General theoretical formulation of image formation in digital Fresnel holography

Interframe-tunable ultrafast differential-displacement holography

We describe the details of a digital holographic microscopy diagnostic capable of quantifying both the topography and velocity of a km/s object with adjustable temporal sensitivity. This technique involves spatially multiplexing a double pulse reflected from a target with reference beams of precisely known temporal separation.

Polarimetric Imaging for Robot Perception: A Review

In recent years, the integration of polarimetric imaging into robotic perception systems has increased significantly, driven by the accessibility of affordable polarimetric sensors. This technology complements traditional color imaging by capturing and analyzing the polarization characteristics of light. This additional information provides robots with valuable insights into object shape, material composition, and other properties, ultimately enabling more robust manipulation tasks. This review aims to provide a comprehensive analysis of the principles behind polarimetric imaging and its diverse applications within the field of robotic perception. By exploiting the polarization state of light, polarimetric imaging offers promising solutions to three key challenges in robot vision: Surface segmentation; depth estimation through polarization patterns; and 3D reconstruction using polarimetric data. This review emphasizes the practical value of polarimetric imaging in robotics by demonstrating its effectiveness in addressing real-world challenges. We then explore potential applications of this technology not only within the core robotics field but also in related areas. Through a comparative analysis, our goal is to elucidate the strengths and limitations of polarimetric imaging techniques. This analysis will contribute to a deeper understanding of its broad applicability across various domains within and beyond robotics.

Viewing-angle expansion in holographic displays implemented with a modulator having finite space-bandwidth

The expansion of viewing angle is a crucial factor in holographic displays implemented with a spatial light modulator having a finite space-bandwidth. A holographic image is reconstructed at a viewing angle dependent on the numerical aperture of digital hologram. However, an interference of high-order noises is inevitable at an angle larger than the diffraction angle by a hologram pixel. This study presents the theoretical foundation for optimizing the Fresnel hologram to recover the low space-bandwidth. The higher spectrum components of the digital hologram beyond the bandwidth exists in the form of their replications. The expansion of angular spectrum by its repetition during optimization procedure increases the image resolution, resulting in a viewing angle that is determined by the hologram numerical aperture. We numerically and experimentally verify our strategy to expand a viewing angle of holographic image.

Decorrelation and anti-correlation from defocus in digital holographic interferometry

This paper presents a theoretical modeling of the speckle noise decorrelation in digital Fresnel holographic interferometry in out-of-focus reconstructed images. The complex coherence factor is derived by taking into account the focus mismatch, which depends on both the sensor-to-object distance and the reconstruction distance. The theory is confirmed by both simulated data and experimental results. The very good agreement between data demonstrates the high relevance of the proposed modeling. The particular phenomenon of anti-correlation in phase data from holographic interferometry is highlighted and discussed.

pyDHM: A Python library for applications in digital holographic microscopy

pyDHM is an open-source Python library aimed at Digital Holographic Microscopy (DHM) applications. The pyDHM is a user-friendly library written in the robust programming language of Python that provides a set of numerical processing algorithms for reconstructing amplitude and phase images for a broad range of optical DHM configurations. The pyDHM implements phase-shifting approaches for in-line and slightly off-axis systems and enables phase compensation for telecentric and non-telecentric systems. In addition, pyDHM includes three propagation algorithms for numerical focusing complex amplitude distributions in DHM and digital holography (DH) setups. We have validated the library using numerical and experimental holograms.

Realistic simulation and real-time reconstruction of digital holographic microscopy experiments in ImageJ

The description, implementation, and validation of an ImageJ plugin that allows the realistic simulation and real-time reconstruction of digital holographic microscopy (DHM) experiments are presented. The simulation module implements a telecentric image-plane DHM recording scheme with fully configurable imaging system, interference, and scaling parameters, including the possibility of defining an estimate of the roughness distribution of the sample to produce realistic coherent-noise affectations. The reconstruction module allows the computation of amplitude, intensity, or phase, from digital holograms' input as either single images or video streams for real-time processing; this module also implements user-defined fine-tuning parameters, allowing subpixel linear phase compensations and digital refocusing of the complex-valued reconstructed fields. In this note, the functionality of the plugin is illustrated by simulating the noisy DHM recording of a phase-only resolution test target and the reconstruction of both the resulting synthetic hologram and an equivalent experimental recording; the results show good agreement between the simulation and the experimental recording, and accurate measurements on the reconstructed information, thus granting the use of either module with full confidence according to needs and possibilities.

Sampling analysis for Fresnel diffraction fields based on phase space representation

Numerical calculation of Fresnel diffraction is widely used in optical applications where wave propagation is needed. Most cases for simulating Fresnel diffraction only consider sampling of complex amplitude fields, which lacks comprehensiveness when different diffraction fields are of interest. In this study, from the perspective of phase space optics, we provide systematical analysis on the sampling of all Fresnel diffraction fields, including the complex amplitude, intensity, amplitude, and phase. The space-bandwidth product of each field is illustrated with the aid of a phase space diagram, which is further demonstrated from a physical point of view. Such comprehensive sampling analysis could provide practical suggestions to the numerical calculations of various diffraction fields according to the requirements. Numerical experiments with discrete Fresnel diffraction are implemented to verify feasibility of the analyses.

