Heteronuclear dipolar recoupling in liquid crystals and solids by PISEMA-type pulse sequences

A pulse sequence is described for the recoupling of heteronuclear dipolar interactions under MAS. The method is similar to the PISEMA experiment, but employs a well-defined amplitude modulation of one of the two radio-frequency fields. The technique is used for measurements of 1H-13C dipolar couplings in unoriented solid and liquid-crystalline samples.     

Frequency offset locking of a synchronously pumped mode-locked dye laser

We have developed a scheme of frequency offset locking for transferring the frequency stability of a commercial dye laser to a synchronously pumped mode-locked dye laser. By observing the beat frequency between the two dye lasers, mode stability of less than 2 MHz has been observed for the mode-locked dye laser mode. The use of the mode-locked system as a spectroscopic tool has been demonstrated by observing Doppler-free multiple pulse two photon spectra of the sodium 3S-4D transition.     

Performance of single spin-echo and doubly refocused diffusion-weighted sequences in the presence of eddy current fields with multiple components

Echo-planar diffusion-weighted images can display significant geometric distortions due to eddy current fields. Several preparation schemes have been proposed, which can null eddy currents with a single time constant. The aim of this work was to compare the performance of three such pulse sequences in the presence of multiple components and investigate whether affine registration is capable of correcting for the resulting distortions. A magnetic resonance imaging simulator was used to eliminate potential confounding factors. The doubly refocused sequences showed substantially reduced effects. Applying affine registration to the single spin-echo images leads to reduced residuals, but not to the level observed for the doubly refocused sequences. Modified versions of the standard single spin-echo and doubly refocused sequences performed better than their original counterparts. Affine registration is not sufficient to correct for strong eddy current effects, which should therefore be minimized at source. When the use of a doubly refocused sequence is not possible, a modified single spin-echo sequence should be considered.     

Rapid calculation of T1 using variable flip angle gradient refocused imaging

We present a method for rapid measurement of T1 relaxation times using gradient refocused images at limited flip angles and short repetition times. This "variable nutation" techniques was investigated using a T1 phantom. There was a high correlation between measurements obtained with the variable nutation and partial saturation techniques. The ability of this method to create calculated T1 images is also demonstrated. We conclude that the variable nutation method may allow measurement of T1 relaxation times with a significant reduction in acquisition time compared to partial saturation techniques.     

Frequency offset and I/Q imbalance estimation for orthogonal frequency division multiplexing based wireless local area networks

The paper proposes a novel joint estimation of the frequency offset, frequency-independent I/Q imbalance, and frequency-dependent I/Q imbalance, using periodic pilots in orthogonal frequency division multiplexing (OFDM) based wireless local area networks (LANs). Although a low-complexity frequency offset estimation in the presence of I/Q imbalance can be obtained via a periodic pilot, the corresponding closed-form solution encounters a sign ambiguity. A method based on a power comparison between the signal and image interference is then proposed to detect the sign. Meanwhile, since one periodic pilot only is unable to estimate I/Q imbalance in the absence of a frequency offset, we also propose an estimation method based on two different periodic pilots available in the preamble. The validity and superiority of the proposed method are demonstrated by computer simulations.     

Joint estimation of frequency offset and chromatic dispersion based on the training sequences in M-ary QAM coherent optical transmission system

A data-aided method of joint frequency offset and chromatic dispersion estimation based on the Chu training sequences is shown in 112 Gb/s polarization-multiplexed （PM） quadrature phase-shift keying and 224 Gb/s PM-16-quadrature amplitude modulation with different pulse-shaping filters, respectively. The proposed method achieves a good accuracy and is verified to be robust to polarization mode dispersion.     

An analysis of the intrinsic resonance offset dependence of magnetization generated by stimulated echo pulse sequences for noncoupled spins.

It is demonstrated that the basic radiofrequency pulse train used to generate stimulated echoes (90x-tau TE-90x-tau TM-90x-tau TE-Acq.) is in general characterized by strong amplitude and phase modulations of the transverse magnetization as a function of the resonance offset. Two dephasing techniques which eliminate the modulations are investigated both theoretically and experimentally, and a simple formula is derived for calculating the relative modulation across a spectrum as a function of gradient strength and duration, echo delay, and spectral linewidth.     

Resonance offset tailored composite pulses

We describe novel composite pulse sequences which act as general rotors and thus are particularly suitable for nuclear magnetic resonance quantum computation. The resonance offset tailoring to enhance nutations approach permits perfect compensation of off-resonance errors at two selected frequencies placed symmetrically around the frequency of the radiofrequency source.     

Carrier-envelope offset dynamics of mode-locked lasers

We investigate coupling mechanisms between the amplitude and the carrier-envelope offset phase in mode-locked lasers. We find that nonlinear beam steering in combination with the intracavity prism compressor is the predominant mechanism that causes amplitude-to-phase conversion in our laser. A second mechanism, induced by self-steepening, is also identified. These mechanisms are important for stabilizing the carrier-envelope offset phase and also explain the extremely low pulse-to-pulse energy fluctuations observed in some lasers with carrier-envelope lock. The coupling mechanisms described have important implications for applications of few-cycle optical pulses.     

