Tomographic three-dimensional imaging of a biological specimen using wavelength-scanning digital interference holography

Implementation of a portable diffraction phase microscope for digital histopathology

Quantitative phase imaging (QPI) has a significant advantage in histopathology as it helps in differentiating biological tissue structures and cells without the need for staining. To make this capability more accessible, it is crucial to develop compact and portable systems. In this study, we introduce a portable diffraction phase microscopy (DPM) system that allows the acquisition of phase map images from various organs in mice using a low-NA objective lens. Our findings indicate that the cell and tissue structures observed in portable DPM images are similar to those seen in conventional histology microscope images. We confirmed that the developed system's performance is comparable to the benchtop DPM system. Additionally, we investigate the potential utility of digital histopathology by applying deep learning technology to create virtual staining of DPM images.

3D Shape reconstruction of normal and cancerous red blood cells using digital holographic tomography: Combination of angular spectrum method and multiplicative technique

Since the red blood cell shape affects the Oxygen transport, so a robust method to reconstruct the 3D shape of an RBC from different projections is presented. A robust one-piece polarizing holographic microscope setup is used to record inline holograms of normal and cancerous red blood cells (RBCs) with high stability. The inline holograms are corrected by flat fielding and windowed Fourier filtering methods to mitigate the zero-order and the defocused twin image due to the inline recording configuration to the least measure. The corrected inline holograms are then reconstructed by the angular spectrum method to extract the 2D wrapping phase-contrast images. The 2D wrapping phase-contrast images are then unwrapped using the graph cuts algorithm to extract the continuous 2D phase-contrast images. The continuous 2D phase-contrast images are reconstructed at different projections by the multiplicative technique to extract the 3D shape of the normal and the cancerous RBCs. Experimental results show that any deformation in the shape of the normal and the cancerous RBCs can be seen clearly at any rotational angle in 3D. This method, which is based on the degree of deformation from the best fitting, can be used as an alternative method of counting method for discrimination between normal and cancerous cells and hence diagnoses the disease easily. This article is protected by copyright. All rights reserved.

Influence of noise-reduction techniques in sparse-data sample rotation tomographic imaging

Data acquisition and processing is a critical issue for high-speed applications, especially in three-dimensional live cell imaging and analysis. This paper focuses on sparse-data sample rotation tomographic reconstruction and analysis with several noise-reduction techniques. For the sample rotation experiments, a live Candida rugosa sample is used and controlled by holographic optical tweezers, and the transmitted complex wavefronts of the sample are recorded with digital holographic microscopy. Three different cases of sample rotation tomography were reconstructed for dense angle with a step rotation at every 2°, and for sparse angles with step rotation at every 5° and 10°. The three cases of tomographic reconstruction performance are analyzed with consideration for data processing using four noise-reduction techniques. The experimental results demonstrate potential capability in retaining the tomographic image quality, even at the sparse angle reconstructions, with the help of noise-reduction techniques.

Adaptive wavefront correction structured illumination holographic tomography

In this study, a novel adaptive wavefront correction (AWC) technique is implemented on a compactly developed structured illumination holographic tomography (SI-HT) system. We propose a mechanical movement-free compact scanning architecture for SI-HT systems with AWC, implemented by designing and displaying a series of computer-generated holograms (CGH) composed of blazed grating with phase Fresnel lens on a phase-only spatial light modulator (SLM). In the proposed SI-HT, the aberrations of the optical system are sensed by digital holography and are used to design the CGH-based AWC to compensate the phase aberrations of the tomographic imaging system. The proposed method was validated using a standard Siemens star target, its potential application was demonstrated using a live candida rugosa sample, and its label-free three-dimensional refractive index profile was generated at its subcellular level. The experimental results obtained reveal the ability of the proposed method to enhance the imaging performance in both lateral and axial directions.

