Optical design of a novel near-infrared phase contrast imaging(NI-PCI)diagnostic on the HL-2A tokamak

The optical design of near-infrared phase contrast imaging(NI-PCI)diagnosis on HL-2A is introduced in this paper.This scheme benefits from the great progress of near-infrared laser technology and is a broadening of traditional phase contrast technology.This diagnostic can work as a keen tool to measure plasma wavenumber spectra by inferring string-integrated plasma density fluctuations.Design of both the front optical path which is the path before the laser transmitting into the tokamak plasma and the rear optics which is the path after the laser passing through the plasma is detailed.The 1550 nm laser is chosen as the probe beam and highprecision optical components are designed to fit the laser beam,in which a phase plate with a 194-nm-deep silver groove is the key.Compared with the conventional 10.6μm laser-based PCI system on HL-2A,NI-PCI significantly overcomes the unwanted phase scintillation effect and promotes the measurement capability of high-wavenumber turbulence with an increased maximal measurable wavenumber from 15 cm^(-1)to 32.6 cm^(-1).

A new method for information retrieval in two-dimensional grating-based X-ray phase contrast imaging

Grating-based X-ray phase contrast imaging has been demonstrated to he an extremely powerful phase-sensitive imaging technique. By using two-dimensional （2D） gratings, the observable contrast is extended to two refraction directions. Recently, we have developed a novel reverse-projection （RP） method, which is capable of retrieving the object information efficiently with one-dimensional （1D） grating-based phase contrast imaging. In this contribution, we present its extension to the 2D grating-based X-ray phase contrast imaging, named the two-dimensional reverse- projection （2D-RP） method, for information retrieval. The method takes into account the nonlinear contributions of two refraction directions and allows the retrieval of the absorption, the horizontal and the vertical refraction images. The obtained information can be used for the reconstruction of the three-dimensionak phase gradient field, and for an improved phase map retrieval and reconstruction. Numerical experiments are carried out, and the results confirm the validity of the 2D-RP method.

Real time high accuracy phase contrast imaging with parallel acquisition speckle tracking

X-ray speckle tracking based methods can provide results with best reported angular accuracy up to 2 nrad. However,duo to the multi-frame requirement for phase retrieval and the possible instability of the x-ray beam, mechanical and background vibration, the actual accuracy will inevitably be degraded by these time-dependent fluctuations. Therefore,not only spatial position, but also temporal features of the speckle patterns need to be considered in order to maintain the superiority of the speckle-based methods. In this paper, we propose a parallel acquisition method with advantages of real time and high accuracy, which has potential applicability to dynamic samples imaging as well as on-line beam monitoring.Through simulations, we demonstrate that the proposed method can reduce the phase error caused by the fluctuations to1% at most compared with current speckle tracking methods. Meanwhile, it can keep the accuracy deterioration within0.03 nrad, making the high theoretical accuracy a reality. Also, we find that waveforms of the incident beam have a little impact on the phase retrieved and will not influence the actual accuracy, which relaxes the requirements for speckle-based experiments.

Wide-Field Vibrational Phase Contrast Imaging Based on Coherent Anti-Stokes Raman Scattering Holography

We propose and implement a wide-field vibrational phase contrast detection to obtain imaging of imaginary components of third-order nonlinear susceptibility in a coherent anti-Stokes Raman scattering （CARS） microscope with full suppression of the non-resonant background. This technique is based on the unique ability of recovering the phase of the generated CARS signal based on holographic recording. By capturing the phase distributions of the generated CARS field from the sample and from the environment under resonant illumination, we demonstrate the retrieval of imaginary components in the CARS microscope and achieve background free coherent Raman imaging.

