Hollow elliptical Gaussian beams generated by a triangular prism

Based on the tensor method, an analytical formula for hollow Gaussian beams (HGBs) generated by a triangular prism has been derived, and the propagation properties have been studied. It is shown that hollow elliptical Gaussian beams (HEGBs) can be obtained in the near field after the prism, and that the eccentricity of the output beam is affected by parameters such as the incidence angle, refractive index of the prism and the order of the HGB. It shows that by using a triangular prism one can conveniently obtain HEGBs.     

Design and analysis of a wideband photonic crystal waveguide with low group-velocity and low dispersion

The slow light propagation in a line waveguide in the two-dimensional triangular photonic crystal has been numerically studied, based on which a wideband photonic crystal waveguide with low group-velocity and low dispersion is proposed. The numerical simulation analysis shows that it is possible to maximize the group index and minimize the group-velocity dispersion in wide bandwidth by increasing the radius of the basic air hole and changing the position of the first two rows of air holes in photonic crysta...     

Numerical heat transfer analysis in turbulent channel flow over a side-by-side triangular prism pair

Heat transfer and flow behavior in a channel fitted with a transverse triangular prism pair is numerically investigated in the turbulent flow regime for the Reynolds number ranging from 10000 to 50000. The aspect ratio of channel height to the prism base is fixed at 4.0 throughout the study. The Navier-Stokes equation, along with the energy equation, is solved using a finite volume method with the SIMPLE technique and the QUICK numerical scheme for coupling the discretized equations while the standard k-ɛ turbulence model is used for closure of the problem. The numerical result reveals that heat transfer augmentation in the channel with the built-in prism pair can be obtained. It is observed that as compared to a channel, the heat transfer is enhanced by about 17% for a single triangular prism and by some 85% for a triangular prism pair mounted on the channel wall. Effects of the clearance between the prisms on the heat transfer augmentation are presented. The heat transfer enhancement is due to the vortex formation or recirculation zone downstream of the prism elements. However, the presence of the prisms also leads to higher values of friction loss over the channel.     

Absorption Measurement Using a Leaky Waveguide Mode

We propose an optical waveguide sensor using a leaky mode for absorption measurement of liquid samples. This sensor uses a single coupling prism. A cladding layer and a waveguide layer are directly deposited on the reflecting surface of the prism. The intensity of the internally reflected beam at an appropriate incident angle is very sensitive to the imaginary part of the sample attached to the surface of the waveguide layer. The sensitivity of this sensor is controlled by the thicknesses of both cladding and waveguide layers. We studied the performance of the sensor by numerical calculation.     

极化聚合物薄膜波导的光学特性

叙述了键合型ＤＡＮＳ聚合物薄膜波导的制备和极化过程。应用集成光学中的棱镜耦合技术,对ＤＮＡＳ聚合物薄膜波导极化前后以及紫外光照射前后的光学特性分别进行了表征。实验结果表明,极化后波导非寻常光折射率明显增大而波导寻常光折射率减小;经过紫外光照射后,波导折射率降低约０．０２。     

Design of a high efficiency wide-band 60° bend for TE polarization

In this paper, a high-efficiency 60° photonic crystal (PC) bend geometry is proposed. The corresponding PC is a two-dimensional triangular lattice of air holes in a dielectric slab. In the proposed bend structure, five additional holes are introduced in the bend. The radii of these holes are changed gradually. Hence the effect of bend discontinuity is decreased and consequently, a wide-band high efficiency bend is obtained. In this structure, the frequency bandwidth for which the transmission coefficient is greater than 0.9 covers 84% of the single-mode bandwidth of the waveguide, while for the best geometry reported in the literature this value does not exceed 56%. For numerical analysis of waveguide bends, the slab PC structure is first replaced by a two-dimensional (2D) PC with an effective refractive index and then, a 2D FDTD method is used to obtain the transmission coefficient of the bend.     

Analysis of the Scattering Characteristic of Several Waveguide Components Using Boundary Element Method

Several kinds of waveguide components such as curved waveguide bends, arbitrary angle waveguide bends and T-junctions have been analyzed with boundary element method in this paper. A new discretization method for the boundary element method to solve the waveguide discontinuities has been given. The numerical results obtained agree well with the experimental results and numerical results in other literature. Especially, the scattering characteristics of Forded E-, H-plane T-junctions in 3mm band have been analyzed using boundary element method and the calculation results are presented.     

