一种测量微波谐振腔品质因数的改进方法

提出了一种测量谐振腔Q值的改进方法,即标量反射系数平均法.该方法根据微波谐振腔在谐振点附近的|г(f)|曲线,采用扫频的方法得到其△|г|(dB)从0(dB)到|г0|(dB)对应的△f,进而推算得到其对应的Q值曲线.选取其中一段受误差影响较小的Q值曲线,采用拟合平均的方法得到其准确可靠的无载品质因数.此方法很好地解决了由于外电路的耦合给传统方法带来的测量误差的问题,并对自由选点测量Q值法在选点方面有一定的参考价值.     

嵌埋材料对分布式布拉格反射镜腔共振模式的影响

低维材料嵌入微腔已经广泛应用于纳米激光器和探测器等。为了实现增益材料和光学微腔之间的有效耦合,需要深入研究嵌埋材料对腔共振模式的影响。本文主要讨论了嵌埋材料的厚度、位置、腔层厚度以及分布式布拉格反射镜的对数对腔供着模式的影响。结果表明,腔共振模式随嵌埋材料位置的不同呈现周期性变化并且在λ/2光程周期内存在最大峰位移。最大峰位移随腔层厚度增加而减小,但与嵌埋材料的厚度成正比。分布式布拉格反射镜的对数不影响腔共振模式。这些结果为光学器件的设计和实验现象的分析提供了指导,并且可以应用于不同波长分布式布拉格反射腔结构。     

垂直外腔面发射半导体激光器的双波长调控研究

报道了一种结构紧凑的垂直外腔面发射激光器（Vertical-External-Cavity Surface-Emitting Laser,VECSEL）及其双波长调控。通过调控泵浦光功率,实现了VECSEL输出的两个激光波长之间的相互转换,双波长的间隔接近50 nm。VECSEL的输出功率曲线呈现明显的两次翻转,翻转点对应了激射波长的转换。这是由于泵浦功率变化改变了增益芯片内部的温度,进而通过热调谐使得发光区增益峰值被调谐到腔模的不同位置。在0℃时,每个激射波长的最大输出功率都在1.5 W以上。随着泵浦功率的改变,激射波长可以在950 nm和1000 nm之间切换,同时还可以在1.5 W以上的功率水平下实现双波长同时激射。这种可切换波长及双波长同时激射的VECSEL器件在光调制、差频等领域有较大应用潜力。     

基于闭式谐振腔的一体化介电常数测量系统

为替代矢量网络分析仪,形成一套专用于闭式谐振腔的测量系统,本文采用频谱分析仪模块和跟踪发生器模块,基于C++和VISA库函数进行系统控制,使两模块可以同步收发射频信号,实现闭式谐振腔谐振频率和Q值的测量功能,最终实现微波介质材料的介电常数的测量,形成一套一体化闭式谐振腔介电常数测量系统。该系统与矢量网络分析仪对比测量微波介质陶瓷材料K65,介电常数的相对误差为5.5×10^-3,tanδ的相对误差为-3.74×10^-2;对比测量材料K35,介电常数的相对误差为-1.69×10^-3,tanδ的相对误差为1.08×10^-1。测量结果相对误差较小,介电常数的相对误差小于0.01,tanδ的相对误差小于0.5,说明一体化介电常数测量系统的测量结果准确,可用于闭式谐振腔方法下的介电常数测量,也可推广用于其他介电常数测量系统。     

Ku波段宽带高增益基片集成腔圆极化阵列天线北大核心CSCD

提出了一款应用于Ku波段的宽带高增益基片集成腔(Substrate Integrated Cavity,SIC)圆极化阵列天线。通过引入沿SIC口径面对角线放置的一对半月形寄生贴片和SIC底部馈电纵缝,使SIC中的TM_(211)和TM_(121)谐振模式幅值相等、相位相差90°,产生高增益圆极化辐射。同时,双寄生贴片还引入了一种背腔缝隙耦合振子圆极化辐射模式,扩宽了天线高增益圆极化辐射带宽。在此基础上,设计了一款2×2单元顺序旋转馈电的SIC圆极化阵列天线。阵列天线采用双层基片集成波导顺序相移馈电网络进行馈电,进一步增大了天线的圆极化带宽。综合考虑天线的-10 dB反射系数带宽、3 dB轴比带宽和3 dB增益带宽,测试结果表明,圆极化阵列天线的有效带宽为10.74-13.30 GHz(21.3%),在通带范围内最大增益为14.50 dBi。     

