Differential Phase Signal Research Articles

Optimization procedure for a Talbot-Lau x-ray phase-contrast imaging system

In Talbot-Lau x-ray imaging, the fringe visibility provided by the interferometer is a crucial quality parameter to preserve high quality images at an acceptable dose level. The noise of the obtained differential phase signal and the dark-field image is directly influenced by the visibility. To optimize the performance of such an interferometer, we use wave-field simulations to investigate the effect of the phase grating G1. Therefore, we varied the grating parameters duty cycle and grating bar height. Each set of these parameters were evaluated for different propagation distances and for multiple x-ray spectra. In this multidimensional space the interferometer configuration with the highest visibility over a wide range of energies was selected to cover a multiple possible x-ray applications. We manufactured the optimized phase grating G1, the corresponding source grating G0 and analyzer grating G2 and compare the experimental results with the expected results obtained from simulations. The presented measurements show outstanding visibilities up to 50% using a broad x-ray spectrum. These measurements are in very good agreement to the simulation results. The achieved visibility is up to two times higher than for a standard-type setup. This enhancement results in high quality images at a reasonable dose level which we exemplarily demonstrate by imaging a foreign object in a pork trotter.

Noise analysis of speckle-based x-ray phase-contrast imaging.

Speckle-based x-ray phase-contrast imaging has drawn increasing interest in recent years as a simple, multimodal, cost-efficient, and laboratory-source adaptable method. We investigate its noise properties to help further optimization on the method and further comparison with other phase-contrast methods. An analytical model for assessing noise in a differential phase signal is adapted from studies on the digital image correlation technique in experimental mechanics and is supported by simulations and experiments. The model indicates that the noise of the differential phase signal from speckle-based imaging has a behavior similar to that of the grating-based method.

A disk wind in AB Aurigae traced with Hαinterferometry

Context. A crucial issue in star formation is understanding the physical mechanism by which mass is accreted onto and ejected by a young star, then collimated into jets. Hydrogen lines are often used to trace mass accretion in young stars, but recent observations suggest that they could instead trace mass outflow in a disk wind.Aims. Obtaining direct constraints on the HI line formation regions is crucial in order to disentangle the different models. We present high angular and spectral resolution observations of the Hα line of the Herbig Ae star AB Aur to probe the origin of this line at sub-AU scales, and to place constraints on the geometry of the emitting region.Methods. We use the visible spectrograph VEGA at the CHARA long-baseline optical array to resolve the AB Aur circumstellar environment from spectrally resolved interferometric measurements across the Hα emission line. We developed a 2D radiative transfer model to fit the emission line profile and the spectro-interferometric observables. The model includes the combination of a Blandford & Payne magneto-centrifugal disk wind and a magnetospheric accretion flow.Results. We measure a visibility decrease within the Hα line, indicating that we clearly resolve the Hα formation region. We derive a Gaussian half width at half maximum between 0.05 and 0.15 AU in the core of the line, which indicates that the bulk of the Hα emission has a size scale intermediate between the disk inner truncation radius and the dusty disk inner rim. A clear asymmetric differential phase signal is found with a minimum of −30° ± 15° towards the core of the line. We show that these observations are in general agreement with predictions from a magneto-centrifugal disk wind arising from the innermost regions of the disk. Better agreement, in particular with the differential phases, is found when a compact magnetospheric accretion flow is included.Conclusions. We resolve the Hα formation region in a young accreting intermediate mass star and show that both the spectroscopic and interferometric measurements can be reproduced well by a model where the bulk of Hα forms in a MHD disk wind arising from the innermost regions of the accretion disk. These findings support similar results recently obtained in the Brγ line and confirm the importance of outflows in the HI line formation processes in young intermediate mass stars.

Beam geometry, alignment, and wavefront aberration effects on interferometric differential wavefront sensing

Heterodyne interferometry is a widely accepted methodology with high resolution in many metrology applications. As a functionality enhancement, differential wavefront sensing (DWS) enables simultaneous measurement of displacement, pitch, and yaw using a displacement interferometry system and a single beam incident on a plane mirror target. The angular change is measured using a weighted phase average between symmetrically adjacent quadrant photodiode pairs. In this paper, we present an analytical model to predict the scaling of differential phase signals based on fundamental Gaussian beams. Several numerical models are presented to discuss the effects of physical beam parameters, detector size, system alignment errors, and beam wavefront aberrations on the DWS technique. The results of our modeling predict rotational scaling factors and a usable linear range. Furthermore, experimental results show the analytically predicted scaling factor is in good agreement with empirical calibration. Our three degree-of-freedom interferometer can achieve a resolution of 0.4 nm in displacement and 0.2 μrad in pitch and yaw simultaneously.

