Wavelength-switchable erbium-doped fiber ring laser combined with a Fabry-Perot laser diode and a sampled grating

We present a wavelength-switchable erbium-doped fiber ring laser consisting of an anti-reflection coated Fabry-Perot laser diode (FP-LD) and a sampled fiber Bragg grating (SFBG). The wavelength switching is obtained easily by the shift of FP mode with respect to the reflection peaks of the SFBG. The proposed laser apparatus not only simultaneously emits two wavelengths but also switches between the two wavelengths by adjusting the modes of FP-LD with a change of the injection current of the FP-LD as little as ∼1 mA. The proposed wavelength-switchable erbium-doped fiber ring laser exhibits high laser performance with side-mode suppression ratio (SMSR) ?50 dB.     

Dynamic sheath expansion and ion current in transient ion sheath experiments

Experimental results on the measurement of current collected by an electrode immersed in a plasma for a pulsed negative bias are presented. The measured current is compared with a model based on the concept of an expanding capacitor. The scaling laws predicted by the model are verified for the measured current which agree each other. The paper emphasizes the role of displacement current in an expanding ion sheath.     

一种实现部分谱的Tailored检测的简便方法

本文介绍一种利用延迟采样实现部分谱的Tailored检测的方法。文章对该方法作了理论上的分析并给出了用于强溶剂峰抑制时的实验结果,获得良好的效果。     

Numerical simulation of the horizontal Bridgman growth. Part I: Two-dimensional flow

We study the generation of periodic velocity and temperature fields in a plane horizontal crucible of molten metal under the action of a horizontal temperature gradient. The geometry and the boundary conditions are similar to those encountered in the Bridgman growth process of semiconductor crystals, although the present paper is limited to two-dimensional flows. We use transient finite difference and finite element algorithms which lead to identical results. We demonstrate the oscillatory mechanism in two different geometries.     

Transient natural convection of low-Prandtl-number fluids in a differentially heated cavity

The transient convective motion in a two-dimensional square cavity driven by a temperature gradient is analysed. The cavity is filled with a low-Prandtl-number fluid and the vertical walls are maintained at constant but different temperatures, while the horizontal boundaries are adiabatic. A control volume approach with a staggered grid is employed to formulate the finite difference equations. Numerically accurate solutions are obtained for Prandtl numbers of 0·001, 0·005 and 0·01 and for Grashof numbers up to 1 × 10⁷. It was found that the flow field exhibits periodic oscillation at the critical Grashof numbers, which are dependent on the Prandtl number. As the Prandtl number is decreased, the critical Grashof number and the frequency of oscillation decrease. Prior to the oscillatory flow, steady state solutions with an oscillatory transient period were predicted. In addition to the main circulation, four weak circulations were predicted at the corners of the cavity.     

无限阶段部分可观察马尔可夫决策规划

本文对[1,2]所考虑的无限阶段折扣费用部分可观察马尔可夫决策规划作了进一步的讨论,澄清了其中的一些模糊概念,补充或纠正了其中的疏漏和错误,特别地,在保持费用函数分片线性的原则下扩大了有限瞬时策略类,最后给出了几个新的结论,并对[1]中的策略迭代算法给出了修正及收敛估计。     

Transient magnetoviscosity of dilute ferrofluids

The magnetic field induced change in the viscosity of a ferrofluid, commonly known as the magnetoviscous effect and parameterized through the magnetoviscosity, is one of the most interesting and practically relevant aspects of ferrofluid phenomena. Although the steady state behavior of ferrofluids under conditions of applied constant magnetic fields has received considerable attention, comparatively little attention has been given to the transient response of the magnetoviscosity to changes in the applied magnetic field or rate of shear deformation. Such transient response can provide further insight into the dynamics of ferrofluids and find practical application in the design of devices that take advantage of the magnetoviscous effect and inevitably must deal with changes in the applied magnetic field and deformation. In this contribution Brownian dynamics simulations and a simple model based on the ferrohydrodynamics equations are applied to explore the dependence of the transient magnetoviscosity for two cases: (I) a ferrofluid in a constant shear flow wherein the magnetic field is suddenly turned on, and (II) a ferrofluid in a constant magnetic field wherein the shear flow is suddenly started. Both simulations and analysis show that the transient approach to a steady state magnetoviscosity can be either monotonic or oscillatory depending on the relative magnitudes of the applied magnetic field and shear rate.     

Z intensity-weighted position self-respiratory gating method for free-breathing 3D cardiac CINE imaging

A free-breathing 3D cine steady-state free precession (SSFP) technique was developed using the z intensity-weighted position (ZIP) which is the center of mass of a projection along the slice direction as a respiratory gating signal. The ZIP signal was continuously acquired using a slice encoded k-space center sampling in every TR. The performance of this gating method was compared with a method using the k-space center signal (KC) and with conventional 2D breath-hold cine SSFP in healthy subjects by measuring image quality and left ventricular function. The preliminary data obtained here demonstrated that the ZIP gating method provided superior respiratory motion artifact suppression when compared to the KC gating and provided left ventricular ejection fractions, and end-diastolic and end-systolic volumes similar to those obtained with the breath-hold 2D cine SSFP acquisition.     

Technical aspects of fast magic-angle turning NMR for dilute spin-1/2 nuclei with broad spectra

For obtaining sideband-free spectra of high-Z spin-1/2 nuclei with large (>1000 ppm) chemical-shift anisotropies and broad isotropic-shift dispersion, we recently identified Gan's modified five-pulse magic-angle turning (MAT) experiment as the best available broadband pulse sequence, and adapted it to fast magic-angle spinning. Here, we discuss technical aspects such as pulse timings that compensate for off-resonance effects and are suitable for large CSAs over a range of 1.8γB₁; methods to minimize the duration of z-periods by cyclic decrementation; shearing without digitization artifacts, by sharing between channels (points); and maximizing the sensitivity by echo-matched full-Gaussian filtering. The method is demonstrated on a model sample of mixed amino acids and its large bandwidth is highlighted by comparison with the multiple-π-pulse PASS technique. Applications to various tellurides are shown; these include GeTe, Sb₂Te₃ and Ag_0.53Pb₁₈Sb_1.2Te₂₀, with spectra spanning up to 190 kHz, at 22 kHz MAS. We have also determined the ¹²⁵Te chemical shift anisotropies from the intensities of the spinning sidebands resolved by isotropic-shift separation.     

Gradient enhanced spectroscopy

This paper provides a brief overview of the personal recollections of the authors regarding their contributions to the introduction of shielded gradient technology into NMR spectroscopy during the late 1980s and early 1990s. It provides some background into early probe design and details some of the early technical progress with the use of shielded magnetic field gradients for coherence selection in high resolution NMR and describes the developments at General Electric, the National Institutes of Health, Georgetown University and Johns Hopkins University School of Medicine that ultimately led to this technology becoming commonplace in modern NMR spectroscopy. Most of this early technical work was published in the Journal of Magnetic Resonance.