Mode energy transformation between two connected multimode general square-law-index optical waveguides

A coherent states method is used to investigate the mode coupling coefficients in the most general buttjoint problem for two multimode general square-law-index optical waveguides, when two different waveguides with elliptical cross-sections are to be butt-jointed with axis displacement, tilt and some turning of symmetry axes with respect to each other. The explicit expressions for the mode coupling coefficients and recurrence relations for the mode overlap integrals are obtained; the sum rules for the mode coupling coefficients are formulated and a method of experimental data analysis is proposed for this general problem. The method of calculating mode coupling coefficients is proposed for the case of the butt-joint problem when the waveguide cross-sections are only partially overlapping and then it is used to investigate the influence of misalignment on mode power distribution and pulse dispersion in the second waveguide. The experimental data are compared and it is shown that the coupling coefficients obtained in square-law-index model can be used in dispersion calculations forα-profile fibres in the case when the valueα is not far from the valueα=2.     

Validity of the Franck-Condon principle in the optical spectroscopy: optical conductivity of the Fröhlich polaron

The optical absorption of the Fr?hlich polaron model is obtained by an approximation-free diagrammatic Monte Carlo method and compared with two new approximate approaches that treat lattice relaxation effects in different ways. We show that: (i) a strong coupling expansion, based on the Franck-Condon principle, well describes the optical conductivity for large coupling strengths (alpha > 10); (ii) a memory function formalism with phonon broadened levels reproduces the optical response for weak coupling strengths (alpha < 6) taking the dynamic lattice relaxation into account. In the coupling regime 6 < alpha < 10, the optical conductivity is a rapidly changing superposition of both Franck-Condon and dynamic contributions.     

半径渐变同轴波导模式耦合系数研究

 为了用耦合波理论研究一些同轴结构的高频特性，推导了半径渐变同轴波导内模式耦合系数的计算公式，发现半径渐变圆波导模式耦合系数只是同轴波导模式耦合系数的特例。对同轴波导中的模式及其本征方程根的特点进行了讨论，通过建立同轴波导模式和矩形波导模式之间的对应关系，给出了同轴波导模式本征方程根的快速计算方法。对所给模式耦合系数公式进行了验证，耦合波理论计算结果与有限元方法计算结果吻合良好。     

The roles of vibronic coupling and the Duschinsky effect in resonance Raman scattering

In most vibronic treatments of resonance Raman scattering it is assumed that the normal coordinates of electronically excited states are not rotated relative to those of the ground state, i.e., the Duschinsky effect is neglected. In the present paper this assumption is critically examined for the case of resonance Raman scattering by a non-totally symmetric vibrational mode. For consistency also the second-order Herzberg-Teller coupling is considered. It is shown that these second-order couplings introduce asymmetry in the excitation profile of appearance similar to that due to nonadiabatic coupling. Also it is shown that even small rotations give rise to noticeable effects in the polarization dispersion curve. Analytical formulas for the Franck-Condon factors and for the Raman tensor (valid for an arbitrary number of normal modes) for the case of small rotations are given, and illustrative calculations of excitation and polarization dispersion curves for a two mode system are presented and discussed.     

非平行波导耦合理论研究

利用耦合模理论对非平行波导耦合特性进行了研究，得到了非平行波导的普遍性耦合方程及耦合系数表达式.进一步采用小角度近似，给出了几种情况下的计算结果，并与目前报道的利用其他方法计算的结果做了比较.研究表明，不同方法得到的波导中光功率虽然在远距离处都趋于稳定不变，但稳定功率的大小有明显的差别；另外由新提出方法得到的近距离处光功率随着传输距离的起伏变化更为明显.由于该方法同时考虑了非平行波导中光场相位和振幅对耦合方程和耦合系数的影响，因而所得结果能更全面地反映非平行波导的耦合特性，对实际应用更有意义. 关键词： 非平行波导 耦合方程 耦合系数     

具有同轴结构的高功率微波弯曲波导设计

 基于模式耦合理论,在理论推导出弯曲同轴波导TEM模和同轴TE11模之间耦合系数显式表达的基础上,报道了可传输同轴TE11模的弯曲同轴波导的设计方法和计算结果,并进行了实例研究。数值仿真结果表明：设计的具有同轴结构的弯曲波导,利用不同的同轴空间,在P,L,S波段中心频率0.680,1.575和3.75 GHz处TE11模单模传输效率超过了99.5%,单模传输效率超过90%的工作带宽分别为0.60～0.83,1.10～2.42和3.10～4.16 GHz。该结构的功率容量在各频段均达到了GW量级。     

应用耦合模理论研究光在圆对称双包层光纤中的传输

为了深入理解双包层光纤的光场传输特性,应用耦合模理论将双包层光纤等效为相互耦合的单模光纤和环形芯光纤,研究了圆对称的双包层光纤的光场传输.通过计算单模光纤LP01模和环形芯光纤导模的耦合系数发现了环形芯光纤LP0n模的耦合系数远大于其它导模的耦合系数,且LP0n模中的高阶模比低阶模的耦合系数大.据此,应用耦合模理论计算得到了该双包层光纤的光传输特性,计算结果发现光场沿光纤轴向呈近似周期分布,且纤芯中光功率变化的平均周期随波长递增,但平均归一化功率与光纤参数紧的选择有关.     

