Digital holography and its multidimensional imaging applications: a review

Digital holography and coherent imaging are explored in the terahertz frequency region at 0.480 Terahertz with highly coherent, frequency-tunable continuous wave sources. The long coherence length of microwave sources suggests that interferometric imaging techniques such as Fresnel off-axis holography can achieve sub-micrometer depth resolution of surfaces and through materials and structures that are transmissive in the terahertz spectral region. Research in this area will provide an important non-destructive imaging tool for the rapidly expanding field of additive manufacturing and composite fabrication. Unfortunately, imaging optics as they are used in confocal imaging and optics in general adversely affect the performance of terahertz imagers as their aperture size if comparable to the wavelength, which artificially limits image resolution according to the diffraction limit and can cause undesirable coherence effects in the image. We report on imaging methods that minimize the use of optics but use signal processing techniques that form images digitally from recorded holograms. This approach is directly applicable to focal plane arrays in contrast to confocal imaging modalities. Furthermore, the performance of image reconstruction at multiple aspect angles compiled into videos as user friendly inspection tool is investigated.

Digital holography (Yamaguchi & Zhang, 1997; Osten et al., 2002; Kreis, 2005; Ferraro et al., 2005; Frauel et al., 2006; Nomura & Imbe, 2010) is a useful technique for recording the fully complex field of a wavefront. In line with advances in imaging devices such as CCDs, digital holography is accessible. The digital holography has been used for lots of applications including encryption (Javidi & Nomura, 2000; Tajauerce & Javidi, 2000; Nomura et al., 2004) and three-dimensional object recognition (Poon & Kim, 1999; Tajauerce et al., 2001; Javidi & Kim, 2005). However, the quality of a reconstructed image of a digital hologram is not good, because it is suffering from speckles. Comparing a conventional film hologram, the extent of a digital hologram is small and the resolution of the hologram is low in single figure. This is why the reconstructed image has noticeable speckles. To reduce the speckles, some signal processing approaches have been proposed (Javidi et al., 2005; Do et al., 2005; Maycock et al., 2005; Kreis & Schluter, 2007). By use of wavelet decomposition, three-dimensional image fusion has been proposed and some speckles reduction has been achieved. Independent component analysis has been also applied to reduce speckles. Aperture synthesis approach is also proposed to enhance the resolution. Somewhat speckle noises have been reduced by the approaches. In this paper, two methods to reduce speckle of the reconstructed image are presented. One is the method based on the superposition of the reconstructed images with different wavelengths. Another is the method based on the multiple holograms. In Section 2, the characteristics of speckles are reviewed briefly. In Section 3, the former method is described. Some experimental results are shown to confirm the proposed method. In Section 4, the latter method is described with some experimental results for confirmation.

Comparison of common-path off-axis digital holography and transport of intensity equation in quantitative phase measurement

Quantitative phase measurements at multiple wavelengths has created an opportunity for exploring new avenues in phase microscopy such as enhancing imaging-depth (1), measuring hemoglobin concentrations in erythrocytes (2), and more recently in tomographic mapping of the refractive index of live cells (3). To this end, quantitative phase imaging has been demonstrated both at few selected spectral points as well as with high spectral resolution (4,5). However, most of these developed techniques compromise imaging speed, field of view, or the spectral resolution to perform interferometric measurements at multiple colors. In the specific application of quantitative phase in studying blood diseases and red blood cells, current techniques lack the required sensitivity to quantify biological properties of interest at individual cell level. Recently, we have set out to develop a stable quantitative interferometric microscope allowing for measurements of such properties for red cells without compromising field of view or speed of the measurements. The feasibility of the approach will be initially demonstrated in measuring dispersion curves of known solutions, followed by measuring biological properties of red cells in sickle cell anemia.

