Temporal Multiplexing Method with Off-axis Digital Holography using Acousto-optic Modulators

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 .

Nonlinear filtering method of zero-order term suppression for improving the image quality in off-axis holography

We describe, throughout a Mach-Zehnder interferometric configuration, a new test sieves calibration method based on off-axis digital holography. The experiment is conducted on a test sieve of square openings. The nominal sieve opening is 1.00 mm with maximum individual opening of 1.14 mm in size. The recorded off-axis hologram is numerically processed using Fresnel transforms to obtain an object wave (amplitude and phase). From the reconstructed phase, the average size of the illuminated openings has been measured precisely. The proposed method can provide a real time solution for calibrating test sieves very precisely and with moderate accuracy.

This paper proposes a quality assessment of focusing criteria for imaging in digital off-axis holography. In literature, several refocus criteria have been proposed in the past to get the best refocus distance in digital holography. As a general rule, the best focusing plane is determined by the reconstruction distance for which the criterion function presents a maximum or a minimum. To evaluate the robustness of these criteria, a set of thirteen criteria is compared with application on both amplitude and phase images from off-axis holographic data. Experimental results lead to define general rule and to exhibit the most robust criteria for accurate and rapid refocusing in digital holography.

Accurately determining the focus plane of the reconstructed image is crucial for obtaining high-quality reconstructed images in the process of digital hologram reconstruction. In this paper, a focusing evaluation function based on S-th power weighted neighborhood correlation coefficient (SPWNCC) is proposed to realize automatic focusing of the reconstructed image in off-axis digital Fresnel holography. The Fresnel transform method is utilized as the off-axis digital holographic reconstruction algorithm. Both the numerical simulation and optical experiment results are given to verify the validity of the proposed autofocusing method. The obtained focusing curve can maintain good unimodality and noise immunity performance over a large search range (approximately 800 mm). The proposed SPWNCC based focusing evaluation function has a certain guiding significance on the automatic focusing of off axis digital holographic reconstructed images of long distance recorded objects.

In this paper, we enhance the steepness of film thickness edge in terahertz off-axis digital holography by employing a reconstruction method based on a direct Fourier and Hermite polynomial function. The method is applied to samples of 110 μm and 80 μm step heights to extract the phase images in real time. Use of this method reduces the resolution factor by around one half, representing around a 50% decrease in critical dimension as compared with conventional measurements. The method is very promising and opens the route to potential applications in the fields of microlithography.

We describe a numerical space-shifting reconstruction approach in digital holography. This method is able to remove the dc term in the reconstruction very effectively by utilizing the periodicity and the space-shifting property of inverse discrete Fourier transform. Since the entire process does not need any additional holograms and specific requirements to recording optics, this approach can be a really convenient, practical, and widely effective way to remove the dc term from in-line or off-axis digital holography.

Multi-wavelength digital holography is a very powerful approach for surface shape measurements. It has the advantage of being contact-less, non-intrusive, and yields full-field surface shape data without any requirement for scanning. When dealing with off-axis digital holography and spatial multiplexing of two-wavelength digital holograms, the method becomes real-time, in the sense that the surface shape can be measured at each time instant at which the holograms are recorded. Thus, phase shifting and sequential recording are suppressed. However, due to the roughness of the inspected surface, speckle decorrelation occurs and noise is included in the final data. The noise amount in the data must be investigated in order to define the best processing approach for holograms. This paper proposes the analysis of the standard deviation of noise in surface-shape data from two-wavelength spatially-multiplexed digital holograms. The influence of noise on the measurements of the surface shape is described by an analytical approach. Relationships to quantify the minimum measurable surface height is given by taking into account the experimental parameters of the set-up. These parameters are related to the spatial bandwidths, modulation of holograms, saturation ratio, number of electrons in pixels, readout noise, quantization noise, photon noise, and speckle decorrelation due to roughness. The theoretical modeling is discussed and analysis when considering practical situation for industrial surface shape measurements.

Conical liquid sheet morphology and 3D droplet distribution of aviation kerosene pressure-swirl spray with digital off-axis holography

In off-axis digital holography, e.g., for detecting and imaging ultrafast phenomena, the interference region is confined to a limited region due to the short extent of the light pulse along the propagation axis. Therefore, the detected area of the object wavefront is limited. A recording method for enlarging the detected area in the above case is proposed in this Letter, in which multiple interferences between the object and the reference waves are obtained by spatial angular multiplexing. Experimental results demonstrate the validity of this method.

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.

This paper provides overall analytical insights on the common-path incoherent digital holography using dual-focusing lens with diffraction gratings. Especially when one tries to seek an off-axis solution using the suggested configuration, the low temporal and spatial coherence require specific conditions on parameters of the set-up. AA mathematical explanation on the off-axis digital holography is described.

Digital holography measures the complex optical field and transfer matrix of a device, polarization-diversity is often achieved through spatial multiplexing. We introduce angular multiplexing, to increase flexibility in the optical setup. Comparatively, similar values for cross-talk and mode-dependent loss are measured for a photonic lantern.

Perforation induced ligament evolution in low-temperature swirl spray via off-axis holography

This paper discusses on off-axis digital holography applied to quantitative imaging of flows. The close link between phase measurement and measurands of interest is discussed. Numerical processing of digital holograms and phase retrieval is summarized. The paper presents different architectures that can be found in literature according to different operating modes: speckle-free mode, speckle mode, reduced-sensitivity, and combined approaches.