Angular Spectrum of Modified Cylindrical Wave-Functions

The plane-wave expansion of the modified cylindrical functions, i.e., the product of the mth-order modified Bessel function K _m and the angular factor exp(im?), is given. This expansion, which is found to involve only evanescent plane waves, may be used in the study of coupling phenomena between circular and planar guiding structures.     

Theoretical research on cascaded terahertz difference-frequency generation based on sphalerite crystals

A theoretical model of cascaded terahertz(THz) difference-frequency generation is established based on one-dimensional coupled-wave equations.The relationships between sphalerite crystals’ wave vector mismatches and difference-frequency pump waves are analyzed.To produce terahertz wave with the frequency of 1.5 THz,80-order cascaded difference-frequency is applied.By introducing crystal absorption,we calculate the optimum crystal length and pump frequency under actual circumstances.It is found that Stokes waves dominate the terahertz waves output in cascaded progress,and cascaded difference-frequency can increase the photon conversion efficiency obviously.     

光折变局域体全息柱透镜变波长读出下的波前转换

研究了用波长为632.8nm的柱面波与平面波记录形成的体全息柱透镜用波长为800nm的平面波重现下的波前转换情况。结合二维耦合波理论,推导了形成于LiNbO3双掺杂晶体中该全息柱透镜的耦合波方程,并求出了其积分形式的解析解,分析了改变读出波长下该透镜的布拉格失配因子的分布情况,讨论了它的几何尺寸和记录柱面光波对其衍射效率的影响,以及柱面衍射光振幅在出射口径上的分布情况。结果表明,记录所形成的全息柱透镜的焦距越长它的衍射效率越高;该透镜厚度的增加会使它的衍射效率增加,但是透镜的衍射口径增加它的衍射效率会降低;衍射光振幅在其出射口径上的分布出现了一定程度的失真,衍射光强度主要集中分布在布拉格失配值较小处对应的位置。最后,进一步讨论了全息透镜记录过程中满足布拉格匹配条件的参考点的选择对光栅衍射性质的影响,结果表明衍射效率不随参考点的改变而改变。     

Radiation from cylindrical leaky waves

Formulas are derived for the far-infrared radiation pattern of cylindrical leaky waves propagating on a planar surface. The formulas can be used to predict the radiation pattern of a general class of leaky-wave antennas, consisting of a finite-size source which excites a radially propagating leaky wave on some planar surface. Leaky-wave antennas consisting of antenna elements embedded in dielectric layers (microstrip elements) fall into this category. Using the equivalence principle, formulas are derived for both transverse electric (TE) and transverse magnetic (TM) leaky waves with arbitrary propagation constants. The formulas allow for radiation from cylindrical apertures of arbitrary size, so that the effect of truncating the supporting planar surface with an absorbing material can be determined. Particular attention is devoted to the case of a leaky wave for which the real and imaginary parts of the complex propagation constant are equal, since this type of wave has been shown to be responsible for broadside radiation in certain leaky-wave antennas comprised of dielectric layers     

螺旋波纹波导中的返波

从螺旋波纹波导的一般性耦合波传输方程出发,根据螺旋波纹波导的模式耦合规则给出该波导内返波的耦合波方程和色散方程并分析其色散特性,由此分析螺旋波纹波导返波管的工作机理,并通过耦合波理论计算出本征模式中场的分布情况.     

Analysis of the stimulated Raman effects in an anisotropic crystal KDP

The semiclassical model of Shen and Bloembergen is applied to discuss the stimulated Raman effects in an anisotropic medium. In crystal KDP when the linear electrooptic effects are suppressed by proper orientation of the laser polarization with respect to the crystal axes, the coupled-wave analysis for Stokes generation give rise to solutions of ten composite Stokes-vibrational waves in the (anisotropic) Raman active medium. These composite waves may be identified as two ordinary forward-traveling waves, two ordinary backward-traveling waves, three extraordinary forward-traveling waves, and three extraordinary backward-traveling waves. When the phonons are nonpropagating, the solutions reduce to only one ordinary and one extraordinary wave in each direction. The thresholds and Stokes generations in this situation are discussed. Owing to the anisotropy of the crystal, it is found that the dispersion of KDP in general allows the momentum-matching condition for the coupling between Stokes and anti-Stokes waves to be nearly satisfied in five different situations.     

Coupled-wave equations for propagation in stratified compressible magnetoplasmas

This letter deals with the propagation of coupled electromagnetic and electroacoustic waves in a magnetised compressible continuously stratified electron plasma. Some first-order coupled-wave equations are derived.     

