Off-axis digital hologram reconstruction: some practical considerations

Full-field noise-correlation elastography for in-plane mechanical anisotropy imaging

Elastography contrast imaging has great potential for the detection and characterization of abnormalities in soft biological tissues to help physicians in diagnosis. Transient shear-waves elastography has notably shown promising results for a range of clinical applications. In biological soft tissues such as muscle, high mechanical anisotropy implies different stiffness estimations depending on the direction of the measurement. In this study, we propose the evolution of a noise-correlation elastography approach for in-plane anisotropy mapping. This method is shown to retrieve anisotropy from simulation images before being validated on agarose anisotropic tissue-mimicking phantoms, and the first results on in-vivo biological fibrous tissues are presented.

Recent Advances and Current Trends in Transmission Tomographic Diffraction Microscopy

Optical microscopy techniques are among the most used methods in biomedical sample characterization. In their more advanced realization, optical microscopes demonstrate resolution down to the nanometric scale. These methods rely on the use of fluorescent sample labeling in order to break the diffraction limit. However, fluorescent molecules' phototoxicity or photobleaching is not always compatible with the investigated samples. To overcome this limitation, quantitative phase imaging techniques have been proposed. Among these, holographic imaging has demonstrated its ability to image living microscopic samples without staining. However, for a 3D assessment of samples, tomographic acquisitions are needed. Tomographic Diffraction Microscopy (TDM) combines holographic acquisitions with tomographic reconstructions. Relying on a 3D synthetic aperture process, TDM allows for 3D quantitative measurements of the complex refractive index of the investigated sample. Since its initial proposition by Emil Wolf in 1969, the concept of TDM has found a lot of applications and has become one of the hot topics in biomedical imaging. This review focuses on recent achievements in TDM development. Current trends and perspectives of the technique are also discussed.

Real-time millimeter wave holography with an arrayed detector

Millimeter and terahertz wave imaging has emerged as a powerful tool for applications such as security screening, biomedical imaging, and material analysis. However, intensity images alone are often insufficient for detecting variations in the dielectric constant of a sample, and extraction of material properties without additional phase information requires extensive prior knowledge of the sample. Digital holography provides a means for intensity-only detectors to reconstruct both amplitude and phase images. Here we utilize a commercially available source and detector array, both operating at room temperature, to perform digital holography in real-time for the first time in the mm-wave band (at 290 GHz). We compare the off-axis and phase-shifting approaches to digital holography and discuss their trade-offs and practical challenges in this regime. Owing to the low pixel count, we find phase-shifting holography to be the most practical and high fidelity approach for such commercial mm-wave cameras even under real-time operational requirements.

Digital hologram reconstruction algorithm based on the Fractional Fourier transform in non-telecentric digital holographic microscopy

A hologram reconstruction algorithm is proposed based on the fractional Fourier transform (FRFT) in non-telecentric digital holographic microscopy. The optimal fractional order representing the recorded hologram is estimated based on an evaluation metric. The FRFT-based hologram reconstruction enables noise robust amplitude and phase imaging with enhanced resolution. The effectiveness of the proposed approach is demonstrated in practical scenarios through both simulation and experimental results.

Complete holography-based system for the identification of microparticles in water samples

Here, we present a comprehensive holography-based system designed for detecting microparticles through microscopic holographic projections of water samples. This system is designed for researchers who may be unfamiliar with holographic technology but are engaged in microparticle research, particularly in the field of water analysis. Additionally, our innovative system can be deployed for environmental monitoring as a component of an autonomous sailboat robot. Our system's primary application is for large-scale classification of diverse microplastics that are prevalent in water bodies worldwide. This paper provides a step-by-step guide for constructing our system and outlines its entire processing pipeline, including hologram acquisition for image reconstruction.

Coded aperture temporal compressive digital holographic microscopy

We report a coded aperture temporal compressive digital holographic microscopy (CATCHY) system to capture high-speed high-resolution samples by integrating snapshot compressive imaging (SCI) into digital holographic microscopy. Specifically, a two-dimensional (2D) detector samples a 4D (x, y, z, t) spatiotemporal data in a compressive manner, and after this, an efficient deep learning-based video SCI reconstruction algorithm is employed to reconstruct the desired 4D data cube. Up to ten high-resolution microscopic images are reconstructed from a snapshot measurement captured by our CATCHY system. Experimental results demonstrate the potential to visualize the 3D dynamic process of micro-nanostructures and imaging biological samples with high spatial and temporal resolution.

