Digital Holograms Research Articles

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.

Microparticle Manipulation and Imaging through a Self-Calibrated Liquid Crystal on Silicon Display

We present in this paper a revision of three different methods we conceived in the framework of liquid crystal on silicon (LCoS) display optimization and application. We preliminarily demonstrate an LCoS self-calibration technique, from which we can perform a complete LCoS characterization. In particular, two important characteristics of LCoS displays are retrieved by using self-addressed digital holograms. On the one hand, we determine its phase-voltage curve by using the interference pattern generated by a digital two-sectorial split-lens configuration. On the other hand, the LCoS surface profile is also determined by using a self-addressed dynamic micro-lens array pattern. Second, the implementation of microparticle manipulation through optical traps created by an LCoS display is demonstrated. Finally, an LCoS display based inline (IL) holographic imaging system is described. By using the LCoS display to implement a double-sideband filter configuration, this inline architecture demonstrates the advantage of obtaining dynamic holographic imaging of microparticles independently of their spatial positions by avoiding the non-desired conjugate images.

Color Image Reconstruction Based On Digital Holography

In this paper, a color image reconstruction algorithm based on digital hologram is proposed. The color image is decomposed into three primary color images, and the corresponding hologram is generated in each channel using the four step phase shift method. Then the image of each channel is reconstructed using the inverse Fourier transform, the reconstructed graph of three channels is superimposed to obtain the final color reconstruction image. The reconstructed image overcomes the zero-order image and the straight-through bright spot generated during Fresnel digital holographic image generation in the coaxial recording mode.

Three-dimensional stimulation and imaging-based functional optical microscopy of biological cells.

A new type of functional optical microscope system called three-dimensional (3D) stimulation and imaging-based functional optical microscopy (SIFOM) is proposed, to the best of our knowledge. SIFOM can precisely stimulate user-defined targeted biological cells and can simultaneously record the volumetric fluorescence distribution in a single acquisition. Precise and simultaneous stimulation of fluorescent-labeled biological cells is achieved by multiple 3D spots generated by digital holograms displayed on a phase-mode spatial light modulator. Single-shot 3D acquisition of the fluorescence distribution is accomplished by common-path off-axis incoherent digital holographic microscopy in which a diffraction grating with a focusing lens is displayed on another phase-mode spatial light modulator. The effectiveness of the proposed functional microscope system was verified in experiments using fluorescent microbeads and human lung cancer cells located at various defocused positions. The system can be used for manipulating the states of cells in optogenetics.

Digital hologram for data augmentation in learning-based pattern classification.

This study proposes a novel data augmentation method based on numerical focusing of digital holography to boost the performance of learning-based pattern classification. To conduct digital holographic data augmentation (DHDA), a complex pattern diffraction approach is used to provide the least separation of confusion in the effective diffraction regime to access the full-field wavefront information of a target sample. By using DHDA, the accessible amount of labeled data is increased to complement the data manifold and to provide various three-dimensional diffraction characteristics for improving the performance of learning-based pattern classification. Experimental results demonstrated that overall accuracy of pattern classification with DHDA (95.1%) was higher than that without DHDA (90.9%).

A Method to Improve the Imaging Quality in Dual-Wavelength Digital Holographic Microscopy.

A digital hologram-optimizing method was proposed to improve the imaging quality of dual-wavelength digital holographic microscopy (DDHM) by reducing the phase noise level. In our previous work, phase noise reduction was achieved by dual-wavelength digital image-plane holographic microscopy (DDIPHM). In the optimization method in this paper, the phase noise was further reduced by enhancing the real-image term and suppressing effects of the zero-order term in the frequency spectrum of a digital hologram. Practically, the carrier frequency of the real-image term has the correspondence with interference fringes in the hologram. Mathematically, the first order intrinsic mode function (IMF1) in bidimensional empirical mode decomposition (BEMD) has similar characteristics to the grayscale values of ideal interference fringes. Therefore, with the combination of DDIPHM and BEMD, by utilizing the characteristics of IMF1, the digital hologram was optimized with purified interference fringes, enhancing the real-image term simultaneously. Finally, the validity of the proposed method was verified by experimental results on a microstep.

