高斯光束在介质波导中的传播

本文考虑轴对称高斯型激光束在均匀介质及类透镜介质中的传播问题。本文采用作者以前所导出的正确方程,计及光束的衍射效应,求出了方程的解析解。在此基础上,讨论了激光束的宽度和波阵面问题。在均匀介质中,激光束将单调变粗;在类透镜介质中,激光束的宽度是z正弦函数,粗细交替出现。     

Propagation of TE Modes in Dielectric Loaded Waveguides

The propagation of TE/sub no/ modes in rectangular wave-guides that contain two dielectric slabs parallel to the narrow wall and extending over the full height of the guide is investigated. Waveguide and dielectric are assumed to be lossless and infinitely long. Apart from these restrictions, the dielectric slabs may have arbitrary thickness, position, and dielectric constant. The analysis is restricted to TE/sub no/ modes with the E-field parallel to the narrow guidewall. The guide containing only one dielectric slab is covered by this analysis. The even modes n = 2, 4, 6, . . . of the guide with two slabs correspond to the odd modes n' = n/2 = 1, 2, 3, . . . of the guide with one slab half the width of the guide with two slabs. For six relative dielectric constants (/spl epsilon/= 2.25, 4.00, 9.00, 12.25, 16.00, 25.00) the cutoff frequencies for TE 10, 20, 30, 40, 60 modes and the normalized propagation constants for TE 10 and TE 20 modes between their respective cutoff frequencies and a frequency slightly above the second- and fourth-order mode cutoff frequency for the empty guide, respectively, have been computed for a large range of slab thicknesses and slab positions. Selected results are presented graphically. These results are discussed. The parametric dependence of field distributions, of normalized characteristic impedances, of the ratio of cutoff frequencies (fractional bandwidth), and of the ratio of magnetic field components (ellipticity) are illustrated.     

Modes and Pseudomodes in Dielectric Waveguides (Correspondence)

It is shown that a medium whose dielectric constant has a square-law distribution with complex coefficients possesses guided mode solutions even under the extreme condition that both the real and the imaginary part of the index of refraction have their lowest value at the axis of the waveguide. However, the resulting modes are unstable (pseudomodes). Stable guided modes exist if the imaginary part of the refractive index has its highest value on axis.     

TE Mode Excitation on Dielectric Loaded Parallel Plane and Trough Waveguides

A theoretical and experimental study of the launching of TE surface wave modes on dielectric loaded parallel plane and trough waveguides has been performed. The source is a linear transverse current filament perpendicular to and extending across the space between the parallel side walls. Families of curves are presented, which show the bidirectional Iaunchlng efficiencies for the dominant TE modes of these two transmission lines as a function of dielectric constant, dielectric slab thickness, and current filament location. Measured bidirectional efficiencies are compared to the theoretically predicted values. Measured unidirectional launching efficiencies as high as 97 per cent were obtained for the case where a short circuit is located on one side of the current filament.     

Coherent Control of Wave Beams Via Unidirectional Evanescent Modes Excitation

Conventional coherent absorption occurs only when two incident beams exhibit mirror symmetry with respect to the absorbing surface, i.e., the two beams have the same incident angles, phases, and amplitudes. This study proposes a more general metasurface paradigm for coherent perfect absorption with impinging waves from arbitrary asymmetric directions. By exploiting excitation of unidirectional evanescent waves, the output can be fixed at one reflection direction for any amplitude and phase of the control wave. It shows theoretically and confirm experimentally that the relative amplitude of the reflected wave can be tuned continuously from zero to unity by changing the phase difference between the two beams, i.e., switching from coherent perfect absorption to full reflection. It hopes that this study will open up promising possibilities for wave manipulation via evanescent waves engineering with applications in optical switches, optical computing, one-side sensing, photovoltaics, and radar cross-section control.     