Flash-profilometry: fullfield lensless acquisition of spectral holograms for coherence scanning profilometry

Flash-profilometry is a novel measurement approach based on the fullfield lensless acquisition of spectral holograms. It is based on spectral sampling of the mutual coherence function and the subsequent calculation of its propagation along the optical axis several times the depth-of-field. Numerical propagation of the entire coherence function, rather than solely the complex amplitude, allows to digitally reproduce a complete scanning white-light interferometric (WLI) measurement. Hence, the corresponding 3D surface profiling system presented here achieves precision in the low nanometer range along an axial measurement range of several hundred micrometers. Due to the lensless setup, it is compact, immune against dispersion effects and lightweight. Additionally, because of the spectral sampling approach, it is faster than conventional coherence scanning WLI and robust against mechanical distortions, such as vibrations and rigid body movements. Flash-profilometry is therefore suitable for a wide range of applications, such as surface metrology, optical inspection, and material science and appears to be particularly suitable for a direct integration into production environments.

Chromatic aberration correction employing reinforcement learning

In fluorescence microscopy a multitude of labels are used that bind to different structures of biological samples. These often require excitation at different wavelengths and lead to different emission wavelengths. The presence of different wavelengths can induce chromatic aberrations, both in the optical system and induced by the sample. These lead to a detuning of the optical system, as the focal positions shift in a wavelength dependent manner and finally to a decrease in the spatial resolution. We present the correction of chromatic aberrations by using an electrical tunable achromatic lens driven by reinforcement learning. The tunable achromatic lens consists of two lens chambers filled with different optical oils and sealed with deformable glass membranes. By deforming the membranes of both chambers in a targeted manner, the chromatic aberrations present in the system can be manipulated to tackle both systematic and sample induced aberrations. We demonstrate chromatic aberration correction of up to 2200 mm and shift of the focal spot positions of 4000 mm. For control of this non-linear system with four input voltages, several reinforcement learning agents are trained and compared. The experimental results show that the trained agent can correct system and sample induced aberration and thereby improve the imaging quality, this is demonstrated using biomedical samples. In this case human thyroid was used for demonstration.

Digital holographic approaches to the detection and characterization of microplastics in water environments

Microplastic (MP) pollution is seriously threatening the environmental health of the world, which has accelerated the development of new identification and characterization methods. Digital holography (DH) is one of the emerging tools to detect MPs in a high-throughput flow. Here, we review advances in MP screening by DH. We examine the problem from both the hardware and software viewpoints. Automatic analysis based on smart DH processing is reported by highlighting the role played by artificial intelligence for classification and regression tasks. In this framework, the continuous development and availability in recent years of field-portable holographic flow cytometers for water monitoring also is discussed.

Compact lensless Fizeau holographic interferometry for imaging domain patterns in ferroelectric single crystals

Domain patterns in ferroelectric single crystals are physical systems that are fascinating from a theoretical point of view and essential for many applications. A compact lensless method for imaging domain patterns in ferroelectric single crystals based on a digital holographic Fizeau interferometer has been developed. This approach provides a large field-of-view image while maintaining a high spatial resolution. Furthermore, the double-pass approach increases the sensitivity of the measurement. The performance of the lensless digital holographic Fizeau interferometer is demonstrated by imaging the domain pattern in periodically poled lithium niobate. To display the domain patterns in the crystal, we have used an electro-optic phenomenon, which, when an external uniform electric field is applied to the sample, produces a difference in refractive index values in domains with different polarization states of the crystal lattice. Finally, the constructed digital holographic Fizeau interferometer is used to measure the difference in the index of refraction in the antiparallel ferroelectric domains in the external electric field. The lateral resolution of the developed method for ferroelectric domain imaging is discussed.