应用耦合模理论研究光在圆对称双包层光纤中的传输(英文)

为了深入理解双包层光纤的光场传输特性,应用耦合模理论将双包层光纤等效为相互耦合的单模光纤和环形芯光纤,研究了圆对称的双包层光纤的光场传输.通过计算单模光纤LP01模和环形芯光纤导模的耦合系数发现了环形芯光纤LP0n模的耦合系数远大于其它导模的耦合系数,且LP0n模中的高阶模比低阶模的耦合系数大.据此,应用耦合模理论计算得到了该双包层光纤的光传输特性,计算结果发现光场沿光纤轴向呈近似周期分布,且纤芯中光功率变化的平均周期随波长递增,但平均归一化功率与光纤参数紧的选择有关.     

光声池径向共振模式耦合系数理论分析

光声光谱法是基于红外吸收光谱原理的一种高灵敏度的微量气体探测技术。它使用声共振腔来实现微弱声信号的共振放大。通过调节激光的调制频率,当它等于腔的某个共振频率时,在腔内形成声驻波,而腔本身的作用相当于一声放大器。共振腔的放大作用取决于当前被激活的共振模式、腔的品质因素、声传感器的状态以及电磁辐射与腔共振模式的耦合作用。值得关注的是,红外激光相对于声共振腔的入射方位不同则激励产生的光声信号幅值也不同。采用理论推导与数值计算相结合的方法,以圆柱形光声池为例,研究了径向共振模式下耦合系数受激光入射方位的影响。研究表明,激光入射角在0～π/2范围变化时耦合系数存在2个零点和2个极大值：入射角为0或tan-1(0.859 2×2R/L)时,耦合系数为零而径向共振失效;入射角为tan-1(0.556 8×2R/L)或tan-1(2R/L)时,耦合系数极大而径向共振最强。此处R为池径而L为池长。结果可用于指导光声池结构优化设计与安装调试,增强光声法检测微量气体的信号幅值,提高检测灵敏度。     

Nonlinear Resonant Interaction of Two Longitudinal Waves and a Transverse Wave in a Hot Magnetized Plasma

By Whitham's method of averaged Lagrangian and using Low's form of Lagrangian, coupled mode equations and coupling coefficients are derived for resonant nonlinear interaction of two longitudinal and one transverse wave in a magnetized plasma, in which the later wave propagates along the external uniform magnetic field. The limiting form of these coupling coefficients are obtained when the external magnetic field vanishes.     

Super-sensitivity in Dynamics of Ising Model with Transverse Field: From Perspective of Franck-Condon Principle

We study the role of Franck-Condon (F-C) principle in the dynamics of a central spin system, which is coupled to an Ising chain in transverse field. The transition process of energy levels caused by the excited central spin is studied to manifest the quantum critical effect through the Franck-Condon principle. The super-sensitivity of this quantum critical system is demonstrated clearly from the properties of Franck-Condon factors. We analytically show how spin numbers, coupling strength and order parameter of the Ising chain sensitively effect on the energy level populations in dynamical evolution near the critical point. This super-sensitivity and criticality are explicitly displayed in absorption spectrum.     

Nonlinear Resonant Interaction of Three Extra-ordinary Waves in a Hot Electron Plasma

The coupled mode equations and coupling coefficients for the resonant nonlinear interaction of three extra-ordinary waves in a hot magnetized plasma are derived by Whitham's method of averaged Lagrangian. The wave vectors of the three waves are taken as noncollinear and perpendicular to the external uniform magnetic field. Of the three waves one is of high frequency mode and the other two are of low frequency mode.     

Franck-Condon blockade and giant Fano factors in transport through single molecules

We show that Franck-Condon physics leads to a significant current suppression at low bias voltages (termed Franck-Condon blockade) in transport through single molecules with strong coupling between electronic and vibrational degrees of freedom. Transport in this regime is characterized by remarkably large Fano factors (10(2)-10(3) for realistic parameters), which arise due to avalanchelike transport of electrons. Avalanches occur in a self-similar manner over a wide range of time scales, leading to power-law dependences of the current noise on frequency and vibrational relaxation rate.     