左手材料介质平板波导的表面模

引入归一化频率、归一化传播常数和非对称量等参数,研究了衬底是左手材料的介质平板波导中的表面模,得到用归一化参数表示的色散关系.用解析法计算了色散曲线,结果表明,材料参数对左手材料平板波导色散特性的影响很大.有的情况下只有TE1表面模存在,而有的情况下TE0表面模和TE1表面模都存在.有时TE1表面模与零阶振荡模圆滑连接,形成一个完整的曲线,有时会出现双重简并现象.分别讨论了TE表面模和TM表面模.     

Excitation and Scattering of Modes on a Dielectric or Optical Fiber

A technique similar to that of Kirchhoff is used to obtain an analytic expression for mode Iaunching on a semi-infinite rod. The approximation is quasi-optical and only valid for small angles of incidence /spl theta/. When /spl theta/ = 0 only HE/sub 1M/ modes are excited; however, for /spl theta/<<1 many modes can be launched. The effect on HE/sub 11/ mode propagation of small imperfections in a dielectric waveguide is analyzed. At the frequency of interest for optical communication (cutoff for the TM/sub 01/ mode) the radiated power is 160 times larger than that scattered into the HE/sub 11/ mode.     

多层平面介质波导两类辐射模之间的正交关系

通过场的平面波表示式导出了多层平面介质波导两类辐射模之间一个简单的正交关系，这一正交关系仅由多层波导最外面两个界面上场的幅度之比及相位之差确定，并与介质的层数多少以及多层介质波导内部各层场的具体分布无关．这是一个普适的正交条件，以前所报道的正交关系都是本文正交关系的一个特例．     

Excitation and Scattering of Guided Modes on a Helically Corrugated Dielectric Cylinder

The excitation and scattering of guided modes on a dielectric cylinder with a helical corrugation are investigated. A finite number of radiatiorr modes and an infinite number of surface modes are considered, with the consequence that both the amplitude and phase of the interacting modes can be evaluated. The effect of the pitch angle on the coupling and conversion between the guided modes and the radiation modes of various polarizations is investigated. It is found that TE/sub 0/ guided modes undergo a Brewster phenomenon in the low permittivity dielectrics. The power conservation relation is developed and the reciprocity relations are treated. The aperture fields are used to study the radiation characteristics of a novel dielectric-helix antenna capable of yielding a circularly, elliptically, or linearly polarized radiation.     

Chasi-Guided Modes and Related Radiation Losses in Optical Dielectric Waveguides with External Higher Index Surroundings

Mode filter actions are found theoretically in an optical dielectric waveguide consisting of a core and a thin cladding layer which is further surrounded by an external higher index region. The propagating waves, which are usually considered to be cutoff modes, can be guided with a small amount of loss under a certain condition. These waves are defined here as quasi-guided modes. These modes tend to the guided modes of the guide when the cladding thickness increases infinitely. A method is given to estimate the losses. As an example, the radiation losses are formulated for a symmetric slab waveguide, and are found to be approximately proportional to the cube of the mode number of the quasi-guided mode. Therefore, losses of the quasi-guided modes depend strongly on the mode number. It is suggested that fibers with large core diameters can be used as quasi-single mode fibers by covering the clad-type multimode fibers with external higher index surroundings and choosing the parameters properly.     

Rays, Modes, and Equivalent Networks (Correspondence)

This correspondence describes certain analogies between ray-optical and modal (equivalent-network) procedures employed in the study of a variety of propagation, guiding, and scattering phenomena.     

Coupling of Modes in Uniform, Composite Waveguides

The principle of coupling of modes is used to compute the phase constant in a uniform waveguide Iilled with two ditferent dielectric materials. The natural modes of two hypothetical waveguides filled with the different dielectrics are computed. The propagation of the combined system is computed by considering the coupling between the two sets of modes. Comparison is made between the approximate theory and an exact theory.     