Single-shot grating-based x-ray differential phase contrast imaging with a modified analyzer grating

X-ray grating interferometer has attracted widely attention in the past years due to its capability in achieving x-ray phase contrast imaging with low brilliance source. However, the widely used phase stepping information extraction method reduces system stability and prolongs data acquisition time by several times compared with conventional x-ray absorption- based imaging. The mechanical stepping can be avoided by using a staggered grating, but at the cost of low vertical spatial resolution. In this paper, employing a modified staggered grating and the angular signal radiography, we proposed a single-shot grating-based x-ray differential phase contrast imaging with decent vertical spatial resolution. The theoretical framework was deduced and proved by numerical experiments. Absorption, phase, and scattering computed tomography can be performed without phase stepping. Therefore, we believe this fast and highly stable imaging method with decent resolution would be widely applied in x-ray grating-based phase contrast imaging.

In-line phase contrast for weakly absorbing materials with a microfocus x-ray source

For weakly absorbing materials, image contrast can be enhanced by phase contrast in formation. The effectiveness of the in-line phase contrast technique relies on its ability to record intensity data which contain information on the x- ray＇s phase shift. Four kinds of approaches to the relationship between intensity distribution and phase shift axe reviewed and discussed. A micro-focal x-ray source with high geometrical magnification is used to acquire phase contrast images. A great improvement on image quality is shown and geometrical parameters axe modified for comparison between different imaging positions.

Simple phase extraction in x-ray differential phase contrast imaging

A fast and simple method to extract phase-contrast images from interferograms is proposed, and its effectiveness is demonstrated through simulation and experiment. For x-ray differential phase contrast imaging, a strong attenuation signal acts as an overwhelming background intensity that obscures the weak phase signal so that no obvious phase-gradient information is detectable in the raw image. By subtracting one interferogram from another, chosen at particular intervals,the phase signal can be isolated and magnified.

Algorithm for phase contrast X-ray tomography based on nonlinear phase retrieval

A new algorithm for phase contrast X-ray tomography under holographic measurement was proposed in this paper. The main idea of the algorithm was to solve the nonlinear phase retrieval problem using the Newton iterative method. The linear equations for the Newton directions were proved to be ill-posed and the regularized solutions were obtained by the conjugate gradient method. Some numerical experiments with computer simulated data were presented. The efficiency, feasibility and the numerical stability of the algorithm were illustrated by the numerical experiments. Compared with the results produced by the linearized phase retrieval algorithm, we can see that the new algorithm is not limited to be only efficient for the data measured in the near-field of the Fresnel region and thus it has a broader validity range.

Wavefront sensing based on phase contrast theory and coherent optical processing

A novel wavefront sensing method based on phase contrast theory and coherent optical processing is proposed. The wavefront gradient field in the object plane is modulated into intensity distribution in a gang of patterns, making highdensity detection available. By applying the method, we have also designed a wavefront sensor. It consists of a classical coherent optical processing system, a CCD detector array, two pieces of orthogonal composite sinusoidal gratings, and a mechanical structure that can perform real-time linear positioning. The simulation results prove and demonstrate the validity of the method and the sensor in high-precision measurement of the wavefront gradient field.

Shifting curves based on the detector integration effect for x-ray phase contrast imaging

In theory, we find that the actual function of the analyzer grating in the Talbot–Lau interferometer is segmenting the self-images of the phase grating and choosing integral areas, which make sure that each period of self-images in one detector pixel contributes the same signal to the detector. Furthermore, in the case of the lack of an analyzer grating, the shifting curves are still existent in theory as long as the number of fringes is non-integral in a detector pixel, which is a sufficient condition for creating shifting curve. The sufficient condition is available for not only the Talbot–Lau interferometer and the inverse geometry of Talbot–Lau interferometer, but also the x-ray phase contrast imaging system based on geometrical optics. In practical applications, we propose a method to improve the performances of the existing systems by employing the sufficient condition. This method can shorten the system length, is applicable to large period gratings, and can use the detectors with large pixels and large field of view. In addition, the experimental arrangement can be simplified due to the lack of an analyzer grating. In order to improve detection sensitivity and resolution, we also give an optimal fringe period.We believe that the theory and method proposed here is a step forward for x-ray phase contrast imaging.

