (these readings are performed annually by the Vavilov State Optical Institute and the Rozhdestvensky Optical Society in St. Petersburg). In September 2011, the author was invited to report the fundamentals of the theory at the conference Goloekspo-2011 in Minsk. The response to these reports showed an interest in the mode theory. This interest prompted this publication, the purpose of which is to prepare the reader for comprehending the original studies. [ABSTRACT FROM AUTHOR]

In the present study, record and readout of holographic transmission diffraction gratings in photopolymer media are investigated with allowance for an arbitrary degree of nonlinearity of photopolymerization, contrast of interference patterns, and absorption and arbitrary relative contributions of polymerization and diffusion recording mechanisms. A theoretical model of ( N + 1) 3-D harmonics of the refractive index grating, based on a solution of the coupled kinetic photopolymerization equations, allows the time dynamics of harmonic amplitudes, 3-D refractive index profile, diffraction efficiency, and angular selectivity of the recorded gratings to be investigated. [ABSTRACT FROM AUTHOR]

The presence of a scattering medium in the imaging path between an object and an observer is known to severely limit the visual acuity of the imaging system. We present an approach to circumvent the deleterious effects of scattering, by exploiting spectral correlations in scattered wavefronts. Our Synthetic Wavelength Holography (SWH) method is able to recover a holographic representation of hidden targets with sub-mm resolution over a nearly hemispheric angular field of view. The complete object field is recorded within 46 ms, by monitoring the scattered light return in a probe area smaller than 6 cm × 6 cm. This unique combination of attributes opens up a plethora of new Non-Line-of-Sight imaging applications ranging from medical imaging and forensics, to early-warning navigation systems and reconnaissance. Adapting the findings of this work to other wave phenomena will help unlock a wider gamut of applications beyond those envisioned in this paper. Here, the authors present Synthetic Wavelength Holography, an approach for Non-Line-of-Sight imaging. By exploiting spectral correlations in scattered light, the authors transform real world surfaces such as walls or scatterers into High-Resolution, Wide-Field-of-View imaging portals that provide holograms of objects obscured from view. [ABSTRACT FROM AUTHOR]

The application of a burst imaging method to digital holography (DH) with multiple image capturing is firstly demonstrated. Nobody has discussed this approach explicitly, although the burst imaging method is not so uncommon. The burst imaging method intermittently captures several images with a high repetition rate followed by a relatively long rest time. If the light wave from an object has a much smaller complex amplitude change in the frame interval of an image sensor than the precision of the DH system, the wave can be regarded as being in a static state. As a result, this solves a fatal problem in DH for an object that moves or deforms while capturing multiple interference images. In this research, the burst imaging method was applied to phase-shifting DH, and it is simply implemented by a high-speed image sensor and a continuous phase shifting which is good for high-speed phase shifting. The measurement error was analytically solved for the movement speed of an object. Finally, the proposed method was experimentally used to observe an evaporating droplet of ethanol on a glass substrate. [ABSTRACT FROM AUTHOR]

Laser Doppler holography is a planar blood flow imaging technique recently introduced in ophthalmology to image human retinal and choroidal blood flow non-invasively. Here we present a digital method based on the Doppler spectrum asymmetry that reveals the local direction of blood flow with respect to the optical axis in out-of-plane vessels. This directional information is overlaid on standard grayscale blood flow images to depict flow moving towards the camera in red and flow moving away from the camera in blue, as in ultrasound color Doppler imaging. We show that thanks to the strong contribution of backscattering to the Doppler spectrum in out-of-plane vessels, the local axial direction of blood flow can be revealed with a high temporal resolution, which enables us to evidence pathological blood flow reversals. We also demonstrate the use of optical Doppler spectrograms to quantitatively monitor retinal blood flow reversals. [ABSTRACT FROM AUTHOR]

Acoustic holograms are the keystone of modern acoustics. They encode three-dimensional acoustic fields in two dimensions, and their quality determines the performance of acoustic systems. Optimisation methods that control only the phase of an acoustic wave are considered inferior to methods that control both the amplitude and phase of the wave. In this paper, we present Diff-PAT, an acoustic hologram optimisation platform with automatic differentiation. We show that in the most fundamental case of optimizing the output amplitude to match the target amplitude; our method with only phase modulation achieves better performance than conventional algorithm with both amplitude and phase modulation. The performance of Diff-PAT was evaluated by randomly generating 1000 sets of up to 32 control points for single-sided arrays and single-axis arrays. This optimisation platform for acoustic hologram can be used in a wide range of applications of PATs without introducing any changes to existing systems that control the PATs. In addition, we applied Diff-PAT to a phase plate and achieved an increase of > 8 dB in the peak noise-to-signal ratio of the acoustic hologram. [ABSTRACT FROM AUTHOR]

