Compact and portable low-coherence interferometer with off-axis geometry for quantitative phase microscopy and nanoscopy

Six-pack holography for dynamic profiling of thick and extended objects by simultaneous three-wavelength phase unwrapping with doubled field of view

Dynamic holographic profiling of thick samples is limited due to the reduced field of view (FOV) of off-axis holography. We present an improved six-pack holography system for the simultaneous acquisition of six complex wavefronts in a single camera exposure from two fields of view (FOVs) and three wavelengths, for quantitative phase unwrapping of thick and extended transparent objects. By dynamically generating three synthetic wavelength quantitative phase maps for each of the two FOVs, with the longest wavelength being 6207 nm, hierarchical phase unwrapping can be used to reduce noise while maintaining the improvements in the 2π phase ambiguity due to the longer synthetic wavelength. The system was tested on a 7 μm tall PDMS microchannel and is shown to produce quantitative phase maps with 96% accuracy, while the hierarchical unwrapping reduces noise by 93%. A monolayer of live onion epidermal tissue was also successfully scanned, demonstrating the potential of the system to dynamically decrease scanning time of optically thick and extended samples.

Off-axis common-path digital holography using a cube beam splitter

An off-axis common-path digital holography is built up by inserting a 45° tilted cube beam splitter (CBS) into a 4f system that is described in this paper. Two apertures are set as the input of the 4f system, where one supports the object, and the other is vacant. The CBS divides the incoming beam into two copies, which are symmetrical with each other along the semi-reflecting layer. Due to the separation of two beams in a Fourier plane and the flipping of the field of view induced by the CBS, an off-axis hologram can be captured. Moreover, the carrier frequency can be easily modulated by translating the CBS perpendicular to the optical axis. The new proposed scheme has high light utilization, a compact setup, and high temporal stability. The experiments are carried out to demonstrate the validity and stability of the proposed method.

Numerical analysis of the effect of reduced temporal coherence in quantitative phase microscopy and tomography

We present the numerical analysis of the effect of the temporarily partially coherent illumination on the phase measurement accuracy in digital holography microscopy (DHM) and optical diffraction tomography (ODT), as reconstruction algorithms tend to assume purely monochromatic conditions. In the regime of reduced temporal coherence, we simulate the hologram formation in two different optical setups, representing classical off-axis two-beam and grating common-path configurations. We consider two ODT variants: with sample rotation and angle-scanning of illumination. Besides the coherence degree of illumination, our simulation considers the influence of the sample normal dispersion, shape of the light spectrum, and optical parameters of the imaging setup. As reconstruction algorithms we employ Fourier hologram method and first-order Rytov approximation with direct inversion and nonnegativity constraints. Quantitative evaluation of the measurement results deviations introduced by the mentioned error sources is comprehensively analyzed, for the first time to the best of our knowledge. Obtained outcomes indicate low final DHM/ODT reconstruction errors for the grating-assisted common-path configuration. Nevertheless, dispersion and asymmetric spectrum introduce non-negligible overestimated refractive index values and noise, and should be thus carefully considered within experimental frameworks.

Single-Shot 3D Topography of Transmissive and Reflective Samples with a Dual-Mode Telecentric-Based Digital Holographic Microscope

Common path DHM systems are the most robust DHM systems as they are based on self-interference and are thus less prone to external fluctuations. A common issue amongst these DHM systems is that the two replicas of the sample’s information overlay due to self-interference, making them only suitable for imaging sparse samples. This overlay has restricted the use of common-path DHM systems in material science. The overlay can be overcome by limiting the sample’s field of view to occupy only half of the imaging field of view or by using an optical spatial filter. In this work, we have implemented optical spatial filtering in a common-path DHM system using a Fresnel biprism. We have analyzed the optimal pinhole size by evaluating the frequency content of the reconstructed phase images of a star target. We have also measured the accuracy of the system and the sensitivity to noise for different pinhole sizes. Finally, we have proposed the first dual-mode common-path DHM system using a Fresnel biprism. The performance of the dual-model DHM system has been evaluated experimentally using transmissive and reflective microscopic samples.

Simplified dual-channel two-wavelength interferometer using a polarized cube beam splitter

Two-wavelength interferometers can extend an unambiguous measurement range; however, they suffer from complex optical configurations. To simplify the optical setup for a two-wavelength common-path off-axis interferometer, we propose a dual-channel two-wavelength interferometer using a polarized cube beam splitter. In contrast with the previously presented two-wavelength common-path off-axis interferometer, the proposed method has a simple setup, in which only one polarized cube beam splitter is inserted into the 4f system. With the help of polarization modulation, two single-wavelength interferograms can be captured simultaneously. Several experimental results are presented to demonstrate the advantages and effectiveness of the proposed method.

