Propagating anti-symmetrically coupled plasmons generation by electron beams

Propagating anti-symmetrically coupled plasmons that usually cannot be excited with incident light and radiate to far field can be efficiently generated by electron beams. An electron beam is proposed as a practical propagating anti-symmetrically coupled plasmon source due to that it couples differently to the surface plasmons than free radiation. Specifically, whispering-gallery anti-symmetrically coupled plasmons with the character of symmetrical coupled dipoles are excited by an electron beam in a nested ringlike waveguide, which is consistent with the dispersion of electron excited plasmons in an infinite-long nanowire pair.     

Coupled mode analysis to study cascading in the QPM Cerenkov regime in waveguides

In this paper we use the coupled mode analysis to study nonlinear phase shifts of the fundamental beam caused by cascaded second order nonlinear effects in the Quasi Phase Matched Cerenkov (QPMC) configuration in waveguides. Under the no-pump depletion approximation which is valid for low conversion efficiencies, we obtain the nonlinear phase shift as a function of length of interaction and grating period. It is observed that the nonlinear phase shift of the fundamental beam can be maximized by choosing a grating period for which the phase matched second harmonic radiation mode is radiated parallel to the film-substrate interface, i.e., for zero Cerenkov angle. Although the phase shifts are smaller than in the case of all guided geometry, QPM Cerenkov configuration is expected to have greater tolerance towards various waveguide parameters and the fundamental wavelength.     

Waveguide lenses with multimode interference for low-loss slab propagation

It is shown that a nearly ideal two-dimensional focusing Gaussian beam can be synthesized by use of a linear combination of the two lowest-order even modes of an optical waveguide. This property can be used to couple laterally guided modes across slab waveguide regions with low loss. The technique is illustrated by use of a conventional multimode interference (MMI) geometry, in which the MMI coupler transforms the fundamental mode of an initial waveguide into a focusing Gaussian beam, which is then fed to a slab region. Two-dimensional beam propagation simulations show that the beam does not initially diverge in the slab region, but rather comes to a focus. A second MMI coupler then transforms the diverging beam back to the initial mode. A structure is designed that can couple the fundamental mode of a 9-microm-wide waveguide across an 88-microm-long slab region with only a 0.036-dB loss. This technique can be applied to improve the performance of small-angle waveguide crossings and integrated turning mirrors.     

Two-dimensional hard x-ray beam compression by combined focusing and waveguide optics

A two-dimensionally confining x-ray channel waveguide structure is combined with a high gain Kirkpatrick-Baez prefocusing mirror system yielding a hard x-ray beam with a cross section of 25 x 47 nm(2) (FWHM). Unlike the previously employed resonant beam coupling scheme, the incoming beam is coupled in from the front side of the waveguide and the waveguided beam is no longer accompanied by spurious reflected or transmitted beams. The field distribution in the waveguide channel has been calculated numerically. The calculated transmission and far-field intensity pattern are in good agreement with the experimental results.     

Multi-mode holographic waveguide coupler

A double-exposure thick-holographic-grating waveguide coupler is described in which two waveguide modes can be excited with a single incident beam. Single-grating decouplers are also designed to selectively remove one of the coupled waves.     

Matching of a Gaussian beam into hollow oversized circular waveguides

A Gaussian beam entering an hollow oversized circular waveguide of dielectric or low-conductivity wall material is considered: a simple formula giving the optimal beam charac-teristics for the lowest losses in the guide is derived from an optical theory. This optimization depends on the geometry and the wall material of the guide. The formula is valid for small beam diameters at the guide entrance and for low losses inside the guide, which are conditions corresponding to usual cases. Physical interpretations are given. Comparisons with some experimental results show a good agreement with the theory. These results are of interest for waveguides lasers and waveguide transmission lines.     

离子交换LiNbO_3:Fe平面光波导中两波混合的研究

本文利用耦合波理论分析推导了单模平面光波导中两波混合的耦合波方程和弱相干光放大率的表示式.并且,在LiNbO_3:Fe单模平面光波导中进行了两波混合的实验研究,得到了与理论分析相符合的结果.     