Suppression of alias and replica noises in phase holograms using fractal topologies

Two-dimensional fractal topologies featuring (scaling) self-similarity, dense set of Bragg (diffraction) peaks, and inherent rotation symmetry, which are not achievable with regular grid-matrix geometries, exhibit optical robustness against structural damage and noise immunity of optical transmission paths. In this work, we numerically and experimentally demonstrate phase holograms using fractal plane-divisions. By taking advantage of the symmetries of the fractal topology, we propose numerical algorithms to design the fractal holograms. This algorithm solves the inapplicability of the conventional iterative Fourier transform algorithm (IFTA) method and enables efficient optimizations of millions of adjustable parameters in the optical element. Experimental samples show that the alias and replica noises in the image plane of fractal holograms are clearly suppressed, facilitating applications for high-accuracy and compact requirements.

Multiple wavelength digital holography for freeform shape measurement and lens alignment

This paper introduces a technique for freeform optical surface measurements using digital holography with multiple discrete wavelengths or wavelength scans. This experimental arrangement, i.e., a Mach-Zehnder holographic profiler, is optimized to achieve maximal theoretical precision and is capable of measuring freeform diffuse surfaces. Moreover, the approach can also be used for diagnostics of precise placement of elements within optical systems.

3D differential interference contrast microscopy using polarisation-sensitive tomographic diffraction microscopy

Tomographic diffraction microscopy (TDM) is a generalisation of digital holographic microscopy (DHM), for which the illumination angle onto the sample is fully controlled, which has become a tool of choice for 3D, high-resolution imaging of unlabelled samples. TDM makes it possible to obtain the optical field in both amplitude and phase for each illumination angle. Proper information reallocation eventually allows for 3D reconstruction of the complex refractive index map. On the other hand, polarisation array sensors (PAS) paves new way for TDM, as vectorial information assessment about the investigated sample. In this contribution, we show an alternative use of this polarisation information based on the phase sensitive nature of TDM. Here, we demonstrated that TDM coupled with PAS can lead to a 3D differential interference contrast (DIC) microscope with almost no experimental configuration modification.

Effects of measurement noise on the construction of a transmission matrix

The effects of time-varying measurement noise on transmission matrix acquisition processes are considered for the first time, to our knowledge. Dominant noise sources are discussed, and the noise properties of a typical interferometer system used for characterizing a multimode fiber transmission matrix are quantified. It is demonstrated that an appropriate choice of measurement basis allows a more accurate transmission matrix to be more quickly obtained in the presence of measurement noise. Finally, it is shown that characterizing the noise figure of the experimental system allows the inverse transmission matrix to be constructed with an ideal amount of regularization, which can in turn be used for optimal image acquisition.

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.

Multimodal image reconstruction from tomographic diffraction microscopy data

Tomographic Diffraction Microscopy (TDM) is a tool of choice for high-resolution, marker-less 3D imaging of biological samples. Based on a generalization of Digital Holographic Microscopy (DHM) with full control of the sample's illumination, TDM measures, from many illumination directions, the diffracted fields in both phase and amplitude. Photon budget associated to TDM imaging is low. Therefore, TDM is not limited by photo-toxicity issues. The recorded information makes it possible to reconstruct 3D refractive index distribution (with both refraction and absorption contributions) of the object under scrutiny, without any staining. In this contribution, we show an alternate use of this information. A tutorial for multimodal image reconstruction is proposed. Both intensity contrasts and phase contrasts are proposed, from the image formation model to the final reconstruction with both 2D and 3D rendering, turning TDM into a kind of "universal" digital microscope. This article is protected by copyright. All rights reserved.

Robust correction of interferometer phase drift in transmission matrix measurements

A complex-valued transmission matrix describing a scattering medium can be constructed from a sequence of many interferometric measurements. A major challenge in such experiments is to correct for rapid phase drift of the optical system during the data acquisition process, especially when the phase drifts significantly between consecutive measurements. Therefore, a new method is presented where the exact phase drift between two measurements is characterized and corrected using a single additional measurement. This approach removes the need to continuously track the phase and significantly relaxes the phase stability requirements of the interferometer, allowing transmission matrices to be constructed in the presence of fast and erratic phase drift.

