抛物反射镜对高斯束变换特性的研究

在准光技术的研究和应用中,利用解析公式设计准光系统具有方便快捷的优点。采用解析公式设计透镜和椭球反射镜组成的准光系统都得到与实际吻合的良好结果。但在使用解析公式计算抛物反射面对高斯束的变换特性时存在较大误差,不满足设计抛物面组成的准光系统的要求。为解决此问题,在Ka波段利用CST 全波分析软件对高斯束经抛物面镜的变换特性进行大量仿真并进行相关的数据处理,对经典解析公式进行了校正。校正后的公式对于在Ka波段和W波段的高斯束斜入射抛物面镜问题上的计算结果与全波仿真结果有较好吻合,可以用于设计相关准光系统,同时也消除了使用全波仿真软件带来的耗时长及占用计算机空间大等问题。     

Design and analysis of quasi-integrated horn antennas formillimeter and submillimeter-wave applications

The authors present a systematic process for the design of multimode quasi-integrated horn antennas, and provide a full range of practical antenna designs for millimeter- and submillimeter-wave applications. The design methodology is based on the Gaussian beam approach and the structures are optimized for achieving maximum fundamental Gaussian coupling efficiency. For this purpose, a hybrid technique is used in which the integrated part of the antennas is treated using full-wave analysis, whereas the machined part is treated using an approximate model. This results in a simple and efficient design process. The design procedure has been applied to the design of 20-, 23-, and 25-dB quasi-integrated horn antennas, all with a Gaussian coupling efficiency exceeding 97%. The designed antennas have been tested and characterized using both full-wave analysis and 91/370-GHz measurements. The quasi-integrated horn antennas are also examined as feed elements for Cassegrain antenna systems and are proved to be comparable to the traditional machined corrugated horn feeds     

High power THz sources and applications at ENEA-Frascati

ENEA has a long term expertise in the development of powerful short-pulse mm-wave and THz radiation sources driven by free electrons. Various electron-wave interaction schemes were successfully tested in the past, including Cerenkov and Smith-Purcell radiators as well as undulator devices. Two THz-FEL sources are currently available, covering altogether the spectral range from 90 GHz to 0.7 THz. Recently a novel Electro-Magnetic pulser, capable of providing both nanosecond THz electromagnetic (EM) radiation pulses and electrostatic (ES) pulses with identical time duration in one device has been designed and is currently under development. The pulser will allow, for the first time, direct comparison of EM and ES pulses on biological systems with peak electric fields that are significantly higher than any reported to date.     

基于电路参数模型的辐射电磁干扰噪声预估方法

针对Hubing电流驱动模型中认为电流在辐射线缆中是均一分布的, 幅值和相位保持不变即未考虑辐射线缆共模电流频率效应给辐射电磁干扰噪声预估带来的误差的问题.文中利用电流传输波动特性建立了辐射线缆长度与共模电流波长为同一数量级时的辐射线缆共模电流分布模型, 并设计电路模型进行测试预估.实验结果表明:采用文中方法预估辐射电磁干扰噪声与Hubing电流驱动模型预估方法相比能提高20.12 dBμV/m, 更加接近标准暗室测试结果, 从而为辐射电磁干扰(Electro Magnetic Interference, EMI)测试与分析提供理论依据.     

A 2D Gaussian-Beam-Based Method for Modeling the Dichroic Surfaces of Quasi-Optical Systems

In this article, we propose an approach in the spectral domain to treat the interaction of a field with a dichroic surface in two dimensions. For a Gaussian beam illumination of the surface, the reflected and transmitted fields are approximated by one reflected and one transmitted Gaussian beams. Their characteristics are determined by means of a matching in the spectral domain, which requires a second-order approximation of the dichroic surface response when excited by plane waves. This approximation is of the same order as the one used in Gaussian beam shooting algorithm to model curved interfaces associated with lenses, reflector, etc. The method uses general analytical formulations for the GBs that depend either on a paraxial or far-field approximation. Numerical experiments are led to test the efficiency of the method in terms of accuracy and computation time. They include a parametric study and a case for which the illumination is provided by a horn antenna. For the latter, the incident field is firstly expressed as a sum of Gaussian beams by means of Gabor frames.     

高阶贝塞尔-高斯光束通过圆孔硬边光阑的传输特性

文章通过把圆孔光阑函数展开为有限复高斯函数和的方法,用惠更斯-菲涅尔衍射积分,推导出了高阶贝塞尔高斯光束通过具有圆孔光阑的近轴ABCD光学系统的传输近似解析公式,并对高阶贝塞尔-高斯光束通过圆孔硬边光阑的传输特性进行了研究.分析结果表明,衍射特性与截断参数、贝塞尔函数的阶数以及光束的菲涅尔数等因素有关.     