结合n模式表示势的含时密度矩阵重正化群求解激发态非辐射衰减速率：理论形式和在薁分子中的应用

本文提出一种来计算多原子分子包含Franck-Condon区势能面非谐效应的无辐射衰减速率的方法. 这种方法结合了n模式表示法,通过构造第一性势能面和近乎精确的含时密度矩阵重正化群法(TD-DMRG),来模拟量子动力学过程. 另外,在TD-DMRG 的框架下,进一步发展了计算末态分辨的速率常数的算法,它对于分析每个振动模式对于跃迁过程的贡献十分有用. 本文采用这种方法研究了考虑基态势能面非谐性后的薁分子的内转换过程. 结果表明,即使对这个半刚性的分子,模式内的非谐性也能够显著的提高内转换速率;在考虑双模式的耦合之后,速率会进一步增加. 其原因是由于C-H振动的黄昆因子接近于0,其在简谐势能面上无法对内转换做出贡献,而非谐性打开了C-H振动接受电子能量的能力.     

单模双包层光纤之间的耦合

本文求得了单模双包层光纤之间耦合系数精确的解析表达式.计算了上升内包层、匹配包层和凹陷内包层光纤耦合系数随归一化频率V的关系曲线.也给出了不同V值的耦合系数随归一化距离(D/α)的关系曲线.该公式不但能够计算x偏振模的耦合系数,而且也能计算y偏振模的耦合系数.它可用于分析折射率差较大的光纤之间能量耦合以及耦合器的偏振特性.     

Adiabatic three-waveguide directional coupler

We propose a three-waveguide directional coupler based on adiabatic evolution of a single normal mode of the system and analyze its switching properties using coupled mode theory. In our scheme the propagation coefficients of the waveguides are constant and the coupling coefficients are variable. This device works in the case that the propagation coefficients of the two outer waveguides are identical and that the coupling coefficients are designed to act in a counterintuitive order.     

Angular spectrum theory to calculate coupling efficiency in rectangular waveguide resonators

Coupling efficiency of the rectangular waveguide resonators are discussed in terms of the method of angular spectrum theory. Under the condition given in the paper, the coupling coefficients for the EH₁₁ mode in the rectangular waveguide resonator are presented as a function of mirror curvature and position. It is shown that there exist two special geometries to provide low coupling efficiency. The method can be applied to the other modes.     

A theoretical analysis of mode-mixing in optical waveguides with nearest-neighbour mode coupling

Properties of the solutions of the coupled equations describing the flow of average mode power in optical waveguides are investigated. All of the important properties and their theoretical foundations are first reviewed for systems with arbitrary coupling and loss coefficients. It is then shown that, for the special class of systems with nearest-neighbour coupling, the solutions are expressible in terms of orthogonal polynomials. In particular, for systems obeying a quasi-uniform loss model and having coupling coefficients with simple dependence on mode number (uniform or linear), the solutions are associated with the classical polynomials. As illustrations of this analysis, the characteristics of optical power flow in three such systems are studied in detail. These include a slab-waveguide with uniform coupling, the corresponding cylindrical waveguide with uniform coupling, and a parabolic-index fibre with linear coupling dependence.     

Effects of poroelastic coefficients on normal vibration modes in vocal-fold tissues

The vocal-fold tissue is treated as a transversally isotropic fluid-saturated porous material. Effects of poroelastic coefficients on eigenfrequencies and eigenmodes of the vocal-fold vibration are investigated using the Ritz method. The study demonstrates that the often-used elastic model is only a particular case of the poroelastic model with an infinite fluid-solid mass coupling parameter. The elastic model may be considered appropriate for the vocal-fold tissue when the absolute value of the fluid-solid mass coupling parameter is larger than 10(5) kg/m(3). Otherwise, the poroelastic model may be more accurate. The degree of compressibility of the vocal tissue can also been described by the poroelastic coefficients. Finally, it is revealed that the liquid and solid components in a poroelastic model could have different modal shapes when the coupling between them is weak. The mode decoupling could cause desynchronization and irregular vibration of the folds.     

Characteristics of the transmission spectrum of the long period fiber gratings based on the coupling of core mode to the higher order cladding modes

The characteristics of the transmission spectrum of the Long-period fiber gratings (LPFGs) based on the coupling of core mode to a higher order cladding mode (HE mode) are investigated using the coupled mode theory. This kind of LPFGs is different from that based on the coupling of core mode to a lower order cladding mode because of the effect of the coupling of core mode to EH cladding mode. When the cladding mode order is higher, the coupling coefficients of core mode to HE and EH cladding modes are comparable and both of the propagation constants of HE and EH cladding modes approach, so the spectrum has an additional loss peak. The bandwidth of LPFG based on the coupling of core mode to different cladding mode differs greatly. With the change of the mode orders from lower to higher, the transmission spectrum changes from narrow to wide and more narrow.