Sperm Inspection for In Vitro Fertilization via Self-Assembled Microdroplet Formation and Quantitative Phase Microscopy

We present a new method for the selection of individual sperm cells using a microfluidic device that automatically traps each cell in a separate microdroplet that then individually self-assembles with other microdroplets, permitting the controlled measurement of the cells using quantitative phase microscopy. Following cell trapping and droplet formation, we utilize quantitative phase microscopy integrated with bright-field imaging for individual sperm morphology and motility inspection. We then perform individual sperm selection using a single-cell micromanipulator, which is enhanced by the microdroplet-trapping procedure described above. This method can improve sperm selection for intracytoplasmic sperm injection, a common type of in vitro fertilization procedure.

Label-free quantitative measurement of cardiovascular dynamics in a zebrafish embryo using frequency-comb-referenced-quantitative phase imaging

SIGNIFICANCE

Real-time monitoring of the heart rate and blood flow is crucial for studying cardiovascular dysfunction, which leads to cardiovascular diseases.

AIM

This study aims at in-depth understanding of high-speed cardiovascular dynamics in a zebrafish embryo model for various biomedical applications via frequency-comb-referenced quantitative phase imaging (FCR-QPI).

APPROACH

Quantitative phase imaging (QPI) has emerged as a powerful technique in the field of biomedicine but has not been actively applied to the monitoring of circulatory/cardiovascular parameters, due to dynamic speckles and low frame rates. We demonstrate FCR-QPI to measure heart rate and blood flow in a zebrafish embryo. FCR-QPI utilizes a high-speed photodetector instead of a conventional camera, so it enables real-time monitoring of individual red blood cell (RBC) flow.

RESULTS

The average velocity of zebrafish's RBCs was measured from 192.5 to 608.8  μm  /  s at 24 to 28 hour-post-fertilization (hpf). In addition, the number of RBCs in a pulsatile blood flow was revealed to 16 cells/pulse at 48 hpf. The heart rates corresponded to 94 and 142 beats-per-minute at 24 and 48 hpf.

CONCLUSIONS

This approach will newly enable in-depth understanding of the cardiovascular dynamics in the zebrafish model and possible usage for drug discovery applications in biomedicine.

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.

Lensless Three-Dimensional Quantitative Phase Imaging Using Phase Retrieval Algorithm

Quantitative phase imaging (QPI) techniques are widely used for the label-free examining of transparent biological samples. QPI techniques can be broadly classified into interference-based and interferenceless methods. The interferometric methods which record the complex amplitude are usually bulky with many optical components and use coherent illumination. The interferenceless approaches which need only the intensity distribution and works using phase retrieval algorithms have gained attention as they require lesser resources, cost, space and can work with incoherent illumination. With rapid developments in computational optical techniques and deep learning, QPI has reached new levels of applications. In this tutorial, we discuss one of the basic optical configurations of a lensless QPI technique based on the phase-retrieval algorithm. Simulative studies on QPI of thin, thick, and greyscale phase objects with assistive pseudo-codes and computational codes in Octave is provided. Binary phase samples with positive and negative resist profiles were fabricated using lithography, and a single plane and two plane phase objects were constructed. Light diffracted from a point object is modulated by phase samples and the corresponding intensity patterns are recorded. The phase retrieval approach is applied for 2D and 3D phase reconstructions. Commented codes in Octave for image acquisition and automation using a web camera in an open source operating system are provided.

Cell and nucleus refractive-index mapping by interferometric phase microscopy and rapid confocal fluorescence microscopy

We present a multimodal technique for measuring the integral refractive index and the thickness of biological cells and their organelles by integrating interferometric phase microscopy (IPM) and rapid confocal fluorescence microscopy. First, the actual thickness maps of the cellular compartments are reconstructed using the confocal fluorescent sections, and then the optical path difference (OPD) map of the same cell is reconstructed using IPM. Based on the co-registered data, the integral refractive index maps of the cell and its organelles are calculated. This technique enables rapidly measuring refractive index of live, dynamic cells, where IPM provides quantitative imaging capabilities and confocal fluorescence microscopy provides molecular specificity of the cell organelles. We acquire human colorectal adenocarcinoma cells and show that the integral refractive index values are similar for the whole cell, the cytoplasm and the nucleus on the population level, but significantly different on the single cell level.

Advantages of Fresnel biprism-based digital holographic microscopy in quantitative phase imaging

SIGNIFICANCE

The hallmarks of digital holographic microscopy (DHM) compared with other quantitative phase imaging (QPI) methods are high speed, accuracy, spatial resolution, temporal stability, and polarization-sensitivity (PS) capability. The above features make DHM suitable for real-time quantitative PS phase imaging in a broad number of biological applications aimed at understanding cell growth and dynamic changes occurring during physiological processes and/or in response to pharmaceutical agents.

