相速和群速

目前,大多数工科物理教学中难以对相速和群速两概念详细论述,但对部分专业却必须明确地将它们区分开来。例如,在研习共用天线电视工程的内容时就必须严格区分相速和群速的概念。本文将按这类专业的需要,阐述电磁波中相速和群速的基本物理概念。     

相速和群速的演示实验

本文介绍了演示相速和群速的演示实验装置和方法。     

《导行电磁波的色散、相速和群速》

本文以在波导管中传播的导行电磁波为例，阐明了导行电磁波色散的概念，得出了电磁波的相速ｖｐ和波长λ的关系．并且指出了波导管作为一种实际的微波传输，当线传输占有一定频带宽度的微波信号时，必须引入群速的概念．以一个调幅信号在波导管中传播的情况为例，导出了群速的计算公式以及群速和相速的普遍关系．最后指出了群速概念的局限性和在微波波段的实用性．     

电子波的群速与相速

电子波的群速与相速董庆祥（北京电子科技学院，北京１０００３９）（收稿日期：１９９６—１０—３１）我们首先回顾一下群速与相速这两个概念．在经典波动中，振幅均为Ａ，频率为ω１、ω２的两列简谐波，ｙ１＝Ａｃｏｓ（ω１ｔ－ｋ１ｘ）；ｙ２＝Ａｃｏｓ（ω２ｔ－ｋ...     

电磁波的群速与能量传播速度

众所周知,在某些场合波的相速可能超过真空中光速C.典型的例子有波导管。其中电磁波的色散关系为式中为截止频率。由式(1)可得按相对论,任何物体或信号的速度不能超过C,否则将导致因果律的破坏.每当教学中遇到的场合,往往要作如下解说:相速不代表信号速度,群速才是波能或信号的传播速度.按式(1)有从而不与因果律冲突。 在色散介质中光的群速与相速之间有如下关系式中 为折射率。在介质的正常色散区;在反常色散区,且数值往往很大,这时不仅它还可能大于。群速超过真空中光速C!这又当如何解释?原来群速的概念只适用于吸收很少的透明波段,此时…     

多模光纤模式场群速与相速的关系

为了从理论上统一射线光学和导波光学关于多模光纤中各模式场传播过程中时延的论述,试从介质波导理论出发,根据模式的特征根U随光纤归一化频率R的变化关系,在弱波导近似下导出了描述模式场相速Vp与群速Vg关系的重要结果:Vp*Vg=(c/n1)2。根据这一结果可以得出结论:对于低阶模式,由于相速Vp慢,所以群速快;高阶模式其相速Vp快,所以群速慢。对于给定的光纤,利用射线光学和波导光学进行了带宽估计,2种理论计算结果的一致性,印证了上述关系的正确性。     

电磁波速度可以大于光速c吗?

我们都知道狭义相对论有任何物体的运动速度小于c的结论,(本文中把真空中的光速简写为c),根据因果关系不能颠倒的要求,狭义相对论又有任何信号传输速度不大于c的结论。我们还知道电磁波在真空中速度为c,在介质中速度为c/n,而n>1,电磁波可以传输信号,能否得出结论:电磁波速度不能大于c呢?本文将通过对波速的讨论,论证在两种情况下,电磁波的相速可以大于c,但是电磁波传输信号的速度仍不大于c。 一、相速和群速 许多光学书上都有相速和群速的讨论,因此,我们只简单地叙述与本文有关的内容。 园频率为。,波长为A;波数为七一2。/几,振幅为人,沿X…     

慢光速研究的理论与实现

大学物理教学中有相速和群速的介绍, 但往往止步于对它们的定义和表达式推导. 如果此时适当地拓 展它们的应用, 会起到事半功倍的作用. 本文介绍慢光速研究的理论和实现途径, 目的: 一为教学提供有价值的案 例, 二为科普     

