共查询到20条相似文献,搜索用时 31 毫秒
1.
Johnson J.L. Dorney T.D. Mittleman D.M. 《IEEE journal of selected topics in quantum electronics》2001,7(4):592-599
We demonstrate a new reflection imaging technique using single-cycle pulses, in which the sample to be imaged is placed at the focus of a lens in one arm of a Michelson interferometer. The detected signal is the superposition of the pulses from the two arms of the interferometer, one with a sample and one with a reference mirror. Because of the Gouy phase shift from the lens in the sample arm, these two pulses are nearly π out of phase, and can destructively interfere. This leads to a strong cancellation of the measured waveform and a nearly background-free imaging method. We demonstrate the technique using terahertz time-domain spectroscopy and show that the destructive interference provides enhanced sensitivity to features in the sample that are much thinner than the coherence length of the radiation. This technique could be valuable in any low-coherence tomographic measurement in which the reflected electric field can be measured directly 相似文献
2.
为了满足太赫兹高分辨检测及实时处理需求,利用光电导天线产生和探测太赫兹时域光谱信号,基于现场可编程门阵列(field programmable gate array, FPGA)实现太赫兹时域光谱的采集、维纳滤波反卷积处理、传输和上位机显示等功能。将采集到的太赫兹时域光谱数据进行维纳滤波反卷积处理,实现还原太赫兹信号、提高时间分辨率以及降噪的效果,将数据由以太网传输的方式传输到上位机进行实时显示,针对实际检测中太赫兹信号反卷积后脉宽较宽,提出在维纳滤波反卷积算法中引入与频率相关的函数对算法进行优化,使信号的脉宽变窄,提高检测精度。优化的维纳滤波反卷积算法处理结果相比于原始算法信噪比增加7 dB,脉宽降低0.2 ps,实现更高的检测分辨能力,算法在FPGA中实现,精度误差小于0.7%,处理效率提升14.29倍,并且减少后期上位机处理时间。 相似文献
3.
Jongjoo Lee Heeseok Lee Sungkyu Yu Joungho Kim 《IEEE journal of selected topics in quantum electronics》2001,7(4):674-682
For the first time, a micromachined near-field probe has been developed, based on picosecond photoconductive sampling using low-temperature-grown GaAs (LT-GaAs). By changing the direction of the probe, it is capable of scanning/mapping independent orthogonal components of free-space electric fields. The micromachined 1-μm-thick LT-GaAs epilayer substrate of the probe provides a minimal loading effect for measuring picosecond electric fields. The use of optical fibers, for guiding laser pulses to the photoconductive switch on the probe and for electrical connections, enables the probe to be positioned freely with uniform sensitivity. We have demonstrated by finite-difference time-domain simulation that the measurement process of the developed probe is dominated by the switching action of the carriers generated in the photoconductive switch of the probe. Using the probe, propagating picosecond pulse waveforms and field distribution images on coplanar transmission lines were successfully obtained. The probe measurement provides a useful understanding, which cannot be obtained from conventional external-port access test instruments, of electromagnetic phenomena related to wave propagation 相似文献
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Duvillaret L. Garet F. Roux J.-F. Coutaz J.-L. 《IEEE journal of selected topics in quantum electronics》2001,7(4):615-623
From classical charge transport equations and dipole radiation equations, we derive a simple analytical model for the signal detected in terahertz time-domain spectroscopy using photoconducting antennas. The validity of our model is checked by comparison to published calculated results and to experimental data we have measured. This expression depends mostly on the carrier lifetime in the antennas' semiconductor material and on the laser pulse duration. The analytical form of the detected terahertz (THz) signal allows us to separate the contribution of the different experimental parameters. Together with a detailed noise analysis in THz time-domain spectroscopy, we optimize the parameter values in order to achieve the largest dynamics of measurement 相似文献
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7.
Terahertz pulsed imaging (TPI) was evaluated for nondestructively characterizing the 3-D internal structures of pharmaceutical tablets. The structural information of a pharmaceutical tablet, such as coating thickness and interface uniformity, was obtained directly from the analysis of the time-domain terahertz waveform. The chemical map of a sample was obtained by using frequency-domain terahertz spectra, together with spectral matching techniques such as cosine correlation mapping. The axial spatial resolution achieved was 30 mum, limited by the confinement of terahertz pulses in time (pulse width); and the lateral spatial resolution was determined to be 150 mum at 90 cm-1 (2.7 THz), limited by the confinement of terahertz pulses in space (focus size, which is diffraction limited and thus frequency dependent). In addition, the buried structure within a tablet was mapped using the TPI, and its chemical composition was successfully identified through spectral-time analysis of the recorded terahertz waveform. We also present a rigorous electromagnetic theory for simulating the terahertz propagation in a multilayered sample, to facilitate terahertz data analysis and interpretation. In conclusion, the TPI is a powerful tool for assaying the tablet coating layer thickness and interface uniformity, and for identifying polymorphs. 相似文献
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9.