Effect of longitudinal space-charge waves of a helical relativisticelectron beam on the cyclotron maser instability

The influence of the longitudinal space-charge waves of a coherently gyrophased helical relativistic electron beam on the cyclotron maser instability is investigated in a cylindrical waveguide configuration using a three-dimensional kinetic theory. A dispersion relation that includes waveguide effects is derived. The stability properties of the cyclotron maser interaction are examined in detail. It is shown that, in general, the effects of space-charge waves on a coherently gyrophased beam are suppressed in a waveguide geometry, in comparison with an ideal one-dimensional cyclotron maser with similar beam parameters     

掺半导体玻璃波导的导波功率限制

首次利用阳极氧化法在掺半导体玻璃上制备了两端带有抛物耦合喇叭的4微米条宽沟道波导,用光栅耦合器将波长0.532微米的YAG倍频激光耦合进波导中,实现了输出光的功率限制.     

Parameters to Define the Electron Beam Trajectory of a Double-Tapered Disc-Loaded Wideband Gyro-TWT in Profiled Magnetic Field

In the method of tapering the cross section of the interaction structure for broadbanding a gyro-TWT, the different portions of the interaction length of the tapered-cross-section waveguide become effective for different frequency ranges if the magnetic field and beam parameters are profiled to maintain the condition of electron cyclotron resonance throughout the interaction length. In the present paper, the study of profiling the magnetic field and beam parameters in steps of the stepped analytical model of a double-tapered disc-loaded circular waveguide was made throughout the steps of the model. In the observed profile, the magnetic flux density in a typical step relative to its value in first-step decreases from first-step (gun-end) to end-step (collector-end) of the model considering the up-tapering schemes, in which structure parameters increase from gun-end to collector-end. Also, the transverse beam velocity in a typical step relative to its value in first-step decreases from gun-end to collector-end. However, the Larmor radius in a typical step relative to its value in first-step as well as the hollow-beam radius in a typical step relative to its value in first-step, both increase from gun-end to collector-end in the model considering the up-tapering schemes.     

Observation of interference in reflection profile resulting from excitation of optical normal modes with focused beams

We report the observation of novel interference patterns in reflected beam profiles resulting from the excitation with a focused laser beam of both the long-range surface plasmon mode and leaky waveguide modes in a prism-coupled thin-film geometry. Many new features of the observation are in good agreement with the theory; namely, the greater than unity normalized peak interference intensities, the monotonically decreasing spacing between the periodic intensity variations, and the reshaping of the reflected profile due to propagation. Implications of these results for submicrometer patterning in nanoscience are pointed out.     

Theory of gyrotwystrons with mixed transverse geometries of thevarious stages

Gyrotwystrons are gyrotron amplifiers with resonant cavities, field free drift regions, and a nonresonant output waveguide. They are expected to be more wideband than gyroklystrons, and more efficient than gyro-travelling wave tubes. A theory of gyrotwystrons has been developed which describes gyrotwystrons with cavities of different transverse geometry than the output waveguide. An analytical approach permits one to estimate in a simple manner the effect of mixed transverse geometry on the interaction between the electron beam and the microwave field. Numerical studies of an NRL gyrotwystron design show it possible to realize a rather high efficiency over a large bandwidth (e,g., 35% efficiency with a bandwidth of 6%)     

Modification of spontaneous emission from a five-level M-type atom in coupled cavity waveguide

We study the spontaneous emission spectrum of a five-level M-type atom driven by a microwave field, in which two upper levels are coupled by the same-coupled cavity waveguide reservoir to a lower level. The spectrum behavior presents a strong non-Lorentzian shape that originates from effective quantum interference in Markovian reservoir, in which the spectral line can be significantly enhanced and eliminated by adjusting the proper parameters of the system. However, for non-Markovian reservoir, it seems that the shape of emission spectrum is quite dependent on the geometry behavior of a coupled cavity waveguide.     

Two-dimensional coupled mode equation for grating waveguide excitation by a focused beam

The problem of grating coupling of a focused incident beam under non-normal incidence into a slab waveguide is given a complete three-dimensional (3D) solution. The diffracted field is expressed as the Fourier integral of a regular part and of a singular part resulting from the existence of the coupled guided mode. A suitable expression of the field in the neighborhood of the pole and a rigorous definition of the modal field lead to a generalized coupled mode equation relating the incident field and the two-dimensional (2D) modal field propagating in the plane of the slab waveguide. The phenomenological parameters involved in the coupled wave equation: the propagation constant, the radiation coefficient as well as the modal field shape are derived from the exact treatment of plane wave diffraction in the same structure. The solution of the complete coupling problem is given in the particular case of a Gaussian incident beam, and of a high index step-index waveguide.     

