Fringe visibility in X-ray interferometer using dual triangular phase gratings

The state of polarization of a specularly reflected beam from metal and dielectric surface is mainly dependent on the optical constants of the materials, wavelength of the illuminating beam, state of polarization and angle of incidence of the incident beam. In the present study the state of polarization of the reflected beam is analyzed with the help of Stokes formalism where the input beam is linearly polarized. The intensity variation of the reflected beam passing through an analyzer is then studied by varying the orientation of the transmission axis of the analyzer and acquiring the image of the metal-dielectric (copper film deposited on microscope slide) surface for each angular position of the analyzer. The degree of linear polarization (DOLP) as well as fractional non-linear polarization (FNTP) of the image of the metal-dielectric surface is evaluated both theoretically and experimentally and for both monochromatic and polychromatic incident illumination. A good agreement is observed between theoretical and experimental results. For estimation of pixel-wise variation of DOLP and FNLP of the metal dielectric surface, the spectral Stokes parameters for each pixel location are computed with the help of measurement matrix. As a practical application of the study, a printed circuit board (vero board) is used as a metal-dielectric (perforated copper strip on glossy dielectric substrate) surface for observing pixel-wise DOLP and FNLP variation. The resultant DOLP and FNLP images for Vero board show the metal-dielectric discrimination capability under polychromatic illumination.

The authors present initial progress toward a clinically compatible x-ray phase contrast CT system, using glancing-angle x-ray grating interferometry to provide high contrast soft tissue images at estimated by computer simulation dose levels comparable to conventional absorption based CT. DPC-CT scans of a joint phantom and of soft tissues were performed in order to answer several important questions from a clinical setup point of view. A comparison between high and low fringe visibility systems is presented. The standard phase stepping method was compared with sliding window interlaced scanning. Using estimated dose values obtained with a Monte-Carlo code the authors studied the dependence of the phase image contrast on exposure time and dose. Using a glancing angle interferometer at high x-ray energy (∼ 45 keV mean value) in combination with a conventional x-ray tube the authors achieved fringe visibility values of nearly 50%, never reported before. High fringe visibility is shown to be an indispensable parameter for a potential clinical scanner. Sliding window interlaced scanning proved to have higher SNRs and CNRs in a region of interest and to also be a crucial part of a low dose CT system. DPC-CT images of a soft tissue phantom at exposures in the range typical for absorption based CT of musculoskeletal extremities were obtained. Assuming a human knee as the CT target, good soft tissue phase contrast could be obtained at an estimated absorbed dose level around 8 mGy, similar to conventional CT. DPC-CT with glancing-angle interferometers provides improved soft tissue contrast over absorption CT even at clinically compatible dose levels (estimated by a Monte-Carlo computer simulation). Further steps in image processing, data reconstruction, and spectral matching could make the technique fully clinically compatible. Nevertheless, due to its increased scan time and complexity the technique should be thought of not as replacing, but as complimentary to conventional CT, to be used in specific applications.

Changeable density pseudocoloring with diffraction of dual phase gratings

When a Young interferometer is fed by a general electromagnetic field, the fringe visibility may change upon insertion of an anisotropic optical element over one of the interferometer pinholes. The maximum visibility that the fringes may exhibit in this way is theoretically known, but no direct experimental check seems to be available. Here we discuss the scheme of an experimental test. In particular, maximum fringe visibility is obtained with a three-component universal SU2 polarization gadget for arbitrary coherence features of the illuminating field. Confirming results obtained with a suitable experimental setup are presented.

This study investigated the dynamic behavior of the first-order diffraction efficiency of gratings formed in polymer-dispersed liquid crystal (PDLC) films doped with a guest-host dye. PDLC films were fabricated using various LC-polymer mixing ratios, and written with various powers. Experimental results indicated that several peaks appeared in the curve of the first-order diffraction efficiency versus time. According to the light scattering study, we believe that the first peak was due to the superposition of density and absorption gratings. The density grating was associated with the spatially varied molecular weight of polymer molecules across the sample, and the absorption grating resulted from the spatially varied density of free electrons. The other peaks were caused by the superposition of the absorption and phase gratings. The phase grating was generated by the formation of a periodic structure of polymer-rich and LC-rich regions in the sample. This study also proposes a model to explain these experimental results. Moreover, the theory derived from this model correlates well with the experimental results, allowing us to determine the amplitude of the final grating.

We investigated the contribution of the absorption and phase gratings to the total diffraction efficiency of volume holographic gratings written in glass-like polymer recording materials based on poly(methyl methacrylate) and its thermostable derivative (copolymer with acrylic acid) with distributed phenanthrenequinone. The typical maximal diffraction efficiency was 0.5%-2.0% for the absorption grating and 22-32% for the phase grating. The modulation of the absorption coefficient varied between 10 cm(-1) and 100 cm(-1) and the modulation of the refractive index was about 10(-4)-10(-3).