Signal processing challenges for digital holographic video display systems

Holography is considered to be the ultimate display technology since it can account for all human visual cues such as stereopsis and eye focusing. Aside from hardware constraints for building holographic displays, there are still many research challenges regarding holographic signal processing that need to be tackled. In this overview, we delineate the steps needed to realize an end-to-end chain from digital content acquisition to display, involving the efficient generation, representation, coding and quality assessment of digital holograms. We discuss the current state-of-the-art and what hurdles remain to be taken to pave the way towards realistic visualization of dynamic holographic content.

Reconstruction research for stitching error immune synthetic aperture digital holograms with seams

A complete reconstruction scheme for synthetic aperture in-line digital holograms with seams is proposed. The width of seams and the stitching error are first estimated by an algorithm combined image matching with phase factor. Then a method based on the shift theorem of the Fourier transform is presented to correct the stitching error of the adjacent sub-holograms. After being correct synthesis, means prefilled synthetic aperture digital holograms are reconstructed by using improved phase retrieval algorithm. The particle detection experiment is conducted. The result shows lost particle in seams are recovered and the correlation coefficient between the reconstructed image and the particles target image tripled in contrast to the traditional method. The feasibility of the scheme is verified.

Analysis of quantitative phase obtained by digital holography on H&E-stained pathological samples

The application of digital holography in cell imaging is gaining attraction as it gives quantitative information related to optical thickness without the need for staining. In contrast, conventional pathology examination uses tissues or cells that are stained to visualize the morphological structure or molecular expression with color. However, the relationship between color information and quantitative phase inside histopathology specimen is not yet well understood. In this study, we developed a system to capture both a color image and digital hologram, and those of H&E (hematoxylin and eosin)-stained liver tissue were acquired. Then, we calculated and analyzed the relationship between the textural features inside the color and phase images for hepatocellular carcinoma (HCC) histopathological specimen. Upon experimental investigation, we found that gray-level co-occurrence matrix (GLCM) textural features in phase images are useful for discriminating cancer and normal tissue, and varies between HCC grades which bring the possibility to be utilized for HCC diagnosis or classification without staining procedure.

Characteristics of vibration frequency measurement based on sound field imaging by digital holography

We present the characteristics of indirect frequency measurement system based on off-axis digital holography (DH) for vibrating objects, which works on phase change characteristics of a medium where sound wave propagates. The sound field measurement method using off-axis DH already proposed by the author’s group is applied to the measurement of vibration frequency of arbitrary sound sources. In DH based sound imaging for frequency measurement, object wave passes near to vibrating object and the interference patterns are recorded in digital holograms as a function of time. After using inverse Fresnel calculation, acousto-optic data processing, and Fourier analysis, vibration frequency of an object of interest can be measured. In this paper, the measurement capability of vibration frequency for different objects and quantitative characteristics of the sound power are evaluated experimentally. The frequency range of the experimental results are from hundreds Hz to tens of kHz that covers audible range. The comparison with microphone recording and the analysis have also been carried out to check the strength and stability of the proposed scheme.

Fourier rainbow holography

We present Fourier rainbow holographic imaging approach. It involves standard laser holographic recording and novel horizontal parallax only holographic display. In the display, the rainbow effect is introduced in an illumination module by high-frequency diffraction grating and white light LED source. The display is addressed by Fourier rainbow digital hologram (FRDH) encoding defocused object field with removed spatial frequency components in one direction by hologram slitting and without spherical phase factor. Theoretically and experimentally it is shown that the method extends the viewing zone of the classical viewing window display in vertical and longitudinal directions, thus the comfort of observation is improved. It is also numerically and experimentally validated that the numerical slitting applied within FRDH generation improves reconstruction depth of the display, here up to 400 mm.