Mars rover positioning technology based on same-beam VLBI

Position measurement of a Mars rover is important for engineering and scientific researches. This paper reports a method to carry out high precision positioning of the rover and orbit determination of the orbiter by using the same- beam VLBI between the rover and the orbiter and the orbiters ranging data. The calculation method of the differential phase delay and signal link of a Mars rover is analyzed. The measurement error of the differential phase delay caused by solar plasma, atmosphere, ionosphere and instruments is estimated to lower than 10 ps. As a successful example, in ChangE-3, the differential phase delay data between the rover and lander are obtained with a random error of 1 ps, and the relative position of the rover is determined with an accuracy of 1 m by using the same beam VLBI, which demonstrates the effectiveness of the method. In the case of the Mars rover and the orbit, the simulation analysis results show that the Mars rover position can be determined with an error of hundreds of meters and the orbit of the orbiter can be determined with an error of tens of meters by using the orbiter ranging and the same-beam VLBI differential phase delay data.

Spline based iterative phase retrieval algorithm for X-ray differential phase contrast radiography.

Differential phase contrast imaging using grating interferometer is a promising alternative to conventional X-ray radiographic methods. It provides the absorption, differential phase and scattering information of the underlying sample simultaneously. Phase retrieval from the differential phase signal is an essential problem for quantitative analysis in medical imaging. In this paper, we formalize the phase retrieval as a regularized inverse problem, and propose a novel discretization scheme for the derivative operator based on B-spline calculus. The inverse problem is then solved by a constrained regularized weighted-norm algorithm (CRWN) which adopts the properties of B-spline and ensures a fast implementation. The method is evaluated with a tomographic dataset and differential phase contrast mammography data. We demonstrate that the proposed method is able to produce phase image with enhanced and higher soft tissue contrast compared to conventional absorption-based approach, which can potentially provide useful information to mammographic investigations.

A single-image method for x-ray refractive index CT

X-ray refraction-based computer tomography imaging is a well-established method for nondestructive investigations of various objects. In order to perform the 3D reconstruction of the index of refraction, two or more raw computed tomography phase-contrast images are usually acquired and combined to retrieve the refraction map (i.e. differential phase) signal within the sample. We suggest an approximate method to extract the refraction signal, which uses a single raw phase-contrast image. This method, here applied to analyzer-based phase-contrast imaging, is employed to retrieve the index of refraction map of a biological sample. The achieved accuracy in distinguishing the different tissues is comparable with the non-approximated approach. The suggested procedure can be used for precise refraction computer tomography with the advantage of a reduction of at least a factor of two of both the acquisition time and the dose delivered to the sample with respect to any of the other algorithms in the literature.

The variable stellar wind of Rigel probed at high spatial and spectral resolution

We present a spatially resolved, high-spectral resolution (R=12000) K-band temporal monitoring of Rigel using AMBER at the VLTI. Rigel was observed in the Bracket Gamma line and its nearby continuum in 2006-2007, and 2009-2010. These unprecedented observations were complemented by contemporaneous optical high-resolution spectroscopy. We analyse the near-IR spectra and visibilities with the 1D non-LTE radiative-transfer code CMFGEN. The differential and closure phase signal exhibit asymmetries that are interpreted as perturbations of the wind. A systematic visibility decrease is observed across the Bracket Gamma. During the 2006-2007 period the Bracket Gamma and likely the continuum forming regions were larger than in the 2009-2010 epoch. Using CMFGEN, we infer a mass-loss rate change of about 20% between the two epochs. We further find time variations in the differential visibilities and phases. The 2006-2007 period is characterized by noticeable variations of the differential visibilities in Doppler position and width and by weak variations in differential and closure phase. The 2009-2010 period is much more quiet with virtually no detectable variations in the dispersed visibilities but a strong S-shape signal is observed in differential phase coinciding with a strong ejection event discernible in the optical spectra. The differential phase signal that is sometimes detected is reminiscent of the signal computed from hydrodynamical models of corotating interaction regions. For some epochs the temporal evolution of the signal suggests the rotation of the circumstellar structures.

