Phase-constrained parallel MR image reconstruction

A generalized method for phase-constrained parallel MR image reconstruction is presented that combines and extends the concepts of partial-Fourier reconstruction and parallel imaging. It provides a framework for reconstructing images employing either or both techniques and for comparing image quality achieved by varying k-space sampling schemes. The method can be used as a parallel image reconstruction with a partial-Fourier reconstruction built in. It can also be used with trajectories not readily handled by straightforward combinations of partial-Fourier and SENSE-like parallel reconstructions, including variable-density, and non-Cartesian trajectories. The phase constraint specifies a better-conditioned inverse problem compared to unconstrained parallel MR reconstruction alone. This phase-constrained parallel MRI reconstruction offers a one-step alternative to the standard combination of homodyne and SENSE reconstructions with the added benefit of flexibility of sampling trajectory. The theory of the phase-constrained approach is outlined, and its calibration requirements and limitations are discussed. Simulations, phantom experiments, and in vivo experiments are presented.     

柱面共形蝶形微带天线的阻抗特性和方向图研究

研究了小曲率半径有限长度圆柱体上共形蝶形微带天线的阻抗特性和方向图.给出了不同曲率半径下的柱面共形蝶形微带天线反射损失和方向图的仿真结果.实际制作和测试了柱面共形微带天线的单元和二元阵列天线,仿真结果与实测结果比较吻合.研究表明:当圆柱半径较小时,不但方向图改变,而且谐振频率下降、阻抗带宽明显减小;当圆柱长度缩短时,方向图的起伏增大.     

用于宽带高功率微波辐射的双层贴片天线特性

研究了用于宽带高功率微波辐射的双层贴片微带天线输入阻抗带宽和辐射特性,提出一种适合于高功率微波辐射的圆锥传输线馈电结构以减小传统馈电探针的引线感抗,使天线的功率容量提高到百MW级,同时作为同轴线到径向传输线的过渡变换,使反射系数降低近50%。通过数值计算分析了不同的介质基底厚度组合和贴片尺寸对天线带宽的影响,优化参数后的天线带宽达到了34.9%(电压驻波比小于3),带内平均增益高于6.5 dB,最大增益8.3 dB,主辐射方向上的远场辐射因子与馈入宽带脉冲幅度比值大于1.3。分析了天线的功率容量,模拟计算表明该天线可用于辐射幅值300 kV的宽带高功率微波。     

同心圆环阵列模态波束形成器设计

针对贝塞尔函数零点造成的模态波束形成器性能损失的问题,提出了基于均匀同心圆环阵列的模态波束形成器设计方法。从阵元域出发,建立了阵元域和圆谐波域之间的转换关系,推导了圆谐波域阵列信号处理表达式,其中重点推导了延时求和、最小方差无失真响应和多约束波束形成器的设计方法。理论证明了在平面各向同性噪声场中的基于同心环阵的圆谐波域最小方差无失真响应波束形成器等价于相位模式波束形成器。在该结论的基础上,综合考虑相位模式波束形成器在一定频率范围内的稳健性和指向性,提出了一种同心圆环阵列的内环半径优化方法。使用16元双环阵列对本文提出的波束形成算法进行仿真和实验,结果表明:同心圆环阵列能够较好地解决贝塞尔函数零点问题;其中多约束波束形成器能够在多个关联的性能指标之间取得合理的折衷,实现优于相位模式和延时求和波束形成器的声源定位性能。对内环半径优化方法进行仿真和分析结果表明,该方法得到了相较于传统方法更优的阵列结构。     

阻抗加载对微带天线辐射和散射的影响

计算了阻抗加载对微带天线RCS的影响，以及加载不同阻抗时天线的辐射性能。结果表明，阻抗加载主要降低天线带内RCS。适当的阻抗加载能有效降低天线RCS，同时还能保证天线的辐射性能。     

The LPF with improved stopband response designed for wideband microwave detectors

To achieve an improved stopband response including expanded stopband width, increased rejection level, and a smooth S₁₁ phase response, a lowpass filter (LPF) is proposed based on the suspended microstrip line with naturally high characteristic impedance and the radial stub with intrinsic transmission zero. The usually unmentioned phase response, are critical to obtaining a wide and flat operating band for the microwave detector, since a smooth phase response exerts small load effects to the diode. As a demonstration, a detector employing the LPF is fabricated and measured. The measured results show that a wide operating band from 1.8 to 13.5 GHz with flat detection outputs of ripple less than 1 dB is obtained for the detector, which coincides with the smooth-phase range of the LPF.     

