Off-axis Holograms Research Articles

Digital holographic phase imaging based on phase iteratively enhanced compressive sensing.

Digital holography has been widely applied in quantitative phase imaging (QPI) for monolayer objects within a limited depth. For multilayer imaging, compressive sensing is employed to eliminate defocused images but with missing phase information. A phase iteratively enhanced compressive sensing (PIE-CS) algorithm is proposed to achieve phase imaging and eliminate defocused images simultaneously. Linear filtering is first applied to the off-axis hologram in Fourier domain, and an intermediate reconstructed complex image is obtained. A periodic phase mask is then superimposed on the intermediate reconstructed image to iteratively eliminate the defocused images and recover the object with phase information. The experimental recovery of amplitude and phase of a two-layer sample with as little as 7% random measurement is demonstrated. The average phase error of the PIE-CS algorithm is analyzed, and the results show the feasibility for QPI.

Image quality improvement of random phase-free holograms by addressing the cause of ringing artifacts.

Holographic projectors utilize holography techniques, although there are several barriers to realizing holographic projections. One such challenge is the deterioration of the hologram image quality caused by speckle noise and ringing artifacts. Several methods designed to reduce the speckle noise and ringing artifacts have been proposed. However, these methods require multiple diffraction calculations and a significant amount of computational time. In this paper, we reveal that ringing artifacts are due to object light being recorded on the edge of the hologram and that the high-frequency component of the original image leaks outside of the recording area of the hologram when the random phase-free method is used. Therefore, this study proposes an object light centering method that prevents object light from being recorded on the edge of the hologram and prevents the high-frequency component of the original image from leaking outside the recording area of the hologram, which removes the ringing artifact and extends the random phase-free method to an off-axis hologram.

HOLOGRAPHIC ASSOCIATIVE MEMORY PROBLEM WITHIN CORRELATION-OPTICAL APPROACH

Проблема голографічної асоціативної пам’яті обговорюється в рамках кореляційно-оптичного підходу. Аналіз моделей безопорної голограми та нелінійно зареєстрованих позаосевих голограм, зчитуваних в асоціативному режимі, показує широкий спектр корисних властивостей таких голограм не лише для відновлення відсутньої частини збереженого масиву даних, а й для детектування малих змін (помилок) у записаній картині, для відновлення повного масиву даних з корекцією помилок за неповною версією масиву, а також для здійснення високоефективного гетероасоціативного відновлення на базі неінтерференційного механізму зв’язку парціальних сигналів. Показано, що візуально оцінювана якість відновленого асоціативного відгуку може наближатись до якості зображення, що досягається у звичайній позаосевій голографії.

Comparative appraisal of global and local thresholding methods for binarisation of off-axis digital holograms

Binary digital and computer-generated holograms are used in a number of digital micromirror device (DMD) applications, including holographic displays, media characterisation, optical encryption, and others. Binarisation is one of the most simple and efficient methods of hologram compression. In this paper, 12 local and 18 global thresholding techniques of different groups were analysed and compared based on attribute similarity, clustering, entropy, histogram shape, and local adaptive factors. Optically recorded off-axis digital holograms of different objects were binarised using these methods. The amplitude quality of the obtained reconstructed images was compared using PSNR and SSIM values. The clustering-based methods achieved the highest quality. The results of binarisation by global and local methods were comparable on average.

Rough surface characterization using off-axis digital holographic microscopy compensated with self-hologram rotation

In this paper, an off-axis digital holographic microscopy compensated with self-hologram rotation is presented. The process is implemented via subtracting the unwrapped phase maps of the off-axis parabolic hologram and its rotation 180° to eliminate the tilt induced by the angle between the spherical object wave O and the plane reference wave R. Merit of the proposed method is that it can be done without prior knowledge of physical parameters and hence can reconstruct a parabolic hologram of 1024 × 768 pixels within tens of milliseconds since it doesn't require a digital reference wave. The method is applied to characterize rough gold bumps and the obtained results were compared with those extracted from the conventional reconstruction method. The comparison showed that the proposed method can characterize rough surfaces with excellent contrast and in real-time. Merit of the proposed method is that it can be used for monitoring smaller biological cells and micro-fluidic devices.

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.

Compression of multiple 3D color scenes with experimental recording and reconstruction

We present for the first time a method for compression of 3D color scenes with experimental recording and reconstruction, including scene multiplexing. An experimental setup allows separately registering three off-axis Fourier holograms corresponding to the RGB color channels of a 3D scene. Then, the optical field data contained in each hologram is extracted and their phase is retained. The phases associated to each color channel are spatially arranged to get a single phase-only optical field for the experimental full color reconstruction in another optical setup. This process is then further developed to include different 3D color scenes by multiplying the arranged phase-only optical fields corresponding to each scene by complementary binary masks. Three sampled scenes are multiplexed giving a compression in the data volume up to 97.81% in our experiments. The technique used in the context of multiplexing allows a substantial data volume reduction and the appropriate reconstruction of each color scene without cross-talk and in a single step.

