Object Wave Research Articles

Electron Beam Lithographic Pixelated Micropolarizer Array for Real-Time Phase Measurement

Pixelated micropolarizer arrays (PMAs) have recently been used as key components to achieve real-time phase measurement. PMA fabrication by electron beam lithography and inductively coupled plasma-reactive ion etching is proposed in this work. A 320 × 240 aluminum PMA with 7.4 μm pitch is successfully fabricated by the proposed technique. The period of the grating is 140 nm, and the polarization directions of each of the 2 × 2 units are 0°, 45°, 90°, and 135°. The scanning electron microscopy and optical microscopy results show that the PMA has a good surface characteristic and polarization performances. When the PMA is applied to phase-shifting interferometry, four fringe patterns of different polarization directions are obtained from only one single frame image, and then the object wave phase is calculated in real time.

Phase Retrieval with a Diffractive Micro Lens Array

Multi-plane phase retrieval is a well established technique for reconstructing both, amplitude and phase of an object wave. This standard technique works best, if the intensity of the object wave changes rapidly along the optical axis. For slowly varying intensities, the iterative procedure may not converge at all. To overcome this limitation we combined the standard technique with a periodic phase element. We demonstrate that a binary diffractive micro lens array with overlapping aperture significantly improves the convergence of phase retrieval and thus the quality of reconstruction. Thus multi-plane phase retrieval can be applied for both rough and smooth phase distributions.

A new estimation method for two-step-only quadrature phase-shifting digital holography

Two-step-only quadrature phase-shifting digital holography can reconstruct the original complex object using only two holograms without the additional recording of the reference wave intensity and the object wave intensity. Its success depends on the accurate estimation of the reference wave intensity from the two acquired holograms. The previous estimation method is relatively computational expensive. In this paper, we present a novel simple and effective method for estimating the reference wave intensity with low computational load. Simulation results are presented to demonstrate the effectiveness and speedup of the proposed method.

Investigation of phase objects using off-axis digital holography with a-priori known information on the reference wave

Off-axis digital holography with a-priori known information on the reference wave allows phase reconstruction from a single interference pattern, thus, being a promising tool for investigations of dynamic processes. The paper presents the method aimed for estimation of angles between the object and reference wave from a single digital hologram. This information is essential for hologram reconstruction using a-priori known data on the reference wave and may also be beneficial in other reconstruction methods. The technique was validated using a specially prepared phase object; the recording was performed at various angles between the object and reference beams. Main factors affecting the reconstructed phase quality were analyzed.

Preprocessing for Interference Fringe of Spherical Object Wave

A simple preprocessing method for interference fringe of holograms generated between spherical object wave fronts and plane wave reference is suggested. The formulae to calculate the diameters of the interference concentric circle and then to get the radius of spherical wave are deduced strictly and the flowchart for the algorithm is given clearly. By using the proposed method the characteristics of a spherical object wave can be achieved directly with only one recording frame and then a simulated one can be gained by numerical simulation. The corresponding experiments show that this technique is simple, effective and convenient for use.

Four-dimensional key design in amplitude, phase, polarization and distance for optical encryption based on polarization digital holography and QR code.

We demonstrate that all parameters of optical lightwave can be simultaneously designed as keys in security system. This multi-dimensional property of key can significantly enlarge the key space and further enhance the security level of the system. The single-shot off-axis digital holography with orthogonal polarized reference waves is employed to perform polarization state recording on object wave. Two pieces of polarization holograms are calculated and fabricated to be arranged in reference arms to generate random amplitude and phase distribution respectively. When reconstruction, original information which is represented with QR code can be retrieved using Fresnel diffraction with decryption keys and read out noise-free. Numerical simulation results for this cryptosystem are presented. An analysis on the key sensitivity and fault tolerance properties are also provided.

Hybridization of Off-Axis and In-line High-Resolution Electron Holography

Reconstruction of the complex-valued wave function that gives rise to the probability density of fast electrons in transmission electron microscopy (TEM) seems, at first glance, to be a solved problem. However, it is still challenging when considered more closely. In conventional TEM experiments, only the intensity (i.e., the square of the amplitude) of the wave function can be measured. Direct information about the phase of the wave, which carries information about electrostatic and magnetic fields that the electron has passed through, is lost during detection. In order to recover the phase information, it is necessary to interfere the electron wave function with a reference wave, in order to create an interference pattern. Denis Gabor introduced an approach that could be used to solve this problem 66 years ago [1]. In Gabor s original setup, which is the pioneering scheme for in-line holography, the wave that has been scattered by the specimen (the object wave) interferes with a reference wave propagated along the same axis. Using laser light, Leith and Upatnieks [2] showed that separation of the axes of propagation of the reference and object waves could be used to solve the twin-image problem. Mollenstedt later translated this idea back to electron microscopy, creating the field of off-axis electron holography [3,4].

