Phase Reconstruction Method Research Articles

Resonant characteristics of microcantilever by using self-mixing interferometer based on phase reconstruction method

The self-mixing interferometer is used to investigate the characteristics of resonant frequency of the microresonator, which is excited by a sinusoidally driven loud-speaker. The detected self-mixing signal is processed by the phase reconstruction method. The 1st-order resonant frequency of the microresonator is measured to be 4.437 kHz with full-width at half-maximum (FWHM) of 0.13 kHz. The measurement results are verified by the sinusoidal phase modulating (SPM) interferometer.

Spatially resolved refractive index profiles of electrically switchable computer-generated holographic gratings

We describe a spatially resolved interferometric technique combined with a phase reconstruction method that provides a quantitative two-dimensional profile of the refractive index and spatial distribution of the optical contrast between the on-off states of electrically switchable diffraction gratings as a function of the external electric field. The studied structures are holographic gratings optically written into polymer/liquid crystal composites through single-beam spatial light modulation by means of computer-generated holograms. The electro-optical response of the gratings is also discussed. The diffraction efficiency results to be dependent on the incident light polarization suggesting the possibility to develop polarization dependent switching devices.

Quantitative phase reconstruction from input-output intensities using a shift-invariant linear imaging system

We formulate an iteration scheme for quantitative two-dimensional phase retrieval of complex scalar wave fields from input-output intensity profiles using a coherent rotationally symmetric shift-invariant linear optical imaging system. This method is a generalization of our previous method for quantitative single-image pure phase reconstruction using an imperfect shift-invariant linear imaging system. The method is somewhat analogous to the Gerchberg-Saxton iterative phase-retrieval algorithm, but typically converges much more rapidly. We have demonstrated the efficacy of the method using simulated data in the presence of noise.

Lorentz microscopy mapping during magnetization process of L10 FePd thin films

Lorentz Transmission Electron Microscopy (LTEM) allows the observation of magnetization evolution in thin magnetic layers. Electron Microscopy which enables observation at a nanometric scale is sensitive to magnetic induction perpendicular to the beam. Here we show that this technique allows the magnetization process in thin layers having perpendicular magnetic anisotropy to be studied. LTEM has been carried out on a JEOL 3010 and a FEI Titan 300kV (equipped with a Lorentz Lens). We determine how the magnetization is distributed along the domain walls (where the magnetization is in plane). Hence we could observe the evolution of a domain configuration during the magnetization process versus the orientation of its boundaries. This study is based on phase reconstruction methods and is completed by Lorentz image simulations.

Blind carrier phase recovery for general quadrature amplitude modulation constellations

A new blind carrier phase reconstruction method that can be applied to both square- and cross-suppressed-carrier L-ary quadrature amplitude modulations (QAMs) is proposed. This method is based on a special phase metric that performs an absolute minimum for the carrier phase offset. In particular, it does not require any modification of the modulation structure. As a result, the non-coherent detection process is easy even for modulations with large QAM constellations. Simulation results will demonstrate the good performance of the estimator described, which is superior to that of conventional blind phase estimation methods.

Strain and composition mapping of epitaxial nanostructures

We have used surface x-ray diffraction and a direct method of phase reconstruction to obtain subangström resolution maps of an ion-beam milled In0.27Ga0.73As∕GaAs(001) thin film exhibiting three-dimensional (3D) epitaxial nanostructures. The 3D electron density was calculated based on the diffraction pattern along the Bragg rods measured with synchrotron radiation, from which the chemical composition, strain profile, and average nanostructure shape were extracted. The film maintained a wetting layer exhibiting a sharp strain gradient, which extended into the substrate. Atop the wetting layer, the ion-beam milled islands possessed an apical shape and were depleted in indium.

A generalized temporal phase unwrapping algorithm for three-dimensional profilometry

A generalized temporal phase unwrapping algorithm is proposed for absolute phase measurement of object surfaces with complex topography. We first make comments on error analysis of various temporal phase unwrapping (TPU) algorithms and point out problems existing in these techniques. We present a generalized TPU algorithm (GTPU), which eliminates some limitations imposed on the fringe sequence for the classical TPU algorithms. So the desired number of fringes can be determined according to practical situation, leading to a flexible method for phase reconstruction. In addition, compared with the classical TPU algorithms, GTPU has better noise immunity and less computational complexity. Theoretical analysis and experiment results are presented to show the validity of the proposed algorithm.