Deep learning speckle de-noising algorithms for coherent metrology: a review and a phase-shifted iterative scheme [Invited]

We present a review of deep learning algorithms dedicated to the processing of speckle noise in coherent imaging. We focus on methods that specifically process de-noising of input images. Four main classes of applications are described in this review: optical coherence tomography, synthetic aperture radar imaging, digital holography amplitude imaging, and fringe pattern analysis. We then present deep learning approaches recently developed in our group that rely on the retraining of residual convolutional neural network structures to process decorrelation phase noise. The paper ends with the presentation of a new approach that uses an iterative scheme controlled by an input SNR estimator associated with a phase-shifting procedure.

Wide viewing-angle holographic display based on enhanced-NA Fresnel hologram

The viewing-angle enlargement of a holographic image is a crucial factor for realizing the holographic display. The numerical aperture (NA) of digital hologram other than a pixel specification has been known to determine the angular field extent of image. Here, we provide a valid foundation for the dependence of viewing angle on the hologram numerical aperture by investigating mathematically the internal structure of the sampled point spread function showing a self-similarity of its modulating curve. The enhanced-NA Fresnel hologram reconstructs the image at a viewing angle larger than a diffraction angle by a hologram pixel pitch where its angle value is expressed in terms of the NA of whole hologram aperture, which is observed systematically by optical experiments. Finally, we found that the aliased replica noises generated in the enhanced-NA Fresnel diffraction regime are effectively suppressed within the diffraction scope by a digitized pixel. This characteristic enables us to overcome the image reduction and to remove the interference of high-order images, which leads to the wide viewing-angle holographic display.

Modeling of speckle decorrelation in digital Fresnel holographic interferometry

This paper presents analytical modeling of the speckle decorrelation noise in digital Fresnel holographic interferometry. The theoretical analysis is carried out by considering the complex coherence factor between two speckled images from two digitally reconstructed holograms at two different instants. The expression giving the modulus of the coherence factor is established and depends on the local surface deformation and parameters from the holographic setup. The analysis is supported by realistic simulations and experiments. Both simulations and experimental results exhibit a very good agreement with the theoretical prediction.

Transform-based phase retrieval techniques from a single off-axis interferogram

Optical phase retrieval (OPR) methods are important because they are used to obtain the transverse phase profile information of a beam. Interference methods are extensively used to convert the phase information into an intensity pattern, which can then be processed further to retrieve the unknown phase. The most widely used interference method involves the interference of the unknown object beam and a known reference beam with an angle between them. There are several algorithms that retrieve the phase information from such a single off-axis interference pattern. For a particular application, the choice of an algorithm for OPR is very important. Therefore, it is necessary to choose between them, depending on the requirements. Three entirely different noniterative, transform-based algorithms, namely the Fourier transform (FT) method, the continuous wavelet transform (CWT) method, and the Hilbert transform (CWT) method, are explained in detail. A quantitative comparison is made using a combination of rms error and standard structural similarity measure. The advantages of using a standard unwrapping algorithm are also validated using the same combination of comparison metrics. We show that the HT method has a better response with object beam with higher spatial frequency content, but with the penalty of affected noise. The FT method and CWT method have better noise immunity, but have the limitation of the spatial frequency range of the object beam. The different constraints, advantages, and some practical limitations of the methods are discussed with the help of a quantitative phase imaging experiment of monodispersed polymethyl methacrylate beads.

Lock-in vibration retrieval based on high-speed full-field coherent imaging

The use of high-speed cameras permits to visualize, analyze or study physical phenomena at both their time and spatial scales. Mixing high-speed imaging with coherent imaging allows recording and retrieving the optical path difference and this opens the way for investigating a broad variety of scientific challenges in biology, medicine, material science, physics and mechanics. At high frame rate, simultaneously obtaining suitable performance and level of accuracy is not straightforward. In the field of mechanics, this prevents high-speed imaging to be applied to full-field vibrometry. In this paper, we demonstrate a coherent imaging approach that can yield full-field structural vibration measurements with state-of-the-art performances in case of high spatial and temporal density measurements points of holographic measurement. The method is based on high-speed on-line digital holography and recording a short time sequence. Validation of the proposed approach is carried out by comparison with a scanning laser Doppler vibrometer and by realistic simulations. Several error criteria demonstrate measurement capability of yielding amplitude and phase of structural deformations.

Behavior of CAD/CAM ceramic veneers under stress: A 3D holographic study

OBJECTIVES

Ceramic veneers restorations may undergo damages, such as cracks, fractures, or debonding. Full-field measurements must be carried out in order to visualize and analyze the strain fields. This paper demonstrates that digital holography permits to investigate the mechanical behavior under stress of a natural incisor and a natural incisor reconstructed with CAD/CAM ceramic veneer.