Adaptive Digital Hologram Binarization Method Based on Local Thresholding, Block Division and Error Diffusion

High-speed optical reconstruction of 3D-scenes can be achieved using digital holography with binary digital micromirror devices (DMD) or a ferroelectric spatial light modulator (fSLM). There are many algorithms for binarizing digital holograms. The most common are methods based on global and local thresholding and error diffusion techniques. In addition, hologram binarization is used in optical encryption, data compression, beam shaping, 3D-displays, nanofabrication, materials characterization, etc. This paper proposes an adaptive binarization method based on a combination of local threshold processing, hologram division into blocks, and error diffusion procedure (the LDE method). The method is applied for binarization of optically recorded and computer-generated digital holograms of flat objects and three-dimensional scenes. The quality of reconstructed images was compared with different methods of error diffusion and thresholding. Image reconstruction quality was up to 22% higher by various metrics than that one for standard binarization methods. The optical hologram reconstruction using DMD confirms the results of the numerical simulations.

Phase unwrapping using the extracted degree of coherence and phase from phase shifting interferometry systems

Current phase unwrapping methods for non-scanning interferometry systems with one wavelength are not robust in the presence of high steps while still having a limited step height and range using two wavelengths configurations. Here, a new phase unwrapping method is proposed, allowing imaging steps with a height up to 15 times the wavelength using one wavelength or up to 1500 times using two wavelengths. It is based on a one-time computational model fitting of calibration measurements that allows to extract the degree of coherence and phase from two phase-shifted images per wavelength, perform phase unwrapping and accurately reconstruct the 3D structure of the sample. The proposed method has a nanometric axial accuracy and can operate in real-time. The algorithms and methodology for one and two wavelengths are presented and confirmed experimentally.

Time-of-flight and noise-correlation-inspired algorithms for full-field shear-wave elastography using digital holography

SIGNIFICANCE

Quantitative stiffness information can be a powerful aid for tumor or fibrosis diagnosis. Currently, very promising elastography approaches developed for non-contact biomedical imaging are based on transient shear-waves imaging. Transient elastography offers quantitative stiffness information by tracking the propagation of a wave front. The most common method used to compute stiffness from the acquired propagation movie is based on shear-wave time-of-flight calculations.

AIM

We introduce an approach to transient shear-wave elastography with spatially coherent sources, able to yield full-field quantitative stiffness maps with reduced artifacts compared to typical artifacts observed in time-of-flight.

APPROACH

A noise-correlation algorithm developed for passive elastography is adapted to spatially coherent narrow or any band sources. This noise-correlation-inspired (NCi) method is employed in parallel with a classic time-of-flight approach. Testing is done on simulation images, experimental validation is conducted with a digital holography setup on controlled homogeneous samples, and full-field quantitative stiffness maps are presented for heterogeneous samples and ex-vivo biological tissues.

RESULTS

The NCi approach is first validated on simulations images. Stiffness images processed by the NCi approach on simulated inclusions display significantly less artifacts than with a time-of-flight reconstruction. The adaptability of the NCi algorithm to narrow or any band shear-wave sources was tested successfully. Experimental testing on homogeneous samples demonstrates similar values for both the time-of-flight and the NCi approach. Soft inclusions in agarose sample could be resolved using the NCi method and feasibility on ex-vivo biological tissues is presented.

CONCLUSIONS

The presented NCi approach was successful in computing quantitative full-field stiffness maps with narrow and broadband source signals on simulation and experimental images from a digital holography setup. Results in heterogeneous media show that the NCi approach could provide stiffness maps with less artifacts than with time-of-flight, demonstrating that a NCi algorithm is a promising approach for shear-wave transient elastography with spatially coherent sources.

Accurate and practical feature extraction from noisy holograms

Quantitative phase imaging using holographic microscopy is a powerful and non-invasive imaging method, ideal for studying cells and quantifying their features such as size, thickness, and dry mass. However, biological materials scatter little light, and the resulting low signal-to-noise ratio in holograms complicates any downstream feature extraction and hence applications. More specifically, unwrapping phase maps from noisy holograms often fails or requires extensive computational resources. We present a strategy for overcoming the noise limitation: rather than a traditional phase-unwrapping method, we extract the continuous phase values from holograms by using a phase-generation technique based on conditional generative adversarial networks employing a Pix2Pix architecture. We demonstrate that a network trained on random surfaces can accurately generate phase maps for test objects such as dumbbells, spheres, and biconcave discoids. Furthermore, we show that even a rapidly trained network can generate faithful phase maps when trained on related objects. We are able to accurately extract both morphological and quantitative features from the noisy phase maps of human leukemia (HL-60) cells, where traditional phase unwrapping algorithms fail. We conclude that deep learning can decouple noise from signal, expanding potential applications to real-world systems that may be noisy.

