Travelling Wave Window with Matching Gratings

At millimeter waves, travelling wave dielectric windows are advantageous over commonly used resonant ones due to the higher power capability. An attractive method to compensate the wave reflections from vacuum-dielectric and dielectric-vacuum boundaries is using anti-mirror reflection gratings (Fig. 1) [1]. In this paper, the power loss due to the finite cross-section of the wave beam and the frequency mismatch relative to the resonant value are calulated for the latter type of window.     

Experimental study of coupled vibration in a finite periodic dual-layered structure with transverse connection

The analytical equations of the transfer matrix method are further derived for the multi-coupled vibration of flexural and longitudinal waves in a periodic dual-layered beam structure with connection branches, with full consideration given to the flexural and longitudinal motions that are tri-coupled at each connection. Measurements of mobilities at the junctions on the uni-layered beam and the cross-layered beam are made. The numerical results agree well with the experimental results at all frequencies from 10 to 2000 Hz, which verifies the theoretical methodology for the multi-coupled vibration in a finite dual-layered beam. The cross-layer energy transmission is calculated, which reveals that the transmitted longitudinal energy is enhanced not only at the longitudinal resonant modes but also at the flexural resonant modes of the connection branches due to the structural wave coupling. The flexural energy is excited by wave coupling and becomes stronger at the longitudinal resonant modes and the flexural resonant modes of the connection branches. The cross-layer vibration motions from coupled waves in the branches can be effectively controlled by the attached cantilevers with mass at the resonance modes. This method can be used to control the structure-borne sound transmission in multi-layer beam structures.     

双层交叉叠合式共振亚波长光栅

提出了一种防伪光栅结构-双层交叉叠合式共振亚波长光栅,其特点是可以展宽共振亚波长光栅的共振波长宽度,使得当将其应用于防伪时能得到更佳的光变效果。分析了半峰全宽展宽的原理,指出共振宽度值的大小依赖于耦合到光栅波导层内一级衍射光的能量。优化设计了此种光栅的结构参量。用矢量衍射理论对该结构的共振特性和制作工艺误差进行了研究。研究表明：这种防伪光栅的共振光谱峰值并不随着入射角的改变而降低,共振半峰全宽最大值约为同等条件下基本共振光栅结构的7倍,该结构的共振性能对光栅制作误差不是很敏感,是一种应用价值较高的防伪光栅结构。     

Beam splitting, combining, and cross coupling through multiple superimposed volume-index gratings

We investigate the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout. The vector synthetic gratings consist of two volume-index gratings with different grating wave vectors, which can be obtained in photorefractive materials, in optical fibers or in Bragg cells with two driving acoustic waves. This structure can realize beam splitting, beam combining, and beam cross coupling, etc. Analytic expressions for the diffraction efficiency as well as the amplitude of the waves involved are obtained. The results are presented and discussed.     

Scintillation of a laser beam propagation through non-Kolmogorov strong turbulence

Atmospheric turbulence causes strong irradiance fluctuations of propagating optical wave under the severe weather conditions in long-distance free space optical communication. In this paper, the scintillation index for a Gaussian beam wave propagation through non-Kolmogorov turbulent atmosphere is derived in strong fluctuation regime, using non-Kolmogorov spectrum with a generalized power law exponent and the extended Rytov theory with a modified spatial filter function. The analytic expressions are obtained and then used to analyze the effect of power law, refractive-index structure parameter, propagation distance, phase radius of curvature, beam width and wavelength on scintillation index of Gaussian beam under the strong atmospheric turbulence. It shows that, with the increasing of structure parameter or propagation distance, scintillation index increases sharply up to the peak point and then decreases gradually toward unity at rates depending on power law. And there exist optimal value of radius of curvature and beam width for minimizing the value of scintillation index and long wavelength for mitigating the effect of non-Kolmogorov strong turbulence on link performance.     

Theory of resonant relativistic oscillator with nonuniform focusing field

The two-dimensional model of millimeter wave resonant O-type oscillator (as orotron, ledatron, resonant BWO, etc.) with a relativistic electron beam is analyzed. The selfconsistent nonlinear simultaneous equations have been obtained for the arbitrary space distribution of the magnetic guide field. The start generation characteristics are analyzed under small-signal conditions with an analytical solution taken for the case of inclined focusing magnetic field. It is found that the efficiency of electron-wave interaction appreciably depends on the focusing field strength and the relativistic mass factor. The results of numerical optimization of the guide field structure are presented to show possibility of improvement of the start characteristics of the oscillator.     

Radiation of a charge moving over the diffraction grating

The states of a charged particle with a finite free path are determined in the field of a resonant electromagnetic wave. The exact resonance conditions, the modulation and beam instability mechanisms, the charge and current densities (Ohm's law) are obtained for the collisionless beam of resonance particles. Quantum theory of radiation is developed for the resonant adiabatic interaction between a particle and a wave taking into account the interaction with a constant magnetic field induced at the grating surface by the charge and nonresonant waves. The radiation power, the spectrum, and the range of generated frequencies are determined. The results obtained can be used in the plasma and solid-state theories and in electronics.     