A Method Extending the Boundary Condition for Analyzing Guided Modes of Dielectric Waveguides of Arbitrary Cross-Sectional Shape

A method based on the extended boundary condition method is presented for analyzing guided modes of dielectric waveguides of arbitrary cross-sectional shape. Numerical integration needed in this method is only over the boundary periphery line of the waveguide. Nevertheless, it is applicable to the waveguides with any refractive index difference between core and cladding ranging from negligibly small to considerably large difference, as well as to certain types of waveguide with inhomogeneous core. Approximate formulas for the case of weakly guiding are also derivable from the general basic set of equations presented. Numerical examples are given to verify the usefulness and accuracy of this method.     

On the Design of Dielectric Loaded Waveguides

Rectangular waveguides partially filled with a dielectric slab in the E plane can provide an alternative to ridged waveguides in broad-band systems. It is shown that by an appropriate choice of the dielectric constant, maximum power handling capacity for a given bandwidth and cutoff wavelength can be achieved. This power-handling capacity is much higher than for ridged waveguides. Attenuation and the effect of the first longitudinal-section-electric (LSE) mode on bandwidth are also discussed.     

Modes in Dielectric-Loaded Waveguides and Resonators

Analysis of nonaxially symmetric modes in circular wave-guides partially filled with high-dielectric constant material is presented. A method for the accurate determination of resonant frequencies of any mode excited in dielectrically loaded wavegnide cavities is described. The method is used to construct mode charts for dielectric resonators. Comparison of the resonant frequency calculations for several cases agrees closely with measurements.     

Application of Rayleigh-Ritz Method to Dielectric Steps in Waveguides

The Rayleigh-Ritz method is applied to obtain approximations to the first N eigenfunctions and corresponding eigenvalues in an inhomogeneously filled rectangular waveguide. These approximate eigenfunctions are then used to obtain a solution for the reflection and transmission coefficients at the junction of an empty and partially filled waveguide. Theoretical and experimental results are given for a dielectric slab which extends completely across the broad dimension of the guide, but only partially across the narrow dimension. The experimental values are within the experimental error of the computed values obtained by considering the dominant mode and only two evanescent modes.     

Coupling Characteristics of Nonradiative Dielectric Waveguides

An analytical method is presented for predicting coupling characteristics of dielectric strips in the nonradiative dielectric waveguide. Starting with an approximate but very accurate expression for the coupling coefficient between parallel dielectric strips, the scattering coefficients for nonuniform coupling structures are derived in simple closed-form expressions by taking the effect of the field deformation at the curved sections into account. The coupling coefficient of the nonradiative dielectric waveguide is found to be so large compared to those of other dielectric waveguides that complete power transfer can be attained with coupled polystyrene bends having a curvature radius as small as 20 mm at 50 GHz. The theory was verified experimentally for various coupling structures. As an application toward millimeter-wave integrated circuits, 0-dB couplers, quadrature hybrid couplers, and an in-phase power divider were constructed based on the present analysis. A comparison of theory and experimental data of these fabricated coupling circuits suggests that the effect of the nondegenerate modes in the straight and curved guides must be included in the analysis to further improve the theory.     

Propagation of Cladded Inhomogeneous Dielectric Waveguides

An approximate theory on the propagation of modes in an arbitrarily inhomogeneous optical waveguide embedded in a homogeneous medium is presented. Simple formulas are given, whereby the propagation constants can be determined assuming that the analytic solution is known in the absence of cladding. The results obtained applying the theory to a truncated parabolic-index profile are shown to be in good agreement with those obtained by the rigorous analysis. The theory is also applied to the propagation of TE and TM waves in truncated near-parabolic-index media.     

Analysis of Circular Chiral Dielectric Waveguides

The general eigenequations and the field expressions for circular waveguides containing chiral material in both the core and the cladding are given in a simple formulation. The results indicate that there are two types of field distribution which is related to the operating wavelength and chiral admittance. When the chiral admittance is larger than a critical value, the fields of one kind of circularly polarized wave in the core become exponentially-like damping in the transverse direction. Based on the eigenequations, the characteristics of a circular waveguide with chiral medium filled in the core are investigated. The relation between the cutoff frequency and the chiral admittance, the dispersion curves for some modes with different angular mode number and the transverse distribution of fields are presented. Some features different from the ordinary circular waveguide are resulted.