METHODS

The facial surface of a maxillary central incisor is prepared to receive a monolithic ceramic reconstruction manufactured using a chairside computer-aided design and computer aided manufacturing (CAD/CAM) system (Cerec AC® system, Sirona Dental System®, Bensheim, Germany). One incisor is kept intact for comparison. The samples are sectioned longitudinally to obtain a planar observation of the region of interest. A mechanical indentation head and digital holographic set-ups permit a full-field, contact-less and single-shot measurement of the three-dimensional displacement fields at the surface of the tooth sample when subjected to load. Stain fields are then estimated and comparison of the results between two samples can be carried out.

RESULTS

3D displacement, fields and strain fields are measured and highlight the behavior of the region of interest in three directions of space for the ceramic veneer and the natural incisor. The strain maps reveal the local behavior, especially the concentration or the sudden change in strain. The transition zones are clearly observed, particularly for the veneered sample.

CONCLUSION

Digital holography highlights the localization of stress concentration zones in regions of interest and yields comparative analysis between samples with different tooth preparations.

SIGNIFICANCE

holography permits to visualize and compare the mechanical response of the ceramic veneer and natural tooth. This helps choosing the mechanical properties of the bonding interface.

Speckle reduction in digital holography with low-dimensional reconstruction

Speckle reduction is a crucial technique since the presence of speckle disturbs the quality of the reconstruction in digital holography. In this paper, we present an easy, fast, and efficient single-shot method to reduce speckle noise in digital holography. The method reconstructs subholograms from a single hologram. Then, sub-reconstruction images are randomly shuffled and divided into several groups and low-dimensional noise-reduced images can be achieved by averaging sub-reconstruction image groups by groups. Next, these low-dimensional noise-reduced images are combined to obtain a noise-reduced image. Finally, the noise-reduced image is processed by a mean filter to obtain a final image, which has substantially less speckle noise while preserving the dimensions of the original image. The experimental results demonstrate the effectiveness of the proposed method and indicate its potential in real-time digital holography.

Noise and bias in off-axis digital holography for measurements in acoustic waveguides

This paper discusses noise and bias in the method of holographic interferometry applied to the study of acoustics phenomena. The influence of noise on the measurement of acoustic pressure is described by an analytical approach. Relationships to quantify the minimum measurable fluid density and acoustic pressure are given by taking into account the experimental parameters of the setup. These parameters are related to the spatial bandwidths, number of electrons in pixels, readout noise, and quantization noise. Experimental results show that theoretical relations are relatively close to experimental data and that the lower pressure measurement limit is on the order of 15 Pa for the acoustics pressure. The case of waveguides excited by an internal or external acoustic source is investigated. Specifically, for the case of studies in thermoacoustics, this paper demonstrates that the parasitic coupling of vibrations can be compensated. The proposed method is based on the determination of the amplitude and phase of the parasitic oscillation, requiring a few assumptions related to the physics of the underlying phenomenon. Successful compensation is obtained and yields experimental data in agreement with the theoretical predictions.

Large volume holographic imaging for biological sample analysis

SIGNIFICANCE

Particle field holography is a versatile technique to determine the size and distribution of moving or stationary particles in air or in a liquid without significant disturbance of the sample volume. Although this technique is applied in biological sample analysis, it is limited to small sample volumes, thus increasing the number of measurements per sample. In this work, we characterize the maximum achievable volume limit based on the specification of a given sensor to realize the development of a potentially low-cost, single-shot, large-volume holographic microscope.

AIM

We present mathematical formulas that will aid in the design and development and improve the focusing speed for the numerical reconstruction of registered holograms in particle field holographic microscopes. Our proposed methodology has potential application in the detection of Schistosoma haematobium eggs in human urine samples.

APPROACH

Using the Fraunhofer holography theory for opaque objects, we derived an exact formula for the maximum diffraction-limited volume for an in-line holographic setup. The proof-of-concept device built based on the derived formulas was experimentally validated with urine spiked with cultured Schistosoma haematobium eggs.

RESULTS

Results obtained show that for urine spiked with Schistosoma haematobium eggs, the volume thickness is limited to several millimeters due to scattering properties of the sample. The distances of the target particles could be estimated directly from the hologram fringes.

CONCLUSION

The methodology proposed will aid in the development of large-volume holographic microscopes.

Spatial bandwidth-optimized compression of image plane off-axis holograms with image and video codecs

Image plane off-axis holograms (IP-OAH) are the most common data captured in digital holographic microscopy and tomography. Due to increasing storage and data transmission requirements, lossy compression of such holograms has been subject of earlier investigations. However, hologram compression can not be allowed to hinder the metrological capabilities of the measurement technique itself. In this work, we present lossy and lossless IP-OAH compression approaches that are based on conventional compression codecs, but optimized with regard to bandwidth of the signal. Both approaches outperform respective conventional codecs, while the lossy approach is shown to uphold the accuracy of holographic phase measurements.