The collimation of strongly diverging laser beams emitted by diode lasers is performed with micro-optical components. In order to obtain a good beam profile high-quality cylindrical micro-lenses with a large numerical aperture compared to conventional lenses have to be applied. The characterization of these components using conventional interferometric techniques is costly or inaccurate with respect to the required accuracy of the lens shape. In conventional interferometry the resulting interferogram has to be imaged onto the CCD-target. The imaging lenses in the set-up can lead to additional wavefront aberrations and therefore to measurement errors. Additionally, conventional techniques are using phase shifting techniques for the evaluation of the interferograms. These techniques require at least three phase shifted interferograms which leads to a higher experimental and temporal effort. Digital Holography is an advanced optical diagnostic tool mainly used for surface deformation analysis, measurement of refractive index variations and particle analysis in transparent media: Holograms are stored electronically without any imaging optics and the reconstruction is performed by numerical methods. Due to the reconstruction process a numerical representation of the recorded wavefront can be evaluated including amplitude and phase from one hologram. In this paper Digital Holography as a measurement tool for the characterization of micro-optical components is presented, which has some advantageous properties with respect to other interferometric techniques. An analysis of the resolution of digital holography is performed in order to optimize the efficiency of this technique for the characterization of microlenses. The application of Digital Holography leads to a simple and robust measurement tool for shape measurement of microlenses which is demonstrated with two exemplary experiments characterising typical microlenses.

Motility, or co-ordinated movement, of cells is fundamental for the development of many processes, such as formation of the embryo, the immune response, inflammation, wound healing, and metastasis of cancer cells 1. Cells often migrate in response to specific external signals, but they can also move spontaneously. Aberrant cell motility can be fatal in cancer diseases when cells start to metastasize 2. Metastasis is strictly associated with tissue invasion of the primary tumor cells, and occurs as a multistep progress 3. The first phase of cancer cell metastasis involves the physical translocation of the cell to a distant organ, whereas the second phase implicates the ability of the cancer cell to develop into a metastatic lesion at that distant site 4. For skin cancer melanoma, the main cause of death is widespread metastases 5. Melanoma cells may travel along external vessel lattices. After settling in the metastatic sites, melanoma cells develop mechanisms that protect them against the attack of the immune system. It is, therefore, of great interest and importance to investigate, monitor, and understand the principles of tumor cell invasion and migration. The development of novel therapeutic applications that have the ability to block migratory processes is crucial. For in vitro studies, the cell culture wound-closure, and the transwell migration and invasion assays can provide data for an understanding how well a particular cell type can migrate 6. The wound healing assay is also called a scratch assay because it is performed by making a scratch in the cell monolayer (Fig. 1A). The transwell migration assay is a commonly used test to study the migratory response of endothelial cells to angiogenic inducers or inhibitors (Fig. 1B). This assay is also known as the Boyden or modified Boyden chamber assay. These assays have been frequently used to study movement of cells. Weckmann et al. developed an interesting system to monitor and quantify neutrophils on single cell level, by utilizing commercially available tissue culture hardware, simple video microscopic equipment, and standardized tracking 7. However, the interest for novel standardized real-time analyses using quantitative phase imaging (QPI) for cell tracking is growing (Fig. 2). More than 10 years ago, the digital holographic (DH) microscopy methodology of QPI became known to a larger audience and the technique has since then successfully been applied for label-free monitoring of live cells undergoing cell division, cell proliferation, cell death, and cell migration over time 8-10. By QPI, studies associated with both cellular thickness and refractive index fluctuations have been performed. The possibilities for utilizing QPI for cancer diagnostics have increased 11. Cellular behavior and phenotypes are other parameters that can be measured 12. For two human osteosarcoma cell types, the motility pattern, but not the proliferative pattern, was different. Early migration studies using DH microscopy showed fibroblast migration 13 and HT-1080 fibrosarcoma cells embedded in a 3D collagen gel 14. Alm et al. and Persson et al. also presented data on movement of different cell types and cell sizes 15 as well as demonstrated the correlation of cell movement with cell morphology 16. In the recent article published in the November 2018 issue of Cytometry Part A by Zhang et al., (pages 1125–1131) commercially available DH cytometry platforms equipped with semi-automated image acquisition, segmentation, and analysis software packages for assessing cell behavior are used and discussed. The motility data was obtained with QPI/cytometry using "Track cells module" and "Wound healing module," respectively, and with transwell migration assays determining migration and invasion, respectively. The different techniques assessing motility were compared. The two melanoma cell lines, WM793 and 1205Lu, used in the study were reported to have different metastatic capacity, resulting in differences in proliferation and motility. The motility measured with the "Track cells module" and "Wound healing module," using DH cytometry equipment housed in an incubator, produced highly correlated relative motilities, even when compared to the standard transwell migration and invasion assays. Images acquired with DH are from the same position in a cell culture vessel, making it possible to study individual cells. This is a strong advantage, since most studies of effects of new drugs on proliferation and migration of cancer cells in culture are performed on a population level. In a recent study by Kamlund et al., cell images were captured every fifth minute for a total of up to 72 h 17. For cell movement studies, the semi-automated algorithm attempts to find each tracked cell in the next frame by selecting the closest cell based on the centroid position. The advantages of the Track cells and Wound healing modules over the transwell migration and invasion assays in the paper by Zhang et al., included the possibility to use the cells in the experiment for other purposes after completing the measurement. DH microscopy also benefits from the fact that morphology analysis can be performed of each cell. Indeed, this opens up for almost unlimited possibilities to perform cell morphology analysis using the QPI methodology, since each image is very rich in cellular information. As discussed in a review by El-Schich et al., automated analysis and machine learning is one approach to handle the large amount of morphological data that can now be generated when analyzing cells with QPI 8. Only the computer resources and the training speed will set the limitations.