A method for phase reconstruction from measurements obtained using a configured detector with a scanning transmission X-ray microscope

We developed a technique for performing quantitative phase reconstructions from differential phase contrast images obtained using a configured detector in a scanning transmission X-ray microscope geometry. The technique uses geometric optics to describe the interaction of the X-ray beam with the specimen, which allows interpretation of the measured intensities in terms of the derivative of the phase thickness. Integration of the resulting directional derivatives is performed using a Fourier integration technique. We demonstrate the approach by reconstructing simulated measurements of a 0.5 - μ m -diameter gold sphere at 7-keV photon energy.

Spatially discordant voltage alternans cause wavebreaks in ventricular fibrillation

Ventricular fibrillation (VF) is characterized by complex ECG patterns emanating from multiple, short-lived, reentrant electrical waves. The incessant breakup and creation of new daughter waves (wavebreaks) perpetuate VF. Dispersion of refractoriness (static or dynamic) has been implicated as a mechanism underlying wavebreaks. The purpose of this study was to investigate the mechanisms underlying wavefront instability in VF by localizing wave fractionation sites (the appearance of multiple waves) and their relationship to local spatial dispersion of voltage (V(m)) oscillations. Wave fractionations were identified by tracking V(m) oscillations optically at unprecedented spatial (100 x 100 pixels) and temporal (2,000 frames per second) resolution using a CMOS camera viewing the surface (1 x 1 cm(2)) of perfused guinea pig hearts (n = 6). VF was induced by burst stimulation, and wavefront dynamics were highlighted using region-based image analysis to automatically detect wavebreaks. Direct detection of wavebreak locations by image analysis was more reliable than the phase reconstruction method because baseline noise obstructed the correct identification of phase singularities by detecting false-positives. Wave fractionations (34 +/- 4 splits/s.cm(2)) fell into three categories: decremental conduction (49% +/- 7%), wave collisions (32% +/- 8%), and wavebreaks (17 +/- 2%). Wavebreaks occurred at a frequency of 5.8 +/- 1 splits/s.cm(2) and did not preferentially occur at anatomic obstacles (i.e., coronary vessels) but coincided with discordant alternans where V(m) amplitudes and durations shifted from high to low to from low to high on opposite sides of wavebreak sites. Spatial discordant alternans cause wavebreaks most likely because they are sites of abrupt dispersion of refractoriness.

Estimation of minimum electron dose necessary to resolve molecular structure of deoxyribonucleic acid by phase transmission electron microscopy

The minimum electron dose that is necessary to resolve the molecular structure of deoxyribonucleic acid (DNA) was estimated based on experimental measurements of information limits and simulated DNA images, considering conditions of a low electron dose. From these results, a dose of ∼400e∕Å2 was found to be necessary to achieve observation of DNA on a molecular scale under the present experimental setup. A DNA molecule was observed by a phase reconstruction method using through-focus images under the limited electron dose. In the reconstructed images, the helical structure and the intervals of the base pairs of DNA were partially resolved.

Molecular-scale imaging of unstained deoxyribonucleic acid fibers by phase transmission electron microscopy

The molecular structure of deoxyribonucleic acid (DNA) fibers was observed by a phase reconstruction method called three-dimensional Fourier filtering using a 200kV transmission electron microscope. The characteristic helical structure and the spacing of adjacent base pairs of DNA were partially resolved due to an improved signal-to-noise ratio and resolution enhancement by the phase reconstruction although the molecular structure was damaged by the electron beam irradiation. In the spherical aberration-free phase images, the arrangements of single atom-sized spots forming sinusoidal curves were sometimes observed, which seem to be the contrast originating in the sulfur atoms along the main chains.

Conjugate phase MRI reconstruction with spatially variant sample density correction

A new image reconstruction method to correct for the effects of magnetic field inhomogeneity in non-Cartesian sampled magnetic resonance imaging (MRI) is proposed. The conjugate phase reconstruction method, which corrects for phase accumulation due to applied gradients and magnetic field inhomogeneity, has been commonly used for this case. This can lead to incomplete correction, in part, due to the presence of gradients in the field inhomogeneity function. Based on local distortions to the k-space trajectory from these gradients, a spatially variant sample density compensation function is introduced as part of the conjugate phase reconstruction. This method was applied to both simulated and experimental spiral imaging data and shown to produce more accurate image reconstructions. Two approaches for fast implementation that allow the use of fast Fourier transforms are also described. The proposed method is shown to produce fast and accurate image reconstructions for spiral sampled MRI.