Noise-free quantitative phase imaging in Gabor holography with conditional generative adversarial network

This paper shows that deep learning can eliminate the superimposed twin-image noise in phase images of Gabor holographic setup. This is achieved by the conditional generative adversarial model (C-GAN), trained by input-output pairs of noisy phase images obtained from synthetic Gabor holography and the corresponding quantitative noise-free contrast-phase image obtained by the off-axis digital holography. To train the model, Gabor holograms are generated from digital off-axis holograms with spatial shifting of the real image and twin image in the frequency domain and then adding them with the DC term in the spatial domain. Finally, the digital propagation of the Gabor hologram with Fresnel approximation generates a super-imposed phase image for the C-GAN model input. Two models were trained: a human red blood cell model and an elliptical cancer cell model. Following the training, several quantitative analyses were conducted on the bio-chemical properties and similarity between actual noise-free phase images and the model output. Surprisingly, it is discovered that our model can recover other elliptical cell lines that were not observed during the training. Additionally, some misalignments can also be compensated with the trained model. Particularly, if the reconstruction distance is somewhat incorrect, this model can still retrieve in-focus images.

Single-shot speckle reduction by elimination of redundant speckle patterns in digital holography

Speckle reduction is a crucial technique, since the presence of speckle disturbs the quality of the reconstruction in digital holography. In this paper, we present a redundant speckle elimination method to suppress the speckle noise. For the same position in each of the reconstructed sub-images, we consider pixels with the same gray value as information with the same speckle distribution. Therefore, a speckle-suppressed gray value can be obtained by extracting pixels with different gray values and then averaging. Through theoretical analysis and experiments, we demonstrate that speckle contrast can be decreased significantly by using the proposed method. Moreover, we show that the despeckle strength of the proposed method highly depends on the number of binary masks. These results indicate the potential of the proposed method for various applications.

RedCap: residual encoder-decoder capsule network for holographic image reconstruction

A capsule network, as an advanced technique in deep learning, is designed to overcome information loss in the pooling operation and internal data representation of a convolutional neural network (CNN). It has shown promising results in several applications, such as digit recognition and image segmentation. In this work, we investigate for the first time the use of capsule network in digital holographic reconstruction. The proposed residual encoder-decoder capsule network, which we call RedCap, uses a novel windowed spatial dynamic routing algorithm and residual capsule block, which extends the idea of a residual block. Compared with the CNN-based neural network, RedCap exhibits much better experimental results in digital holographic reconstruction, while having a dramatic 75% reduction in the number of parameters. It indicates that RedCap is more efficient in the way it processes data and requires a much less memory storage for the learned model, which therefore makes it possible to be applied to some challenging situations with limited computational resources, such as portable devices.

Resolution and contrast in terahertz pulse time-domain holographic reconstruction

Here, we present a comprehensive study of the reconstruction quality in terahertz (THz) pulse time-domain holography. We look into single wavelength reconstructions, as well as broadband recovery enabled by the ultrabroadband nature of radiation and coherent detection enabled by electro-optic or photoconductive sensing. We demonstrate the transverse resolution dependence for amplitude and phase objects on the solid angle of the inline recorded time-domain THz hologram, and then turn to the contrast of reconstructed binary amplitude objects, and further to longitudinal resolution of phase objects. We show that transverse resolution can reach values comparable to the wavelength of the radiation used, and longitudinally, phase objects can be resolved with even greater precision. We compare the obtained resolution with theoretical estimates and show that THz pulse time-domain holography is a powerful non-contact imaging tool.

Digital holographic interferometry and speckle interferometry applied on objects with heterogeneous reflecting properties

In digital holographic and speckle interferometry devoted to solid object displacement measurement, the reflecting properties of the object under study are of importance in designing the observation and laser illumination systems. In practical cases, the objects can show separate zones in which the surface property can simultaneously cause either scattering or specular reflectivity. We present strategies for dealing with both reflectivity types at a time in digital holographic and speckle interferometers. The scattered surface is illuminated with a point source whereas the specular one is illuminated by a diffuser. Both types of surfaces visible across the field-of-view give rise to a specific interferogram with gaps in between, which in turn are interpreted separately related to the sensitivity vector, the latter being defined differently for scattering and specular areas.

Visualization of internal structure and internal stress in visibly opaque objects using full-field phase-shifting terahertz digital holography

We construct a full-field phase-shifting terahertz digital holography (PS-THz-DH) system by use of a THz quantum cascade laser and an uncooled, 2D micro-bolometer array. The PS-THz-DH enables us to separate the necessary diffraction-order image from unnecessary diffraction-order images without the need for spatial Fourier filtering, leading to suppress the decrease of spatial resolution. 3D shape of a visibly opaque object is visualized with a sub-millimeter lateral resolution and a sub-µm axial resolution. Also, the digital focusing of amplitude image enables the visualization of internal structure with the millimeter-order axial selectivity. Furthermore, the internal stress distribution of an externally compressed object is visualized from the phase image. The demonstrated results imply a possibility for non-destructive inspection of visibly opaque non-metal materials.