π‐Conjugation Enlargement Toward the Creation of Multi‐Porphyrinic Systems with Large Two‐Photon Absorption Properties

Recent progress in the synthesis of covalently linked porphyrin arrays with large two‐Photon absorption (TPA) cross‐section values has been reviewed with a particular focus on the relation of TPA properties with molecular structures. Covalently linked porphyrin arrays continue to be important and useful for the creation of functional materials owing to their chemical robustness, fine‐tuning, and easy manipulation. More importantly, the porphyrin electronic systems are quite susceptible to periphery conjugative perturbations, hence allowing facile fabrications to extensively delocalized systems. This property has been used for exploration of porphyrin‐based molecular systems with large TPA values, demonstrating a general trend that enhancement in electronic interactions leads to large TPA cross‐section values. As a representative example, the porphyrin tapes exhibit larger TPA values owing to the fully delocalized nature of the π‐electrons. This Focus Review will help understand the structural requirements of porphyrin arrays with large TPA values, which will be useful for future applications in optical communication in the IR region.     

The use of low power air-cooled lasers in conjunction with an intensified diode array for Raman spectroscopy

A range of air-cooled lasers operating with a single phase electricity supply has been evaluated for use in laser-Raman spectroscopy. Three main types of laser are available — helium-neon, argon, and helium-cadmium. Such lasers should prove valuable for applications such as on-line sampling in hostile environments, where their robust nature and low cost give distinct advantages over conventional water-cooled gas ion lasers.     

Infrared negative luminescent devices and higher operating temperature detectors

Infrared LEDs and negative luminescent devices, where less light is emitted than in equilibrium, have been attracting an increasing amount of interest recently. They have a variety of applications, including as a ‘source’ of IR radiation for gas sensing; radiation shielding for, and non-uniformity correction of, high sensitivity staring infrared detectors; and dynamic infrared scene projection. Similarly, infrared (IR) detectors are used in arrays for thermal imaging and, discretely, in applications such as gas sensing. Multi-layer heterostructure epitaxy enables the growth of both types of device using designs in which the electronic processes can be precisely controlled and techniques such as carrier exclusion and extraction can be implemented. This enables detectors to be made which offer good performance at higher than normal operating temperatures, and efficient negative luminescent devices to be made which simulate a range of effective temperatures whilst operating uncooled. In both cases, however, additional performance benefits can be achieved by integrating optical concentrators around the diodes to reduce the volume of semiconductor material, and so minimise the thermally activated generation-recombination processes which compete with radiative mechanisms. The integrated concentrators are in the form of Winston cones, which can be formed using an iterative dry etch process involving methane/hydrogen and oxygen. We present results on negative luminescence in the mid- and long-IR wavebands, from devices made from indium antimonide and mercury cadmium telluride, where the aim is sizes greater than 1 cm×1 cm. We also discuss progress on, and the potential for, operating temperature and/or sensitivity improvement of detectors, where very high-performance imaging is anticipated from systems which require no mechanical cooling.     

Preparation and properties of Ni-doped ZnO rod arrays from aqueous solution

Ni-doped ZnO rod arrays were successfully prepared on glass substrate from the aqueous solution at a temperature of 80 °C. The densities, diameters, and lengths of the rods can easily be well controlled through the concentrations of dopants, and the dopant Ni ions were incorporated into the wurtzite-structure of ZnO crystal. Room temperature photoluminescence spectrum of rod arrays show a strong emission band at 410 nm, and the oxygen deficiencies in ZnO structures were significantly reduced with Ni doped in ZnO rod arrays. The field dependence of magnetization measured at room temperature exhibited the obvious ferromagnetic properties.