AIM

The insertion of a Fresnel biprism (FB) in the image space of a light microscope potentially turns any commercial system into a DHM system enabling QPI with the five desired features in QPI simultaneously: high temporal sensitivity, high speed, high accuracy, high spatial resolution, and PS. To the best of our knowledge, this is the first FB-based DHM system providing these five features all together.

APPROACH

The performance of the proposed system was calibrated with a benchmark phase object. The PS capability has been verified by imaging human U87 glioblastoma cells.

RESULTS

The proposed FB-based DHM system provides accurate phase images with high spatial resolution. The temporal stability of our system is in the order of a few nanometers, enabling live-cell studies. Finally, the distinctive behavior of the cells at different polarization angles (e.g., PS capability) can be observed with our system.

CONCLUSIONS

We have presented a method to turn any commercial light microscope with monochromatic illumination into a PS QPI system. The proposed system provides accurate quantitative PS phase images in a new, simple, compact, and cost-effective format, thanks to the low cost (a few hundred dollars) involved in implementing this simple architecture, enabling the use of this QPI technique accessible to most laboratories with standard light microscopes.

Holographic virtual staining of individual biological cells

We present a method for virtual staining for morphological analysis of individual biological cells based on stain-free digital holography, allowing clinicians and biologists to visualize and analyze the cells as if they have been chemically stained. Our approach provides numerous advantages, as it 1) circumvents the possible toxicity of staining materials, 2) saves time and resources, 3) optimizes inter- and intralab variability, 4) allows concurrent staining of different types of cells with multiple virtual stains, and 5) provides ideal conditions for real-time analysis, such as rapid stain-free imaging flow cytometry. The proposed method is shown to be accurate, repeatable, and nonsubjective. Hence, it bears great potential to become a common tool in clinical settings and biological research.

Many medical and biological protocols for analyzing individual biological cells involve morphological evaluation based on cell staining, designed to enhance imaging contrast and enable clinicians and biologists to differentiate between various cell organelles. However, cell staining is not always allowed in certain medical procedures. In other cases, staining may be time-consuming or expensive to implement. Staining protocols may be operator-sensitive, and hence may lead to varying analytical results, as well as cause artificial imaging artifacts or false heterogeneity. We present a deep-learning approach, called HoloStain, which converts images of isolated biological cells acquired without staining by holographic microscopy to their virtually stained images. We demonstrate this approach for human sperm cells, as there is a well-established protocol and global standardization for characterizing the morphology of stained human sperm cells for fertility evaluation, but, on the other hand, staining might be cytotoxic and thus is not allowed during human in vitro fertilization (IVF). After a training process, the deep neural network can take images of unseen sperm cells retrieved from holograms acquired without staining and convert them to their stainlike images. We obtained a fivefold recall improvement in the analysis results, demonstrating the advantage of using virtual staining for sperm cell analysis. With the introduction of simple holographic imaging methods in clinical settings, the proposed method has a great potential to become a common practice in human IVF procedures, as well as to significantly simplify and radically change other cell analyses and techniques such as imaging flow cytometry.

High-resolution 4-D acquisition of freely swimming human sperm cells without staining

We present a new acquisition method that enables high-resolution, fine-detail full reconstruction of the three-dimensional movement and structure of individual human sperm cells swimming freely. We achieve both retrieval of the three-dimensional refractive-index profile of the sperm head, revealing its fine internal organelles and time-varying orientation, and the detailed four-dimensional localization of the thin, highly-dynamic flagellum of the sperm cell. Live human sperm cells were acquired during free swim using a high-speed off-axis holographic system that does not require any moving elements or cell staining. The reconstruction is based solely on the natural movement of the sperm cell and a novel set of algorithms, enabling the detailed four-dimensional recovery. Using this refractive-index imaging approach, we believe that we have detected an area in the cell that is attributed to the centriole. This method has great potential for both biological assays and clinical use of intact sperm cells.

Quantitative phase imaging by wide-field interferometry with variable shearing distance uncoupled from the off-axis angle

We introduce a new shearing interferometry module for digital holographic microscopy, in which the off-axis angle, which defines the interference fringe frequency, is not coupled to the shearing distance, as is the case in most shearing interferometers. Thus, it enables the selection of shearing distance based on the spatial density of the sample, without losing spatial frequency content due to overlapping of the complex wave fronts in the spatial frequency domain. Our module is based on a 4f imaging unit and a diffraction grating, in which the hologram is generated from two mutually coherent, partially overlapping sample beams, with adjustable shearing distance, as defined by the position of the grating, but with a constant off-axis angle, as defined by the grating period. The module is simple, easy to align, and presents a nearly common-path geometry. By placing this module as an add-on unit at the exit port of an inverted microscope, quantitative phase imaging can easily be performed. The system is characterized by a 2.5 nm temporal stability and a 3.4 nm spatial stability, without using anti-vibration techniques. We provide quantitative phase imaging experiments of silica beads with different shearing distances, red blood cell fluctuations, and cancer cells flowing in a micro-channel, which demonstrate the capability and versatility of our approach in different imaging scenarios.