Resolution gain in space-time digital holography by self-assembling of the object frequencies

Space-time digital holography (STDH) exploits the object motion to record the hologram in a hybrid ST domain. This representation adds new capabilities to conventional DH, e.g., unlimited field of view and variable phase shifting. This is the best candidate for imaging biological samples flowing in microfluidic channels. Here, we show that STDH is able to improve the spatial resolution as well. Different from other super-resolution approaches, stitching between holograms or their spectra is no longer required. Moreover, we introduce a new oblique STDH modality to record and process hybrid ST representations. This allows improving resolution in 2D with one single object scan, paving the way to the use of STDH for super-resolution imaging onboard microfluidic devices.

Strategies for reducing speckle noise in digital holography

Digital holography (DH) has emerged as one of the most effective coherent imaging technologies. The technological developments of digital sensors and optical elements have made DH the primary approach in several research fields, from quantitative phase imaging to optical metrology and 3D display technologies, to name a few. Like many other digital imaging techniques, DH must cope with the issue of speckle artifacts, due to the coherent nature of the required light sources. Despite the complexity of the recently proposed de-speckling methods, many have not yet attained the required level of effectiveness. That is, a universal denoising strategy for completely suppressing holographic noise has not yet been established. Thus the removal of speckle noise from holographic images represents a bottleneck for the entire optics and photonics scientific community. This review article provides a broad discussion about the noise issue in DH, with the aim of covering the best-performing noise reduction approaches that have been proposed so far. Quantitative comparisons among these approaches will be presented.

Off-axis digital holography with multiplexed volume Bragg gratings

We report on an optical imaging design based on common-path off-axis digital holography, using a multiplexed volume Bragg grating. In the reported method, a reference optical wave is made by deflection and spatial filtering through a volume Bragg grating. This design has several advantages, including simplicity, stability, and robustness against misalignment.

Resolution optimization of an off-axis lensless digital holographic microscope

Microscopes aimed at detecting cellular life in extreme environments such as ocean-bearing solar system moons must provide high resolution in a compact, robust instrument. Here, we consider the resolution optimization of a compact off-axis lensless digital holographic microscope (DHM) that consists of a sample placed between an input point-source pair and a detector array. Two optimal high-resolution regimes are identified at opposite extremes-a low-magnification regime with the sample located near a small-pixel detector array, and a high-magnification regime with the sample near the input plane. In the former, resolution improves with smaller pixels, while in the latter, the effect of the finite pixel size is obviated, and the spatial resolution improves with detector array size. Using an off-axis lensless DHM with a 2 k×2 k array of 5.5 μm-pixels in the high-magnification regime, and standard aberration correction software, a resolution of ∼0.95  μm has been demonstrated, a factor of 5.8 smaller than the pixel size. Our analysis further suggests that with yet larger detector arrays, a lensless DHM should be capable of near wavelength-scale resolution.

Endowing a plain fluidic chip with micro-optics: a holographic microscope slide

Lab-on-a-Chip (LoC) devices are extremely promising in that they enable diagnostic functions at the point-of-care. Within this scope, an important goal is to design imaging schemes that can be used out of the laboratory. In this paper, we introduce and test a pocket holographic slide that allows digital holography microscopy to be performed without an interferometer setup. Instead, a commercial off-the-shelf plastic chip is engineered and functionalized with this aim. The microfluidic chip is endowed with micro-optics, that is, a diffraction grating and polymeric lenses, to build an interferometer directly on the chip, avoiding the need for a reference arm and external bulky optical components. Thanks to the single-beam scheme, the system is completely integrated and robust against vibrations, sharing the useful features of any common path interferometer. Hence, it becomes possible to bring holographic functionalities out of the lab, moving complexity from the external optical apparatus to the chip itself. Label-free imaging and quantitative phase contrast mapping of live samples are demonstrated, along with flexible refocusing capabilities. Thus, a liquid volume can be analyzed in one single shot with no need for mechanical scanning systems.

Label free imaging of cell-substrate contacts by holographic total internal reflection microscopy

The study of cell adhesion contacts is pivotal to understand cell mechanics and interaction at substrates or chemical and physical stimuli. We designed and built a HoloTIR microscope for label-free quantitative phase imaging of total internal reflection. Here we show for the first time that HoloTIR is a good choice for label-free study of focal contacts and of cell/substrate interaction as its sensitivity is enhanced in comparison with standard TIR microscopy. Finally, the simplicity of implementation and relative low cost, due to the requirement of less optical components, make HoloTIR a reasonable alternative, or even an addition, to TIRF microscopy for mapping cell/substratum topography. As a proof of concept, we studied the formation of focal contacts of fibroblasts on three substrates with different levels of affinity for cell adhesion.