Scattering of electron stream waves on metal-dielectric periodic structures

The results of the theoretical and experimental research of the surface waves scattering of an electron stream and a planar dielectric waveguide on metal-dielectric structures of the dielectric prism — band diffraction grating-type are presented. Within the approximation of the given current there has been carried out a numerical analysis of energy characteristics of diffraction-Cerenkov radiation for a wide interval of the main system parameters change. The results of numerical analysis are confirmed by the data of experimental simulation.     

Wave growth rate in a cylindrical metal waveguide with ion-channel guiding of a relativistic electron beam

This paper addresses the formulae and numerical issues related to the possibility that fast wave may be grown when a relativistic electron beam through an ion channel in a cylindrical metal waveguide.To derive the dispersion equations of the beam-wave interaction,it solves relativistic Lorentz equation and Maxwell’s equations for appropriate boundary conditions.It has been found in this waveguide structure that the TM 0m modes are the rational operating modes of coupling between the electromagnetic modes and the betatron modes.The interaction of the dispersion curves of the electromagnetic TM 0m modes and the upper betatron modes is studied.The growth rates of the wave are obtained,and the effects of the beam radius,the beam energy,the plasma frequency,and the beam plasma frequency on the wave growth rate are numerically calculated and discussed.     

一种柱-锥盘荷波导的色散关系

 导出了柱-锥盘荷波导的色散方程，并通过数值计算分析了这种慢波结构的几何参数对相对论行波管运行频率和通带的影响，与柱形盘荷波导相比，这种慢波结构的上截止频率高于柱形盘荷波导,因此，可以得到较高的运行频率和较大的通带,为设计这种慢波器件提供了依据。     

Modal analysis and cutoff frequencies of a doubly clad plasma loaded waveguide having a lemniscates of Bernoulli-type core cross-section

The modal dispersion characteristics of EM waves in a new unconventional plasma loaded doubly clad waveguide having a shape of the lemniscates of Bernoulli-type core cross-section have been studied analytically. The proposed waveguide has three parts namely the core with slighter high index, the inner cladding with plasma and the outer cladding with air. Now using the necessary orthogonal coordinates for the proposed structure and imposing the boundary conditions under the weak guidance condition, the modal characteristics equation has been obtained considering variations in plasma width and plasma frequency, the dispersion curves and cutoff frequencies have been obtained and presented in this paper. It is noted that as the width of plasma layer increases, the cutoff frequency also increases considerably in all considered cases. This shows that using plasma width as a new parameter we can control any particular mode on our wish.     

基于等几何分析方法求解任意截面波导本征问题

基于等几何分析方法具有自由度花费少、高精度、高阶连续性等特点,通过加权余量法对椭圆波导本征问题的亥姆霍兹方程等几何离散得出等几何分析方程.解决了传统数值方法的求解域与几何模型的非一致性问题,实现了将问题的分析计算构架于精确几何模型基础之上.分析任意截面波导的本征问题,对不同偏心率的椭圆波导以及三角形和五边形波导的截止波数的求解结果显示等几何分析方法求解波导本征问题的高效及高精度特性.与传统方法相比,此方法以较少的自由度消耗便会达到较高的求解精度,并且数值解的收敛率较快.     

TE0导模干涉刻写周期可调亚波长光栅理论研究

通过棱镜耦合激发非对称金属包覆介质波导结构中的TE₀导波模式, 利用两束TE₀模的干涉从理论上实现了周期可调的亚波长光栅刻写. 分析了TE₀模式的色散关系, 刻写亚波长光栅的周期与激发光源、棱镜折射率、光刻胶薄膜厚度及折射率之间的关系. 用有限元方法数值模拟了金属薄膜、光刻胶薄膜和空气多层结构中TE₀导模的干涉场分布. 研究发现, 激发光源波长越短, TE₀ 模干涉刻写的亚波长光栅周期越小; 光刻胶越厚, 刻写的亚波长光栅周期越小; 高折射率光刻胶有利于更小周期亚波长光栅的刻写. 相较于表面等离子体干涉光刻, 基于TE₀ 模的干涉可在厚光刻胶条件下通过改变激发光源、棱镜折射率、光刻胶材料折射率、特别是光刻胶薄膜的厚度等多种方式实现对亚波长光栅周期的有效调控.     