Objective Quantum Gravity, Its Possible Relation to Gauge Theories and Strings

In this paper the model of quantum gravity for the higher dimensional Lorentzian space-times, in the sense of the analogy with the Arnowitt–Deser–Misner decomposition well-known from General Relativity, is presented. The model is constructed through making use of the quantum geometrodynamics supplemented by the global onedimensionality conjecture, and considers the objective wave functionals. The framework of quantum field theory is applied in order to establish the phenomenological efficiency in accordance with high energy physics. The empirical deductions on the spatial dimensionality are presented as the relationship between the model and gauge theories, especially string theory.

Quantitative phase imaging using a deep UV LED source.

We propose a method for high resolution phase imaging of biological and non-biological samples using an incoherent deep ultraviolet (DUV) LED source. The diffraction pattern of the object wave is recorded at different axial planes and the phase is retrieved by propagation of the angular spectrum. To maintain enough light intensity, we avoided using a pinhole or spectral filter for increasing the coherence of the DUV LED source. This makes the setup very simple and cost effective. The short wavelength (285nm) of the DUV light, tuned to the absorption peak of the biological samples, allows simultaneously high resolution and high contrast images. The experimental results are presented to verify this principle.

Image-quality improvement in space-bandwidth capacity-enhanced digital holography

A method for improving the quality of the reconstructed image in space-bandwidth capacity-enhanced (SPACE) digital holography is proposed. SPACE digital holography is based on an off-axis configuration and is a technique for capturing an object wave with wide space-bandwidth product by introducing high spatial-carrier frequency, intentional aliasing, periodicity of a digital signal, and the optimal arrangements of the angle difference and object wave spectrum. In this paper, the severe spectrum attenuation caused by the integral of fine interference fringes in a pixel according to the sinc function is corrected by using spatial frequency analysis. Numerical and experimental results show the effectiveness of the proposed method.

Digital holographic microscopy with phase-shift-free structured illumination

When structured illumination is used in digital holographic microscopy (DHM), each direction of the illumination fringe is required to be shifted at least three times to perform the phase-shifting reconstruction. In this paper, we propose a scheme for spatial resolution enhancement of DHM by using the structured illumination but without phase shifting. The structured illuminations of different directions, which are generated by a spatial light modulator, illuminate the sample sequentially in the object plane. The formed object waves interfere with a reference wave in an off-axis configuration, and a CCD camera records the generated hologram. After the object waves are reconstructed numerically, a synthetic aperture is performed by an iterative algorithm to enhance the spatial resolution. The resolution improvement of the proposed method is proved and demonstrated by both simulation and experiment.

Imaging properties of discrete holograms II How structural modification of the hologram and a high spatial carrier frequency of the hologram structure that exceeds the Nyquist frequency affects the image reconstruction

This paper discusses the imaging properties of discrete holograms with modified structure—i.e., discrete holograms characterized by an amplitude transmission function that does not contain a term that describes the halo—i.e., the term caused by the mutual interference of the waves generated by various points of the object. Starting from the requirement of spatial separation of the orders of diffraction of such a hologram, the limiting allowable frequency bands are established for the object wave and the hologram as a whole. It is shown that it is possible to increase the resolving power of discrete holograms by modifying their structure. It is established and theoretically substantiated that it is impossible in principle to record discrete holograms with spatial frequencies that exceed the Nyquist frequency of the hologram recorder—for example, a photodetector array. Recommendations are given for putting into practice the effect of substitution of spatial frequencies intrinsic to discrete holograms when recording digital holograms and reconstructing synthesized holograms.

Superresolution of interference fringes in parallel four-step phase-shifting digital holography

A superresolution method for interference fringes obtained by parallel four-step phase-shifting digital holography is proposed. A complex amplitude distribution of an object wave is derived from a recorded hologram by parallel phase-shifting interferometry using two pixels without any interpolation procedures. Multiple distributions are derived by changing one of the two pixels when conducting phase-shifting interferometry. The angular spectrum distribution of the object wave is obtained by both the Fourier transforms and synthesis of the spectrum distribution from the Fourier-transformed images in the spatial frequency domain. Available space bandwidth is extended to half of that of an image sensor.

Dynamic imaging through turbid media based on digital holography

Imaging through turbid media using visible or IR light instead of harmful x ray is still a challenging problem, especially in dynamic imaging. A method of dynamic imaging through turbid media using digital holography is presented. In order to match the coherence length between the dynamic object wave and the reference wave, a cw laser is used. To solve the problem of difficult focusing in imaging through turbid media, an autofocus technology is applied. To further enhance the image contrast, a spatial filtering technique is used. A description of digital holography and experiments of imaging the objects hidden in turbid media are presented. The experimental result shows that dynamic images of the objects can be achieved by the use of digital holography.