En face optical coherence tomography: a technology review [Invited]

A review on the technological development of en face optical coherence tomography (OCT) and optical coherence microscopy (OCM) is provided. The terminology originally referred to time domain OCT, where the preferential scanning was performed in the en face plane. Potentially the fastest realization of en face image recording is full-field OCT, where the full en face plane is illuminated and recorded simultaneously. The term has nowadays been adopted for high-speed Fourier domain approaches, where the en face image is reconstructed from full 3D volumes either by direct slicing or through axial projection in post processing. The success of modern en face OCT lies in its immediate and easy image interpretation, which is in particular of advantage for OCM or OCT angiography. Applications of en face OCT with a focus on ophthalmology are presented. The review concludes by outlining exciting technological prospects of en face OCT based both on time as well as on Fourier domain OCT.

Optical coherence tomography in Optics Express [Invited]

Optical coherence tomography (OCT) is one of the most successful technologies in the history of biomedical optics. Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express. On the occasion of the 20th anniversary of the journal, this review analyzes the reasons for the success of OCT papers in Optics Express and discusses possible motivations for researchers to submit some of their best OCT papers to the journal.

Integrated dual-tomography for refractive index analysis of free-floating single living cell with isotropic superresolution

Digital holographic microtomography is a promising technique for three-dimensional (3D) measurement of the refractive index (RI) profiles of biological specimens. Measurement of the RI distribution of a free-floating single living cell with an isotropic superresolution had not previously been accomplished. To the best of our knowledge, this is the first study focusing on the development of an integrated dual-tomographic (IDT) imaging system for RI measurement of an unlabelled free-floating single living cell with an isotropic superresolution by combining the spatial frequencies of full-angle specimen rotation with those of beam rotation. A novel ‘UFO’ (unidentified flying object) like shaped coherent transfer function is obtained. The IDT imaging system does not require any complex image-processing algorithm for 3D reconstruction. The working principle was successfully demonstrated and a 3D RI profile of a single living cell, Candida rugosa, was obtained with an isotropic superresolution. This technology is expected to set a benchmark for free-floating single live sample measurements without labeling or any special sample preparations for the experiments.

Non-Destructive Analysis of the Internal Anatomical Structures of Mosquito Specimens Using Optical Coherence Tomography

The study of mosquitoes and analysis of their behavior are of crucial importance in the on-going efforts to control the alarming increase in mosquito-borne diseases. Furthermore, a non-destructive and real-time imaging technique to study the anatomical features of mosquito specimens can greatly aid the study of mosquitoes. In this study, we demonstrate the three-dimensional imaging capabilities of optical coherence tomography (OCT) for structural analysis of Anopheles sinensis mosquitoes. The anatomical features of An. sinensis head, thorax, and abdominal regions, along with the morphology of internal structures, such as foregut, midgut, and hindgut, were studied using OCT imaging. Two-dimensional and three-dimensional OCT images, used in conjunction with histological images, proved useful for anatomical analysis of mosquito specimens. By presenting this work as an initial study, we demonstrate the applicability of OCT for future mosquito-related entomological research, and also in identifying changes in mosquito anatomical structure.

Wavefront shaping for imaging-based flow velocity measurements through distortions using a Fresnel guide star

Imaging-based flow measurement techniques, like particle image velocimetry (PIV), are vulnerable to time-varying distortions like refractive index inhomogeneities or fluctuating phase boundaries. Such distortions strongly increase the velocity error, as the position assignment of the tracer particles and the decrease of image contrast exhibit significant uncertainties. We demonstrate that wavefront shaping based on spatially distributed guide stars has the potential to significantly reduce the measurement uncertainty. Proof of concept experiments show an improvement by more than one order of magnitude. Possible applications for the wavefront shaping PIV range from measurements in jets and film flows to biomedical applications.