Excitation of a system of electromagnetic oscillators in the field of a remote microwave radiation source. Part 2. Low-Q systems

The paper presents the results of simulation and experimental testing of the method of forming a periodic temperature profile on the surface of a dielectric body by a regular multisystem of low-Q resistive electromagnetic (EM) oscillators heated in the field of a quasi-optical beam of external microwave radiation. It is shown that, in a system of low-Q oscillators, excitation of a multimode EM-field structure is less pronounced than in the case of high-Q elements, which facilitates formation of a uniform temperature profile with a required contrast on the model??s surface. The efficiency and drawbacks of systems of EM oscillators based on high-Q and low-Q elements are compared, and recommendations on their practical application in an aerodynamic experiment are given.     

A closed-form CAD-oriented model for the high-frequency conductorattenuation of symmetrical coupled coplanar waveguides

Closed-form CAD-oriented conformal-mapping approximations are presented for the HF attenuation of two-conductor symmetrical coupled coplanar waveguides (CCPWs). The expressions are compared with the results obtained from a full-wave electromagnetic (EM) simulator and found to yield acceptable agreement. A modified form is also presented, which partly overcomes the low-frequency limitations of the skin-effect approximation     

Calculation of a Hyperbolic Corrugated Horn Converting the TEM<Subscript>00</Subscript> Mode to the HE<Subscript>11</Subscript> Mode

Corrugated waveguide transmission lines are in use to transmit high power mm-wave radiation from gyrotrons to the plasma for electron cyclotron plasma heating in tokamaks such as ITER. The coupling efficiency of the gyrotron output radiation formed as a quasi-Gaussian beam to the waveguide mode is a critical issue. A hyperbolic corrugated horn serves as a converter of the TEM₀₀ Gaussian mode to the HE₁₁ mode of a corrugated waveguide. We report the design of a hyperbolic horn for application in the ITER transmission line at 170 GHz. The theoretical conversion efficiency of the horn is higher than 0.995.     

Accurate analysis and modeling of laminated multilayered 3-D optical waveguides

The theoretical investigation of waveguides having nonuniform cross-sections is an attractive and challenging problem which deserves serious interest. In this paper, we present a novel analysis of laminated multilayered three-dimensional waveguide, based on two modes: 1) a new coupled transmission line approach that considers the sloping of the layers along the longitudinal direction and 2) a transmission line matrix integral equation (TLMIE) modeling that complete and extends the investigation of the field propagation. In method 1), we neglect radiation modes and their EM coupling. All physical effects instead are accounted for by the full-wave TLMIE method. By using TLMIE, we validate the EM analysis and calculate TE/TM losses, arising from radiation modes.     

Optimal design of an MMI coupler for broadening the spectral response of an AWG demultiplexer

Optimal design of a multimode interference (MMI) coupler for broadening the spectral response of an arrayed-waveguide grating (AWG) demultiplexer is considered. By using a Gaussian beam approximation, an analytical expression for the spectral response of an AWG demultiplexer (with an MMI section) is obtained. A simple analytical formula is derived to relate the optimal MMI length to the separation between the peaks of the twofold images in the MMI region. This peak separation is related to the width of the MMI section. In the proposed design method, a required 1-dB passband width determines the peak separation, which then determines the optimal value for the length of the MMI section according to the analytical formula. The designed flat-top AWG demultiplexer is verified by the beam propagation method.     

Radiation patterns of the HE11 mode and Gaussian approximations

The problem of the approximation of the HE₁₁ radiation pattern by a Gaussian distribution is discussed. A numerical comparison between the HE₁₁ far-field theoretical pattern, and the Gaussian approximations derived by Abrams and by Crenn, permits and evaluation of the precision of these approximations. A new optimized HE₁₁ Gaussian approximation is calculated: the value of r_o=0.421a (or w_o=0.596a) for the beam radius at the waist is demonstrated to give the best HE₁₁ Gaussian approximation in the far-field and is very close to the result given by Crenn, while the Abrams value is less precise. The calculations are extended to the near-field. Universal curves for intensity, amplitude and power distribution are given for the HE₁₁ radiated mode. These results are of interest for laser waveguide applications and for plasma ECRH transmission systems.     

高斯波束对水中柱形粒子的声辐射力研究

根据级数展开理论,将入射高斯波束用柱函数展开,计算得到高斯波束的波束因子。根据声波辐射力理论,推导了水中位于高斯波束中心的柱形粒子在声场中的辐射力的解析表示式,对不同材料的柱形粒子在高斯波束作用下的声辐射力进行了数值模拟,讨论了高斯波束的束腰半径的变化对柱形物体所受声波辐射力的影响。仿真结果表明,高斯波束的束腰半径对声辐射力有影响,随着束腰半径的增大,位于波束中心的柱形粒子所受的声辐射力逐渐变大,当束腰半径远大于声波在水中的波长时,声辐射力的大小接近平面波声场的声辐射力。     

Closed-form transmission line model for radiated susceptibility in metallic enclosures

This work presents an approximate frequency domain mathematical model based on the transmission line (TL) theory for field-to-wire coupling in a rectangular metallic enclosure. The currents and voltages at the terminations of a TL induced by known electromagnetic (EM) field sources are expressed in closed form. Validity limits and applicability of the model are discussed by comparing the analytical TL-based predictions with the outputs of a full-wave numerical analysis of the overall structure using a three-dimensional finite integration technique. Deviations from the full-wave solution, due to the scattered field from the TL, have been identified, analyzed, and discussed. The proposed analytical model proves to be generally suited for accurate prediction of radiated susceptibility of single-ended interconnections in closed environments.     