An efficient split-step time-domain dynamic modeling of DFB/DBRlaser diodes

A novel and efficient approach for the numerical solution of time-dependent coupled-wave equations, which are frequently used for the modeling of distributed-feedback, distributed Bragg reflector, and Fabry-Perot laser diodes, is proposed. In this approach, the coupled-wave equations are split into two sets of equations. One of two sets of equations contains only the phase factors and time derivatives, and the other contains only the coupling terms. The separate sets of equations are solved exactly in their split form successively. This new numerical scheme, which we call the split-step time-domain model, is found to require an order of magnitude smaller number of subsections to get more accurate results than previous methods while the computation time for each time step is comparable to previous methods     

3-Dimensionally Integrated Planar Optics for 100 Gb/s Optical Packet Address Detection

We propose a novel planar optical interconnection scheme for 100 Gb/s optical packet address detection, which consists of waveguide grating couplers and a diffractive microlens integrated on a glass substrate 3-dimensionally. Length and duty cycle of the grating couplers have been determined on the bases of the ray-optic propagation-mode analysis in a slab waveguide and of the rigorous coupled-wave diffraction analysis for out-coupled radiation-modes. The 3-dimensionally integrated planar optics makes it possible to connect each address bit-signals of TE₀-waveguide mode to the detector with a power uniformity of 6.4 % and a total coupling efficiency of 72.3 %.     

Artificial Microwave Volume Holograms Based on Printed Disk Media: Theory and Measurement of Single-Grating Holograms

A new type of complex electromagnetic structure, the artificial microwave volume hologram (AMVH), has been developed. The structure consists of cascaded planar lattices of metallic circular patches with varying size and can be designed to have an effective dielectric modulation that follows a holographic interference pattern. Under the illumination of certain electromagnetic waves, an AMVH can reproduce a required wave field pattern based on its design, just like an optical volume hologram. A theoretical model, namely the self-consistent dynamic-dipole interaction theory, has been presented to characterize AMVHs for wave scattering and beam reconstruction. It can be used for designing and optimizing AMVHs. Comparisons with the coupled-wave theory (CWT) and a full-wave finite element method analysis have been made to validate the proposed theory. A prototype AMVH has been fabricated and measured, which has confirmed the AMVH concept as well as the theoretical model     

Numerical simulation of a pulsed laser pumped distributed-feedback waveguided dye laser by coupled-wave theory

A dynamic model of pulsed laser pumped distributed-feedback (DFB) waveguided dye laser based on a coupled-wave theory is described. Due to the periodical distribution of the intensities of pump source and stimulated emission along the waveguide, the rate equations of the population densities are turned into the equations of the Fourier coefficients. Coupled-wave equations of optical fields are used to simulate the laser oscillation. Besides the temporal evolution of the output intensity, the spectra can also be obtained by the Fourier transform of the optical fields. Two different configurations of the waveguided dye laser, prefabricated DFB (mainly index coupling), first- and second-order holographic DFB (dynamic gain-coupling), are considered in the model. The simulation shows that: 1) the temporal waveforms of the holographic DFB consist of sharp spikes; 2) the broadened spectral widths resulted from the possible nonuniformities in propagation constant or grating period are less than 50 pm except for the second-order holographic DFB; and 3) strong parasitic oscillations can be observed in the second-order holographic DFB with terminal reflection. These results and the comparisons of some of them to the experiments are reported.     

波在暖电离层中的传播和耦合

本文研究了波在暖电离层中的传播。文中导出了用拆射率n表示的波的色散关系,并由此推导出分层暖电离层中q的方程。所有这些方程都被用于用射线方法研究波在暖电离层中的传播,模式联接和波的线性耦合。 本文还讨论了波方程的奇点、等离子体波与电磁波的耦合。在波耦合区中,波的场可以通过求解波方程在奇点附近的解而求得。文中比较了用射线方法和全波方程研究波耦合时电波传播的结果。 本文所得结果对研究用无线电波加热电离层实验中的吸收区域是有用的。     

RCS reduction of canonical targets using genetic algorithmsynthesized RAM

Radar cross section (RCS) reduction of canonical (planar, cylindrical, and spherical) conducting targets is the focus of this paper. In particular, a novel procedure is presented for synthesizing radar absorbing materials (RAM) for RCS reduction in a wide-band frequency range. The modal solutions of Maxwell's equations for the multilayered planar, cylindrical, and spherical canonical structures is integrated into a genetic algorithm (GA) optimization technique to obtain the best optimal composite coating. It Is shown that by using an optimal RAM, the RCS of these canonical structures can be significantly reduced. Characteristics of bistatic RCS of coated cylindrical and spherical structures are also studied and compared with the conducting structures without coating. It is shown that no optimal coating can be found to reduce the RCS in the deep shadow region. An in-depth study has been performed to evaluate the potential usage of the optimal planar coating as applied to the curved surfaces. It is observed that the optimal planar coating can noticeably reduce the RCS of the spherical structure. This observation was essential in introducing a novel efficient GA with hybrid planar/curved surface implementation using as part of its initial generation the best population obtained for the planar RAM design. These results suggest that the optimal RAM for a surface with arbitrary curvature may be efficiently determined by applying the GA with hybrid planar/curved surface population initialization     

Equations for wave propagation in a stratified compressible electron?ion magnetoplasma

This letter deals with the propagation of coupled electromagnetic, electron-acoustic and ion-acoustic waves in a horizontally stratified horizontally magnetised compressible electron?ion plasma. Propagation transverse to the magneto-static field is considered, and some first-order coupled-wave equations are derived.     