Phase characterisation of metalenses

Metalenses have emerged as a new optical element or system in recent years, showing superior performance and abundant applications. However, the phase distribution of a metalens has not been measured directly up to now, hindering further quantitative evaluation of its performance. We have developed an interferometric imaging phase measurement system to measure the phase distribution of a metalens by taking only one photo of the interference pattern. Based on the measured phase distribution, we analyse the negative chromatic aberration effect of monochromatic metalenses and propose a feature size of metalenses. Different sensitivities of the phase response to wavelength between the Pancharatnam-Berry phase-based metalens and propagation phase-reliant metalens are directly observed in the experiment. Furthermore, through phase distribution analysis, it is found that the distance between the measured metalens and the brightest spot of focusing will deviate from the focal length when the metalens has a low nominal numerical aperture, even though the metalens is ideal without any fabrication error. We also use the measured phase distribution to quantitatively characterise the imaging performance of the metalens. Our phase measurement system will help not only designers optimise the designs of metalenses but also fabricants distinguish defects to improve the fabrication process, which will pave the way for metalenses in industrial applications.

Singular value decomposition approach to coherent averaging in digital holography

We present a new approach to coherent averaging in digital holography using singular value decomposition (SVD). Digital holography enables the extraction of phase information from intensity measurements. For this reason, SVD can be used to statistically determine the orthogonal vectors that align the complex-valued measurements from multiple frames and group common modes accounting for constant phase shift terms. The SVD approach enables the separation of multiple signals, which can be applied to remove undesired artifacts such as scatter in retrieved images. The advantages of the SVD approach are demonstrated here in experiments through fog-degraded holograms with spatially incoherent and coherent scatter.

Full-field passive elastography using digital holography

Off-axis digital holography is an imaging technique that allows direct measurement of phase and amplitude from one image. We utilize this technique to capture displacements induced by a diffuse shear wave field with high sensitivity. A noise-correlation-based algorithm is then used to measure mechanical properties of samples. This approach enables full-field quantitative passive elastography without the need of contact or a synchronized source of a mechanical wave. This passive elastography method is first validated on agarose test samples mimicking biological tissues, and first results on an ex vivo biological sample are presented.

Diffraction-limited axial scanning in thick biological tissue with an aberration-correcting adaptive lens

Diffraction-limited deep focusing into biological tissue is challenging due to aberrations that lead to a broadening of the focal spot. The diffraction limit can be restored by employing aberration correction for example with a deformable mirror. However, this results in a bulky setup due to the required beam folding. We propose a bi-actuator adaptive lens that simultaneously enables axial scanning and the correction of specimen-induced spherical aberrations with a compact setup. Using the bi-actuator lens in a confocal microscope, we show diffraction-limited axial scanning up to 340 μm deep inside a phantom specimen. The application of this technique to in vivo measurements of zebrafish embryos with reporter-gene-driven fluorescence in a thyroid gland reveals substructures of the thyroid follicles, indicating that the bi-actuator adaptive lens is a meaningful supplement to the existing adaptive optics toolset.

Wavelet compression of off-axis digital holograms using real/imaginary and amplitude/phase parts

Compression of digital holograms allows one to store, transmit, and reconstruct large sets of holographic data. There are many digital image compression methods, and usually wavelets are used for this task. However, many significant specialties exist for compression of digital holograms. As a result, it is preferential to use a set of methods that includes filtering, scalar and vector quantization, wavelet processing, etc. These methods in conjunction allow one to achieve an acceptable quality of reconstructed images and significant compression ratios. In this paper, wavelet compression of amplitude/phase and real/imaginary parts of the Fourier spectrum of filtered off-axis digital holograms is compared. The combination of frequency filtering, compression of the obtained spectral components, and extra compression of the wavelet decomposition coefficients by threshold processing and quantization is analyzed. Computer-generated and experimentally recorded digital holograms are compressed. The quality of the obtained reconstructed images is estimated. The results demonstrate the possibility of compression ratios of 380 using real/imaginary parts. Amplitude/phase compression allows ratios that are a factor of 2-4 lower for obtaining similar quality of reconstructed objects.

