Influence of the chirp on the intensity distributions of an apertured pulse

Starting from the Huygens-Fresnel diffraction integral and the Fourier transform, the propagation expression of a chirped pulse passing through a hard-edged aperture is derived. Using the obtained expression, the intensity distributions of the pulse with different chirp in the near and far fields are analyzed in detail. Due to the modulation of the aperture, many intensity peaks emerge in the intensity distributions of the chirped pulse in the near field. However, the amplitudes of the intensity peaks decrease on increasing the chirp, which results in the smoothing effect in the intensity distributions. The beam smoothing brought by increasing the chirp is explained physically. Also, it is found that the radius of the intensity distribution of the chirped pulse decreases when the chirp increases in the far field. (c) 2005 Elsevier GmbH. All rights reserved.

Fields of apertured polychromatic laser beams with Gaussian and Hermite-Gaussian transverse modes

Starting from the Huygens-Fresnel diffraction integral, the field expressions of apertured polychromatic laser beams with Gaussian and Hermite-Gaussian transverse modes are derived. Influence of the bandwidth on the intensity distributions of the laser beams is analyzed. It is found that when the bandwidth increases, the amplitudes and numbers of the intensity spikes decrease and beam uniformity is improved in the near field and the width of transverse intensity distribution of the apertured beams decreases in the far field. Thus, the smoothing and narrowing effects can be achieved by increasing the bandwidth. Also, these effects are found in the laser beams with Hermite-Gaussian transverse modes as the bandwidth increases.(c) 2006 Elsevier Ltd. All rights reserved.

谱色散平滑与透镜列阵联用实现均匀照明

为满足激光惯性约束聚变中靶面激光辐照不均匀性低于5%的要求, 在目前使用透镜列阵基础上, 提出了谱色散平滑与透镜列阵联用方案, 对其进行数值计算并分析其平滑效果和应用可行性。结果表明:焦斑的不均匀性从单独使用透镜列阵时的14%降低到与谱色散平滑结合后的3%;对焦斑点功率谱的分析表明谱色散平滑通过抑制焦斑中高频的频谱强度达到平滑效果。该方案可以进一步提高焦斑平滑效果, 计算结果对实际应用有着重要的参考意义。

利用二维光谱色散和透镜列阵改善靶面辐照均匀性

利用二维光谱色散平滑技术和透镜列阵(LA)来改善激光驱动器中靶面的辐照均匀性。通过消衍射透镜列阵可得到包络陡峭且中小空间尺度均匀性较好的焦斑。当在光路中加入二维光谱色散平滑单元后,光束在两个互相垂直的方向发生光谱色散,多光束干涉所引起的细密条纹也将在很大程度上被抹平,如果把横向热传导平滑效应也考虑在内,高空间频率的强度波动可进一步被消除。二维理论模拟结果表明采用该方案可获得顶部平坦边缘陡峭的焦斑,而且该方案无需仔细调整靶面的位置,实际应用较方便。

Two-dimensional performance of uniform irradiation with the use of an edge-softened lens array and spectral dispersion

A lens array composed of edge-softened elements is used to improve on-target irradiation uniformity in the Shenguang II Laser Facility, with which a Fresnel pattern of suppressed diffraction peaks is obtained. Additional uniformity can be reached by reducing short-wavelength interference speckles inside the pattern when the technique of smoothing by spectral dispersion is also used. Two-dimensional performance of irradiation is simulated and the results indicate that a pattern of steeper edges and a flat top can be achieved with this joint technique. (c) 2007 Optical Society of America.

光谱色散后的相位调制光束衍射特性研究

用数值方法研究了激光驱动器系统中使用光谱色散平滑单元后光束的衍射特性、模拟结果表明，光谱色散会使光束衍射光斑变大，近场空间强度均匀性改善，而远场光斑内部存在光强接近于均匀分布的区域．进一步分析了光谱色散平滑单元中相位调制器的调制深度、调制频率及光栅色散系数等主要元件参数对光束传输特性的影响，发现在一定情况下光斑内部会出现较强的强度调制。

光谱色散平滑的实验研究

为提高国内惯性约束聚变（ICF）装置的焦斑平滑能力，在对光谱色散平滑（SSD）技术理论分析的基础上，开展相关实验并研究其平滑机制。实验结合另一种焦斑平滑技术——分布式相位板（DPP）比较分析光谱色散平滑技术应用下的焦斑均匀性的变化。实验表明，利用光谱色散平滑技术中的正弦相位调制对光谱展宽并色散后，在色散方向上的焦斑均匀性有明显改善，120μm内的光强调制被平滑，其平滑能力依赖于光谱色散平滑技术参量的选择；焦斑的低频光谱强度变化很小，但中高频光谱强度下降，焦斑质量有良好的改善，焦斑的不均匀性从40％降低到1