光波导技术同步测量棱镜及波导薄膜参数

依据全反射理论和棱镜耦合原理,实现了对棱镜折射率及波导薄膜材料折射率和厚度的同步测量。使用高准直半导体激光器激光入射到棱镜内部与波导膜的分界面上,逐步旋转棱镜或改变棱镜的入射角,得到棱镜耦合M线,曲线前面几组的波谷为波导模激发,在M线左侧收尾处有一个不完整波峰,其反射光强随入射角迅速衰减,为全反射时的临界点,由此可实现棱镜及波导薄膜参数的同步测量;用此法测量了棱镜耦合一体化平面波导棱镜的折射率和聚甲基丙烯酸甲酯(PMMA)聚合物波导薄膜的折射率和厚度。测量棱镜折射率精度为±1.9×10-4,波导薄膜折射率和厚度的精度分别为±6.2×10-4 μm和±1.6×10-2 μm。     

A Multilayer Waveguide Window for Wide-Bandwidth Millimeter Wave Tubes

A multilayer waveguide window is demonstrated to exhibit wide bandwidth and high transmission for applications in high-frequency microwave tubes. A transfer matrix approach is employed to discretize the dielectric function profile of the multilayer heterostructure in a rectangular waveguide. The closed form has been obtained and the corresponding reflection and transmission characteristics have been carried out. The analytical calculation is also compared with the result of numerical simulation via the finite-element code HFSS. The exact calculation agrees with the numerical simulation very well. By comparison, the approach not only enhances the accuracy and efficiency, but also gives a good criterion for the design. The results show that the bandwidth for a transmission of 99%, i.e., S ₁₁ below –20 dB, can be optimized to be about 8.75 GHz at a central frequency of 35 GHz or about 25%. A wide-bandwidth waveguide window can be easily designed for Ka-band and W-band tubes, and even for higher frequency ones.     

Optical uniform/gradient waveguide sensor structure - characterization

The paper presents the characterization of a composite uniform/gradient waveguide sensor structure. The results of theoretical analysis and experimental studies have been presented. The influence of sensor structure parameters on homogenous sensitivity and on surface sensitivity has been analyzed. Gradient layers for composite sensor structures were produced using the ion-exchange method, and the uniform layers, using the sol-gel method. In the experimental studies, involving the produced sensor structures, a prism coupler and a grating coupler were applied. Excellent agreement between the results of theoretical analysis and experimental studies has been achieved.     

Wave propagation in a waveguide with continuous right-angled corners: Numerical simulations and experiment measurements

This paper studied the acoustic wave propagation in a waveguide with continuous right-angled corners, with emphasis on the effect brought by the distance between the corners. The numerical analyses showed that at middle to high frequencies, the transmission loss (TL) of a multi-cornered waveguide was 2–5 dB higher than that of single-cornered and varied with frequency. To explain the performance at peaks and dips in the TL curve, analyses on eigenmodes and sound intensity distribution were conducted. The performance of multi-cornered waveguides was experimentally investigated, which fit well with the numerical results. The present study indicates that, for a waveguide with continuous corners, its acoustic performance is not simply a “summation” of two individual single-cornered ones. Both the standing wave modes and the evanescent modes between the corners lead to its complicated frequency performance.     

Embossable grating couplers for planar evanescent wave sensors

The paper presents the characterization of a composite uniform/gradient waveguide sensor structure. The results of theoretical analysis and experimental studies have been presented. The influence of sensor structure parameters on homogenous sensitivity and on surface sensitivity has been analyzed. Gradient layers for composite sensor structures were produced using the ion-exchange method, and the uniform layers, using the sol-gel method. In the experimental studies, involving the produced sensor structures, a prism coupler and a grating coupler were applied. Excellent agreement between the results of theoretical analysis and experimental studies has been achieved.     

Propagation properties of elastic waves in semi-infinite phononic crystals and related waveguides

The transmission and reflection coefficients of two-dimensional semi-infinite solid-solid phononic crystal systems and fluid-fluid phononic waveguide structures have been investigated. The numerical results show that the transmission spectra for longitudinally and transversally polarized incident waves are different, and the spectra of the transmission and reflection coefficients of the semi-infinite system agree well with the band structure. The numerical results show that when a guided wave incident, localized modes are excited, and different polarities have different coupling efficiencies with the incident guided wave. At the same time, far from the cutoff frequency, the guided wave couples out of semi-infinite waveguide highly efficiently.     

Effect of coherence property of incident light on prism waveguide coupling

The accurate reflection coefficient of prism waveguide coupler that takes coherence property of incident light into account is obtained at first. Based on the reflection coefficient, the effect of coherence property of incident light on prism coupling is discussed in this work. It is shown that extra damping in waveguide caused by the coherence property is larger for high-order resonant mode than that for low-order mode. For 100 fs Gaussian pulses incidence, the reflected spectra at different resonant modes are obtained theoretically and experimentally, which show that coherence property has a strong effect on prism coupling at high-order modes.