Mapping misoriented fibers using X-ray dark field tomography

X-ray grating interferometers produce three distinct signals; an absorption signal, a differential phase signal and a dark field signal. Until now a method for successfully creating dark field tomograms of nonisotropic samples has not been demonstrated. In this paper we test a method for creating such tomograms on a highly nonisotropic sample, i.e. a five layer “sandwich” of oriented carbon fibers. The fibers are parallel within the individual sandwich layers, but perpendicular to the fibers in the adjacent layers. We show that by choosing a rotation axis parallel to the grating stepping direction (i.e. a horizontal rotation axis in most setup configurations) it is possible to produce a darkfield tomogram where fibers parallel to the probed scattering direction appear to have no dark field signal. The method produces a tomogram in the form of a scalar field of dark field scattering values.

AMBER and CRIRES observations of the binary sgB[e] star HD 327083: evidence of a gaseous disc traced by CO bandhead emission

HD 327083 is a sgB[e] star that forms a binary system with an orbital semi-major axis of ~1.7 AU. Our previous observations using the VLTI and AMBER in the medium resolution K-band mode spatially resolved the environment of HD 327083. The continuum visibilities obtained indicate the presence of a circumbinary disc. CO bandhead emission was also observed. However, due to the limited spectral resolution of the previous observations, the kinematic structure of the emitting material was not constrained. In this paper, we address this and probe the source of the CO emission with high spectral resolution and spatial precision. We have observed HD 327083 with high spectral resolution (25 & 6 km/s) using AMBER and CRIRES. The observations are compared to kinematical models to constrain the source of the emission. It is shown that the CO bandhead emission can be reproduced using a model of a Keplerian disc with an inclination and size consistent with our previous VLTI observations. The model is compared to AMBER differential phase measurements, which have a precision as high as 30-micro-arcseconds. A differential phase signal corresponding to 0.15 milli-arcseconds (~5 sigma) is seen over the bandhead emission, which is in excellent agreement with the model that fits the CRIRES observations. In comparison, a model of an equatorial outflow, as envisaged in the standard sgB[e] scenario, does not reproduce the observations well. The excellent agreement between the disc model and observations in the spatial and spectral domains is compelling evidence that the CO bandhead emission of HD 327083 originates in a circumbinary Keplerian disc. In contrast, the model of an equatorial outflow cannot reproduce the observations well. This suggests that the standard sgB[e] scenario is not applicable to HD 327083, which supports the hypothesis that the B[e] behaviour of HD 327083 is due to binarity (ABRIDGED).

Effect of Faraday rotation on L-band interferometric and polarimetric synthetic-aperture radar data

Electromagnetic waves traveling through the ionosphere undergo a Faraday rotation of the polarization vector, which modifies the polarization and phase characteristics of the electromagnetic signal. Using L-band (/spl lambda/=24 cm), polarimetric synthetic aperture radar (SAR) data from the shuttle imaging radar C (SIR-C) acquired in 1994, the author simulates the effect of a change in the Faraday rotation angle /spl psi/ on spaceborne interferometric and polarimetric data. In one experiment, it was found that phase coherence is reduced by up to 33% when /spl psi/ changes between successive data acquisitions. If /spl psi/ changes by more than 40/spl deg/, a differential phase signal, which varies from field to field, appears in the interferogram and impairs the mapping of surface topography and/or the detection of ground deformation. This signal is caused by phase differences between horizontal-polarized and vertical-polarized radar signals from intermediate levels of vegetation canopy, similar to the phase difference measured between H-polarized and V-polarized signals on a single date. In a second experiment, data from the Japanese Earth Resources Satellite (JERS-1) L-band radar acquired in an area of active deforestation in Rondonia, Brazil, are compared with SIR-C L-band polarimetric data acquired at the same incidence, two weeks later, but from a lower orbiting altitude. Large differences in scattering behavior are recorded between the two datasets in the areas of slash and burn forest, which are difficult to reconcile with surface changes. A simulation with SIR-C polarimetric data, however, suggests that those differences are consistent with a Faraday rotation angle of about 30/spl plusmn/10/spl deg/ in the JERS-1 data and 0/spl deg/ in the SIR-C data. Based on these two experiments and on Global Positioning System (GPS) records of ionospheric activity, it is concluded that Faraday rotation should not affect the analysis of L-band spaceborne data during periods of low ionospheric activity (solar minima).