BBOⅠ类相位匹配非共线参量产生中的群速自匹配

利用重复频率为1kHz的钛宝石激光的倍频蓝光抽运Ⅰ类相位匹配的偏硼酸钡(BBO)晶体，获得了可见光区的非共线参量超荧光.给出了最强光强处参量超荧光的非共线角，波长随相位匹配角的变化.理论计算了在参量超荧光中三波的群速失配随非共线角和相位匹配角的变化规律.结果表明，Ⅰ类非共线相位匹配的参量超荧光关于抽运光中心对称.并且沿光强最强的参量超荧光方向三波的群速失配最小，有效互作用长度最大，即参量超荧光的强度角分布是三波的相速和群速自匹配的结果.该理论和实验研究为减小飞秒光参量放大中三波的群速失配提供了一个理想的几何模型，为获得高增益、窄脉冲宽度的参量光输出提供了理论依据和实验验证. 关键词： 群速自匹配 参量超荧光 非共线相位匹配 飞秒光参量放大     

小于1的折射率

介质折射率小于１的事实导致这样的疑问：介质中光速会比真空中光速ｃ还大吗？事实上，光在介质中的传播速度可分为三种：相速ｖ、群速ｖｇ、信号速度ｖσ．ｖ和ｖｇ可以等于Ｃ，也可以小于Ｃ或大于Ｃ．ｖσ恒小于ｃ，其极限值为ｃ．     

左手介质中电磁波的传播速度

在同向介质中,电磁波的群速度一般是能量传播的方向,并且电磁波的相速度和群速度在大部分情况下都满足瑞利关系.为了研究左手介质的电磁特性,本文从理论上导出了左手介质中电磁波的相速度、群速度及其关系,得出左手介质中电磁波的群速度为负,并介绍了左手介质的一些应用前景.     

On the orthogonality of the phase velocity and its feasibility for plane waves

Phase and group velocities of plane waves may have different directions in a linear and homogeneous medium. This difference may lead to interesting situations such as the phase reversal propagation and the orthogonal phase velocity. Although the former has been analyzed in depth and it has found many applications, the latter has been left as a hypothesis. Actually, due to our best knowledge there is no evidence of such media where the phase velocity is orthogonal to the group velocity. In this paper we discuss this situation thoroughly. We show that a plane wave with orthogonal phase velocity cannot possess linear momentum. Moreover, we show that a plane wave without the linear momentum cannot propagate at all, so the orthogonality of the phase velocity does not take place in reality for electromagnetic plane waves.     

Phase and group velocity considerations for dynamic modulus measurement in anisotropic media

In anisotropic media, the direction of energy propagation does not necessarily coincide with the wave normal, i.e. the energy flux vector does not coincide with the wave normal. Since, experimentally, one measures group velocity not phase velocity, one must therefore be careful in interpreting ultrasonic wave speed measurements in anisotropic media. This is of particular importance in elastic property reconstruction where acoustic velocity measurements are used as the basis for determining anisotropic material properties. In this work, the consequences of energy flux deviation from the wave normal are considered for typical experimental geometries. Particular attention is devoted to developing appropriate relationships between the phase velocity and ultrasonic transit time measurements, as these relations are most useful for elastic property reconstruction. In all the cases considered, it is shown that the phase velocity can be directly calculated from appropriate time delay measurements.     

水平线列阵方位—相速度联合的卡尔曼滤波方法*

传统的水下被动测向方法通过波束形成估计目标角度,水平线列阵波束形成中的参考声速应使用声传播的相速度,在被动测向中,由于声源距离未知,因此在对目标角度估计时选取的参考声速与接收阵处的相速度往往存在偏差,从而影响测向精度。本文提出了一种水平线列阵方位-相速度联合的纯方位扩展卡尔曼滤波方法,该方法引入相速度作为估计状态量以此校准参考声速,提高测向精度,进而改善了由于测向误差较大引起的纯方位扩展卡尔曼滤波算法跟踪结果发散的问题。浅海传播条件下的数值仿真结果表明,改进方法较常规纯方位扩展卡尔曼滤波算法具有更高的跟踪精度及稳健的跟踪性能。     