Raman imaging through multimode sapphire fiber

We report on a sapphire fiber Raman imaging probe's use for challenging applications where access is severely restricted. Small-dimension Raman probes have been developed previously for various clinical applications because they show great capability for diagnosing disease states in bodily fluids, cells, and tissues. However, applications of these sub-millimeter diameter Raman probes were constrained by two factors: first, it is difficult to incorporate filters and focusing optics at such small scale; second, the weak Raman signal is often obscured by strong background noise from the fiber probe material, especially the most commonly used silica, which has a strong broad background noise in low wavenumbers (<500-1700 cm^-1). Here, we demonstrate the thinnest-known imaging Raman probe with a 60 μm diameter Sapphire multimode fiber in which both excitation and signal collection pass through. This probe takes advantage of the low fluorescence and narrow Raman peaks of Sapphire, its inherent high temperature and corrosion resistance, and large numerical aperture (NA). Raman images of Polystyrene beads, carbon nanotubes, and CaSO₄ agglomerations are obtained with a spatial resolution of 1 μm and a field of view of 30 μm. Our imaging results show that single polystyrene bead (~15 µm diameter) can be differentiated from a mixture with CaSO₄ agglomerations, which has a close Raman shift.

Wave front control with SLM and simulation of light wave diffraction

With the development of microfabrication technology, it has become possible to modulate the optical wavefront with high accuracy by a device such as a liquid crystal spatial light modulator (SLM). In this study, we conducted a theoretical analysis and experimental study on the generation of Laguerre Gaussian beam of vortex light wave using phase modulation SLM. Numerical simulation and experimental results on the generation of Laguerre Gaussian beam by both binary and phase only modulation are discussed based on the angular spectrum method and the Fresnel transformation method in hologram (CGH) diffraction. The experimental results show that Fresnel diffraction calculation based on Fourier transform method can simulate the light diffraction of SLM very well. This method is very suitable for the action simulation of digital optical wavefront control devices such as SLM. It can be imagined that the simulation method in this study can also play an important role in the design and development of digital optical diffraction elements.

Multimode fiber Φ-OTDR with holographic demodulation

We propose and demonstrate a method to perform quantitative phase-sensitive optical time domain reflectometry (Φ-OTDR) using multimode fiber. While most Φ-OTDR sensors use single-mode fiber, multimode fiber exhibits higher thresholds for non-linear effects, a larger capture fraction of Rayleigh backscattered light, and the potential to avoid signal fading by detecting many spatial modes in parallel. Previous multimode fiber based OTDR sensors discarded most of the backscattered light and thus failed to take advantage of these noise-reducing factors. Here, we show that by performing off-axis holography with a high-speed camera, we can record the entire Rayleigh backscattered field, maximizing the detected light level and making the sensor immune to fading. The sensor exhibits a high degree of linearity, a minimum phase noise of -80 dB [rel. rad²/Hz], and 20 kHz bandwidth.

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.

Surface relief and refractive index gratings patterned in chalcogenide glasses and studied by off-axis digital holography

Surface relief gratings and refractive index gratings are formed by direct holographic recording in amorphous chalcogenide nanomultilayer structures As₂S₃-Se and thin films As₂S₃. The evolution of the grating parameters, such as the modulation of refractive index and relief depth in dependence of the holographic exposure, is investigated. Off-axis digital holographic microscopy is applied for the measurement of the photoinduced phase gratings. For the high-accuracy reconstruction of the wavefront (amplitude and phase) transmitted by the fabricated gratings, we used a computational technique based on the sparse modeling of phase and amplitude. Both topography and refractive index maps of recorded gratings are revealed. Their separated contribution in diffraction efficiency is estimated.