Near-field analysis of a thick lens and horn combination: theoryand measurements

At millimeter-waves quasi-optical systems are commonly designed based on the Gaussian beam and thin lens approximation. The accuracy of the Gaussian beam and thin lens approximation was studied in the case of a corrugated horn and a thick Teflon lens (focal length to lens diameter ratio f/D=1.2) combination at 87 GHz. A special near-field measurement system was constructed. A large disagreement between the measured and theoretical values with the approximative method was obtained. The measurements showed that the thin lens approximation breaks down when the distance from the input beam waist to the lens is less than 1.5 D. Theoretical values obtained with a thick lens model based on ray tracing and use of exact Huygens' aperture integration principle agree well with the measurements     

Performance evaluation of FSK and CPFSK optical communicationsystems: a stable and accurate method

The modified moments method for evaluating the performance of coherent optical FSK and CPFSK systems is presented. Since the classical procedure becomes ill-conditioned as the order of the moments increases, we consider the construction of Gaussian quadrature rules (GQR) from the modified moments. The analysis accounts for the influences of IF bandwidth, transmitter and local oscillator laser phase noise, postdetection filters, and additive Gaussian noise. It is found that the proposed approach is a highly reliable and efficient method for calculating the error probability. A comparison with results obtained from the Gaussian quadrature rule, Gaussian approximation method, and analytical approximation formulas shows that this technique is very accurate. Analytical expressions are derived for FSK and CPFSK receivers which include polarization and phase diversity techniques. The use of numerical programming to avoid many unnecessary computations is discussed. This evaluation method can be used to account for the effects of crosstalk in multichannel systems and the influence of error-control codes     

Emerging trends in millimeter-wave CAD

This paper discusses three emerging trends in the area of millimeter-wave (mm-wave) electromagnetic (EM) simulation and computer-aided design (CAD). Diakoptics techniques for frequency- and time-domain EM-simulation methods are needed for mm-wave circuit design. Development of artificial neural network (ANN) models is likely to result in an efficient use of EM simulators. Possible applications of knowledge-based tools are suggested for initial stages of the design process     

Accurate evaluation of multiple-access performance in TH-PPM and TH-BPSK UWB systems

A characteristic function method is proposed for precisely calculating the bit-error probability of time-hopping (TH) ultra-wideband (UWB) systems with multiple-access interference in an additive white Gaussian noise environment. The analytical expressions are validated by simulation and used to assess the accuracy of the Gaussian approximation. The Gaussian approximation is shown to be inaccurate for predicting bit-error rates (BERs) for medium and large signal-to-noise ratio (SNR) values. The performances of TH pulse position modulation (PPM) and binary phase-shift keying (BPSK) modulation schemes are accurately compared in terms of the BER. It is shown that the BPSK system outperforms the binary PPM system for all values of SNR. The sensitivity of the performance of the modulation schemes to the system parameters is also addressed through numerical examples.     

一种准光模式变换器的设计和实验

准光模式变换器是实现高功率回旋管高效输出的重要部件。该文针对140 GHz, TE_28,8模回旋振荡管研制,开展以Denisov型辐射器和3个准光镜面构成的准光模式变换器设计与实验研究。利用标量衍射法优化辐射器辐射口径处的场分布,其与理想高斯场之间的矢量相关性为96.2%;基于几何光学和高斯波束匹配方法设计了聚焦镜面与波束整形镜面,采用3维全波分析软件Surf3D获得各个镜面上及输出窗处的场分布,对所设计的镜面系统进行了仿真验证,在输出窗处获得了高斯模式含量为96.67%的输出波束,整个模式变换器的功率转换效率为93.98%。以自行研制的TE_28,8模激励器作为准光模式变换器的输入,通过对模式变换器转换性能仿真结果验证,在严格控制加工精度及装配和实验过程的基础上,完成了准光模式变换器转换性能的冷测实验。实验结果表明,设计和实验具有合理的一致性,可以作为准光模式变换器工程应用设计和验证手段。     

斜程大气传输激光束的平均光强与短期光束扩展

基于广义惠更斯-菲涅耳原理、修正von Karman 湍谱、贝塞耳权重函数的高斯函数近似和误差小于4%的波结构函数平方近似,研究得出了包含湍流尺度影响的湍流大气中传输高斯光束的平均光强新关系和包含湍流外尺度影响的光束短期扩展因子。研究了高斯光束等效半径与传输距离、初始光束束径和光波波长间的关系,结果表明,在给定折射率结构参量和传输距离情况下,存在极小短期光束扩展对应的特征初始束径和特征光波波长。