Optical path design of phase contrast imaging on HL-2A tokamak

A phase contrast imaging（PCI） diagnostic has recently been developed on HL-2 A tokamak. It can diagnose plasma density fluctuations with maximum wave number of 15 cm^（-1） and wave number resolution of 2 cm^（-1）. The time resolution reaches 2 μs. A 10.6 μm CO_2 laser is expanded to a beam with a diameter of 30 mm and injected into the plasma as an incident beam,injecting into plasma. The emerging scattered and unscattered beams are contrasted by a phase plate. The ideas of optical path design are presented in this paper, together with the parameters of the main optical components. The whole optical path of PCI is not only carefully designed, but also constructed on HL-2 A. First calibration results show the ability of this system to catch plasma turbulence in a wide frequency domain.

Research on the Influence of X-ray Phase Contrast Imaging Parameters with Micro-focus Source

X-ray in-line phase contrast imaging enables weakly to absorb specimens to be imaged successfully with high resolution and definition. In this paper we use computer simulation method to analyze how each parameter influences the quality of the image. It can avoid wasting unnecessary time and materials in the course of experiment to get ideal images.

Nondestructive X-ray imaging of inner structure of soft tissues in phase contrast

An experimental study on nondestructive X-ray imaging of inner structure of soft tissues in phase con-trast has been conducted with Beijing Synchrotron Radiation Facility (BSRF). Modification to the beamline setupwas made to enlarge the X-ray beam section and consequently larger samples could be imaged. In-line setup was em-ployed for experiments. Results on a series of samples were given and soft-tissue details of less than 50 μm inside afresh goldfish were obtained. Diagnosis of tumor in its early stage was also investigated taking SD rats as the model.Tumor at the size of ～ 100μm was observed. Potential of this technique in clinic diagnosis was discussed.

Progress of plasma density fluctuation measurements with phase contrast imaging on HL-2A tokamak

A CO2 laser-based phase contrast imaging(PCI) diagnostic has been developed on HL-2A tokamak.It can detect line integrated plasma density fluctuations by measuring the phase shift of laser beam after being scattered by the bulk plasma.The diagnosed radial region ranges from ρ≡r/a =0.625 to 0.7.32-channel HgCdTe detectors with alternative-current biased amplifiers are arranged in line at the imaging plane of the optical path.This PCI is able to diagnose density fluctuations with wavenumbers ranging from 2 to 15 cm-1 and the time resolution is better than 2 μs.The first experimental data were achieved in 2018 spring campaign of HL-2A tokamak.High performance is confirmed in different discharging configurations and makes it a keen tool in broadband turbulence investigations.

Observation of cavitation and water-refilling processes in plants with X-ray phase contrast microscopy

With the spatial coherence of X-rays and high flux and brightness of the 3rd generation synchrotron radiation facility,X-ray phase contrast microscopy(XPCM)at Shanghai Synchrotron Radiation Facility(SSRF)can provide high resolution dynamic imaging of low electron density materials in principle.In this paper,we investigated the cavitation and water-refilling processes in rice and bamboo leaves utilizing XPCM at SSRF.The occurrence of xylem cavitation was recorded in vivo.The study also revealed that under different dehydration conditions,cavitation occurs in different degrees,and therefore,the refilling process is different.The results demonstrate that SSRF can provide high enough fluxes to study dynamic processes in plants in real-time,and XPCM is expected to be a promising method to reveal the mechanisms of cavitation and its repair in plants nondestructively.