Direct method of three-dimensional imaging using the multiple-wavelength range-gated active imaging principle

The tomography executed with mono-wavelength active imaging systems uses the recording of several images to restore a three-dimensional (3D) scene. Thus, in order to show the depth in the scene, a different color is attributed to each recorded image. Therefore, the 3D restoration depends on the video frame rate of the camera. By using a multiple-wavelength range-gated active imaging system, it is possible to restore the 3D scene directly in a single image at the moment of recording with a video camera. Each emitted light pulse with a different wavelength corresponds to a visualized zone at a different distance in the scene. The camera shutter opens just once during the emission of light pulses with the different wavelengths. Thus, the restoration can be executed in real time with regard to the video frame rate of the camera. From an analytical model and from a graphical approach, we demonstrated the feasibility of this new method of 3D restoration. The non-overlapping conditions between two consecutive visualized zones are analyzed. The experimental test results confirm these different conditions and validate the theoretical principle to directly restore the 3D scene in a color image with a multiple-wavelength laser source, an RGB filter, and a triggerable intensified camera.

Functional imaging for regenerative medicine

In vivo imaging is a platform technology with the power to put function in its natural structural context. With the drive to translate stem cell therapies into pre-clinical and clinical trials, early selection of the right imaging techniques is paramount to success. There are many instances in regenerative medicine where the biological, biochemical, and biomechanical mechanisms behind the proposed function of stem cell therapies can be elucidated by appropriate imaging. Imaging techniques can be divided according to whether labels are used and as to whether the imaging can be done in vivo. In vivo human imaging places additional restrictions on the imaging tools that can be used. Microscopies and nanoscopies, especially those requiring fluorescent markers, have made an extraordinary impact on discovery at the molecular and cellular level, but due to their very limited ability to focus in the scattering tissues encountered for in vivo applications they are largely confined to superficial imaging applications in research laboratories. Nanoscopy, which has tremendous benefits in resolution, is limited to the near-field (e.g. near-field scanning optical microscope (NSNOM)) or to very high light intensity (e.g. stimulated emission depletion (STED)) or to slow stochastic events (photo-activated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM)). In all cases, nanoscopy is limited to very superficial applications. Imaging depth may be increased using multiphoton or coherence gating tricks. Scattering dominates the limitation on imaging depth in most tissues and this can be mitigated by the application of optical clearing techniques that can impose mild (e.g. topical application of glycerol) or severe (e.g. CLARITY) changes to the tissue to be imaged. Progression of therapies through to clinical trials requires some thought as to the imaging and sensing modalities that should be used. Smoother progression is facilitated by the use of comparable imaging modalities throughout the discovery and trial phases, giving label-free techniques an advantage wherever they can be used, although this is seldom considered in the early stages. In this paper, we will explore the techniques that have found success in aiding discovery in stem cell therapies and try to predict the likely technologies best suited to translation and future directions.

In-line digital holographic imaging in volume holographic microscopy

A dual-plane in-line digital holographic imaging method incorporating volume holographic microscopy (VHM) is presented to reconstruct objects in a single shot while eliminating zero-order and twin-image diffracted waves. The proposed imaging method is configured such that information from different axial planes is acquired simultaneously using multiplexed volume holographic imaging gratings, as used in VHM, and recorded as in-line holograms where the corresponding reference beams are generated in the fashion of Gabor's in-line holography. Unlike conventional VHM, which can take axial intensity information only at focal depths, the proposed method digitally reconstructs objects at any axial position. Further, we demonstrate the proposed imaging technique's ability to effectively eliminate zero-order and twin images for single-shot three-dimensional object reconstruction.

Depth resolved imaging by digital holography with an illumination of constantly changing curvature

In this Letter, a single wavelength digital holographic method is proposed to achieve depth resolved imaging by recording a series of holograms in the reflection geometry with an illumination of constantly changing curvature. A proper algorithm is employed to selectively generate the images of the object at different depths, including the phase and the modulus information. Theoretical analysis is supported by a visible light experiment to illustrate the feasibility of the proposed method.

Speckle noise suppression in digital holography by angular diversity with phase-only spatial light modulator

A speckle noise suppression method in digital holography is proposed by the angular diversity with a phase-only spatial light modulator (SLM). The minimal angular difference of illumination beams is quantitatively analyzed to ensure the noncorrelation of any two speckle patterns, and then the phase-only SLM is employed to generate a series of tilted illumination beams. Comparing with the typical methods, the tilted illumination beams are controlled dynamically and accurately, which makes it possible to record a large number of holograms. Finally, using an image-plane digital holographic system, 117 holograms are recorded respectively, and the synthesized reconstructed images are obtained with the greatly suppressed speckle noise which is in good agreement with the theoretical results. The experimental results demonstrate the effectiveness, repeatability, and practicability of the proposed approach.