Image reconstruction using minimal measured information has been a long-standing open problem in many computational imaging approaches, in particular in-line holography. Many solutions are devised based on compressive sensing (CS) techniques with handcrafted image priors or supervised deep neural networks (DNN). However, the limited performance of CS methods due to lack of information about the image priors and the requirement of an enormous amount of per-sample-type training resources for DNNs has posed new challenges over the primary problem. In this study, we propose a single-shot lensless in-line holographic reconstruction method using an untrained deep neural network which is incorporated with a physical image formation algorithm. We demonstrate that by modifying a deep decoder network with simple regularizers, a Gabor hologram can be inversely reconstructed via a minimization process that is constrained by a deep image prior. The outcoming model allows to accurately recover the phase and amplitude images without any training dataset, excess measurements, or specific assumptions about the object's or the measurement's characteristics. [ABSTRACT FROM AUTHOR]

An approach to determining the fraction of inhomogeneities of the optical nonlinear properties of a transparent medium characterized by a spatial variation of nonlinear coefficient of refractive index n2 has been considered. The data has been recorded by the method of inline digital holography with femtosecond time resolution, during which, as the time delay between the pump and the probe pulses changes, a series of inline digital probe holograms has been recorded in the Fresnel diffraction zone. The digital holograms contain Fresnel diffraction patterns with the information about induced degenerate noncollinear phase modulation formed in the presence of inhomogeneities of optical nonlinear properties. 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. [ABSTRACT FROM AUTHOR]

Phase retrieval seeks to reconstruct the phase from the measured intensity, which is an ill-posed problem. A phase retrieval problem can be solved with physical constraints by modulating the investigated complex wavefront. Orbital angular momentum has been recently employed as a type of reliable modulation. The topological charge l is robust during propagation when there is atmospheric turbulence. In this work, topological modulation is used to solve the phase retrieval problem. Topological modulation offers an effective dynamic range of intensity constraints for reconstruction. The maximum intensity value of the spectrum is reduced by a factor of 173 under topological modulation when l is 50. The phase is iteratively reconstructed without a priori knowledge. The stagnation problem during the iteration can be avoided using multiple topological modulations. [ABSTRACT FROM AUTHOR]

In mode-division multiplexing systems, the transmission quality is restricted by the differential mode delay (DMD). We previously proposed a spatial mode exchange technique using volume holograms (VHET) to reduce the DMD. VHET can be used for the exchange of multiple spatial modes with different transmission speeds using a single device. However, in VHET, incidents of cross-talk (XT) caused by non-target holograms severely degrade the exchange performance. To address this issue, we propose a spatial mode exchange technique that uses volume holograms in combination with a random optical diffuser. By scattering the intensity distribution of the signal beam uniformly using a random optical diffuser, high exchange performance can be attained due to the reduction in the influence of non-targeted holograms. We confirmed the basic operation of the proposed scheme by considering the crosstalk component coupled to the few-mode fiber in a mode-division multiplexing system. The simulation results show that the maximum XTs are significantly suppressed to − 15.8 dB. [ABSTRACT FROM AUTHOR]

Imaging of cells is a challenging problem as they do not appreciably change the intensity of the illuminating light. Interferometry-based methods to do this task suffer from high sensitivity to environmental vibrations. We introduce scanning diffractometry as a simple non-contact and vibration-immune methodology for quantitative phase imaging. Fresnel diffractometry by a phase step has led to several applications such as high-precision measurements of displacement. Additional scanning may lead to 3D imaging straightforwardly. We apply the technique to acquire 3D images of holographic grating, red blood cell, neuron, and sperm cell. Either visibility of the diffraction fringes or the positions of extrema may be used for phase change detection. The theoretical analysis through the Fresnel diffraction from one-dimensional phase step is presented and the experimental results are validated with digital holographic microscopy. The presented technique can be suggested to serve as a robust device for 3D phase imaging and biomedical measurements. [ABSTRACT FROM AUTHOR]