《Dielectrics and Electrical Insulation, IEEE Transactions on》2008,15(6):1634-1639
Among the different solutions which allow onsite Partial Discharge measurement in energized power transformers, the UHF technique is gaining general interest. In order to apply this method in existing transformers, it is considered advantageous to design a UHF sensor to be fitted inside the transformer tank. This paper describes the constraints affecting the development of such sensor and the process followed to optimize its design. During this process, different solutions for broadband UHF antennas were analysed. The selection was based on computer simulation and experimental results. Computer simulation was used to evaluate differences in radiation pattern, antenna impedance, gain and effective area. Measurements of the antenna response to different partial discharge sources in oil were performed using an oil dielectric breakdown test set in an electromagnetic shielded laboratory. A group of selected antennas were then tested in a power transformer simulating the broadband signals generated by partial discharges by injecting controlled voltage pulses in the transformer. Finally, a sensor based on a conical monopole antenna structure was built and attached to a specially designed housing to be fitted inside the transformer tank through the drain valve. This final prototype was benchmarked during a high voltage factory test of a power transformer which showed a significant partial discharge activity. 相似文献
10.
Z. D. Zaharis 《Electrical Engineering (Archiv fur Elektrotechnik)》2008,90(4):301-311
A new technique of shaping the radiation pattern of an antenna array is presented. The technique aims at finding the appropriate
geometry of the antenna array as well as the excitation of the array elements in order to produce a radiation pattern of the
desired shape. A suitable optimization procedure is applied using a particle swarm optimization algorithm. The technique has
been successfully applied to collinear arrays and seems to be suitable for beam- forming of antenna arrays used by mobile
communications base stations. 相似文献
11.
In this paper, a probe‐fed rectangular microstrip patch antenna with metallic frequency selective surface as superstrate has been analyzed and simulated at terahertz frequency. In addition to this, a novel technique has been proposed to predict the directivity of the antenna at submillimeter and terahertz waves where far‐field measurement is a challenging task. Moreover, the effect of the size of the frequency selective surface array and the ground plane on the directivity of the antenna has been discussed. The concept of using two lateral sidewalls to improve the electrical performance of the antenna is also presented. The theoretical analysis has been supported by two different commercially available 3D electromagnetic simulators: (i) CST microwave studio (CST, Framingham, MA, USA) based on the finite integral technique; and (ii) Ansoft hfss (Ansoft Corporation, Pittsburgh, PA, USA) based on the finite element technique. Analytical and simulated results are comparable with each other. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
12.
为了进一步论证等效磁流法的近场-远场变换算法在平面近场测量中的实用性与准确性,本文首先根据矩量法(MOM)建立了平面天线口径面上等效磁流的辐射电场方程,利用波导探头采集待测天线平面近场的电场分布,通过共轭梯度法求解矩阵方程,得到天线口径面上的等效磁流分布,之后由格林函数计算出天线远场方向图。最后,通过两种喇叭天线进行实验测试验证。同时将等效磁流法计算出的远场方向图以及由传统的平面波展开法计算出的远场方向图与紧缩场天线测试暗室的测量结果进行对比,结果发现本文算法反演出的远场方向图比平面波展开法在相同测试数据下反演出的远场方向图与紧缩场测量结果相比一致性更好,其中线性角锥喇叭天线的E面和H面方向图吻合度均达到,双脊喇叭天线的E面方向图吻合度达到,H面方向图吻合度达到,验证了等效磁流法算法在平面近场测量中的准确性。 相似文献
13.
Dual-band rectangular microstrip patch antenna at terahertz frequency for surveillance system 总被引:1,自引:0,他引:1
In this paper, a rectangular microstrip patch antenna on two-layer substrate materials has been analyzed and simulated at
the terahertz frequency regime for the surveillance system. The proposed antenna has been simulated at 600 and 800 GHz frequencies
by using CST Microwave Studio a commercially available simulator based on finite integral technique. This antenna structure
is also simulated by using finite element method based simulator Ansoft HFSS and the results are compared with former. 相似文献
14.
New optical sampler based on TOAD and data postprocessing for subpicosecond pulse resolution 总被引:2,自引:0,他引:2
Bogoni A. Ponzini F. Scaffardi M. Ghelfi P. Poti L. 《IEEE journal of selected topics in quantum electronics》2004,10(1):186-191
A low-complexity terahertz optical asymmetric demultiplexer-based optical sampler with simple data postprocessing for subpicosecond pulse shape characterization is proposed. Comparisons with commercial instruments demonstrate the accuracy of the implemented scheme reproducing optical pulses with a resolution of few hundreds of femtoseconds. Nevertheless, the proposed technique allows us to go down to femtosecond accuracy, slightly increasing the data postprocessing complexity. A further validation of the proposed optical sampler is numerically provided. 相似文献
15.