Off-axis setup taking full advantage of incoherent illumination in coherence-controlled holographic microscope

Coherence-controlled holographic microscope (CCHM) combines off-axis holography and an achromatic grating interferometer allowing for the use of light sources of arbitrary degree of temporal and spatial coherence. This results in coherence gating and strong suppression of coherent noise and parasitic interferences enabling CCHM to reach high phase measurement accuracy and imaging quality. The achievable lateral resolution reaches performance of conventional widefield microscopes, which allows resolving up to twice smaller details when compared to typical off-axis setups. Imaging characteristics can be controlled arbitrarily by coherence between two extremes: fully coherent holography and confocal-like incoherent holography. The basic setup parameters are derived and described in detail and experimental validations of imaging characteristics are demonstrated.

Real-time in vivo computed optical interferometric tomography

High-resolution real-time tomography of scattering tissues is important for many areas of medicine and biology^1-6. However, the compromise between transverse resolution and depth-of-field in addition to low sensitivity deep in tissue continue to impede progress towards cellular-level volumetric tomography. Computed imaging has the potential to solve these long-standing limitations. Interferometric synthetic aperture microscopy (ISAM)^7-9 is a computed imaging technique enabling high-resolution volumetric tomography with spatially invariant resolution. However, its potential for clinical diagnostics remains largely untapped since full volume reconstructions required lengthy postprocessing, and the phase-stability requirements have been difficult to satisfy in vivo. Here we demonstrate how 3-D Fourier-domain resampling, in combination with high-speed optical coherence tomography (OCT), can achieve high-resolution in vivo tomography. Enhanced depth sensitivity was achieved over a depth-of-field extended in real time by more than an order of magnitude. This work lays the foundation for high-speed volumetric cellular-level tomography.

Tomographic imaging via spectral encoding of spatial frequency

Three-dimensional optical tomographic imaging plays an important role in biomedical research and clinical applications. We introduce spectral tomographic imaging (STI) via spectral encoding of spatial frequency principle that not only has the capability for visualizing the three-dimensional object at sub-micron resolution but also providing spatially-resolved quantitative characterization of its structure with nanoscale accuracy for any volume of interest within the object. The theoretical basis and the proof-of-concept numerical simulations are presented to demonstrate the feasibility of spectral tomographic imaging.

General temperature field measurement by digital holography

This paper presents a digital holographic method for measurement of periodic asymmetric temperature fields. The method is based on a modified Twyman-Green setup having double sensitivity. For measurement only one precisely synchronized and triggered digital camera is used. The periodicity and self-similarity of each cycle of the measured phenomenon combined with the precisely synchronized camera capture allow one to obtain data later used for three-dimensional (3D) measurement. The reconstruction of 3D temperature field is based on tomographic approach.

High-precision three-dimensional shape reconstruction via digital refocusing in multi-wavelength digital holography

Three-dimensional (3D) shape reconstructions and metrology measurements are often limited by depth-of-field constraints. Current focus-detection-based techniques are insufficient to profile out-of-focus 3D objects with high axial accuracy. Extended-focus imaging (EFI) techniques can improve the range and precision of such measurements. By incorporating digital refocusing with multiwavelength interferometry, a holographic imaging solution is presented in this paper to accurately measure 3D objects over a large depth range. Accuracy and repeatability of the proposed EFI technique are validated by digital simulations and refocusing experiments. A reconstruction example demonstrates the feasibility of high-precision 3D measurements of objects deeper than the system's classical depth of field.