A new technique for the assessment of the optical quality of multifocal intraocular lenses (MIOLs) under monochromatic and polychromatic illumination is presented. The system provides, in a totally automated procedure, the modulation transfer function (MTF) of the lens under test for different axial positions of the object. The artificial eye admits different artificial corneas, to optimise the axial resolution in the sampling of the MTF of the MIOL under test, and different pupils, to test the dependence of the optical performance of the MIOL on the eye pupil diameter. The performance and sensitivity of the apparatus is tested with different commercial MIOLs. The through-focus MTF for a refractive MIOL was measured at different moments during the process of hydration of a hydrophilic lens. Next, to show the performance of our proposal, two commercial refractive and diffractive MIOLs were evaluated. We have designed a precise and robust optical method for testing MIOLs in vitro. The proposed method represents a valuable technical improvement to make the procedure of MIOL evaluation more versatile, efficient and trustworthy.

Higher-order harmonics in bleached silver halide holograms

Real-time diffraction efficiency is measured during hologram formation in undeveloped dichromated gelatin (DCG) film. The main features of real-time diffraction efficiency of undeveloped DCG reveal double peaks with increasing exposure time. Adopting the grating translation technique, we show that the first peak originates from the absorption grating and that the latter is related to the phase grating. To explain such complicated hologram-formation dynamics, we present a phenomenological model for mixed gratings associated with photoinduced refractive-index changes and absorption changes. We have compared the model results with the experimental results, which show good agreement.

Several techniques for determination of grating phase shifts relative to intensity grating have been developed by introducing artificial phase mismatch which includes by moving the nonlinear material, by phase shifting one of the two writing beams or by applying external electric fields.(1) In this work, we performed two kinds of experiment for two-wave energy couplings in a photopolymer film: (i) measurement of grating phase shifts of phase grating (PG) and absorption gratings (AG) by translation of the material along the grating wave vector K, (ii) experimental observation of quasi-periodic dynamic self-diffraction oscillations without introducing artificial phase mismatch.

Phase-shifting Jamin double-shearing interferometer based on polarization beam splitting film

The mutual optical intensity (MOI) model is extended to the simulation of the interference pattern produced by extreme ultraviolet lithography with partially coherent light. The partially coherent X-ray propagation through the BL08U1B beamline at Shanghai Synchrotron Radiation Facility is analysed using the MOI model and SRW (Synchrotron Radiation Workshop) method. The fringe intensity at the exposure area is not uniform but has similar envelope lines to Fresnel diffraction, which is explained by the diffraction from the finite grating modelled as a single aperture. By balancing the slit size and photon stop size, the fringe visibility, photon flux and intensity slope can be optimized. Further analysis shows that the effect of pink light on the aerial images is negligible, whereas the third-harmonic light should be considered to obtain a balance between high fringe visibility and high flux. Two grating interference exposure experiments were performed in the BL08U1B beamline. The aerial image depth showed that the polymethyl methacrylate photoresist depth was determined by the X-ray coherence properties.

We present a self-filtering and high-resolution sapphire fiber sensor based on an extrinsic Fabry-Pérot interferometer for high-temperature sensing. The sensor has a high fringe visibility and signal-to-noise ratio. Two well-polished sapphire fibers are aligned side by side to realize shunting transmission between the background light and the interference signal. Heterogeneous fiber splicing between the sapphire fiber and the multimode silica fiber is realized for long-distance transmission of signals. A C-plane double-sided polished sapphire wafer is mounted perpendicularly to the tip of sapphire fibers as the Fabry-Pérot cavity. The crosstalk noise from input light caused by heterogeneous fiber splicing and fiber tip reflection is filtered out to reach a higher signal quality. A series of comparison experiments from 100 to 1080 °C were carried out for the difference in characteristics between self-filtering sensors and traditional sapphire fiber Fabry-Pérot sensors. The self-filtering sensor's fringe visibility was above 43.96%, and its signal-to-noise ratio was above 20.14 dB. The temperature resolution of the self-filtering sensor was better than 0.25 °C.

All-fiber multipath Mach⬜Zehnder interferometer based on a four-core fiber for sensing applications

What is the maximum visibility attainable in double-slit interference by an electromagnetic field if arbitrary - but reversible - polarization and spatial transformations are applied? Previous attempts at answering this question for electromagnetic fields have emphasized maximizing the visibility under local polarization transformations. I provide a definitive answer in the general setting of partially coherent electromagnetic fields. An analytical formula is derived proving that the maximum visibility is determined by only the two smallest eigenvalues of the 4×4 two-point coherency matrix associated with the electromagnetic field. This answer reveals, for example, that any two points in a spatially incoherent scalar field can always achieve full interference visibility by applying an appropriate reversible transformation spanning both the polarization and spatial degrees of freedom - without loss of energy. Surprisingly, almost all current measures predict zero-visibility for such fields. This counter-intuitive result exploits the higher dimensionality of the Hilbert space associated with vector - rather than scalar - fields to enable coherency conversion between the field's degrees of freedom.