Phase resonances in obliquely illuminated compound gratings

The existence of phase resonances in obliquely illuminated, perfectly conducting compound gratings is investigated. The diffraction problem of a p-polarized plane wave impinging on the structure is solved using the modal approach. The results show that even under oblique illumination, where no symmetry is imposed by the incident field, there are resonant wavelengths that are clearly associated with a certain degree of symmetry in the phase distribution of the magnetic field inside the cavities. New configurations of this phase distribution take place, that were not allowed under normal incidence conditions. It was found that the interior field is intensified in the resonances, and the specularly reflected efficiency is maximized. In particular, this efficiency is optimized for Littrow mount.     

On the interaction of a beam of polar molecules with a static and a resonant RF field as a source of molecular interferences

A molecular beam interference model is presented based on a two-state interaction between a polar molecule and a resonant RF field as it occurs in the so-called C-field of a typical molecular beam electric resonant spectrometer. The treatment shows the onset of interferences in the beam transmission spectra as well as in its transverse profile. It is demonstrated how the molecular interferences are originated by the wavefunction phase shift introduced by the resonant RF field. Furthermore it is shown that for a given beam velocity and oscillating field frequency the fringes’ visibility depends on the strength of the RF field, i.e. the Rabi frequency, in the transmission spectra. Likewise the presence of a RF field gradient in the perpendicular beam direction gives rise to a peak structure in the transverse beam profile. The theoretical model was applied to simulate a variety of beam transmission spectra under resonant conditions as well as some experimental data already published by our group showing a satisfactory agreement between experimental and simulated data. Finally, the potentiality of this internal state molecular interferometer to carry out studies in matter-wave interferometry is remarked.     

速调型相对论返波管理论研究

对速调型相对论返波管慢波结构色散特性及束波相互作用进行了理论研究。色散特性研究表明:器件工作模式为TM01模,近点,耦合阻抗较高。色散特性预测的工作频率与粒子模拟结果非常接近。慢波结构峰值增长率相对较小,这与电子束与慢波结构相距较大有关,因而器件从起振到饱和的时间较长。在束波相互作用理论中,全面考虑了电子束与慢波结构前向波基波、反向波－1次空间谐波及空间电荷场相互作用、谐振反射器对电子束进行的束流调制和能量调制作用,以及调制腔和提取腔处引入的耦合阻抗及轴向波数突变。稳态和非稳态计算结果均获得了超过40%的束波转换效率。     

On the design of structural junctions for the purpose of hybrid passive-active vibration control

A theoretical investigation of wave scattering and the active modification of wave scattering at structural junctions is presented. A resonant and a non-resonant Euler-Bernoulli beam are coupled, and an external force is introduced at the junction. The external force is intended for feedforward control in order to manipulate the scattering properties at the junction. The purpose of the investigated control law is to make the junction non-reflective in the case of an incident bending wave. The control effort and the resulting power flow are investigated for different properties of the beams. By introducing damping in the resonant beam all incidence wave power is absorbed either passively, in the resonant beam, or actively, by the force. The results form the basis for a discussion of the possible benefits of using such a configuration for hybrid passive-active vibration control. The results show that for certain ratios of bending stiffness and mass the presented hybrid passive-active solution may offer advantages compared to purely passive or purely active solutions.     

Optimization of a Waveguide Grating for Normal TM Mode Coupling

The conditions for maximum longitudinal electric field of a TM mode at the surface of a step index slab waveguide, and the value of this maximum are given in a normalized analytical form. They are applied to a grating coupling structure corresponding to a resonant laser mirror or to optical interconnects involving a normally incident free space wave and a singlemode slab waveguide.     

Experimental investigation of a 140 GHz gyrotron-backward wave oscillator

We report the experimental operation of a voltage tunable gyrotron backward wave oscillator (gyro-BWO) in the frequency range near 140 GHz. Voltage tunability is an important feature of the gyro-BWO for application as a fast tuning source for driving high power free electron lasers or cyclotron autoresonance maser amplifiers. The gyro-BWO operated in an overmoded cylindrical waveguide structure in the TE_1,2 mode. The electron beam source was a Pierce-wiggler gun producing an 80 kV, 6.2 A beam. Frequency tuning with voltage between 134 and 147 GHz was achieved in the TE_1,2 mode with constant magnetic field. However, this tuning was found to be discontinuous. Output powers of up to 2 kW and 2% efficiency were found, significantly below theoretical predictions for a cold beam. The theoretical beam velocity spread was modeled by a 3D beam transport code. The code results show that space charge forces, coupled with the wiggler-induced helical motion and the short cyclotron wavelength of the beam, produce large increases in velocity spread in the magnetic compression region. A beam with smaller velocity spread would be needed to make the gyro-BWO operate at the desired efficiency.     