Digital holography is a powerful tool for noncontact quantitative phase imaging. According to the relative incident angle between the object beam and the reference beam, digital holography is grouped into on-axis and off-axis digital holography, The measurable area is narrow in off-axis digital holography, on-axis digital holography suffers from image blurring. Phase-shifting technique is usually used to obtain the high-quality object image. However, the phase shifting technique requires to record multiple phase-shifted holograms. The most conventional holography configuration requires a separately generated reference and object beams that result in a low stability. The paper presents an One-shot common-path phase-shifting holography based on micro polarizer camera and large-shearing Wollaston Prism. The system employs a commercial micro polarizer camera and a doubly-refractive prism with large shearing. The Wollaston prism separates the incoming beam into two orthogonally polarized components ,brings the reference and object from the two-windows to overlap at the lateral shearing region. The two light beams transmit through the quarter wave plate(QWP) and pixelated micro polarizer array(PMA) camera, QWP is used to transform the orthogonally polarized light into orthogonal circular components, The circular polarizations interfere at CCD after passing through the micro polarizer array. The data captured by PMA camera can be parsed into four phase shifting fringe images corresponding to each direction of the four polarizations. The interpolation method is used to obtain the same resolution as the original image. The phase distribution of the specimen can be retrieved using the four-step algorithm. Finally, experiments are conducted to prove the validity of the proposed method. The results demonstrate the capability and applicability of the system .

We show that sharp imaging and quantitative phase-contrast microcopy is possible in microfluidics in flowing turbid media by digital holography. In fact, in flowing liquids with suspended colloidal particles, clear vision is hindered and cannot be recovered by any other microscopic imaging technique. On the contrary, using digital holography, clear imaging is possible thanks to the Doppler frequency shift experienced by the photons scattered by the flowing colloidal particles, which do not contribute to the interference process, i.e. the recorded hologram. The method is illustrated and imaging results are demonstrated for pure phase objects, i.e. biological cells in microfluidic channels.

We propose a quantitative phase imaging technique with single-path phase-shifting digital holography using a light-emitting diode (LED). A reference wave is generated from an object wave in the Fourier plane using a single-path interferometer, based on self-reference digital holography. The object wave interferes with the reference wave, and the quantitative phase information of the object wave is recorded as a digital hologram. Quantitative phase images of objects are obtained by applying a phase-shifting interferometry technique. All the light diffracted from the objects can be utilized to generate a digital hologram. Its validity is experimentally demonstrated by constructing an optical system with a wide-field optical microscope.

Life sciences are devoted to the research of all aspects of living organisms. As the basic unit of a living organism, biological cells are diverse and different. These differences are a direct reflection of their corresponding biological characteristics, so "seeing the cell more clearly" has been the goal of scientists at home and abroad. Based on the principle of coherent imaging, digital holographic quantitative phase microscopy imaging technology adopts digital recording and numerical reconstruction to realize phase retrieval and three-dimensional reconstruction. It has become one of the most promising methods in cellular research with better imaging efficiency, reconstruction accuracy, and stability. However, in the band-pass filtering process, tilt aberrations are often introduced due to manual frame selection of the +1 order spectrum, which is detrimental to the imaging quality of the system. To address this problem, this paper proposes a method to automatically determine the spectral center based on the surface fitting of a two-dimensional Hann window function, which improves the phase retrieval accuracy and imaging quality without reducing the computational speed. A more accurate spectral center can be obtained by conducting the sub-pixel spectral center of surface fitting. In addition, the accuracy is sub-pixel level and requires sub-pixel displacement correction of the tilted phase plane to obtain better quantitative phase retrieval results and imaging quality.