A comparison of iterative Fourier transform algorithms for image quality estimation

A comparison was established between two iterative Fourier transform algorithms (IFTAs), such as the original Gerchberg–Saxton (GS) and the mixed-region amplitude freedom (MRAF) algorithms, for the hologram reconstruction of different target images through the full reference image quality estimation (IQE) and pixel homogeneity in the Fourier plane presented theoretically and experimentally. The comparison was applied depending upon both algorithms based on a computer-generated hologram (CGH) implemented utilizing a reflective phase-modulated liquid-crystal spatial light modulator (LC-SLM) to obtain the digital kinoform holograms of the desired intensity distributions. These digital holograms were applied to reconstruct the intensity patterns for 852 nm, which represents a laser beam source. The theoretical and experimental results of the reconstructed patterns obtained using the MRAF algorithm were found to be smoother and better than the patterns obtained using the GS algorithm. Unmodulated light beam (dc term) is removed from the reconstructed patterns attributed to digital kinoform holograms of MRAF algorithm as an alternative to the theoretical and experimental results without using any additional optic equipment at the light path. Moreover, this paper discussed the full reference objective quality estimations, such as mean square error (MSE), peak signal-to-noise ratio (PSNR), structural content (SC), normalized absolute error (NAE), normalized cross correlation (NK), and homogeneity of pixels, through the contrast (Cont) and inverse difference moment (IDM) for numerical and experimental results. According to the two desired intensity distributions processed theoretically and experimentally, the results of MRAF algorithm were found to be in the highly accurate recovered phase, the quality of image was enhanced, and the dc term was decreased. Image quality estimation of full reference objective relay on the feedback algorithms experimental attestation has not been implemented yet.

Nondestructive evaluation of residual stress via digital holographic photoelasticity

The nondestructive evaluation of residual stress forms a very important aspect of structural engineering. In this context, as per the theory underlying digital holographic photoelasticity, the residual stress field in a transparency can be efficiently and nondestructively evaluated with high accuracy by varying the polarization direction of the reference light beam. In this study, we use a setup to experimentally verify the above mentioned hypothesis. We first record digital holograms before and after heating optically transparent specimens at four different corresponding positions of the reference light polarization. We next solve the four resulting light intensity equations after digital image processing of the four digital holograms to obtain the residual stress. A comparison of our experimental results with those of the drilling method (the conventional approach to determine residual stress) indicates that our digital holographic photoelasticity method can be suitably applied to the nondestructive evaluation of residual stress in transparencies.

EHoloNet: a learning-based end-to-end approach for in-line digital holographic reconstruction.

It is well known that in-line digital holography (DH) makes use of the full pixel count in forming the holographic imaging. But it usually requires phase-shifting or phase retrieval techniques to remove the zero-order and twin-image terms, resulting in the so-called two-step reconstruction process, i.e., phase recovery and focusing. Here, we propose a one-step end-to-end learning-based method for in-line holography reconstruction, namely, the eHoloNet, which can reconstruct the object wavefront directly from a single-shot in-line digital hologram. In addition, the proposed learning-based DH technique has strong robustness to the change of optical path difference between reference beam and object light and does not require the reference beam to be a plane or spherical wave.

Review of fast methods for point-based computer-generated holography [Invited

Computer-generated holography (CGH) is a technique for converting a three-dimensional (3D) object scene into a two-dimensional (2D), complex-valued hologram. One of the major bottlenecks of CGH is the intensive computation that is involved in the hologram generation process. To overcome this problem, numerous research works have been conducted with the aim of reducing arithmetic operations involved in CGH. In this paper, we shall review a number of fast CGH methods that have been developed in the past decade. These methods, which are commonly referred to as point-based CGH, are applied to compute digital Fresnel holograms for an object space that is represented in a point cloud model. While each method has its own strength and weakness, trading off conflicting issues, such as computation efficiency and memory requirement, they also exhibit potential grounds of synergy. We hope that this paper will bring out the essence of each method and provide some insight on how different methods may crossover into better ones.