Direct observation of a "devil's staircase" in wave-particle interaction

We report the experimental observation of a "devil's staircase" in a time-dependent system considered as a paradigm for the transition to large-scale chaos in the universality class of Hamiltonian systems. A test electron beam is used to observe its non-self-consistent interaction with externally excited wave(s) in a traveling wave tube (TWT). A trochoidal energy analyzer records the beam energy distribution at the output of the interaction line. An arbitrary waveform generator is used to launch a prescribed spectrum of waves along the slow wave structure (a 4 m long helix) of the TWT. The resonant velocity domain associated to a single wave is observed, as well as the transition to large-scale chaos when the resonant domains of two waves and their secondary resonances overlap. This transition exhibits a "devil's staircase" behavior for increasing excitation amplitude, due to the nonlinear forcing by the second wave on the pendulum-like motion of a charged particle in one electrostatic wave.     

Analysis of coherent beam combining through superposed volume gratings by coupled wave theory

To address the problem of coherent beam combining through superposed volume gratings, a comprehensive mathematical model has been established and its analytical formula has been derived using coupled wave theory. The effect of each of the individual optical parameters involved in the model on the beam combining were numerically analyzed in detail. The sufficient and necessary conditions to realize efficient beam combining are summarized and discussed.     

复合局域共振梁结构带隙研究

该文将周期局域共振梁结构中的基体结构由单一材料发展成多相材料,建立了周期复合局域共振梁结构。由共振单元在单胞中的排列位置,考虑了三种构型,并利用传递矩阵方法计算了在弹性基底作用下这些结构产生的局域共振带隙性质。结果表明,与局域共振梁结构相比,复合局域共振梁结构产生的局域共振带隙对应的频率范围更宽,禁带范围内弹性波的衰减峰值和衰减均值更高。除此之外,该文还分析了局域共振结构、单胞尺寸、基体材料和局域共振结构中的阻尼对弹性波在空间中传播和衰减特性的影响,为满足特定的隔声隔振要求的结构设计提供了依据。     

Axial distribution of Gaussian beam limited by a hard-edged aperture

In this letter, the axial distribution of Gaussian beam limited by a hard-edged aperture is studied. We theoretically analyze the axial diffraction of Gaussian beam limited by a hard-edged aperture, and give the simpler formulas of the axial diffraction intensities of Gaussian beam in Fresnel diffraction field and Fraunhofer diffraction field. The corresponding numerical calculation of axial diffraction intensity distribution of Gaussian beam with different wave waist is provided and the evolution of the diffraction distribution with the wave waist of Gaussian beam is explained. As the especial cases of the truncated Gaussian beam,the Gaussian beam in free space and the parallel light limited by the aperture are discussed too, and the system parameters of the truncated Gaussian beam which can cause it to equal to these cases are given.The theoretical results conform to the numerical analysis.     

多振子梁弯曲振动中的局域共振带隙

从梁的弯曲振动方程出发,利用传递矩阵法,给出了无限周期结构的一维多振子声子晶体梁的弯曲振动能带结构,并利用有限元方法计算了有限周期结构梁的弯曲振动频率响应.建立了多振子声子晶体梁的简化模型,推导出带隙起始截止频率公式.结果表明:一维多振子声子晶体梁具有比单振子声子晶体梁更宽更丰富的振动带隙,可应用于呈倍频关系的减振降噪中;振动在带隙频率范围内频率响应具有明显的衰减;所建立的简化模型与理论模型结果符合较好.研究工作为梁类结构的减振提供一种新的思路.     

High-power relativistic microwave sources based on the backward wave oscillator with a modulating resonant reflector

Results of theoretical and experimental investigations into a relativistic backward wave oscillator with a modulating resonant reflector are generalized. The modulating resonant reflector is used to reflect a counter propagating wave and guide it toward an electron collector. It is shown that premodulation of the electron beam near the reflector may have a significant effect on the starting conditions of oscillation; selective properties of the oscillator; and its efficiency, which may reach 40% when a high-current beam is transported by a strong magnetic field. In the reduced magnetic fields that were employed in the pulsed-periodic regime and were 1.5–2.0 times lower than those at which cyclotron resonance with the counter propagating wave is observed, the oscillator efficiency (30–35% at a wavelength of 8 mm) is limited by position and velocity spreads of particles. Mechanical pulsewise frequency tuning within about 10% at a repetition rate of 1–50 Hz and a multigigawatt microwave power, as well as a rise in the power and energy of microwave pulses via an increase in the cross-sectional dimensions of the slow-wave structure, are demonstrated to be feasible.     

An extended interaction oscillator based on a complex resonatorstructure

We report experimental investigation of a novel scheme for efficient interaction between a linear electron beam and an electromagnetic wave in a complex extended interaction structure. The structure of interest consists of a high R/Q, five-gap, coupled-cavity resonator that incorporates a coaxial section of a quarter plasma wavelength placed between the first and second cavities. In the coaxial section, beam and wave propagate in separate channels. The first cavity, strongly coupled to the other cavities through the wave channel of the coaxial section, serves as a buncher cavity. An inner channel running through the center conductor of the coaxial section provides a cutoff drift space for ballistic bunching of electrons, an effect that is shown to significantly enhance the interaction efficiency. Oscillation power of 2.2 kW at 16.6 GHz was demonstrated with an interaction efficiency of 30%. The total efficiency was further increased to 41% by incorporation of a two-stage depressed collector