Ray-optics analysis of the coupling efficiency from a gaussian beam to a rectangular multimode embedded strip waveguide

The modified ray-optics method is used to analyze the coupling efficiency from a Gaussian beam to a rectangular multimode embedded strip waveguide. It is convenient for analyzing the case of a simple waveguide structure because of reduced computation time. The conditions for determining which optical rays can be coupled into the waveguide to become the waveguiding modes, as well as the effective receiving area of the waveguide, are derived in this work. The data obtained by this method are compared with those calculated by the well-known overlap integral theorem and Marcatili's models for investigating the validity of the modified ray-optics method.     

利用耦合波导列提高光子晶体波导辐射

将耦合波导列应用于光子晶体单模波导,提出一种提高光辐射的光子晶体结构. 基于时域有限差分方法的理论研究表明,当将耦合波导列附加到单模光子晶体波导出口端的适当位置,使出射光分成若干强弱不一的光束,这些光束在传播空间通过干涉形成一定程度的汇聚,大大提高了光子晶体波导在水平方向的光辐射效率. 另外,当耦合波导列的行数大于某固定值(2N=8)时,辐射质量基本保持不变,由此可获得最紧凑的器件结构. 这种类型光子晶体在近场光学和集成光学等诸多方面有潜在的应用价值. 关键词： 光子晶体波导 光辐射 波导列 耦合波导     

相对论行波管慢波结构几何参数研究

 推导了无引导磁场下具有离子通道的波纹波导中的色散方程,并通过数值计算分析了该慢波结构的几何参数如波纹周期、波纹深度以及波导平均半径对相对论行波管的带宽及增益的影响,为相对论行波管的设计提供了一定的依据。     

Nonlinear band gap transmission in optical waveguide arrays

The effect of nonlinear transmission in coupled optical waveguide arrays is theoretically investigated and a realistic experimental setup is suggested. The beam is injected in a single boundary waveguide, linear refractive index of which (n(0)) is larger than refractive indexes (n) of other identical waveguides in the array. Particularly, the effect holds if omega(n(0)-n)/c>2Q, where Q is a linear coupling constant between array waveguides, omega is a carrier wave frequency, and c is a light velocity. Numerical experiments show that the energy transfers from the boundary waveguide to the waveguide array above a certain threshold intensity of the injected beam. This effect is due to the creation and the propagation of gap solitons in full analogy with a similar phenomenon in sine-Gordon lattice [Phys. Rev. Lett. 89, 134102 (2002)]].     

Selective excitation of high-order modes in circular waveguides

The excitation of very high-order modes in circular waveguides has been performed in a cavity with a connected up-taper with a geometry similar to those used in gyrotrons. A Gaussian beam was coupled to the cavity which was made translucent by an array of holes. With the help of a special optics, the amplitude as well as the phase distribution of the beam was matched to the mode to be excited in the resonant cavity. By simple rotation of one mirror to adjust the phase distribution together with the change of frequency to match the resonance condition, a large number of modes could be produced with one experimental set-up. Field measurements in the output waveguide show a high mode purity of the radiation and confirm the calculations. The method can be used for cold tests of electrodynamic systems operating with these modes, e.g. quasi-optical converters for gyrotrons.     

Coupled tapering/uptapering of soliton pairs in nonlinear media

Phenomenon of coupled tapering/uptapring of two mutually incoherent beams coaxially co-propagating in a nonlinear medium with small gain or loss has been investigated in this paper using standard parabolic equation approach (PEA) and the results are compared with the results obtained by Beam Propagation Method (BPM), i.e., by direct simulations of the underlying Nonlinear Schrödinger Equation (NLSE). The PEA results are shown to be in excellent agreement with the BPM results. It is seen that both beams of the pair induce uptapering in each other in presence of losses and tapering in presence of gain. When the medium offers gain to the first beam and losses to the other, both beams taper. When the medium offers gain/absorption to only one of the two beams, the beam undergoes self-tapering/self-uptapering and induces a taperd/uptaperd waveguide. The other beam (for which the medium is lossless) uptapers/tapers due to the taperd/uptaperd waveguide created by the first beam.