Binary Phase Shaping and SHG Contrast Ratios

Finding binary sequences with Large SHG ratios is very important in the field of ultrafast science, biomedical optics, high-resolution microscopy and label-free imaging. In this paper, we have demonstrated the relation between the SHG contrast ratio and the traditional Merit Factor values. And in the light from known results in Merit Factor Problems, we have shown that Legendre Sequences or Jacobi Sequences, are still the best candidates to obtain binary sequences with large SHG contrast ratios. The authors also discussed the SHG behaviors on some sequences obtained from cyclotomic classes over the finite field GF (2l) .

Theory and verification of moiréfringes for x-ray three-phase grating interferometer

Dual-phase and three-phase grating x-ray interference is a promising new technique for grating-based x-ray differential phase contrast imaging.Dual-phase grating interferometers have been relatively completely studied and discussed.In this paper,the corresponding imaging fringe formula of the three-phase grating interferometer is provided.At the same time,the similarities and differences between the three-phase grating interferometer and the dual-phase grating interferometer are investigated and verified,and that the three-phase grating interferometer can produce large-period moiréfringes without using the analyzing grating is demonstrated experimentally.Finally,a simple method of designing three-phase grating and multi-grating imaging systems from geometric optics based on the thin-lens theory of gratings is presented.These theoretical formulas and experimental results provide optimization tools for designing three-phase grating interferometer systems.

A new method of detecting interferogram in differential phase-contrast imaging system based on special structured x-ray scintillator screen

An x-ray scintillator screen with a special structure, functioning as detector and analyser grating, was proposed for collecting the interferogram of differential phase contrast imaging without absorption grating and difficulty of fabrication by a state of the art technique. On the basis of phase grating diffraction, a detecting model of the scintillator screen was built for analysing the phase and absorption information of objects. According to the analysis, a new method of phase retrievals based on two-images and the optimal structure of screen were presented.

Phase Contrast Micro-Computed Tomography of Biological Sample at SSRF

In line X-ray phase contrast micro-computed tomography （IL-XPCT）, which can be implemented at third generation synchrotron radiation sources or by using a micro-focus X-ray tube, is a powerful technique for non-destructive, high-resolution investigations of a broad variety of materials. At the Shanghai Synchrotron Radiation Facility （SSRF）, the X-ray Imaging and Biomedical Applications Beamline was built and started regular user operation in May 2009. Both qualitative （without phase retrieval） and quantitative （with phase retrieval） three-dimensional IL-XPCT experimental techniques have been established at the beamline IL-XPCT experiments of a test sample （plastic pipes） used to evaluate the technique, and of a biological sample （locust） at the beamline are reported. Two series of images, qualitative and quantitative, including tomographic slices and three-dimensional rendering images were obtained. In qualitative images, there is a strong edge-enhancement which leads to very clear sample contours, while in quantitative images, the edge-enhancement fades but quantitative measurement of sample＇s phase information could be achieved. The experiments demonstrate that the combination of qualitative and quantitative images is useful for biological sample studies.

Denoising method of X-ray phase contrast DR image for TRISO-coated fuel particles

TRISO （tristructural-isotropic） fuel is a type of micro fuel particles used in high-temperature gas-cooled reactors （HTGRs）. Among the quality evaluation methods for such particles, inqine phase contrast imaging technique （PCI） is more feasible for nondestructive measurement. Due to imaging hardware limitations, high noise level is a distinct feature of PCI images, and as a result, the dimensional measurement accuracy of TRISO-coated fuel particles decreases. Therefore, we propose an improved denoising hybrid model named as NL P-M model which introduces non-local theory and retains the merits of the Perona-Malik （P-M） model. The improved model is applied to numerical simulation and practical PCI images. Quanti- tative analysis proves that this new anisotropic diffusion model can preserve edge or texture information effectively, while ruling out noise and distinctly decreasing staircasing artifacts. Especially during the process of coating layer thickness measurement, the NL P-M model makes it easy to obtain continuous contours without noisy points or fake contour segments, thus enhancing the measurement accuracy. To address calculation complexity, a graphic processing unit （GPU） is adopted to realize the acceleration of the NL P-M denoising.