Rotational scanning and multiple-spot focusing through a multimode fiber based on digital optical phase conjugation

We demonstrate, for the first time, the rotational memory effect of a multimode fiber (MMF) based on digital optical phase conjugation (DOPC) to achieve multiple-spot focusing. An implementation interferometer is used to address the challenging alignments in DOPC. By rotating the acquired phase conjugate pattern, rotational scanning through a MMF could be achieved by recording a single off-axis hologram. The generation of two focal spots through a MMF is also demonstrated by combining the rotational memory effect with the superposition principle. The results may be useful for ultrafast scanning imaging and optical manipulation of multiple objects through a MMF.

Coefficients Quantization at Off-axis Digital Hologram Wavelet Compression

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Simultaneous three-wavelength unwrapping using external digital holographic multiplexing module.

We present an external interferometric setup that is able to simultaneously acquire three wavelengths of the same sample instance without scanning or multiple exposures. This setup projects onto the monochrome digital camera three off-axis holograms with rotated fringe orientations, each from a different wavelength channel, without overlap in the spatial-frequency domain, and thus allows the full reconstruction of the three complex wavefronts from the three wavelength channels. We use this new setup for three-wavelength phase unwrapping, allowing phase imaging of thicker objects than possible with a single wavelength, but without the increased level of noise. We demonstrate the proposed technique for micro-channel profiling and label-free cell imaging.

Single-shot dual-wavelength in-line and off-axis hybrid digital holography

We propose an in-line and off-axis hybrid holographic real-time imaging technique. The in-line and off-axis digital holograms are generated simultaneously by two lasers with different wavelengths, and they are recorded using a color camera with a single shot. The reconstruction is carried using an iterative algorithm in which the initial input is designed to include the intensity of the in-line hologram and the approximate phase distributions obtained from the off-axis hologram. In this way, the complex field in the object plane and the output by the iterative procedure can produce higher quality amplitude and phase images compared to traditional iterative phase retrieval. The performance of the technique has been demonstrated by acquiring the amplitude and phase images of a green lacewing's wing and a living moon jellyfish.

Spatially incoherent common-path off-axis color digital holography.

We describe a new method for recording spatially incoherent common-path off-axis color digital holograms. We present the theoretical and experimental evidence to demonstrate an incoherent common-path off-axis color digital holographic (ICOCH) system capable of capturing information from three-dimensional color objects under incoherent illumination, both in transmission and reflection modes. Fresnel incoherent correlation holography (FINCH), a common-path system, is a frequently used incoherent holography technique. Our proposed system is conceptually similar to an advanced form of FINCH; moreover, it has three advantages over this advanced form of FINCH. First, removal of the spatial light modulator makes our system simpler and more cost-effective. Second, removal of the polarizer or analyzer allows for greater light throughput. Third, the off-axis optical configuration enables separation of zero-order and twin images with only a single exposure per color rather than requiring three exposures per color for in-line holography FINCH. Therefore, we believe that this simple and cost-effective system with high light throughput can acquire incoherent holograms for different colors involving single exposure for each color, which makes the ICOCH system suitable for many applications.

Four dimensional phase unwrapping of dynamic objects in digital holography

We present a new four-dimensional phase unwrapping approach for time-lapse quantitative phase microscopy, which allows reconstruction of optically thick objects that are optically thin in a certain temporal point and angular view. We thus use all four dimensions of the dynamic quantitative phase profile acquired, including the angular dimension and the temporal dimension, in addition to the x-y dimensions. We first demonstrate the capabilities of this algorithm on simulative data, enabling the quantification of the reconstruction quality relative to both the ground truth and existing unwrapping approaches. Then, we demonstrate the applicability of the proposed four-dimensional phase unwrapping algorithm by experimentally capturing a dual-angular dynamic off-axis hologram with simultaneous recording of two angular views, using multiplexing of two off-axis holograms into a single multiplexed hologram.