The refocused INADEQUATE MAS NMR experiment in multiple spin-systems: interpreting observed correlation peaks and optimising lineshapes

The robustness of the refocused INADEQUATE MAS NMR pulse sequence for probing through-bond connectivities has been demonstrated in a large range of solid-state applications. This pulse sequence nevertheless suffers from artifacts when applied to multispin systems, e.g. uniformly labeled (13)C solids, which distort the lineshapes and can potentially result in misleading correlation peaks. In this paper, we present a detailed account that combines product-operator analysis, numerical simulations and experiments of the behavior of a three-spin system during the refocused INADEQUATE pulse sequence. The origin of undesired anti-phase contributions to the spectral lineshapes are described, and we show that they do not interfere with the observation of long-range correlations (e.g. two-bond (13)C-(13)C correlations). The suppression of undesired contributions to the refocused INADEQUATE spectra is shown to require the removal of zero-quantum coherences within a z-filter. A method is proposed to eliminate zero-quantum coherences through dephasing by heteronuclear dipolar couplings, which leads to pure in-phase spectra.     

Evaluation of steady and pulsatile flow with dynamic MRI using limited flip angles and gradient refocused echoes

Magnetic resonance imaging sequences utilizing limited flip angles and gradient echoes yield rapid (less than 2 min) dynamic images of the cardiovascular system. These images contain both accurate anatomical and functional information. Using a gradient refocused acquisition in the steady state (GRASS) in the CINE mode, we studied the relationship between gradient echo signal intensity and velocity of steady and pulsatile flow in a phantom simulating medium to large vessels. Images were acquired on a 1.5 Tesla system (repetition TIME = 21 ms, echo TIME = 12 ms, flip ANGLE = 30 degrees). Data from each pulse interval were sorted in 16 images. Signal intensities from flow tube lumina and surrounding stationary water jacket were used to calculate contrast ratios which were compared to velocity measurements made with electromagnetic (EM) flow probes outside the magnet room. During steady flow, signal intensity contrast ratios increased with increasing flow and in a 10 mm thick slice, reached a peak at 48 cm/s, and declined for velocities up to 90 cm/s. Changes in instantaneous velocity during pulsatile flow correlated well (r > .88) with signal intensity changes up to a maximum mean velocity of 17 cm/s. Total signal intensity from the lumen for an “R to R” interval correlated extremely well (r > .97) with mean pulsatile flow velocities up to 30 cm/s. The excellent correlation between gradient echo signal intensity and actual flow velocities suggests that this imaging sequence might be useful for evaluating normal and pathologic flow phenomena.     

Calculation of band offset in chalcopyrite system

A recently reformulated tight binding method is used to calculate valence band offset (VBO) at the CuInSe₂/CuGaSe₂ heterojunction. The hybrid energy is calculated in the s²p² configuration and a new model for the average hybrid energy is used. The theoretical VBO value of 0.05 eV is in good agreement with recent experimental value of 0.04 eV. The value of conduction band offset is 0.60 eV giving a type I alignment. The VBO varies linearly with bond length difference (l), as VBO=(0.24)l.     

Passive signal enhancement in spatially offset Raman spectroscopy

We demonstrate experimentally, for the first time, the feasibility of enhancing signals in Spatially Offset Raman Spectroscopy (SORS) using a dielectric bandpass filter, building on our earlier experimental work on the enhancement of transmission Raman signals. The method is shown to lead to the enhancement of both the surface and subsurface Raman layer signal improving the signal‐to‐noise ratio of Raman spectra from the deep areas of samples, thus enhancing the technique's sensitivity and penetration depth. The filter is placed over the laser illumination zone, on the sample surface acting as a ‘unidirectional’ mirror transmitting the collimated laser beam on one side and reflecting photons escaping from the sample back into it. This enhances the degree of coupling of laser radiation into the medium and associated generated Raman signal. The feasibility study was performed on a two‐layer sample with the second layer located at the limit of the penetration depth of the method for this sample. The sample consisted of a 2.2‐mm over‐layer of a thinned paracetamol tablet followed by a 2‐mm layer of trans‐stilbene powder. The Raman signal was collected from a spatially offset region through a hole fabricated within the filter. The experiments demonstrate the presence of an enhancement of the Raman signal from both the layers by a factor of 4.4–4.5 and the improved signal‐to‐noise ratio of sublayer signal by a factor of 2.2, in agreement with photon shot noise dominated signal. Copyright © 2008 John Wiley & Sons, Ltd.     

Triboelectrification of paper in the offset printing process

In offset printing, the printouts experience various complicated processes and operations before the final product is ready to be sent forward. In addition to the actual printing, these unit operations involve cutting, folding, stacking, labelling and various other operations which all include some kind of frictional contact either between the printed sheets themselves, or between the sheets and the machinery. In these contacts, charge exchange takes place which often leads to problems in the handling of lightweight prints. Machine speeds are ever increasing which has increased the importance of electrostatic problems. In this study, we present various measurements which have been performed on coating powders and coated papers both in a laboratory and in a pilot-scale facility.     

Continuously balanced pulse-DC ioniser to minimise the offset voltage

In our previous work, the AC type of ionisers was employed to demonstrate a prospective theory obtained by numerical simulations in which continuously emitting quasi-neutralised positive and negative ions from corona ionisers can transport ions effectively to a charged object and yield zero offset voltage. In this paper, we show that this can also be applied to pulse-DC ionisers, resulting in significant improvements in the offset voltage as well as charge-decay times. In addition, we present an implementation method with an electronic circuit design to create such an ion emission from a pulse-DC ioniser.     

基于图像分析的摄像头轴偏量标定方法

针对摄像头在焦距调节过程中,视频图像的中心点在物体上滑移的问题,进行了理论建模分析,将传统的分段测量、曲线拟合的解决方法进行了改进,提出了一种基于图像分析的轴偏量的标定方法,利用矩形图像确定不同焦距下摄像头的轴偏量,并在实验验证中进行了优化处理。结果表明,本方法可以较为精确地标定出摄像头轴偏量,经过修正补偿后的摄像头,轴偏抖动可以控制在视场角的0.6%,可以满足较高精度闭环跟踪场合的要求。