PhUn-Net: ready-to-use neural network for unwrapping quantitative phase images of biological cells

We present a deep-learning approach for solving the problem of 2π phase ambiguities in two-dimensional quantitative phase maps of biological cells, using a multi-layer encoder-decoder residual convolutional neural network. We test the trained network, PhUn-Net, on various types of biological cells, captured with various interferometric setups, as well as on simulated phantoms. These tests demonstrate the robustness and generality of the network, even for cells of different morphologies or different illumination conditions than PhUn-Net has been trained on. In this paper, for the first time, we make the trained network publicly available in a global format, such that it can be easily deployed on every platform, to yield fast and robust phase unwrapping, not requiring prior knowledge or complex implementation. By this, we expect our phase unwrapping approach to be widely used, substituting conventional and more time-consuming phase unwrapping algorithms.

Quantitative Phase and Intensity Microscopy Using Snapshot White Light Wavefront Sensing

Phase imaging techniques are an invaluable tool in microscopy for quickly examining thin transparent specimens. Existing methods are limited to either simple and inexpensive methods that produce only qualitative phase information (e.g. phase contrast microscopy, DIC), or significantly more elaborate and expensive quantitative methods. Here we demonstrate a low-cost, easy to implement microscopy setup for quantitative imaging of phase and bright field amplitude using collimated white light illumination.

Single-shot two-frame π-shifted spatially multiplexed interference phase microscopy

Single-shot, two-frame, π-shifted spatially multiplexed interference microscopy (π-SMIM) is presented as an improvement to previous SMIM implementations, introducing a versatile, robust, fast, and accurate method for cumbersome, noisy, and low-contrast phase object analysis. The proposed π-SMIM equips a commercially available nonholographic microscope with a high-speed (video frame rate) enhanced quantitative phase imaging (QPI) capability by properly placing a beam-splitter in the microscope embodiment to simultaneously (in a single shot) record two holograms mutually phase shifted by π radians at the expense of reducing the field of view. Upon subsequent subtractive superimposition of holograms, a π-hologram is generated with reduced background and improved modulation of interference fringes. These features determine superior phase retrieval quality, obtained by employing the Hilbert spiral transform on the π-hologram, as compared with a single low-quality (low signal-to-noise ratio) hologram analysis. In addition, π-SMIM enables accurate in-vivo analysis of high dynamic range phase objects, otherwise measurable only in static regime using time-consuming phase-shifting. The technique has been validated utilizing a 20  ×    /  0.46 NA objective in a regular Olympus BX-60 upright microscope for QPI of different lines of prostate cancer cells and flowing microbeads.

Quantitative phase imaging based on simple Michelson-type lateral shearing interferometry with rotational right-angle prisms

By using a kind of simple Michelson-type lateral shearing interferometry, in this paper, the precise quantitative phase measurement of transparent microscopic objects is realized successfully. For this interferometry, on the basis of the fundamental structure of the traditional Michelson interferometer, the two plane mirrors are replaced with two ordinary right-angle prisms. In the beginning, the ridges of the two right-angle prisms are set to align with the optical axis and be in the vertical direction. Subsequently, to achieve the lateral shear, one of these two right-angle prisms is rotated around its ridge. Furthermore, the goal to obtain more lateral shear can be achieved by introducing a bigger rotating angle or rotating another prism simultaneously. In addition, owing to the simple structure of the Michelson interferometer and the inexpensive optical components used, the system is compact, portable, easy to operate, and low cost. The experimental results show the practicability of this system.

A multi-mode digital holographic microscope

We present a transmission-mode digital holographic microscope that can switch easily between three different imaging modes: inline, dark field off-axis, and bright field off-axis. Our instrument can be used: to track through time in three dimensions microscopic dielectric objects, such as motile micro-organisms; localize brightly scattering nanoparticles, which cannot be seen under conventional bright field illumination; and recover topographic information and measure the refractive index and dry mass of samples via quantitative phase recovery. Holograms are captured on a digital camera capable of high-speed video recording of up to 2000 frames per second. The inline mode of operation can be easily configurable to a large range of magnifications. We demonstrate the efficacy of the inline mode in tracking motile bacteria in three dimensions in a 160 μm × 160 μm × 100 μm volume at 45× magnification. Through the use of a novel physical mask in a conjugate Fourier plane in the imaging path, we use our microscope for high magnification, dark field off-axis holography, demonstrated by localizing 100 nm gold nanoparticles at 225× magnification up to at least 16 μm from the imaging plane. Finally, the bright field off-axis mode facilitates quantitative phase microscopy, which we employ to measure the refractive index of a standard resolution test target and to measure the dry mass of human erythrocytes.