Optimal spatial bandwidth capacity in multiplexed off-axis holography for rapid quantitative phase reconstruction and visualization

We present new methods for optimizing the spatial bandwidth capacity in off-axis holography using spatial multiplexing. We use optimal spatial multiplexing of off-axis holograms to fill the entire spatial frequency domain, including the space previously occupied by the intensity of the sample. Our approach enables spatial digital compression of eight off-axis holograms into a single real-valued multiplexed hologram, having the same number of pixels as each of the input holograms, but still allowing their full reconstruction without resolution or magnification loss in the reconstructed complex wave fronts. This new method allows 33% improvement in usage of the spatial bandwidth capacity compared to the best available off-axis holography real-value multiplexing method. Since the output multiplexed hologram contains only real values, it can be used for rapid display of eight wave front reconstructions at once, which is useful for real-time visualization, when the hologram display device is slower than the acquiring camera. We further generalize this technique to digital multiplexing of 16 real-valued holograms into a single complex-valued hologram by simple arithmetic operations in the hologram domain. Then, the extraction of the 16 wave fronts includes a single 2-D discrete Fourier transform to access the spatial frequency domain, allowing fast reconstruction, which is useful for real-time processing of off-axis holograms, with improved processing rate compared to current hologram processing algorithms. These new approaches allow the full reconstruction of all compressed data without loss of resolution or magnification, even though the samples are dense such that their frequency content employs the entire range. Both multiplexing architectures are then demonstrated for experimentally-acquired off-axis holograms for quantitative phase imaging of biological cells.

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

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

Full field-of-view digital lens-free holography for weak-scattering objects based on grating modulation.

Grating-based single-shot digital lens-free holography with spatial spectral multiplexing is proposed to realize full field-of-view (FOV) imaging for weak-scattering objects. Multiple object waves are generated by a one-dimensional grating that is placed in near contact with the object to avoid the cross talk among different diffraction orders during reconstruction. A multiplexed off-axis hologram is created by interference between the object waves and reference wave and captured by an image sensor in one shot. Multiple imaging areas corresponding to the captured object waves can be simultaneously retrieved during reconstruction. A formula which guarantees full FOV imaging without cross talk or information loss is presented. The imaging experiments of a USAF resolution target are presented to demonstrate the feasibility of this method.

Quality of reconstruction of compressed off-axis digital holograms by frequency filtering and wavelets.

Compression of digital holograms can significantly help with the storage of objects and data in 2D and 3D form, its transmission, and its reconstruction. Compression of standard images by methods based on wavelets allows high compression ratios (up to 20-50 times) with minimum losses of quality. In the case of digital holograms, application of wavelets directly does not allow high values of compression to be obtained. However, additional preprocessing and postprocessing can afford significant compression of holograms and the acceptable quality of reconstructed images. In this paper application of wavelet transforms for compression of off-axis digital holograms are considered. The combined technique based on zero- and twin-order elimination, wavelet compression of the amplitude and phase components of the obtained Fourier spectrum, and further additional compression of wavelet coefficients by thresholding and quantization is considered. Numerical experiments on reconstruction of images from the compressed holograms are performed. The comparative analysis of applicability of various wavelets and methods of additional compression of wavelet coefficients is performed. Optimum parameters of compression of holograms by the methods can be estimated. Sizes of holographic information were decreased up to 190 times.

Complex object wave extraction using time-multiplexing in off-axis digital holography.

In this paper, we present an algorithm for complex object wave extraction in off-axis digital holography using a time-multiplexing and frequency spectrum-shifting technique. The proposed approach utilizes the digital time-multiplexing technique, in which two 90-deg-rotated off-axis holograms are recorded in sequence, and corresponding spectra are subtracted in the computed Fourier domain to eliminate the DC term. Then, the two subtracted holograms are digitally multiplexed into one complex hologram in the same plane, and by shifting the spatial frequency spectra of the subtracted hologram in the spatial frequency domain, one of the two cross-correlations can be obtained in the center. This technique simply extracts the spectrum of the real image in the frequency domain than the spatial filtering method.

Six-pack off-axis holography.

We present a new holographic concept, named six-pack holography (6PH), in which we compress six off-axis holograms into a single multiplexed off-axis hologram without loss of magnification or resolution. The multiplexed hologram contains straight off-axis fringes with six different orientations, and can be generated optically or digitally. We show that since the six different complex wavefronts do not overlap in the spatial frequency domain, they can be fully reconstructed. 6PH allows more than 50% improvement in the spatial bandwidth consumption when compared to the best multiplexing method proposed so far. We expect the 6PH concept to be useful for a variety of applications, such as field-of-view multiplexing, wavelength multiplexing, temporal multiplexing, multiplexing for super-resolution imaging, and others.

Axicon Lens for Electrons Using a Magnetic Vortex: The Efficient Generation of a Bessel Beam.

We demonstrate experimentally an efficient electron axicon lens using a magnetic vortex. We show that naturally occurring magnetic vortices with circular magnetic moment distributions in a soft-magnetic thin film create conical phase shifts for fast electrons. Such radially symmetric linear phase ramps are equivalent to ideal light optical axicons. We apply this lens to generate efficient nondiffracting electron Bessel beams, which we observe experimentally in through-focus Lorentz images as well as in propagated off-axis electron holograms. This highlights the potential for using magnetic nanostructures as highly efficient and flexible phase plates for crafting desired electron beam shapes.