Biogeography and molecular phylogeny of the genus Schizothorax (Teleostei : Cyprinidae) in China inferred from cytochrome b sequences

Aim: To test a vicariant speciation hypothesis derived from geological evidence of large-scale changes in drainage patterns in the late Miocene that affected the drainages in the south-eastern Tibetan Plateau. Location: The Tibetan Plateau and adjacent areas. Methods: The cytochrome b DNA sequences of 30 species of the genus Schizothorax from nine different river systems were analysed. These DNA sequences were analysed using parsimony, maximum likelihood and Bayesian methods. The approximately unbiased and Shimodaira-Hasegawa tests were applied to evaluate the statistical significance of the shortest trees relative to alternative hypotheses. Dates of divergences between lineages were estimated using the nonparametric rate smoothing method, and confidence intervals of dates were obtained by parametric bootstrapping. Results: The phylogenetic relationships recovered from molecular data were inconsistent with traditional taxonomy, but apparently reflected geographical associations with rivers. Within the genus Schizothorax, we observed a divergence between the lineages from the Irrawaddy-Lhuit and Tsangpo-Parlung rivers, and tentatively dated this vicariant event back to the late Miocene (7.3-6.8 Ma). We also observed approximately simultaneous geographical splits within drainages of the south-eastern Tibetan Plateau, the Irrawaddy, the Yangtze and the Mekong-Salween rivers in the late Miocene (7.1-6.2 Ma). Main conclusions: Our molecular evidence tentatively highlights the importance of palaeoriver connections and the uplift of the Tibetan Plateau in understanding the evolution of the genus Schizothorax. Molecular estimates of divergence times allowed us to date these vicariant scenarios back to the late Miocene, which agrees with geological suggestions for the separation of these drainages caused by tectonic uplift in south-eastern Tibet. Our results indicated the substantial role of vicariant-based speciation in shaping the current distribution pattern of the genus Schizothorax.

Nanoelectronic devices-resonant tunnelling diodes grown on InP substrates by molecular beam epitaxy with peak to valley current ratio of 17 at room temperature

This paper reports that lnAs/In0.53Ga0.47As/AlAs resonant tunnelling diodes have been grown on InP substrates by molecular beam epitaxy. Peak to valley current ratio of these devices is 17 at 300K. A peak current density of 3kA/cm(2) has been obtained for diodes with AlAs barriers of ten monolayers, and an In0.53Ga0.47As well of eight monolayers with four monolayers of InAs insert layer. The effects of growth interruption for smoothing potential barrier interfaces have been investigated by high resolution transmission electron microscope.

Statistical investigation on morphology development of gallium nitride in initial growth stage

Morphology of Gallium Nitride (GaN) in initial growth stage was observed with atomic force microscopy (AFM) and scanning electron microscopy (SEM), It was found that the epilayer developed from islands to coalesced film. Statistics based on AFM observation was carried out to investigate the morphology characteristics. It was found that the evolution of height distribution could be used to describe morphology development. Statistics also clearly revealed variation of top-face growth rate among islands. Indium-doping effect on morphology development was also statistically studied. The roughening and smoothing behavior in morphology development was explained. (C) 2002 Elsevier Science B.V. All rights reserved.

Pipeline Architecture and Parallel Computation-Based Real-Time Stereovision Tracking System for Surgical Navigation

This paper studies the development of a real-time stereovision system to track multiple infrared markers attached to a surgical instrument. Multiple stages of pipeline in field-programmable gate array (FPGA) are developed to recognize the targets in both left and right image planes and to give each target a unique label. The pipeline architecture includes a smoothing filter, an adaptive threshold module, a connected component labeling operation, and a centroid extraction process. A parallel distortion correction method is proposed and implemented in a dual-core DSP. A suitable kinematic model is established for the moving targets, and a novel set of parallel and interactive computation mechanisms is proposed to position and track the targets, which are carried out by a cross-computation method in a dual-core DSP. The proposed tracking system can track the 3-D coordinate, velocity, and acceleration of four infrared markers with a delay of 9.18 ms. Furthermore, it is capable of tracking a maximum of 110 infrared markers without frame dropping at a frame rate of 60 f/s. The accuracy of the proposed system can reach the scale of 0.37 mm RMS along the x- and y-directions and 0.45 mm RMS along the depth direction (the depth is from 0.8 to 0.45 m). The performance of the proposed system can meet the requirements of applications such as surgical navigation, which needs high real time and accuracy capability.