Real-time quantitative visualization of 3D structural information

We demonstrate a novel approach for the real time visualization and quantification of the 3D spatial frequencies in an image domain. Our approach is based on the spectral encoding of spatial frequency principle and permits the formation of an image as a color map in which spatially separated spectral wavelengths correspond to the dominant 3D spatial frequencies of the object. We demonstrate that our approach can visualize and analyze the dominant axial internal structure for each image point in real time and with nanoscale sensitivity to structural changes. Computer modeling and experimental results of instantaneous color visualization and quantification of 3D structures of a model system and biological samples are presented.

Off-axis digital hologram reconstruction: some practical considerations

Holographic rendering of off-axis intensity digital holograms is discussed. A review of some of the main numerical processing methods, based either on the Fourier transform interpretation of the propagation integral or on its linear system counterpart, is reported. Less common methods such as adjustable magnification reconstruction schemes and Fresnelet decomposition are presented and applied to the digital treatment of off-axis holograms. The influence of experimental parameters on the classical hologram reconstruction methods is assessed, offering guidelines for optimal image rendering regarding the hologram recording conditions.

Focus detection criterion for refocusing in multi-wavelength digital holography

The majority of focus detection criteria reported is based on amplitude contrast. Due to phase wrapping, phase contrast was previously reported unsuitable for focus finding tasks. By taking the advantage of multi-wavelength digital holography, we propose a new focus detection criterion based on phase contrast. Experimental results are presented to prove the feasibility of the developed criterion. Possible applications of the developed technology include inspecting machined surfaces in the auto industry.

Terahertz digital holography using angular spectrum and dual wavelength reconstruction methods

Terahertz digital off-axis holography is demonstrated using a Mach-Zehnder interferometer with a highly coherent, frequency tunable, continuous wave terahertz source emitting around 0.7 THz and a single, spatially-scanned Schottky diode detector. The reconstruction of amplitude and phase objects is performed digitally using the angular spectrum method in conjunction with Fourier space filtering to reduce noise from the twin image and DC term. Phase unwrapping is achieved using the dual wavelength method, which offers an automated approach to overcome the 2π phase ambiguity. Potential applications for nondestructive test and evaluation of visually opaque dielectric and composite objects are discussed.

Optical sectioning with a low-coherence phase-shifting digital holographic microscope

The properties of a low-coherence phase-shifting digital holographic microscope are first studied and analyzed. We then demonstrate en face imaging with transverse resolution of 3 μm and axial resolution of 10 μm through a thickness of 300 μm onion membrane. In addition, with the above said resolutions, optical sectioning of the eye and spine of a live zebra fish has been demonstrated. To the best of our knowledge, this is the first time that a short coherence phase-shifting holographic microscope has been applied to the internal structure visualization of a biological specimen under an in vivo environment.

Quantitative imaging of cellular adhesion by total internal reflection holographic microscopy

Total internal reflection (TIR) holographic microscopy uses a prism in TIR as a near-field imager to perform quantitative phase microscopy of cell-substrate interfaces. The presence of microscopic organisms, cell-substrate interfaces, adhesions, and tissue structures on the prism's TIR face causes relative index of refraction and frustrated TIR to modulate the object beam's evanescent wave phase front. We present quantitative phase images of test specimens such as Amoeba proteus and cells such as SKOV-3 and 3T3 fibroblasts.

Swept-source digital holography to reconstruct tomographic images

We present what we believe to be a new method of swept-source digital holography using a superluminescent diode (SLD) as a broadband light source and an acousto-optic tunable filter (AOTF) as a frequency tunable device. The swept source consists of an SLD as a broadband source in conjunction with the AOTF as the frequency tuning device in the wavelength range of 800-870 nm. Since the AOTF is an electronically controlled device, frequency tuning can be achieved without mechanical movement . The angular spectrum approach to the scalar diffraction theory is used to reconstruct the images for each wavelength. Applications of a broadband source ensure an increased axial resolution of reconstructed images. The proposed swept-source system provides a sufficiently broad range of tunability and can increase the axial range and the resolution of reconstructed tomographic images using digital holography. The system was tested using a semireflecting glass substrate; a character "B" is written on it with black ink. Experimental results are presented.

