A note on the coupling efficiency between an antenna and a Gaussian beam

In this communication a note on the coupling efficiency between an antenna and a Gaussian beam is presented. The calculations of the coupling efficiency from both methods, a nomorlized inner product between the antenna radiation pattern and the Gaussian-beam pattern or alternatively, between the electric field at the antenna aperture and a two-dimensional field representation of a Gaussian beam, are compared for different aperture size in wavelength. How a correct solution can be gotten is stated.     

Application of the Beam Mode Expansion to the Analysis of Noise Reduction Structure (Short Papers)

The beam mode expansion method used to discuss the distraction problem by an aperture is applied to the analysis of the noise reduction structure consisting of two aperture stops. The incident field is a fundamental wave beam whose amplitude distribution is Gaussian. The transmitted field through the structure can be represented as a sum of beam mode functions and is regarded as a signal. The noise which is originated from the spontaneous emission is added to the incident Gaussian wave beam. The signal-to-noise ratio (SNR) in the output is discussed and optimum conditions are obtained numerically.     

Comparison of dc electric field and tapered wiggler free electron laser efficiency enhancement schemes

A constant parameter wiggler free electron laser using a dc electric field for efficiency enhancement is compared to several tapered wiggler efficiency enhancement schemes. Analytical expressions for the efficiency in the plane wave, infinitesimally small radius electron beam limit are derived and compared. Numerical simulations for a Gaussian radiation field and finite radius electron beam with an output radiation wavelength of 2 μm are presented. For finite radius electron beams, the extraction efficiency using a dc electric field is somewhat greater than or equal to that using proposed tapering schemes. While the dc field offers flexibility in efficient, tunable laser design, the required field strengths for visible radiation are in the megavolt/meter range.     

Radiation patterns of circular apertures with Gaussian illumination

We analyze the radiation pattern from circular apertures with Gaussian electric field distribution. Patterns are calculated using the Fraunhofer diffraction integral and we obtain a convenient expression for the FWHM beamwidth as a function of the edge taper. The aperture efficiency can be calculated for blocked and unblocked apertures in analytic form. The beam efficiency must be computed numerically, but can be obtained in a straightforward way from the beamwidth.     

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。     

The hexagonal horn as an efficient Gaussian beam launcher

A new type of feed horn-the hexagonal horn-is proposed and investigated by means of Gaussian-Hermite beam-mode analysis. The electric-field distribution at the horn aperture plane is approximated. The polarization efficiency is 88.66%. The fractional power in the fundamental beam mode is about 85%. The near- and far-field radiation patterns are calculated. The high fractional power in the fundamental beam mode of the horn indicates that it can be used as an efficient Gaussian beam launcher in quasi-optical systems     

传输波束对无线光通信误码率性能的影响分析

在无线光通信中,研究通信系统性能时多数是在平面波、球面波条件下进行的,而实际无线光通信系统发射的激光是高斯波束,所以研究不同波束对通信系统性能的影响具有重要意义。本文从孔径平均效应出发,运用理论分析和实验仿真的方法,在不考虑湍流内、外尺度影响的前提下,首先描述了高斯波束条件下,孔径平均因子随孔径尺寸的变化趋势,并且比较了不同湍流强度条件下孔径平均效应的差异;在OOK调制和非相干检测条件下,比较不同传输波束在弱湍流大气中传输时,孔径平均在改善误码率性能方面存在的差异。结果表明:高斯波束对误码率的影响明显,最先达到饱和状态,球面波次之,平面波较为平缓。     

Diffraction by apodized apertures

This paper investigates the diffraction of light by apodized apertures. The diffraction equation is solved numerically starting with an apertured plane wave or Gaussian beam. A function chosen to characterize spatial intensity modulation is then plotted versus Fresnel number for differing amounts of apodization so that the benefit gained thereby may be quantitatively assessed. In addition, the energy loss incurred due to insertion of an apodized aperture in front of an existing hard aperture is determined.     

高斯光束对颗粒衍射测量的影响

理论推导了高斯光束的圆孔夫琅和费衍射结果并与平行平面波的圆孔夫琅和费衍射结果进行比较,数值计算和理论分析表明,当激光光束的束宽半径远大于圆孔半径时,两者的衍射结果是非常接近的;当激光光束的束宽半径接近于圆孔半径时,高斯光束的衍射偏离平行光衍射。考虑光强的高斯分布,给出了某一粒度颗粒群的衍射结果。     

金属栅厚度对介质波导漏波天线辐射特性的影响

本文研究了厚金属栅加载介质插入波导漏波天线的辐射特性,运用电磁场的网络解析法的推广模型建立起本征值方程。数值结果表明,金属栅厚度对漏波天线波求方向影响较小,但对漏波天线的辐射效率和波束宽度有明显的影响。     

Gabor's expansion of an aperture field in exponential elementarybeams

Gabor's expansion of an aperture field in a discrete two-dimensional superposition of Gaussian elementary beams is extended to incorporate exponential beams. The resultant expansion scheme has an advantage over the commonly used Gaussian beam representation in situations involving an exponential aperture distribution such as the modal field in a thin slab laser amplifier     

Scattering of an incident Gaussian beam by a perfectly conductingrough surface

The modifications of the regular full wave theory for rough surface scattering by an incident Gaussian beam instead of a plane wave are developed. The Gaussian beam is produced by a large circular aperture that has a Gaussian illumination. It is shown that provided the incident Gaussian beam has a plane phase front over the extent of the rough surface patch and the linear dimensions of the rough patch are large compared with the surface height correlation length, the normalized scattering cross-section is the same as for an incident plane wave     