Copyright protection for elemental image array by hypercomplex Fourier transform and an adaptive texturized holographic algorithm

In practical applications of three-dimensional integral imaging, the captured elemental image array (EIA) needs to be stored and delivered through the Internet. Therefore, there is an urgent need for protecting the copyright of EIA against piracy and malicious manipulation. In our work, we propose a copyright protection algorithm for EIA by combining the use of the modified hypercomplex Fourier transform (HFT) and the adaptive texturized holographic algorithm. The modified HFT can accurately extract the features from each elemental image. According to these features, we embed watermark into the visually less noticeable regions of the EIA to increase the visual perception. In addition, an adaptive texturized holographic algorithm is proposed to increase the robustness. Finally, the analytical performances are contrasted with simulation results where the imperceptibility and robustness of the proposed method are evaluated against standard attacks.

Quantitative study on a resampling mask method for speckle reduction with amplitude superposition

One-shot digital holographic imaging has the advantages of high stability and low temporal cost. However, its reconstruction is degraded severely by the laser speckle. A rectangle, ellipse, and diamond resampling mask method in spatial domain for speckle reduction is proposed. The effectiveness of the method for speckle reduction is explained successfully. In the method, one hologram recorded in a certain size is divided into N=S×T sub-holograms. Angular spectrum transform is applied to the holographic reconstruction of a diffuse object. N reconstructed amplitude images are calculated from the corresponding sub-holograms. Benefitting from speckle's random distribution, superimposing these N uncorrelated amplitude images would lead to a final reconstructed image with reduced speckle. Normalized relative standard deviation values of the reconstructed image are in good agreement with the asymptotical law. The maximum relative errors between the experiment data and the theoretical values are below 7.2%. The effect of the method on the spatial resolution of the reconstructed image is also quantitatively evaluated. Experimental and simulation results prove the feasibility and effectiveness of the proposed method.

Automatic full compensation of quantitative phase imaging in off-axis digital holographic microscopy

An automatic method that fully compensates the quantitative phase measurements in off-axis digital holographic microscopy (DHM) is presented. The two main perturbations of the quantitative phase measurements in off-axis DHM are automatically removed. While the curvature phase flaw introduced by the microscope objective is avoided by the use of an optimized telecentric imaging system for the recording of the holograms, the remaining phase perturbation due to the tilt of the reference wave is removed by the automatic computation of a digital compensating reference wave. The method has been tested on both nonbiological and biological samples with and improving on the quality of the recovered phase maps.

Starlet transform applied to digital Gabor holographic microscopy

In this paper, we show how the starlet transform can be used to process holograms from a digital Gabor holographic microscope. The starlet transform is an undecimated wavelet transform with the property that when performing reconstruction, we only need to add all scales without the use of a synthesis filter bank. When the starlet transform is applied to a hologram, we divide the hologram into a certain number of scales, process them separately, and propagate each one using a numerical diffraction method. After diffraction propagation, we perform processing on complex amplitudes that correspond to individual scales. With the aforementioned procedure, it is possible to reduce the background and effects of parasitic fringes caused by high coherence of a laser, enhance the contrast, and reduce the effects of the twin image. Experimental results confirming the method are presented.

Remote monitoring of building oscillation modes by means of real-time Mid Infrared Digital Holography

Non-destructive measurements of deformations are a quite common application of holography but due to the intrinsic limits in the interferometric technique, those are generally confined only to small targets and in controlled environment. Here we present an advanced technique, based on Mid Infrared Digital Holography (MIR DH), which works in outdoor conditions and provides remote and real-time information on the oscillation modes of large engineering structures. Thanks to the long wavelength of the laser radiation, large areas of buildings can be simultaneously mapped with sub-micrometric resolution in terms of their amplitude and frequency oscillation modes providing all the modal parameters vital for all the correct prevention strategies when the functionality and the health status of the structures have to be evaluated. The existing experimental techniques used to evaluate the fundamental modes of a structure are based either on seismometric sensors or on Ground-based Synthetic Aperture Radar (GbSAR). Such devices have both serious drawbacks, which prevent their application at a large scale or in the short term. We here demonstrate that the MIR DH based technique can fully overcome these limitations and has the potential to represent a breakthrough advance in the field of dynamic characterization of large structures.

A review of snapshot multidimensional optical imaging: measuring photon tags in parallel

Multidimensional optical imaging has seen remarkable growth in the past decade. Rather than measuring only the two-dimensional spatial distribution of light, as in conventional photography, multidimensional optical imaging captures light in up to nine dimensions, providing unprecedented information about incident photons' spatial coordinates, emittance angles, wavelength, time, and polarization. Multidimensional optical imaging can be accomplished either by scanning or parallel acquisition. Compared with scanning-based imagers, parallel acquisition-also dubbed snapshot imaging-has a prominent advantage in maximizing optical throughput, particularly when measuring a datacube of high dimensions. Here, we first categorize snapshot multidimensional imagers based on their acquisition and image reconstruction strategies, then highlight the snapshot advantage in the context of optical throughput, and finally we discuss their state-of-the-art implementations and applications.