Submicrometer tomography of cells by multiple-wavelength digital holographic microscopy in reflection

We present first results on a method enabling mechanical scanning-free tomography with submicrometer axial resolution by multiple-wavelength digital holographic microscopy. By sequentially acquiring reflection holograms and summing 20 wavefronts equally spaced in spatial frequency in the 485-670 nm range, we are able to achieve a slice-by-slice tomographic reconstruction with a 0.6-1 microm axial resolution in a biological medium. The method is applied to erythrocytes investigation to retrieve the cellular membrane profile in three dimensions.

Digital holography of total internal reflection

We introduce a new microscopy technique termed total internal reflection holographic microscopy (TIRHM). Quantitative phase microscopy by digital holography is used to image the phase profile of light in total internal reflection, which is modulated by the materials present on or near the surface of internal reflection. The imaging characteristics are theoretically modeled and imaging capabilities are experimentally demonstrated.

Quantitative Phase Microscopy of microstructures with extended measurement range and correction of chromatic aberrations by multiwavelength digital holography

Quantitative Phase Microscopy (QPM) by interferometric techniques can require a multiwavelength configuration to remove 2pi ambiguity and improve accuracy. However, severe chromatic aberration can affect the resulting phase-contrast map. By means of classical interference microscope configuration it is quite unpractical to correct such aberration. We propose and demonstrate that by Digital Holography (DH) in a microscope configuration it is possible to clear out the QPM map from the chromatic aberration in a simpler and more effective way with respect to other approaches. The proposed method takes benefit of the unique feature of DH to record in a plane out-of-focus and subsequently reconstruct numerically at the right focal image plane. In fact, the main effect of the chromatic aberration is to shift differently the correct focal image plane at each wavelength and this can be readily compensated by adjusting the corresponding reconstruction distance for each wavelength. A procedure is described in order to determine easily the relative focal shift among different imaging wavelengths by performing a scanning of the numerical reconstruction along the optical axis, to find out the focus and to remove at the same time the chromatic aberration.

Influence of shot noise on phase measurement accuracy in digital holographic microscopy

Digital Holographic Microscopy (DHM) is a single shot interferometric technique, which provides quantitative phase images with subwavelength axial accuracy. A short hologram acquisition time (down to microseconds), allows DHM to offer a reduced sensitivity to vibrations, and real time observation is achievable thanks to present performances of personal computers and charge coupled devices (CCDs). Fast dynamic imaging at low-light level involves few photons, requiring proper camera settings (integration time and gain of the CCD; power of the light source) to minimize the influence of shot noise on the hologram when the highest phase accuracy is aimed. With simulated and experimental data, a systematic analysis of the fundamental shot noise influence on phase accuracy in DHM is presented.

Numerical parametric lens for shifting, magnification, and complete aberration compensation in digital holographic microscopy

The concept of numerical parametric lenses (NPL) is introduced to achieve wavefront reconstruction in digital holography. It is shown that operations usually performed by optical components and described in ray geometrical optics, such as image shifting, magnification, and especially complete aberration compensation (phase aberrations and image distortion), can be mimicked by numerical computation of a NPL. Furthermore, we demonstrate that automatic one-dimensional or two-dimensional fitting procedures allow adjustment of the NPL parameters as expressed in terms of standard or Zernike polynomial coefficients. These coefficients can provide a quantitative evaluation of the aberrations generated by the specimen. Demonstration is given of the reconstruction of the topology of a microlens.

Submicrometer optical tomography by multiple-wavelength digital holographic microscopy

We present a method for submicrometer tomographic imaging using multiple wavelengths in digital holographic microscopy. This method is based on the recording, at different wavelengths equally separated in the k domain, in off-axis geometry, of the interference between a reference wave and an object wave reflected by a microscopic specimen and magnified by a microscope objective. A CCD camera records the holograms consecutively, which are then numerically reconstructed following the convolution formulation to obtain each corresponding complex object wavefront. Their relative phases are adjusted to be equal in a given plane of interest and the resulting complex wavefronts are summed. The result of this operation is a constructive addition of complex waves in the selected plane and destructive addition in the others. Tomography is thus obtained by the attenuation of the amplitude out of the plane of interest. Numerical variation of the plane of interest enables one to scan the object in depth. For the presented simulations and experiments, 20 wavelengths are used in the 480-700 nm range. The result is a sectioning of the object in slices 725 nm thick.