Numerical Comparison of Heterogeneous Combustion Waves in Horizontal and Vertical Porous Objects under Free Convection

Using numerical experiment one-dimensional unsteady processes of heterogeneous combustion in porous object under free convection in horizontal case have been investigated and compared with vertical case. It is shown that in the case of the horizontal porous object the oxidizer supply, which is the result of natural convection, is not sufficient for the appearance of stable wave of heterogeneous combustion. In contrast to the vertical position of the object, there are no long-lived combustion waves in the horizontal case, which completely burns down the solid combustible material. When the ignition zone is on the border of the object, the combustion wave near the vicinity of the boundary burns the oxygen and becomes extinct. When the ignition zone is in the center of the porous object, two combustion waves arise which may move very slowly, burn oxygen and become extinct.

Spatial and temporal characteristics of a four-wave radiation converter in a transparent medium based on electrostriction and dufour effect

Dynamics of spatial spectrum of the object wave has been analyzed for four-wave radiation converter in an optically transparent medium. It has been shown, that such four-wave radiation converter performs filtering of high spatial frequencies. Over time the spatial frequency bandwidth cut out by four-wave radiation converter decreases leaving the steady-state value.

基于光程差扫描的低相干离轴数字全息术

A recording method based on scanning the optical path difference was proposed in low coherence off-axis digital holography,due to the coherence length of the light source limit the detected area of the object wave.First,the different areas of detected plane were scaned by changing the optical path difference between the reference wave and the object wave.Then,the phase distribution of each scanning area was obtained by numerical reconstruction for each scanning area of digital hologram.At last,phase stitching technology was applied to stitch each scanning area to achieve the full field of the view of the object wave.

ワンショット位相シフトデジタルホログラフィによる三次元計測装置の開発

In these years, demand for 3D measurement technology is increasing. However, in many of the conventional 3D measurement methods, such as phase-shift patterns projection method, it is difficult to measure shape of specular reflective and transparent objects. This is because power of reflected light is too high from specular reflective objects or too low from transparent objects, so that reflected light cannot be captured accurately by digital camera. Digital Holography has been used for resolving this problem. In digital holography, interference fringe of object wave and reference wave is recorded by digital camera. Phase of object wave can be calculated using Fresnel diffraction, and shape of object can be calculated using interferometry. Although shape of specular reflective and transparent objects can be measured using digital holography, DC and virtual term exist in single interference fringe as noises, decreases measurement accuracy. For higher accuracy measurement, phase-shift digital holography has been developed. However, for generating four phase-shift interference fringes, measurement system is expensive and complex. Moreover, phase-shift interference fringes are instable due to vibration of measurement environment. For these reasons, conventional phase-shift digital holography is difficult to apply. We propose a new method of generating phase-shift interference fringes using wave detection principle, and developed a low price measurement system using the proposal method to solve the problem. This paper uses a measuring method of phase-shift digital holography using single interference fringe. The experimental result shows how the stable 3D measurement system using single-shot phase-shift digital holography can achieve with low-cost and high accuracy.

Analysis and improvement of reconstruction phase error in the image plane digital holography

The image plane digital holography is commonly used in digital holographic measurement and imaging. The methods of the discrete Fourier transform and the frequency filtering are usually used for reconstructing the object wave in image plane digital holography, and the influence of these algorithms on phase reconstruction is discussed in this paper. Firstly, the influence of the frequency spectrum leakage on phase reconstruction error is analyzed. The results show that the error of the phase reconstruction is very small when the sampling periodicity is an integer, so high accuracy of phase reconstruction can be achieved in this case. But when the sampling periodicity is not an integer, the phase reconstruction error is increased obviously. In order to reduce the phase error caused by the frequency spectrum leakage, the Hanning window is used in the pretreatment of the digital hologram. The result shows that the Hanning window can improve the accuracy of the reconstruction phase effectively.

Method for calculating the dynamic phase delay in holographic interferometry without phase unwrapping

A method for calculating the difference between two arbitrary spatial phase distributions reconstructed from digital holograms is presented. The method is promising for application in double-exposure digital holographic interferometry, especially for the evaluation of phase variations in heavily noised or speckle-structure-modulated digital holograms, because it does not use the phase unwrapping procedure for each reconstructed spatial phase distribution. Instead, by analogy with classical double-exposure holographic interferometry, an interference pattern formed by two reconstructed object waves is calculated first. The interferogram thus obtained is then processed as a digital hologram by the same method that was used for reconstruction of physically recorded holograms. The advantages of the proposed method are demonstrated by both by the numerical computation of the difference of the phase distributions of speckle-structure-modulated wavefronts and the experimental observation of the laser-induced temperature gradients in an aqueous solution of a photosensitizer.