Pipeline Architecture and Parallel Computation-Based Real-Time Stereovision Tracking System for Surgical Navigation

This paper studies the development of a real-time stereovision system to track multiple infrared markers attached to a surgical instrument. Multiple stages of pipeline in field-programmable gate array (FPGA) are developed to recognize the targets in both left and right image planes and to give each target a unique label. The pipeline architecture includes a smoothing filter, an adaptive threshold module, a connected component labeling operation, and a centroid extraction process. A parallel distortion correction method is proposed and implemented in a dual-core DSP. A suitable kinematic model is established for the moving targets, and a novel set of parallel and interactive computation mechanisms is proposed to position and track the targets, which are carried out by a cross-computation method in a dual-core DSP. The proposed tracking system can track the 3-D coordinate, velocity, and acceleration of four infrared markers with a delay of 9.18 ms. Furthermore, it is capable of tracking a maximum of 110 infrared markers without frame dropping at a frame rate of 60 f/s. The accuracy of the proposed system can reach the scale of 0.37 mm RMS along the x- and y-directions and 0.45 mm RMS along the depth direction (the depth is from 0.8 to 0.45 m). The performance of the proposed system can meet the requirements of applications such as surgical navigation, which needs high real time and accuracy capability.

IMPROVEMENT OF SELECTIVITY WITH NUMERICAL DERIVATIVE TECHNIQUES IN INDUCTIVELY COUPLED PLASMA ATOMIC EMISSION-SPECTROMETRY

One of the most attractive features of derivative spectrometry is its higher resolving power. In the present power, numerical derivative techniques are evaluated from the viewpoint of increase in selectivity, the latter being expressed in terms of the interferent equivalent concentration (IEC). Typical spectral interferences are covered, including flat background, sloped background, simple curved background and various types of line overlap with different overlapping degrees, which were defined as the ratio of the net interfering signal at the analysis wavelength to the peak signal of the interfering line. the IECs in the derivative spectra are decreased by one to two order of magnitudes compared to those in the original spectra, and in the most cases, assume values below the conventional detection limits. The overlapping degree is the dominant factor that determines whether an analysis line can be resolved from an interfering line with the derivative techniques. Generally, the second derivative technique is effective only for line overlap with an overlapping degree of less than 0.8. The effects of other factors such as line shape, data smoothing, step size and the intensity ratio of analyte to interferent on the performance of the derivative techniques are also discussed. All results are illustrated with practical examples.

Molecular systematics and biogeography of Crawfurdia, Metagentiana and Tripterospermum (Gentianaceae) based on nuclear ribosomal and plastid DNA sequences

Background and Aims The systematic position of the genus Metagentiana and its phylogenetic relationships with Crawfurdia, Gentiana and Tripterospermum have not been explicitly addressed. These four genera belong to one of two subtribes (Gentianinae) of Gentianeae. The aim of this paper is to examine the systematic position of Crawfurdia, Metagentiana and Tripterospermum and to clarify their phylogenetic affinities more clearly using ITS and trnL intron sequences.Methods Nucleotide sequences from the internal transcribed spacers (ITS) of nuclear ribosomal DNA and the plastid DNA trnL (UAA) intron were analysed phylogenetically. Ten of fourteen Metagentiana species were sampled, together with 40 species of other genera in the subtribe Gentianinae.Key Results The data support several previously published conclusions relating to the separation of Metagentiana from Gentiana and its closer relationships to Crawfurdia and Tripterospermum based on studies of gross morphology, floral anatomy, chromosomes, palynology, embryology and previous molecular data. The molecular clock hypothesis for the tested sequences in subtribe Gentianinae was not supported by the data (P < 0.05), so the clock-independent non-parametric rate smoothing method was used to estimate divergence time. This indicates that the separation of Crawfurdia, Metagentiana and Tripterospermum from Gentiana occurred about 11.4-21.4 Mya (million years ago), and the current species of these three genera diverged at times ranging from 0.4 to 6.2 Mya.Conclusions The molecular analyses revealed that Crawfurdia, Metagentiana and Tripterospermum do not merit status as three separate genera, because sampled species of Crawfurdia and Tripterospermum are embedded within Metagentiana. The speciation and rapid radiation of these three genera is likely to have occurred in western China as a result of upthrust of the Himalayas during the late Miocene and the Pleistocene.