Improved particle position accuracy from off-axis holograms using a Chebyshev model

Side scattered light from micrometer-sized particles is recorded using an off-axis digital holographic setup. From holograms, a volume is reconstructed with information about both intensity and phase. Finding particle positions is non-trivial, since poor axial resolution elongates particles in the reconstruction. To overcome this problem, the reconstructed wavefront around a particle is used to find the axial position. The method is based on the change in the sign of the curvature around the true particle position plane. The wavefront curvature is directly linked to the phase response in the reconstruction. In this paper we propose a new method of estimating the curvature based on a parametric model. The model is based on Chebyshev polynomials and is fit to the phase anomaly and compared to a plane wave in the reconstructed volume. From the model coefficients, it is possible to find particle locations. Simulated results show increased performance in the presence of noise, compared to the use of finite difference methods. The standard deviation is decreased from 3-39 μm to 6-10 μm for varying noise levels. Experimental results show a corresponding improvement where the standard deviation is decreased from 18 μm to 13 μm.

3D image distortion problem in digital in-line holographic microscopy and its effective solution

Digital in-line holographic microscopy (DIHM) has attracted attention because of its simple but powerful three-dimensional (3D) imaging capability. To improve the spatial resolution, 3D image reconstruction algorithms use numerical magnification, which generates distortions in the generated images. We propose a method to overcome this problem by using the simple relation between the object and image positions in 3D space. Several holograms were taken while translating a resolution target at different axial positions by a motorized stage. We demonstrated the effectiveness of our method by reconstructing the 3D positions of 3-μm-diameter polymer beads on a tilted slide glass from a single measured hologram.

Off-axis holographic laser speckle contrast imaging of blood vessels in tissues

Laser speckle contrast imaging (LSCI) has become one of the most common tools for functional imaging in tissues. Incomplete theoretical description and sophisticated interpretation of measurement results are completely sidelined by a low-cost and simple hardware, fastness, consistent results, and repeatability. In addition to the relatively low measuring volume with around 700 ?? ? m of the probing depth for the visible spectral range of illumination, there is no depth selectivity in conventional LSCI configuration; furthermore, in a case of high NA objective, the actual penetration depth of light in tissues is greater than depth of field (DOF) of an imaging system. Thus, the information about these out-of-focus regions persists in the recorded frames but cannot be retrieved due to intensity-based registration method. We propose a simple modification of LSCI system based on the off-axis holography to introduce after-registration refocusing ability to overcome both depth-selectivity and DOF problems as well as to get the potential possibility of producing a cross-section view of the specimen.

Sub-millimeter depth-resolved digital holography

We present sub-millimeter full-field depth from focus digital holography of surface topography of rough objects. For each pixel, the depth of the object is calculated from the variance of the intensity image over a set of reconstruction distances. First, we theoretically describe the axial resolution of this method and show that sub-millimeter resolution is feasible. Second, using a digital holography setup without magnifying optics or lateral scanning we experimentally demonstrate 100 μm axial resolution depth ranging and surface topography imaging. This is significantly better than what has previously been reported using digital holography and could make this technique useful for rapid large-area characterization of surface topography of objects.

Gated holographic imaging for structured illumination through obscurations

We describe a pulsed phase conjugator system that projects a laser beam possessing a scene-dependent energy distribution. We expand upon the inherent properties of optical phase conjugation (OPC) to include not only corrections for phase aberrations but to provide targeted laser beam illumination of partially obscured objects. For instance, if incorporated into a foliage-penetrating lidar system, OPC could significantly enhance performance. We demonstrate this OPC beam projection concept experimentally.

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.

An ultra-compact multiplexed holographic microscope using a multiple-pinhole aperture

In this paper, we present a low-cost and ultra-compact holographic microscope with multiple imaging areas. Instead of a dual-pinhole aperture as presented in our previous work, a multi-pinhole aperture is employed to filter the light source and to generate a reference wave as well as multiple object waves. The reference wave and the object waves interfere at the digital sensor and form multiplexed off-axis holograms without any lenses, splitters or combiners. The optimal number of object waves is determined which does not only fit our system but also brings some inspiration for traditional multiplexed off-axis holography. Our new system is tested to be able to retrieve quantitative phase images along with the amplitude images of multiple imaging areas at the same time with a lateral resolution of ∼ 2.2µm and an accuracy of the optical path of tens of nanometers.