Is multiplexed off-axis holography for quantitative phase imaging more spatial bandwidth-efficient than on-axis holography? [Invited]

Digital holographic microcopy is a thriving imaging modality that attracts considerable research interest due to its ability not only to create excellent label-free contrast but also to supply valuable physical information regarding the density and dimensions of the sample with nanometer-scale axial sensitivity. Three basic holographic recording geometries currently exist, including on-axis, off-axis, and slightly off-axis holography, each of which enables a variety of architectures in terms of bandwidth use and compression capacity. Specifically, off-axis holography and slightly off-axis holography allow spatial hologram multiplexing, enabling one to compress more information into the same digital hologram. In this paper, we define an efficiency score to analyze the various possible architectures and compare the signal-to-noise ratio and the mean squared error obtained using each of them, thus determining the optimal holographic method.

Flipping interferometry with doubled imaging area

We present a new external off-axis holographic module that doubles the acquired complex wavefront field of view, based on using both holographic flipping and multiplexing. In contrast to previous designs, this design does not require spatial filtering (no pinhole or lenses) to create the reference beam externally. In addition, the overlap area between the fields of view, as well as the off-axis angle between the sample and reference beams, can be fully controlled. As we demonstrate experimentally, this approach is useful for quantitative phase microscopy of extended stationary and dynamic samples, such as cancer cells during rapid flow and beating cardiomyocytes.

Stain-free interferometric phase microscopy correlation with DNA fragmentation stain in human spermatozoa

Acridine orange (AO) staining is used to diagnose DNA fragmentation status in sperm cells. Interferometric phase microscopy (IPM) is an optical imaging method based on digital holographic microscopy that provides quantitative morphological and refractive index imaging of cells in vitro without the need for staining. We have imaged sperm cells using stain-free IPM in order to estimate different cellular parameters, such as acrosome dry mass and size, in addition to an embryologist evaluation according to the World Health Organization (WHO)-2010 criteria. Following this, the same sperm cells were stained by AO, imaged using a fluorescence confocal microscope and assessed by the AO-emitted color, forming five DNA fragmentation groups. These DNA fragmentation groups were correlated with the embryologist-based classification and the IPM-based morphological parameters. Our results indicate on significant differences in the IPM-based parameters between groups with different fragmentation levels. Based on the validation with AO, we conclude that stain-free IPM images analyzed digitally may assist in selecting sperm cells with intact DNA prior to intracytoplasmic sperm injection. This information may potentially increase percentage of successful pregnancies.

Wavelength-multiplexed digital holography for quantitative phase measurement using quantum dot film

We propose an enhanced quantitative three-dimensional measurement system using wavelength-multiplexed digital holography. To simplify the configuration, a dual-peak quantum dot wavelength converter, combined with a blue LED, is adapted as a single low-coherence light source. Rather than a conventional dual-wavelength method, which records and reconstruct the object wave for each wavelength, the proposed system can capture the holograms of two wavelengths simultaneously with fewer acquisitions, simple setup, and low noise. To verify the system's performance, the measurements of the step height sample are presented.

Simultaneous off-axis multiplexed holography and regular fluorescence microscopy of biological cells

We present a new technique for obtaining simultaneous multimodal quantitative phase and fluorescence microscopy of biological cells, providing both quantitative phase imaging and molecular specificity using a single camera. Our system is based on an interferometric multiplexing module, externally positioned at the exit of an optical microscope. In contrast to previous approaches, the presented technique allows conventional fluorescence imaging, rather than interferometric off-axis fluorescence imaging. We demonstrate the presented technique for imaging fluorescent beads and live biological cells.

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.

Hilbert-Huang single-shot spatially multiplexed interferometric microscopy

Hilbert-Huang single-shot spatially multiplexed interferometric microscopy (H2S2MIM) is presented as the implementation of a robust, fast, and accurate single-shot phase estimation algorithm with an extremely simple, low-cost, and highly stable way to convert a bright field microscope into a holographic one using partially coherent illumination. Altogether, H2S2MIM adds high-speed (video frame rate) quantitative phase imaging capability to a commercially available nonholographic microscope with improved phase reconstruction (coherence noise reduction). The technique has been validated using a 20×/0.46  NA objective in a regular Olympus BX-60 upright microscope for static, as well as dynamic, samples showing perfect agreement with the results retrieved from a temporal phase-shifting algorithm.