A radar coal thickness sensor is being developed by the Bureau of Mines to measure both the dielectric constant and thickness of a coal seam as part of a sensor array supporting a computer-assisted coal mining machine. The noncontacting stepped-CW radar sensor measures the complex reflection of coefficient of the coal-shale interface to resolve the coal thickness to sub-wavelength accuracies. The technique uses a monostatic antenna configuration. Transfer function errors are identified by using spatial modulation created by antenna motion. Synthetic range gating is used for clutter rejection. Finally, polarimetric scattering matrix techniques are used to calculate the dielectric constant and thickness of the multilayered coal/shale media. Data taken both in the laboratory and in an underground mine, in the 0.6-1.4 GHz range, with a network analyzer and dipole antenna, have validated the technique. Accurate in situ measurements were made of a coal seam 6 in (15.2 cm) thick with a dielectric constant of 4. The coal thickness was confirmed by physical measurement. An independent measurement of the coal dielectric constant confirmed the accuracy of the noncontacting dielectric measurement technique 相似文献
16.
水分是电力设备油纸绝缘寿命的首要劣化因素。将太赫兹时域光谱系统(THz-TDS)引入到油纸绝缘微水含量测试中,利用太赫兹时域光谱系统对不同微水含量绝缘纸板样品进行测试,得到了不同太赫兹时域及频域响应信号结果并进行拟合,从而获得了可以初步估计纸板中水分含量的计算公式;最后结合太赫兹成像技术对纸板平面的水分不均匀分布情况进行了检测。由于水分子中的氢键在太赫兹波照射下会产生强烈的共振与吸收,在太赫兹透射波中产生明显的反馈作用,基于此原理可以将太赫兹检测技术运用于对绝缘纸板水分含量与分布的评估中。 相似文献
17.
田晓霞 《国外电子测量技术》2000,(4):13-16
本文描述了天线方向图的一种测量方法。当一根细金属导线在天线口径面上移动时,馈源处的反射系数会产生变化。如果天线口径场分布是保分的,那么它的反射系数的变化就与导线位置处积分口径场的平方成比例。以金属导线散射信号引起的反射系数变化的平方根作为导线位置处的函数,可得到积分口径场,由此可推导出天线远场辐射方向图。这种测量系统现已建成,其测量结果与传统的远场测量结果相比较一致性很好。这种测量方法能使我们快速 相似文献
18.
Liye LIU Yi SU Junjie MAO 《Frontiers of Electrical and Electronic Engineering in China》2008,3(1):90-95
One of the most critical hardware components of a ground-penetrating radar (GPR) is the antenna system. Important parameters
of antennas, such as antenna bandwidth, radiation waveform and cross coupling determine the GPR system performance. The modified
TEM horn antenna with distributed resistor load is presented in this paper, and the radiation properties of the antenna with
the shields and absorbers are studied through the three-dimensional finite-difference time-domain (FDTD) scheme. Simulations
show that the direct signal coupled from the transmitter is decreased by means of the shields and absorbers. Therefore, using
the antenna in the GPR system can improve the signal-to-clutter ratio and the dynamic range of the system.
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Translated from Chinese Journal of Radio Science, 2006, 21(3): 422–427 [译自: 电波科学学报] 相似文献
19.
The authors discuss the antenna test range at the US Naval Ocean Systems Center, which provides solutions to some formidable electromagnetics problems on ships. They describe the measurement approach; the pattern range, a 160 ft diameter ground plane constructed of prestressed concrete covered with a very thin coating of lead; the impedance range, which has a subterranean instrument room that houses the measurement equipment and personnel during the test; and the time-domain measurement range. The authors then examine computer modelling of antennas and analysis of the EMC (electromagnetic compatibility) problem. The modelling approach as presented is not limited to ships; it can work for power lines, buildings, and even mountains 相似文献
20.
探地雷达、隔墙雷达、人体接触式微波治疗探测设备的天线设计,要考虑应用场景并加入应用场景一并设计或仿真计算,才能获得最佳的应用效果。本文采用应用背景下的设计方法,设计了一种应用于接触式微波检测系统的圆极化介质天线。该天线工作在L波段,采用带状线缝隙耦合馈电。通过不等长十字耦合缝隙实现圆极化。研究了辐射背景在一定范围内变化时,对天线回波损耗,轴比和方向图的影响。并制作了实物,VSWR<2的带宽,仿真结果为16.6%,实测结果为29.7%。 相似文献