Comparison of Thermal Detector Arrays for Off-Axis THz Holography and Real-Time THz Imaging

In terahertz (THz) materials science, imaging by scanning prevails when low power THz sources are used. However, the application of array detectors operating with high power THz sources is increasingly reported. We compare the imaging properties of four different array detectors that are able to record THz radiation directly. Two micro-bolometer arrays are designed for infrared imaging in the 8-14 μm wavelength range, but are based on different absorber materials (i) vanadium oxide; (ii) amorphous silicon; (iii) a micro-bolometer array optimized for recording THz radiation based on silicon nitride; and (iv) a pyroelectric array detector for THz beam profile measurements. THz wavelengths of 96.5 μm, 118.8 μm, and 393.6 μm from a powerful far infrared laser were used to assess the technical performance in terms of signal to noise ratio, detector response and detectivity. The usefulness of the detectors for beam profiling and digital holography is assessed. Finally, the potential and limitation for real-time digital holography are discussed.

Microtomography imaging of an isolated plant fiber: a digital holographic approach

This paper describes a method for optical projection tomography for the 3D in situ characterization of micrometric plant fibers. The proposed approach is based on digital holographic microscopy, the holographic capability being convenient to compensate for the runout of the fiber during rotations. The setup requires a telecentric alignment to prevent from the changes in the optical magnification, and calibration results show the very good experimental adjustment. Amplitude images are obtained from the set of recorded and digitally processed holograms. Refocusing of blurred images and correction of both runout and jitter are carried out to get appropriate amplitude images. The 3D data related to the plant fiber are computed from the set of images using a dedicated numerical processing. Experimental results exhibit the internal and external shapes of the plant fiber. These experimental results constitute the first attempt to obtain 3D data of flax fiber, about 12  μm×17  μm in apparent diameter, with a full-field optical tomography approach using light in the visible range.

Quality assessment of combined quantization-shot-noise-induced decorrelation noise in high-speed digital holographic metrology

this paper discusses on the influence of decorrelation noise induced by quantization and shot-noise when recording digital holograms at very high frame rate. A criterion based on the coherence factor of the hologram phase difference is proposed. The main parameters of interest are the ratio between the reference and the object waves and the sensor dynamics, depending on the photo-electron capacity of pixels. The study is based on a full numerical simulation of the holographic process, which provides useful rules. This leads to define the optimal conditions for recording at very-high frame rate with minimization of the decorrelation noise. Experimental results obtained with frame rate at 50kHz confirm the proposed approach.

High numerical aperture holographic microscopy reconstruction with extended z range

A holographic microscopy reconstruction method compatible with a high numerical aperture microscope objective (MO) up to NA=1.4 is proposed. After off-axis and reference field curvature corrections, and after selection of the +1 grating order holographic image, a phase mask that transforms the optical elements of the holographic setup into an afocal device is applied in the camera plane. The reconstruction is then made by the angular spectrum method. The field is first propagated in the image half-space from the camera to the afocal image of the MO optimal plane (the plane for which the MO has been designed) by using a quadratic kernel. The field is then propagated from the MO optimal plane to the object with the exact kernel. Calibration of the reconstruction is made by imaging a calibrated object such as a USAF resolution target for different positions along z. Once the calibration is done, the reconstruction can be made with an object located in any plane z. The reconstruction method has been validated experimentally with a USAF target imaged with a NA=1.4 microscope objective. Near-optimal resolution is obtained over an extended range (±50  μm) of z locations.

Real-time terahertz digital holography with a quantum cascade laser

Coherent imaging in the THz range promises to exploit the peculiar capabilities of these wavelengths to penetrate common materials like plastics, ceramics, paper or clothes with potential breakthroughs in non-destructive inspection and quality control, homeland security and biomedical applications. Up to now, however, THz coherent imaging has been limited by time-consuming raster scanning, point-like detection schemes and by the lack of adequate coherent sources. Here, we demonstrate real-time digital holography (DH) at THz frequencies exploiting the high spectral purity and the mW output power of a quantum cascade laser combined with the high sensitivity and resolution of a microbolometric array. We show that, in a one-shot exposure, phase and amplitude information of whole samples, either in reflection or in transmission, can be recorded. Furthermore, a 200 times reduced sensitivity to mechanical vibrations and a significantly enlarged field of view are observed, as compared to DH in the visible range. These properties of THz DH enable unprecedented holographic recording of real world dynamic scenes.

A general model for resolution of digital holographic microscopy

For digital holographic microscopic imaging, the resolution in the reconstructed image is one of the most important parameters. To optimize the lateral resolution, a general model for the resolution of digital holographic microscopy (DHM) is proposed in this work, in which the effects of the sizes of each pixel, total area of the charge coupled device (CCD) and the microscopic objective lens are taken into account. Comparison between our model and others was carried out by calculating the point spread function (PSF) of DHM at different reconstruction distances and with different fill factors. It is shown that the effect of fill factors on the resolution of DHM becomes significant when the reconstruction distance is long. For high resolution DHM imaging the influence of fill factors must be taken into account when estimating the resolution of the reconstructed image. Furthermore, It is also demonstrated that the sidelobe of PSF can be cut effectively choosing appropriate values of the fill factors. Finally, the reconstructions of polyethylene microspheres have been implemented to demonstrate the theoretical analysis. These results obtained are helpful for estimation of the resolution and design of the DHM systems.