Application of up-sampling and resolution scaling to Fresnel reconstruction of digital holograms

Fresnel transform implementation methods using numerical preprocessing techniques are investigated in this paper. First, it is shown that up-sampling dramatically reduces the minimum reconstruction distance requirements and allows maximal signal recovery by eliminating aliasing artifacts which typically occur at distances much less than the Rayleigh range of the object. Second, zero-padding is employed to arbitrarily scale numerical resolution for the purpose of resolution matching multiple holograms, where each hologram is recorded using dissimilar geometric or illumination parameters. Such preprocessing yields numerical resolution scaling at any distance. Both techniques are extensively illustrated using experimental results.

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.

In-line particle holography with an astigmatic beam: setup self-calibration using an "inverse problems" approach

The use of digital in-line holography for the characterization of confined flows in cylindrical geometry confinements (e.g., cylindrical pipe or cylindrical capillaries) is discussed. Due to cylindrical geometry of the walls, the illuminating laser wave can be strongly astigmatic, which renders the use of classical reconstruction techniques impossible. Contrary to plane wave holography setup, the diffraction pattern of the particles strongly depends on the axial distance of the latter to the entry face of the confinement structure. To address this reconstruction issue, we propose to use an "inverse problems" approach. This approach amounts to finding the best match (least squares solution) between a diffraction pattern model and the captured hologram. For this purpose, a direct imaging model for astigmatic holograms, based on the use of transfer matrices, is presented and validated by comparing experimental and simulated holograms. The accuracy of the "inverse problems" reconstruction is then used to calibrate the experimental setup adjustable parameters. Finally, the approach is tested through experimental astigmatic hologram reconstruction, thus paving the way to its use in pipe flow studies.

Simple, portable, and low-cost microscope based on off-axis digital holography using two spherical waves

In this Letter, we present a highly compact and low-cost holographic microscope which is especially suitable for observing transparent samples with certain specific supports such as microchannels. This microscope employs only an inexpensive laser diode, a dual precision round aperture, and a digital light sensor. The total cost of the system except for the digital sensor is less than 400 US dollars, and a hand-held system can be made based on our setup. Besides the simple, cheap, and compact setup, this system can capture the off-axis interference pattern of two spherical waves and reconstruct the quantitative phase profile along with the amplitude image of the observed sample with the twin image eliminated in real time. Experimental results show that the resolution of our system is better than 2 μm, and the 3D structure of blood cells can be retrieved.

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.

Off-axis digital holographic microscopy: practical design parameters for operating at diffraction limit

The utilization of microscope objectives (MOs) in digital holographic microscopy (DHM) has associated effects that are not present in conventional optical microscopy. The remaining phase curvature, which can ruin the quantitative phase imaging, is the most evident and analyzed. As phase imaging is considered, this interest has made possible the development of different methods of overcoming its undesired consequences. Additionally to the effects in phase imaging, there exist a set of less obvious conditions that have to be accounted for as MOs are utilized in DHM to achieve diffraction-limit operation. These conditions have to be considered even in the case in which only amplitude or intensity imaging is of interest. In this paper, a thorough analysis of the physical parameters that control the appropriate utilization of MOs in DHM is presented. A regular DHM system is theoretically modeled on the basis of the imaging theory. The Fourier spectrum of the recorded hologram is analyzed to evaluate the performance of the DHM. A set of the criteria that consider the microscope features and the recording parameters to achieve DHM operation at the diffraction limit is derived. Numerical modeling and experimental results are shown to validate our findings.

Absolute measurement of small-amplitude vibrations by time-averaged heterodyne holography with a dual local oscillator

We report a demonstration of the measurement of the ratio between an optical modulation sideband component and the nonshifted light component by time-averaged heterodyne holography in off-axis and frequency-shifting configuration, through coherent frequency-division multiplexing with a dual optical local oscillator. Experimental results of sinusoidal vibration sensing are reported. This technique enables absolute measurements of subnanometric out-of-plane vibration amplitudes.