Shot-noise influence on the reconstructed phase image signal-to-noise ratio in digital holographic microscopy

In digital holographic microscopy, shot noise is an intrinsic part of the recording process with the digital camera. We present a study based on simulations and real measurements describing the shot-noise influence in the quality of the reconstructed phase images. Different configurations of the reference wave and the object wave intensities will be discussed, illustrating the detection limit and the coherent amplification of the object wave. The signal-to-noise ratio (SNR) calculation of the reconstructed phase images based on the decision statistical theory is derived from a model for image quality estimation proposed by Wagner and Brown [Phys. Med. Biol. 30, 489 (1985)]. It will be shown that a phase image with a SNR above 10 can be obtained with a mean intensity lower than 10 photons per pixel and per hologram coming from the observed object. Experimental measurements on a glass-chrome probe will be presented to illustrate the main results of the simulations.

Full-field time-encoded frequency-domain optical coherence tomography

Ultrahigh axial resolution surface profiling as well as volumetric optical imaging based on time encoded optical coherence tomography in the frequency domain without any mechanical scanning element is presented. A frequency tuned broad bandwidth titanium sapphire laser is interfaced to an optical microscope (Axioskop 2 MAT, Carl Zeiss Meditec) that is enhanced with an interferometric imaging head. The system is equipped with a 640 x 480 pixel CMOS camera, optimized for the 800 nm wavelength tuning range for transmission and reflection measurements of a microscopic sample. Sample volume information over 1.3 x 1 x 0.2 mm(3) with ~3 mum axial and ~4 mum transverse resolution in tissue is acquired by a single wavelength scan over more than 100 nm optical bandwidth from <760 to >860 nm with 128-2048 equidistant optical frequency steps with an acquisition time of 1 to 50 ms per step. Topography and tomography with a signal to noise ratio of 83 dB is demonstrated on test surfaces and biological specimen respectively. This novel OCT technique promises to enable high speed, three dimensional imaging by employing high frame rate cameras and state of the art tunable lasers in a mechanically stable environment, due to lack of moving components while reducing the intensity on the sample.

Movies of cellular and sub-cellular motion by digital holographic microscopy

Background

Many biological specimens, such as living cells and their intracellular components, often exhibit very little amplitude contrast, making it difficult for conventional bright field microscopes to distinguish them from their surroundings. To overcome this problem phase contrast techniques such as Zernike, Normarsky and dark-field microscopies have been developed to improve specimen visibility without chemically or physically altering them by the process of staining. These techniques have proven to be invaluable tools for studying living cells and furthering scientific understanding of fundamental cellular processes such as mitosis. However a drawback of these techniques is that direct quantitative phase imaging is not possible. Quantitative phase imaging is important because it enables determination of either the refractive index or optical thickness variations from the measured optical path length with sub-wavelength accuracy.

Digital holography is an emergent phase contrast technique that offers an excellent approach in obtaining both qualitative and quantitative phase information from the hologram. A CCD camera is used to record a hologram onto a computer and numerical methods are subsequently applied to reconstruct the hologram to enable direct access to both phase and amplitude information. Another attractive feature of digital holography is the ability to focus on multiple focal planes from a single hologram, emulating the focusing control of a conventional microscope.

Methods

A modified Mach-Zender off-axis setup in transmission is used to record and reconstruct a number of holographic amplitude and phase images of cellular and sub-cellular features.

Results

Both cellular and sub-cellular features are imaged with sub-micron, diffraction-limited resolution. Movies of holographic amplitude and phase images of living microbes and cells are created from a series of holograms and reconstructed with numerically adjustable focus, so that the moving object can be accurately tracked with a reconstruction rate of 300ms for each hologram. The holographic movies show paramecium swimming among other microbes as well as displaying some of their intracellular processes. A time lapse movie is also shown for fibroblast cells in the process of migration.