Lorentz study of magnetic domains in Heusler-type ferromagnetic shape memory alloys

We report on magnetic domain observations using in-situ Lorentz microscopy of the ferromagnetic shape memory alloys of the type Ni 2 MnGa and Co 2 NiGa. The domain configurations are analyzed by means of the Transport-of-Intensity phase reconstruction method.

Polarization-phase mapping and reconstruction of biological tissue architectonics during diagnosis of pathological lesions

Specific features of the formation of local and statistical polarization structures of laser radiation scattered in phase-inhomogeneous layers (PIL) of biological tissue (BT) were studied. The distribution of azimuth and eccentricity of boundary field polarization was found to correlate with the orientation-phase structure of multifractal PIL. A method of polarization phase reconstruction of BT architectonics was suggested.

In-situ Lorentz TEM cooling study of magnetic domain configurations in Ni/sub 2/MnGa

Magnetic domain configurations in the ferromagnetic shape memory alloy Ni/sub 2/MnGa are analyzed by means of Lorentz microscopy and noninterferometric phase reconstruction methods. Domain structures in the cubic phase consist of cross-tie walls in the thinnest portions of the foil, and more complex configurations in thicker regions. At low temperature, the magnetization configurations change as the structure transforms martensitically to a tetragonal phase. A simple model for the magnetization changes is proposed.

Overlap-add methods for time-scaling of speech

In this tutorial on time-scaling we follow one particular line of thought towards computationally efficient high quality methods. We favor time-scaling based on time–frequency representations over model based approaches, and proceed to review an iterative phase reconstruction method for time-scaled magnitude spectrograms. The search for a good initial phase estimate leads us to consider synchronized overlap-add methods which are further optimized to eventually arrive at WSOLA, a technique based on a waveform similarity criterion.

Phase-compensation experiment with a 37-element adaptive optics system

An experimental study on phase compensation for turbulent effects with a 37-element adaptive optics system is performed in both a simulated turbulence cell and in a real atmosphere. The experimental results demonstrate that the compensated Strehl ratio S(0), which is influenced mainly by the deformable-mirror fitting error, has a functional form S(0) = exp[-kappa(d/r(0))(5/3)], where r(0) is Fried's coherence length and d is the average interval of the actuators on the deformable mirror. The fitting parameter kappa is 0.45. Numerical simulations are also performed with the experimental parameters. The numerical results are in agreement with data obtained in the experiment, which shows that the direct-tilt phase-reconstruction method used in our four-dimensional simulation code is reasonable.

Phase reconstruction from reflectivity in uniform fiber Bragg gratings

We propose a method for phase, time-delay, and impulse-response reconstruction from spectral power reflectance or reflectivity and apply it to uniform fiber Bragg gratings. This method is based on causality conditions and uses Hilbert and Wiener-Lee transforms. Results show a good agreement between exact and reconstructed functions.

Phase reconstruction from reflectivity in fiber Bragg gratings

We propose a method for phase, time delay, and impulse response reconstruction from the spectral power reflectance or reflectivity in fiber Bragg gratings (FBGs). This method is based on causality and stability conditions and uses the Hilbert and Wiener-Lee transforms. In order to obtain a complete study of the algorithm, we have applied it to uniform and nonuniform fiber Bragg gratings. Finally, we get a complete characterization of a experimental uniform fiber Bragg grating, from the measured reflectivity, by means of the phase reconstruction algorithm. Results show a good agreement between exact and reconstructed functions.

Analysis of lateral shearing interferograms by use of Zernike polynomials

A modal phase-reconstruction method for wave-front analysis in lateral shearing interferometry is presented. Pseudo-Zernike polynomial functions describe the differential wave fronts and are related to a Zernike polynomial description of the original wave front. We show that this reconstruction is robust for shear ratios in the range 0.15-0.50. The error propagation properties of this differential Zernike polynomial matrix-inversion method are discussed on the basis of both analysis and simulation. It is concluded that the method allows wave-front analysis with an absolute inaccuracy of 2 mλ rms for diffraction-limited wave fronts and with 1% relative inaccuracy for more strongly aberrated wave fronts.