Design of all-optical tunable filter based on two-dimensional photonic crystals for WDM (wave division multiplexing) applications

In this paper, all-optical tunable filters based on two-dimensional photonic crystals with very small dimensions for optical telecommunication of WDM technology are designed and simulated. The structure is made of air holes in a dielectric background. The tuning is done by changing resonant defect angle. The channels obtained for this structure will be set in wavelength range of 1550 nm. Created channels are at wavelengths of 1550, 1551, 1552, and 1565 (16 channels); the distance between adjacent channels is 1 nm. Design and simulation of this filter is done by RSOFT software. Quality factor, transmission efficiency, and band gap shows that filter performance is very good.     

数值模拟技术在气体辅助注射成型过程中的应用

基于粘性流体力学基本方程,针对气体辅助注射成型的特点,提出合理的假设并进行必要的简化,建立描述气体辅助注射成型充模流动过程的数学模型,进而采用有限元／有限差分混合算法进行数值求解。在对移动边界的处理采用控制体积法对充模过程中的两类移动边界：熔体前沿、熔体—气体边界进行跟踪,从而实现气体辅助注射成型充模过程的数值模拟。     

Waveguide filter with improved stopband response using transversal slots

In this article, the design and realization of waveguide filter with improved stop‐band response is presented. By inserting transversal slots at the proper position of cavities, without disturbing the field distribution of the dominant mode, the second longitudinal resonant modes are radiated out. This caused the out‐of‐band frequency response of the filter to be improved. The validity of the proposed technique is confirmed by comparison between measurement values and simulation results from the finite element method through the commercial software (high frequency structure simulator). Also, to assure electromagnetic compatibility the slot surfaces are covered by absorbing material. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.     

A class of high-resolution algorithms for incompressible flows

We present a class of a high-resolution Godunov-type algorithms for solving flow problems governed by the incompressible Navier-Stokes equations. The algorithms use high-resolution finite volume methods developed in LeVeque (SIAM J Numer Anal 1996;33:627-665) for the advective terms and finite difference methods for the diffusion and the Poisson pressure equation. The high-resolution algorithm advects the cell-centered velocities using the divergence-free cell-edge velocities. The resulting cell-centered velocity is then updated by the solution of the Poisson equation. The algorithms are proven to be robust for constant-density flows at high Reynolds numbers via an example of lid-driven cavity flow. With a slight modification for the projection operator in the constant-density solvers, the algorithms also solve incompressible flows with finite-amplitude density variation. The strength of such algorithms is illustrated through problems like Rayleigh-Taylor instability and the Boussinesq equations for Rayleigh-Bénard convection. Numerical studies of the convergence and order of accuracy for the velocity field are provided. While simulations for two-dimensional regular-geometry problems are presented in this study, in principle, extension of the algorithms to three dimensions with complex geometry is feasible.     

A numerical algorithm coupling a bifurcating indicator and a direct method for the computation of Hopf bifurcation points in fluid mechanics

This paper deals with the computation of Hopf bifurcation points in fluid mechanics. This computation is done by coupling a bifurcation indicator proposed recently (Cadou et al., 2006) [1] and a direct method (Jackson, 1987; Jepson, 1981) [2] and [3] which consists in solving an augmented system whose solutions are Hopf bifurcation points. The bifurcation indicator gives initial critical values (Reynolds number, Strouhal frequency) for the direct method iterations. Some classical numerical examples from fluid mechanics, in two dimensions, are studied to demonstrate the efficiency and the reliability of such an algorithm.