Tunable mode-locked Tm-doped fiber laser based upon cross-phase modulation.

We demonstrate the generation of soliton and dissipative soliton in an ultrafast thulium (Tm) doped fiber laser based upon cross-phase modulation (XPM) induced mode-locking. The mode-locking is realized by periodically modulating the 2-µm signal through XPM that is activated by an injected 1.5-µm pulsed laser. Such a mechanism enables the laser to be mode-locked in various operation regimes without any real or artificial saturable absorbers. Thanks to the XPM pulling effect, the wavelength of the Tm-doped fiber laser can be tuned by adjusting the repetition frequency of the 1.5-µm pulsed laser. The maximum tuning ranges achieved in this work for the soliton and dissipative soliton regimes are respectively 11 nm and 15 nm. The outcomes of this work not only provide a continuously and controllably wavelength-tunable ultrafast laser but also offer a passively synchronized dual-color fiber laser system, which is promised for many important applications such as Raman spectroscopy, nonlinear frequency conversion systems, and multi-color pump-probe systems.

Accurate PSF determination in x ray image restoration.

When a source's focal spot is known, an x ray image can be significantly improved by a deconvolution algorithm with the point spread function (PSF). We propose a simple method to measure the PSF for image restoration using x ray speckle imaging. In this method, the PSF is reconstructed with intensity and total variation constraints from a single x ray speckle of an ordinary diffuser. Compared with the traditional time-consuming measurement with a pinhole camera, the speckle imaging method is fast and easy to execute. When the PSF is available, we use a deconvolution algorithm to reconstruct the sample's radiography image, and the result provides more structural details than the original images.

Quantitative analysis of fringe visibility in grating-based x-ray phase-contrast imaging.

The newly developed x-ray differential phase-contrast imaging technique has attracted increasing research interest. In this study, we quantitatively analyze the fringe visibility obtained in differential phase-contrast imaging. Numerical results of the visibility for polychromatic x rays with different structure heights of absorption gratings are shown and discussed. Furthermore, the fringe visibility of the nonabsorption grating approach is calculated, and based on the results, we conclude that this approach can potentially be applied for higher x-ray photon energies. These analytic results will be useful for designing a differential phase-contrast imaging system for different applications.

Sampling grating approach for X-ray differential phase contrast imaging.

Grating-based X-ray differential phase contrast imaging (GDPCI) typically employs the phase-stepping technique to extract an object's phase information. This method requires heavy radiation dosage and is time consuming. Another potential approach is the reverse projection (RP) method, which, however, relies on a synchrotron radiation source to obtain highly sensitive differential phase contrast(DPC) signal. Here, we present an alternative approach that enables the RP method to be used with a conventional X-ray source and substantially improves the sensitivity of the DPC signal by replacing the analyzer grating of the GDPCI with a sampling grating. This development represents a significant step towards obtaining fast and low-dosage DPC images in medical, biological, and industrial applications.

Non-absorption grating approach for X-ray phase contrast imaging.

We demonstrate a non-absorption grating approach for X-ray phase contrast imaging based-on grating interferometry. This technique overcomes the limitations imposed by absorption gratings, provides another choice for X-ray phase contrast imaging and potentially improves the image quality for higher X-ray photon energies. We constructed the key devices, established the system and obtained phase contrast images with a field of view larger than 5 centimeters, which is the limitation imposed by the size of our current CCD detector. This method has no need for absorption gratings, which represents a significant development for future promising applications in medicine and industry.

A low cost method for hard x-ray grating interferometry.

Grating interferometry is advantageous over conventional x-ray absorption imaging because it enables the detection of samples constituted by low atomic number elements (low-Z materials). Therefore, it has a potential application in biological science and medical diagnostics. The grating interferometry has some critical optics components such as absorption gratings which are conventionally manufactured by the lithography, electroplating, and molding (LIGA) technique and employing gold as the absorbent material in it. However, great challenge lies in its implementations for practical applications because of the cost and difficulty to achieve high aspect ratio absorbing grating devices. In this paper, we present a low-cost approach that involves using the micro-casting technique with bismuth (Bi) as the absorber in source grating and as well as filling cesium iodide thallium(CsI:Tl) in a periodically structured scintillator. No costly facilities as synchrotron radiation are required and cheap material is used in our approach. Our experiment using these components shows high quality complementary images can be obtained with contrast of absorption, phase and visibility. This alternative method conquers the limitation of costly grating devices for a long time and stands an important step towards the further practical application of grating interferometry.