Conclusion

Digital holography and movies of digital holography are seen to be useful new tools for visualization of dynamic processes in biological microscopy. Phase imaging digital holography is a promising technique in terms of the lack of coherent noise and the precision with which the optical thickness of a sample can be profiled, which can lead to images with an axial resolution of a few nanometres.

Characterization of microlenses by digital holographic microscopy

We demonstrate the use of digital holographic microscopy (DHM) as a metrological tool in micro-optics testing. Measurement principles are compared with those performed with Twyman-Green, Mach-Zehnder, and white-light interferometers. Measurements performed on refractive microlenses with reflection DHM are compared with measurements performed with standard interferometers. Key features of DHM such as digital focusing, measurement of shape differences with respect to a perfect model, surface roughness measurements, and optical performance evaluation are discussed. The capability of imaging nonspherical lenses without any modification of the optomechanical setup is a key advantage of DHM compared with conventional measurement tools and is demonstrated on a cylindrical microlens and a square lens array.

Wavelength-scanning digital interference holography for tomographic three-dimensional imaging by use of the angular spectrum method

A tomographic imaging system based on wavelength-scanning digital interference holography is developed by applying the angular spectrum method. Compared to the well-known Fresnel diffraction formula, which is subject to a minimum distance requirement in reconstruction, the angular spectrum method can reconstruct the wave field at any distance from the hologram plane. The new system allows three-dimensional tomographic images to be extracted with an improved signal-to-noise ratio, a more flexible scanning range, and an easier specimen size selection. Experiments are performed to demonstrate the effectiveness of the method.

Wavelength scanning digital interference holography for variable tomographic scanning

We present a novel technique of variable tomographic scanning capable of reconstructing tomographic images of an object volume along any arbitrarily tilted plane. The method is based on wavelength scanning digital interference holography, using a series of holograms generated with a range of scanned wavelengths. From each hologram, the object field is reconstructed in a number of selected tilted planes. The desired tomographic images are then reconstructed from the numerical superposition of the object fields. Thus the tomographic images can be generated along variable planes without the need for physically repeating the scanning and recording processes. Experimental results are presented to verify the proposed concept.

Full-color three-dimensional microscopy by wide-field optical coherence tomography

We demonstrate a method of optical tomography for surface and sub-surface imaging of biological tissues, based on the principle of wide field optical coherence tomography and capable of providing full-color three-dimensional views of a tissue structure. Contour or tomographic images are obtained with an interferometric imaging system using broadband light sources. The interferometric images are analyzed in the three color channels and recombined to generate 3D microscopic images of tissue structures with full natural color representation. In contrast to most existing three-dimensional microscopy methods, the presented technique allows monitoring of tissue structures close to its natural color, and can provide critical information for the physiological and pathological applications.

Compensation of the inherent wave front curvature in digital holographic coherent microscopy for quantitative phase-contrast imaging

An approach is proposed for removing the wavefront curvature introduced by the microscope imaging objective in digital holography, which otherwise hinders the phase contrast imaging at reconstruction planes. The unwanted curvature is compensated by evaluating a correcting wave front at the hologram plane with no need for knowledge of the optial parameters, focal length of the imaging lens, or distances in the setup. Most importantly it is shown that a correction effect can be obtained at all reconstruction planes. Three different methods have been applied to evaluate the correction wave front and the methods are discussed in detail. The proposed approach is demonstrated by applying digital holography as a method of coherent microscopy for imaging amplitude and phase contrast of microstructures.

Numerical reconstruction of digital holograms with variable viewing angles

Here we describe a new method for numerically reconstructing an object with variable viewing angles from its hologram(s) within the Fresnel domain. The proposed algorithm can render the real image of the original object not only with different focal lengths but also with changed viewing angles. Some representative simulation results and demonstrations are presented to verify the effectiveness of the algorithm.