Integral refractive index imaging of flowing cell nuclei using quantitative phase microscopy combined with fluorescence microscopy

We suggest a new multimodal imaging technique for quantitatively measuring the integral (thickness-average) refractive index of the nuclei of live biological cells in suspension. For this aim, we combined quantitative phase microscopy with simultaneous 2-D fluorescence microscopy. We used 2-D fluorescence microscopy to localize the nucleus inside the quantitative phase map of the cell, as well as for measuring the nucleus radii. As verified offline by both 3-D confocal fluorescence microscopy and 2-D fluorescence microscopy while rotating the cells during flow, the nucleus of cells in suspension that are not during division can be assumed to be an ellipsoid. The entire shape of a cell in suspension can be assumed to be a sphere. Then, the cell and nucleus 3-D shapes can be evaluated based on their in-plain radii available from the 2-D phase and fluorescent measurements, respectively. Finally, the nucleus integral refractive index profile is calculated. We demonstrate the new technique on cancer cells, obtaining nucleus refractive index values that are lower than those of the cytoplasm, coinciding with recent findings. We believe that the proposed technique has the potential to be used for flow cytometry, where full 3-D refractive index tomography is too slow to be implemented during flow.

Off-axis tilt compensation in common-path digital holographic microscopy based on hologram rotation

We present a simple and effective compensation method for the off-axis tilt in common-path digital holographic microscopy (CPDHM) by introducing a rotating operation on the hologram. The proposed method mainly requires a digital reference hologram (DRH), which is a rotated version of the original one; it is assumed to be easy to obtain by rotating the specimen's hologram 180°. In this way, the off-axis tilt could be removed by subtracting the retrieved phase of DRH from the retrieved phase of the original hologram, but without any complex spectrum centering judgment, fitting procedures, or prior knowledge of the system. This highly automatic and efficient performance makes our approach available for real-time quantitative phase imaging (QPI), although it limits the field of view (FOV) of the specimen. Some experimental results of microlens array and phase plate demonstrate the feasibility and effectiveness of the proposed method.

Compensation of aberration in quantitative phase imaging using lateral shifting and spiral phase integration

We present a simple and effective method to eliminate system aberrations in quantitative phase imaging. Using spiral phase integration, complete information about system aberration is calculated from three laterally shifted phase images. The present method is especially useful when measuring confluent samples in which acquisition of background area is challenging. To demonstrate validity and applicability, we present measurements of various types of samples including microspheres, HeLa cells, and mouse brain tissue. Working conditions and limitations are systematically analyzed and discussed.

Single-shot dual-wavelength off-axis quasi-common-path digital holography using polarization-multiplexing

We present a dual-wavelength off-axis quasi-common-path digital holography for quantitative phase imaging using polarization-multiplexing in a single shot. Employing an off-axis nearly common-path configuration, our approach separates the two-wavelength information using a polarizing beam splitter, while modulates the orthogonal fringe directions for each wavelength using two retro-reflector mirrors, and thus generates a single multiplexed off-axis interferogram on a monochrome CCD camera. The information of a specimen, including phase and height, is reconstructed through a division algorithm for dual wavelengths with the help of a specimen-free multiplexed interferogram. The experimental results obtained on a square step target and a circular step target illustrate the validity and stability of our setup.

Localized measurements of physical parameters within human sperm cells obtained with wide-field interferometry

We developed a new method to identify the separate cellular compartments in the optical path delay (OPD) maps of un-labeled spermatozoa. This was conducted by comparing OPD maps of fixed, un-labeled spermatozoa to bright field images of the same cells following labeling. The labeling enabled us to identify the acrosomal and nuclear compartments in the corresponding OPD maps of the cells. We then extracted the refractive index maps of fixed cells by dividing the OPD maps of spermatozoa by the corresponding thickness maps of the same cells, obtained with AFM. Finally, the dry mass of the head, nucleus and acrosome of un-labeled immobile spermatozoa, was measured. This method provides the ability to quantitatively measure the dry mass of cellular compartments within human spermatozoa. We expect that these measurements will assist label-free selection of sperm cells for fertilization.

Double-channel angular-multiplexing polarization holography with common-path and off-axis configuration

We propose a double-channel angular-multiplexing polarization holographic imaging system with common-path and off-axis configurations. In the system, its input plane is spatially divided into three windows: an object window and two reference windows, and two orthogonal linear polarizers are attached, respectively, on the two reference windows; a two-dimensional cross grating is inserted between the input and output planes of the system. Thus the object beam passing through the object window and the two orthogonal polarized reference beams passing through the two reference windows can overlap each other at the output plane of the system and form a double-channel angular-multiplexing polarization hologram (DC-AM-PH). Using this system, the complex amplitude distributions of two orthogonal polarized components from an object can be recorded and reconstructed by one single-shot DC-AM-PH at the same time. Theoretical analysis and experimental results demonstrated that the system can be used to measure the Jones matrix parameters of polarization-sensitive or birefringent materials.