飞秒光参量放大中三波群速失配的补偿

为了消除群速失配对参量放大的不利影响，描述了利用脉冲波面倾斜与非共线相位匹配相结 合，完全补偿飞秒光参量放大（OPA）中三波群速失配的新方法.计算了在BBOⅠ类、Ⅱ类相 位匹配条件下， 三波实现群速匹配时，相位匹配角、脉冲波面倾斜角以及非共线角随信号 光波长的变化.并分析了三波群速匹配对空间走离长度、参量增益和参量带宽的影响.结果表 明，在BBOⅠ类、Ⅱ类相位匹配条件下，利用该方法均能实现飞秒OPA连续调谐时三波的群速 匹配，从而大大增加了三波的有效互作用长度，为能够获得高增益，窄脉宽的参量光脉冲提 供了理论依据和指导. 关键词： 群速匹配 脉冲波面倾斜 非共线相位匹配 飞秒光参量放大     

光波位相耦合色散效应与固态介质中室温下的光速调控

文章介绍了作者利用光波位相耦合的色散效应调控光速的新机制，并利用光折变位相耦合的色散效应在室温下在Bi12SO20晶体中实现极慢光速和超光速现象．进一步的研究表明，在Bi12SO20晶体中传播的、由位相耦合色散效应引起的慢光和快光由于光折变光强耦合效应而得到了相干放大．     

Band structure,group velocity,effective group index and effective phase index of one dimensional plasma photonic crystal

In this present communication, we study the reflection behavior of electromagnetic in one-dimensional photonic crystal of electromagnetic waves in microwave region of electromagnetic spectrum. The Plasma Photonic Crystal structure is composed of alternate layers of thin micro-plasma and dielectric material electromagnetic (EM) spectrum in one-dimensional Plasma Photonic Crystals (PPCs). In order to obtain the characteristic equation (dispersion relation), we have solved the Maxwell scalar wave equation. Beside this we study the anomalous behavior of group velocity and effective index of refraction of PPCs in microwave region. We have adopted Transfer Matrix Method (TMM) for calculating reflectance and dispersion relation of PPCs. In order to calculate the expression for effective refractive index inside the PPCs structure we have used the concept of the group velocity instead of phase velocity.     

Group velocity of the light pulse in an open V-type system

We investigate the group velocity of the probe light pulse in an open V-type system with spontaneously generated coherence. We find that, not only varying the relative phase between the probe and driving pulses can but varying the atomic exit rate or incoherent pumping rate also can manipulate dramatically the group velocity, even make the pulse propagation switching from subluminal to superluminal; the subliminal propagation can be companied with gain or absorption, but the superluminal propagation is always companied with absorption.     

高透射率慢光速的光子晶体耦合腔波导的研究

将二维三角晶格光子晶体波导和微腔结构结合,优化设计了一种二维三角晶格光子晶体共振耦合腔波导,运用时域有限差分法(FDTD)模拟共振耦合腔波导TE偏振光的透射谱,通过透射谱得到传输光的透射率和群速度。结果表明,合适参数的二维三角晶格共振耦合腔波导在波长1.551μm处的群速度为c/130、透射率为20.1%,在波长1.502μm处的群速度为c/50、透射率为29.2%。运用平面波展开法(PWE)计算的该波导的能带结构对慢光特性进行了分析。这种慢光特性的光子晶体波导将在光存储、光延迟及光子集成等方面有潜在的应用价值。     

Correlations of group velocity, phase velocity, and dispersion with bone density in bovine trabecular bone

The present study investigated the correlations of the group velocity, the phase velocity, and the velocity dispersion with the apparent bone density in bovine trabecular bone in vitro. The phase velocity exhibited the negative dispersion, consistent with the behavior in human trabecular bone. The group and the phase velocities were found to increase with increasing apparent bone density, respectively, exhibiting similar high correlations of r=0.94 and 0.96. The negative dispersion rate exhibited a decreasing dependence on the apparent bone density, with a significant correlation of r=-0.86.