Recent advances in the field of digital sensors and computational facilities have turned digital holography (DH) into a powerful tool for many applications such as quantitative phase imaging, optical metrology, evaluation of cell parameters, optical cryptography, and optical pattern recognition among others. The transport of intensity equation (TIE) is one of the deterministic phase-retrieval methods, which does not use interferometric geometry. In this method, the phase is calculated directly from a set of intensities rather than iteratively approximating a solution. Recently, use of the TIE in the DH reconstruction process has been reported as a phase-retrieval method. Combining both DH and TIE provides better solution to the phase-retrieval. Refocusing property of the DH helps exploit the translation issue of digital sensor along the optical axis while capturing the intensity distributions at different depths. The issue of phase unwrapping is solved through the use of TIE. Hence, both the methods complement each other. In this paper, we review briefly the applicability of the combination of DH with TIE.

We report the use of an amplified femtosecond laser for single-shot two-photon exposure of the commercial photoresist SU-8. By scanning of the focal volume through the interior of the resist, three-dimensional (3-D) structures are fabricated on a shot-by-shot basis. The 800-nm two-photon exposure and damage thresholds are 3.2 and 8.1TW/cm(2), respectively. The nonlinear nature of the two-photon process allows the production of features that are smaller than the diffraction limit. Preliminary results suggest that Ti:sapphire oscillators can achieve single-shot two-photon exposure with thresholds as low as 1.6TW/cm(2) at 700 nm, allowing 3-D structures to be constructed at megahertz repetition rates.

The digital holography has been widely employed in the microscopic imaging, which enables quantitative phase analysis. The phase image is obtained in the numerical reconstruction procedure. The in-focus reconstruction distance could be determined according to the extremum of the autofocusing criterion function, which is commonly applied to the amplitude image of the object. With regard to a pure phase sample, such as the living cell, its in-focus amplitude image should be a uniform bright field, so the minimum value needs be found. Then the in-focus reconstruction distance for the phase image is considered to be equal to the one for the amplitude image. However, in the experiment, the in-focus amplitude image is often not an ideal uniform bright field, so this method will result in some deviation. In this contribution, two derivatives-based criterion functions are applied to the phase image directly to accomplish the in-focus phase contrast imaging, which is proved to be more intuitive and precise. In our experiments, two optical configurations of the digital holography are established firstly. Then the standard phase grating and the living cervical carcinoma cells are detected. The phase aberration is corrected by two-step algorithm. The final autofocusing results verify the principle proposed in this paper.

Quantitative phase microscopy by digital holography provides direct access to the phase profile of a transparent subject with high precision. This is useful for observing phenomena that modulate phase, but are otherwise difficult or impossible to detect. In this letter, a carefully constructed digital holographic apparatus is used to measure optically induced thermal lensing with an optical path difference precision of less than 1 nm. Furthermore, by taking advantage of the radial symmetry of a thermal lens, such data are processed to determine the absorption coefficient of transparent media with precisions as low as 1 \times 10^{-5} cm^{-1} using low power (30 mW) continuous wave (CW) excitation.

In digital holography, extracting the +1-order spectrum accurately and making full utilization of the spatial bandwidth of the CCD sensor are essential for high-resolution and artifacts-free quantitative phase imaging. In this paper, using the light intensity symmetry of the Gaussian laser beam, we delicately eliminate the zero-order spectrum by means of subtraction of two off-axis hologram spectra acquired by symmetrically deflecting the reference beam. Therefore, the +1-order spectrum can be extracted accurately even if it is completely overlapped with the zero-order spectrum. Compared with phase-shifting methods, such as pi-phase and random phase, which require accurate control or calculation of the phase-shifting amount, this proposed method does not need to precisely control the deflection angle of reference beam. Being achievable the maximum utilization of half-space bandwidth of the CCD sensor, the proposed method has realized high-resolution imaging demonstrated by the experimental results of three specimens. This method has general applications in digital holography, such as eliminating the zero-order spectrum and extracting the +1-order spectrum.