Quasi-periodic complex-coupled distributed-feedback structures withan exponential-like gradient of coupling

We investigate theoretically the linear properties of quasi-periodic complex-coupled distributed-feedback (DFB) structures in which the coupling constants experience a longitudinal variation along the propagation axis according to an exponential-like relationship (exp, sh, ch, ···). The results of the simulations are in good accordance with what could be expected at first sight from the particular form taken in this case by the coupled-wave equations. In particular, the gradient of coupling can provide a shift, either of the apparent average absorption (respectively, gain) or of the detuning from Bragg resonance, depending upon its exact form, Moreover, we show that this shift is asymmetric with respect to the direction of propagation, leading to a nonreciprocal behavior in reflection. We extend our analysis up to the laser modes at threshold     

Analysis of two dimensions sparse multicylinder scattering problem using DD-FDTD method

An algorithm of two-dimensional (2-D) domain decomposition finite-difference time-domain (DD-FDTD) using in sparse multicylinders scattering problem is proposed in this paper. The idea of domain decomposition is introduced to divide the sparse problem domain into several subdomains. All of subdomains are connected by means of the 2-D time domain Green's function. As a result, a great deal of meshes memory between cylinders is removed, especially when the distances between cylinders become large. Furthermore, the coupling between cylinders can be regarded as the equivalent cylindrical wave irradiations. The incident signals of the equivalent cylindrical waves are expressed as cylindrical wave input field array (CWIFA) according to Huygens principle. Then the calculation time is significantly reduced. The near-field to far-field transformation is used to obtain the equivalent cylindrical wave; as a result, the calculation time can be reduced further. The new method has been demonstrated in 2-D multicylinders scattering problem. Numerical results are in good agreement with the results obtained using classical FDTD method and moment of methods (MM).     

Analysis and design of combined distributed-feedback/Fabry-Perotstructures for surface-emitting semiconductor lasers

We propose combined distributed-feedback/Fabry-Perot (DFB/FP) structures for surface-emitting semiconductor lasers. The analysis is based on coupled-wave equations modified for surface-emitting lasers. The proposed structures, which exhibit enhanced resonance due to a matching between the gain and field distributions resulting in a reduced threshold compared with simple FP structures, are formed by placing the DFB structure between two DBR mirrors of an FP resonant cavity and introducing phase layers between the DFB region and the mirrors. It was found that the periodic-gain structures are a special case of the combined DFB/FP structures in which the index coupling effect is assumed to be negligible due to a small fill factor or a small refractive-index difference. The effect of complex (gain and index) coupling on the design and the threshold characteristics of the structures is clearly illustrated. Some important design considerations that were neglected in the previous papers are addressed     

I-V characteristics by radial tunneling in double-barrier tunnelingdiodes with cylindrical barriers

Radial tunneling transport in a double-barrier tunneling diode (DBTD) with cylindrical barriers has been theoretically studied. Calculations use a generalization of the transfer matrix method. Fine oscillation is demonstrated in transmission spectra and I-V characteristics. It is attributed to the coherence of traveling waves associated with cylindrical geometry. Various Al_xGa_1-x As-GaAs DBTD's with cylindrical barriers have been examined at absolute zero and room temperatures. Results are compared with DBTD's with planar geometry. It is recognized that different bias direction induces asymmetric I-V characteristics. Devices that operate on this inborn asymmetry are proposed. The device characteristics could be less sensitive to temperature than that of planar structures due to the fundamental operation principle. The results are useful to understand charge transport by the fast tunneling in quantum-well and superlattice structures with different geometries     

Reconstructing permittivity profiles using integral transforms andimproved renormalization techniques

Some new ideas for reconstructing permittivity profiles in planar and cylindrical objects illuminated by TEM-, TE- or TM-polarized waves are presented in this paper. For a planar medium, an improved renormalization technique along with a revised version of the nonlinear Riccati differential equation describing the direct problem are introduced. A nonlinear Riccati-similar differential equation for the cylindrical case has also been derived here for the first time, which helps reconstructing radially varying permittivity profiles in a way parallel to that of the planar case. The above-mentioned renormalization technique has been used for the cylindrical case as well to solve the inverse problem making use of a Hankel transform. The method represents fundamental bases for a three-dimensional generalization, which is essential for microwave imaging used, e.g., in biomedical applications and for the diagnostic of diseases in trees and vegetation. A known permittivity profile has been taken to generate synthetic reflection-coefficient data by solving the nonlinear equations describing the direct problems using MATLAB. These data have been used in conjunction with the proposed technique to reconstruct the permittivity profile. About 50-100 data points over the wavelength range from a minimum value (ranging from one-tenth to one-fifth of a typical length in the structure) to infinity have been used for the reconstruction. Reconstructed profiles have been compared to the original ones for a number of cases. Deviations of less than 2% have been achieved