Holographic microscopy reconstruction in both object and image half-spaces with an undistorted three-dimensional grid

We propose a holographic microscopy reconstruction method that propagates the hologram in the object half-space in the vicinity of the object. The calibration yields reconstructions with an undistorted reconstruction grid, i.e., with orthogonal x, y, and z axes and constant pixel pitch. The method is validated with a USAF target imaged by a ×60 microscope objective (MO), whose holograms are recorded and reconstructed for different USAF locations along the longitudinal axis: -75 to +75  μm. Since the reconstruction numerical phase mask, the reference phase curvature, and the MO form an afocal device, the reconstruction can be interpreted as occurring equivalently in the object or image half-space.

Single-shot speckle reduction in numerical reconstruction of digitally recorded holograms

A single-shot method to reduce the speckle noise in the numerical reconstructions of electronically recorded holograms is presented. A recorded hologram with the dimensions N×M is split into S=T×T sub-holograms. The uncorrelated superposition of the individually reconstructed sub-holograms leads to an image with the speckle noise reduced proportionally to the 1/S law. The experimental results are presented to support the proposed methodology.

Multi-point vibrometer based on high-speed digital in-line holography

This paper describes a digital holographic setup based on in-line holography and a high-speed recording to get a multipoint vibrometer. The use of a high-speed sensor leads to specificities that enable the in-line configuration to be used. The case of transient vibrations is investigated through a full simulation of the holographic process. The simulation shows that the first instants are critical since distortion may occur, resulting in errors in the phase measurement. Experimental results are provided by exciting an aluminum beam with a transient signal. A comparison with the velocity measured by a pointwise vibrometer is provided. Frequency response functions are extracted and the experimental results confirm the ability of the method to provide full-field contactless measurements at the high-speed time scale evolution of the vibration.

Stochastic digital holography for visualizing inside strongly refracting transparent objects

This paper presents a digital holographic method to visualize and measure refractive index variations, convection currents, or thermal gradients, occurring inside a transparent and refracting object. The proof of principle is provided through the visualization of refractive index variation inside a lighting bulb. Comparison with transmission and reflection holography is also provided. A very good agreement is obtained, thus validating the proposed approach.

Holographic laser Doppler imaging of microvascular blood flow

We report on local superficial blood flow monitoring in biological tissue from laser Doppler holographic imaging. In time-averaging recording conditions, holography acts as a narrowband bandpass filter, which, combined with a frequency-shifted reference beam, permits frequency-selective imaging in the radio frequency range. These Doppler images are acquired with an off-axis Mach-Zehnder interferometer. Microvascular hemodynamic components mapping is performed in the cerebral cortex of the mouse and the eye fundus of the rat with near-infrared laser light without any exogenous marker. These measures are made from a basic inverse-method analysis of local first-order optical fluctuation spectra at low radio frequencies, from 0 Hz to 100 kHz. Local quadratic velocity is derived from Doppler broadenings induced by fluid flows, with elementary diffusing wave spectroscopy formalism in backscattering configuration. We demonstrate quadratic mean velocity assessment in the 0.1-10 mm/s range in vitro and imaging of superficial blood perfusion with a spatial resolution of about 10 micrometers in rodent models of cortical and retinal blood flow.

Terahertz holography for imaging amplitude and phase objects

A non-monochromatic THz Quantum Cascade Laser and an uncooled micro-bolometer array detector with VGA resolution are used in a beam-splitter free holographic set-up to measure amplitude and phase objects in transmission. Phase maps of the diffraction pattern are retrieved using the Fourier transform carrier fringe method; while a Fresnel-Kirchhoff back propagation algorithm is used to reconstruct the complex object image. A lateral resolution of 280 µm and a relative phase sensitivity of about 0.5 rad are estimated from reconstructed images of a metallic Siemens star and a polypropylene test structure, respectively. Simulations corroborate the experimental results.

Video-rate computational heterodyne holography

We present a versatile computational image rendering software of optically acquired holograms. The reported software can process 4 Megapixel 8-bit raw frames from a sensor array acquired at a sustained rate of 80 Hz. Video-rate image rendering is achieved by streamline image processing with commercial computer graphics hardware. For time-averaged holograms acquired in off-axis optical configuration with a frequency-shifted reference beam, wide-field imaging of one tunable spectral component is permitted. This software is validated by phase-stepped hologram rendering, and noncontact monitoring of surface acoustic waves by single and dual sideband hologram rendering. It demonstrates the suitability of holography for video-rate computational laser Doppler imaging in a heterodyne optical configuration.