Optimum aperture monopulse excitations

A unified formulation of the optimization of monopulse antenna performance indices for a specified sidelobe envelope function and/or specified nulls of the pattern is presented. The performance indices considered are beam efficiency, gain factor, and angular sensitivity factor of rectangular and circular apertures. The unconstrained optimization of beam efficiency result in an integral equation, the solutions of which are prolate spheroidal wave functions for rectangular aperture and hyperspheroidal wave functions for circular aperture. These functions reduce, respectively, to Legendre and Zernike polynomials in the case of gain factor and angular sensitivity factor. The double orthogonality properties of these eigenfunctions are used for constrained optimization. The results obtained by this technique for the near-in sidelobes constrained at a uniform level are shown to be in agreement with the earlier works. The method is applicable for other aperture surfaces such as elliptical, ellipsoidal, and spherical.     

Gaussian beam techniques for illuminating reflector antennas

Simple design procedures are presented for use when a Gaussian beam is used to illuminate a classical reflector antenna. Displacement of the location of the beamwaist toward the focusing element in the case of electrically small antennas where the aperture is in the near field of the feed was calculated together with modification of the required beamwaist radius. Dual reflector antennas were discussed and design procedures appropriate for systems with large and small focal length to diameter ratio developed. Cases where a reflector or subreflector is electrically small, or in the near field of a feed, are readily treated. For elliptical beam antennas, a simple illumination system using only a scalar horn and a single cylindrical lens can generally be found; this has no ray optics analogue. A configuration of this type is discussed, with a practical case study of a 28-by-80-λ elliptical Cassegrain antenna operating at a wavelength of 3 mm. The design process for designing the feed system is discussed in detail. Despite the small size and relatively large aperture blockage, an aperture efficiency of 0.48 was measured, which compared quite well with the expected efficiency of 0.53, thus verifying the validity of the Gaussian beam design approach     

高斯光束经含光阑失调光学系统的传输特性

为分析光学系统中圆孔光阑和透镜失调对高斯光束传输特性的影响,利用硬边椭圆光阑的近似展开式和适用于失调光学系统的广义衍射公式,推导了高斯光束经含失调圆孔光阑的失调光学系统传输的近似解析式,得出了输出光束场分布与光束参量、孔径尺寸、ABCD矩阵元、光阑和光学系统失调量间的关系。针对特定光学系统,定量分析了各失调量对输出光束场分布的影响。     

激光扩束系统衍射现象数值计算

本文在标量衍射理论的基础上 ,应用柯林斯公式数值计算了高斯光束经过通光口径较小的扩束光学系统后的光强分布 ,结果表明出射激光光束的束散角虽然可以压缩 ,但往往不是高斯光束 ,而是具有亮暗相间的多个环状光束。本文的分析对于激光扩束光学系统及其它光学系统的设计具有一定的参考价值     

平面近场测试高斯波束束腰半径的方法

高斯波束理论广泛应用于准光学、毫米波领域,束腰半径作为高斯波束最重要的参数,在工程应用中具有重要价值,有必要对其进行精确测量。提出了一种基于平面近场测试高斯波束束腰半径的方法,对准光学、毫米波装置的出射波束进行平面近场测试,根据采集的近场数据,反演远场方向图计算出束腰半径大小。进一步对近场电场数据进行高斯函数拟合及后处理,并利用高斯波束传输理论计算出束腰半径位置。本方法完全借用平面近场测试条件,通用性好,特别是解决了大口径装置束腰半径难以测试的难题。     

激光驻波场中铬原子的三维沉积特性分析

利用近共振激光驻波场操纵中性原子实现纳米级条纹沉积是一种新型的研制纳米结构长度标准传递方法,但仅通过一维和二维形式的仿真不能给出激光驻波场作用下中性原子沉积纳米光栅的全部信息。利用半经典模型,从铬原子在高斯激光驻波场中的运动方程出发,通过四阶Rungo-Kutta法模拟了铬原子在高斯激光驻波场中的三维运动轨迹以及三维沉积条纹结构,并分析了原子束发散、色差和球差等因素对三维运动轨迹及沉积条纹结构的影响。结果表明,利用三维仿真形式模拟高斯激光驻波场中铬原子的运动得到的结果与一维和二维形式下相比可以直观地表现出其较为详细的本质。     

会聚高斯光束光轴上的光场强度分布

根据惠更斯一菲涅耳原理，推导出会聚病折光束经圆形孔径衍射后光轴上的场分布及强度分布表达式。讨论了轴上点光强最大值相对会聚高斯光束束腰的偏移量同衍射孔径及高斯光束参数的关系。     

Inversion of roughness profile of heterogeneous fractal surface using Gaussian beam incidence at low grazing angle

As a Gaussian beam is incident upon a rough surface at low grazing angle, the Helmholts scalar wave equation may be replaced by the parabolic approximate equation. As the incident field is known, the scattered field and surface current give the Volterra integral equation. Surface roughness profile can be formulated by the integral equation of the surface currents. These two coupled equations are applied to invert the roughness profile of heterogeneous fractal surface. Using Monte Carlo method, the fractal rough surfaces with a band-limited Weistrass-Manderbrot function are numerically simulated and the scattered fields along a line parallel to the mean surface are solved. The Gaussian beam incidence and scattered fields are used to progressively invert the surface roughness profile. Reconstructed profile and its inverted fractal dimension, roughness variance and correlation length are well matched with the simulated surfaces.