Characterization of Spatial Light Modulator Based on the Phase in Fourier Domain of the Hologram and Its Applications in Coherent Imaging

Although digital holography is used widely at present, the information contained in the digital hologram is still underutilized. For example, the phase values of the Fourier spectra of the hologram are seldom used directly. In this paper, we take full advantage of them for characterizing the phase modulation of a spatial light modulator (SLM). Incident plane light beam is divided into two beams, one of which passes the SLM and interferes with the other one. If an image with a single grey scale loads on the SLM, theoretical analysis proves that the phase of the Fourier spectra of the obtained hologram contains the added phase and a constant part relative to the optical distance. By subtracting the phase for the image with the grey scale of 0 from that for the image with other grey scales, the phase modulation can be characterized. Simulative and experimental results validate that the method is effective. The SLM after characterization is successfully used for coherent imaging, which reconfirms that this method is exact in practice. When compared to the traditional method, the new method is much faster and more convenient.

Twin-image reduction method using a diffuser for phase imaging in-line digital holography.

Phase imaging by digital holography is widely used. An in-line optical setup is preferable for a practical use. A twin image, however, is a serious obstacle to achievement of high-quality phase imaging in in-line digital holography. For in-line digital holography, a time-division phase-shifting method or a wave-splitting phase-shifting method is used to eliminate the twin image. The former requires multiple exposures, and the latter requires specially designed optical elements. To solve these problems, a twin-image reduction method using a diffuser is proposed. In the proposed method, recording a digital hologram using a diffuser and signal processing enable reduction of the twin image on a reconstructed plane. A preliminary experimental result confirms the feasibility of the proposed method.

Continuous-wave off-axis and in-line terahertz digital holography with phase unwrapping and phase autofocusing

Continuous-wave terahertz digital holography is a practical tool to obtain the complete wave-front information of a sample in the terahertz region that has offered unique advantages in various fields. The propagation distance plays a decisive role in the numerical reconstruction of both in-line and off-axis digital holograms. Only when a terahertz hologram is propagated back to a focal object plane can the authentic amplitude and phase distribution be reconstructed, which is actually a troublesome task. In this manuscript, the combination of hologram enhancement, spectrum filtering, phase retrieval and phase unwrapping algorithms is illustrated and organized to benefit the decision-making process of the autofocusing method not only for the amplitude distribution but also for the phase image. To guarantee the accuracy of the autofocusing results, the noise of the hologram, the intrinsic twin image in the amplitude and phase results and the wrapped ambiguity in the phase distribution are suppressed or lessened by using the four techniques stated above. The focused amplitude and unwrapped phase distributions of the sample are reconstructed, and experimental verification confirms that the present hybrid autofocusing method is a feasible, effective and promising technology with broad application prospects.

Quantitative phase imaging system with slightly-off-axis configuration and suitable for objects both larger and smaller than the size of the image sensor.

We propose a quantitative phase imaging system with exact slightly-off-axis configuration and suitable for objects both smaller and larger than the size of the effective recording region of the image sensors. In this system, the object is illuminated by a convergent spherical beam and a specially designed aperture filter is placed on the spatial frequency plane of the object wave; at the same time, a point source emitting from the edge of the aperture is taken as the reference beam, so that the optimal frequency condition for reconstruction of slightly-off-axis digital holograms can be always guaranteed for both large and small objects as well as different magnification (or the field of view) configurations. At the same time, a 1x2 single-mode optical fiber splitter is used for generating the reference and the illumination beams. Benefited from such fiber-based slightly-off-axis design, the proposed system provides a low-cost way to convert a regular microscope into a slightly-off-axis holographic one for microbiological specimens with a high spatial resolution.

Achromatic phase shifting self-interference incoherent digital holography using linear polarizer and geometric phase lens.

A simple Fresnel-type self-interference incoherent digital holographic recording system is proposed. The main part of the system consists of the two linear polarizers and geometric phase lens. The geometric phase lens is employed as a polarization selective common-path interferometer. One of the polarizers is rotated by the motor and serves as a phase-shifter with the geometric phase lens, to eliminate the bias and twin image noise. A topological phase is obtained by the relative angle between the polarizer and geometric phase lens. Since this phase shifting method does not depend on the change of the optical path length, the phase shifting performance is almost constant in the broad spectral range. Using the proposed achromatic phase shifting method, a simultaneous three-color phase shifting digital hologram recording under the incoherent light source is demonstrated.