Compact lensless off-axis transmission digital holographic microscope

Current compact lensless holographic microscopes are based on either multiple angle in-line holograms, multiple wavelength illumination or a combination thereof. Complex computational algorithms are necessary to retrieve the phase image which slows down the visualization of the image. Here we propose a simple compact lensless transmission holographic microscope with an off-axis configuration which simplifies considerably the computational processing to visualize the phase images and opens the possibility of real time phase imaging using off the shelf smart phone processors and less than $3 worth of optics and detectors, suitable for broad educational dissemination. This is achieved using a side illumination and analog hologram gratings to shape the reference and signal illumination beams from one light source. We demonstrate experimentally imaging of cells with a field of view (FOV) of ~12mm², and a resolution of ~3.9μm.

Lateral shearing common-path digital holographic microscopy based on a slightly trapezoid Sagnac interferometer

We propose a compact and easy-to-align lateral shearing common-path digital holographic microscopy, which is based on a slightly trapezoid Sagnac interferometer to create two laterally sheared beams and form off-axis geometry. In this interferometer, the two beams pass through a set of identical optical elements in opposite directions and have nearly the same optical path difference. Without any vibration isolation, the temporal stability of the setup is found to be around 0.011 rad. Compared with highly simple lateral shearing interferometer, the off-axis angle of the setup can be easily adjusted and quantitatively controlled, meanwhile the image quality is not degraded. The experiments for measuring the static and dynamic specimens are performed to demonstrate the capability and applicability.

Compact interferometric module for full-field interferometric phase microscopy with low spatial coherence illumination

We propose a compact and external off-axis interferometric module that can achieve interference with low spatial coherence illumination over the entire field of view. The interferometer is easy to align and stable and can be connected to the output of an existing microscope illuminated with a low spatial coherence light source, thus allowing quantitative phase imaging with a low degree of spatial noise. We demonstrate the imaging and the interference properties of the proposed interferometric module and use it for quantitative phase imaging of reflective samples.

Angular phase unwrapping of optically thick objects with a thin dimension

We present a new phase unwrapping approach, which allows reconstruction of optically thick objects that are optically thin from at least one viewing angle, by considering the information stored in the object phase maps captured from consecutive angles. Our algorithm combines 1-D phase unwrapping in the angular dimension with conventional 2-D phase unwrapping, to achieve unwrapping of the object from the optically thick perspective. We thus obtain quantitative phase imaging of objects that were previously impossible to image in certain viewing angles. To demonstrate our approach, we present both numerical simulation and experimental results for quantitative phase imaging of biological cells.

Rapid 3D Refractive-Index Imaging of Live Cells in Suspension without Labeling Using Dielectrophoretic Cell Rotation

A major challenge in the field of optical imaging of live cells is achieving rapid, 3D, and noninvasive imaging of isolated cells without labeling. If successful, many clinical procedures involving analysis and sorting of cells drawn from body fluids, including blood, can be significantly improved. A new label-free tomographic interferometry approach is presented. This approach provides rapid capturing of the 3D refractive-index distribution of single cells in suspension. The cells flow in a microfluidic channel, are trapped, and then rapidly rotated by dielectrophoretic forces in a noninvasive and precise manner. Interferometric projections of the rotated cell are acquired and processed into the cellular 3D refractive-index map. Uniquely, this approach provides full (360°) coverage of the rotation angular range around any axis, and knowledge on the viewing angle. The experimental demonstrations presented include 3D, label-free imaging of cancer cells and three types of white blood cells. This approach is expected to be useful for label-free cell sorting, as well as for detection and monitoring of pathological conditions resulting in cellular morphology changes or occurrence of specific cell types in blood or other body fluids.

Quadriwave lateral shearing interferometric microscopy with wideband sensitivity enhancement for quantitative phase imaging in real time

Real-time quantitative phase imaging has tremendous potential in investigating live biological specimens in vitro. Here we report on a wideband sensitivity-enhanced interferometric microscopy for quantitative phase imaging in real time by employing two quadriwave lateral shearing interferometers based on randomly encoded hybrid gratings with different lateral shears. Theoretical framework to analyze the measurement sensitivity is firstly proposed, from which the optimal lateral shear pair for sensitivity enhancement is also derived. To accelerate the phase retrieval algorithm for real-time visualization, we develop a fully vectorized path-independent differential leveling phase unwrapping algorithm ready for parallel computing, and the framerate for retrieving the phase from each pair of two 4 mega pixel interferograms is able to reach 47.85 frames per second. Experiment results demonstrate that the wideband sensitivity-enhanced interferometric microscopy is capable of eliminating all the periodical error caused by spectral leaking problem and reducing the temporal standard deviation to the half level compared with phase directly retrieved by the interferogram. Due to its high adaptability, the wideband sensitivity-enhanced interferometric microscopy is promising in retrofitting existing microscopes to quantitative phase microscopes with high measurement precision and real-time visualization.

Simultaneous two-wavelength phase unwrapping using an external module for multiplexing off-axis holography

We present a dual-wavelength external holographic microscopy module for quantitative phase imaging of 3D structures with extended thickness range. This is done by simultaneous acquisition of two off-axis interferograms, each at a different wavelength, and generation of a synthetic wavelength, which is larger than the sample optical thickness, allowing two-wavelength unwrapping. The simultaneous acquisition is carried out by using optical multiplexing of the two interferograms onto the camera, where each of them has orthogonal off-axis interference fringe direction in relation to the other one. We used the system to quantitatively image a 7.96 μm step target and 30.5 μm circular copper pillars.

High-speed phase-shifting common-path quantitative phase imaging with a piezoelectric actuator

We present a phase-shifting quantitative phase imaging technique providing high temporal and spatial phase stability and high acquisition speed. A piezoelectric microfabricated phase modulator allows tunable modulation frequencies up to the kHz range. After assessing the quantitative phase accuracy with technical samples, we demonstrate the high acquisition rate while monitoring cellular processes at temporal scales ranging from milliseconds to hours.

Label-free optical quantification of structural alterations in Alzheimer's disease

We present a wide-field quantitative label-free imaging of mouse brain tissue slices with sub-micrometre resolution, employing holographic microscopy and an automated scanning platform. From the measured light field images, scattering coefficients and anisotropies are quantitatively retrieved by using the modified the scattering-phase theorem, which enables access to structural information about brain tissues. As a proof of principle, we demonstrate that these scattering parameters enable us to quantitatively address structural alteration in the brain tissues of mice with Alzheimer’s disease.

Differential interference contrast tomography

We present a new approach to optical tomography of phase objects that is referred to as differential interference contrast tomography (DICT). The main feature of DICT is that the result of tomographic reconstruction is a 3D DIC image. This image is described by partial derivative of 3D refractive index distribution in one direction. The DICT setup consists of a lateral shearing phase-shifting interference microscope with low-coherent LED illumination. To create projections of the sample at various illumination angles, an angular scanning beam was used. 3D DIC tomograms of a white blood cell are presented. The comparison between the reconstructed DIC tomogram slices and the conventional DIC images of the same sample at the same depths are also represented.

Video-rate processing in tomographic phase microscopy of biological cells using CUDA

We suggest a new implementation for rapid reconstruction of three-dimensional (3-D) refractive index (RI) maps of biological cells acquired by tomographic phase microscopy (TPM). The TPM computational reconstruction process is extremely time consuming, making the analysis of large data sets unreasonably slow and the real-time 3-D visualization of the results impossible. Our implementation uses new phase extraction, phase unwrapping and Fourier slice algorithms, suitable for efficient CPU or GPU implementations. The experimental setup includes an external off-axis interferometric module connected to an inverted microscope illuminated coherently. We used single cell rotation by micro-manipulation to obtain interferometric projections from 73 viewing angles over a 180° angular range. Our parallel algorithms were implemented using Nvidia's CUDA C platform, running on Nvidia's Tesla K20c GPU. This implementation yields, for the first time to our knowledge, a 3-D reconstruction rate higher than video rate of 25 frames per second for 256 × 256-pixel interferograms with 73 different projection angles (64 × 64 × 64 output). This allows us to calculate additional cellular parameters, while still processing faster than video rate. This technique is expected to find uses for real-time 3-D cell visualization and processing, while yielding fast feedback for medical diagnosis and cell sorting.

Flipping interferometry and its application for quantitative phase microscopy in a micro-channel

We present a portable, off-axis interferometric module for quantitative phase microscopy of live cells, positioned at the exit port of a coherently illuminated inverted microscope. The module creates on the digital camera an interference pattern between the image of the sample and its flipped version. The proposed simplified module is based on a retro-reflector modification in an external Michelson interferometer. The module does not contain any lenses, pinholes, or gratings and its alignment is straightforward. Still, it allows full control of the off-axis angle and does not suffer from ghost images. As experimentally demonstrated, the module is useful for quantitative phase microscopy of live cells rapidly flowing in a micro-channel.

White-light quantitative phase imaging unit

We introduce the white-light quantitative phase imaging unit (WQPIU) as a practical realization of quantitative phase imaging (QPI) on standard microscope platforms. The WQPIU is a compact stand-alone unit which measures sample induced phase delay under white-light illumination. It does not require any modification of the microscope or additional accessories for its use. The principle of the WQPIU based on lateral shearing interferometry and phase shifting interferometry provides a cost-effective and user-friendly use of QPI. The validity and capacity of the presented method are demonstrated by measuring quantitative phase images of polystyrene beads, human red blood cells, HeLa cells and mouse white blood cells. With speckle-free imaging capability due to the use of white-light illumination, the WQPIU is expected to expand the scope of QPI in biological sciences as a powerful but simple imaging tool.