Lossless dielectric tapers with three-dimensional geometry

General properties of two-dimensional tapers for the cylindrical coordinate systems are described. A criterion to determine the index distribution of lossless tapers is given in terms of the phase shift. The design method of lossless tapers with three dimensional geometry is presented. Design equations of tapers for spherical, conical and ellipsoidal coordinate systems are described, and some specific design examples are presented. Tapers for a complicated refractive index can be solved by a numerical method based on the design equations. It is shown that equations for the ellipsoidal coordinates can be reduced to those for coordinate systems with high symmetry     

Surface-wave radiation loss from curved dielectric slabs and fibers

A newly developed systematic procedure to calculate the real and imaginary parts of the change in propagation constant of a surface-wave mode on a curved open waveguide of general cross section is used here to determine these quantifies for the TE modes of an asymmetric slab waveguide and for all the modes of an optical fiber. Comparison of these results with the existing literature points up the care which must be taken in making approximations in this problem, since errors of significant factors in the radiation loss have been made in previous work.     

太赫兹频段一维介质光子晶体中的史密斯-帕塞尔辐射特性模拟

基于Smith-Purcell(SP)效应,采用粒子模拟的方法探讨了电子束团激发一维介质光子晶体中的SP辐射特性.模拟研究了单个束团激发一维介质圆柱光子晶体产生的SP辐射现象,并对周期束团激发的THz频段的相干SP辐射进行了模拟分析.研究表明,提高介质的相对介电常数和增加光子晶体的层数都可使辐射强度增加,选择合适的参数能够有效地增强THz频段的相干SP辐射强度.     

Ultraspherical tapers

Tapered transmission lines for impedance matching at microwave frequencies are obtained from quarter-wave transformers using ultraspherical polynomials. It is shown that a free parameter can be suitably chosen so as to optimise energy transmission.     

Evaluation of dielectric optical waveguides from their far-field radiation patterns

An experimental technique for determining the characteristics of dielectric optical waveguides is presented. The far-field pattern of a passive waveguide excited by an external source is measured and compared with a theoretical pattern. The presence of spurious light in the fax field complicates the measurement, but this difficulty can be minimized for many structures. Application to semiconductor laser structures is discussed. Determining the waveguiding properties of laser structures prior to processing can result in the early detection of faults for increased yield.     

Legendre tapers

A new class of tapered lines using Legendre polynomials is discussed. The main advantage of such a device is the high value of the major/minor lobes ratio for the reflection coefficient, especially for frequencies much higher than the cutoff value.     

Lateral wave contribution to the radiation from a dielectric halfmedium

Two methods are presented for the computation of the radiation at microwave frequencies in a thin dielectric medium from a source located in a dense dielectric medium. One is based on geometrical optics and the other on physical optics. The geometrical optics approach encounters some difficulties in the evaluation of the radiation in the thin dielectric medium near grazing-incidence condition, due to excitation of a lateral wave, which is not taken in consideration by this approach. The physical optics method, which considers the lateral wave contribution, approaches this problem successfully and gives a better approximation of the radiated field near grazing. Numerical results, which compare the two methods, and experimental data are offered to validate the theoretical approach     

Symmetrical low cross-polar radiation from a dielectric sphere loaded horn

The measured radiation pattern of a dielectric sphere fed from a conical horn loaded with a concentric dielectric cylinder is given. The matching arrangement of a double taper leads to multimode excitation. Pattern symmetry in the principal planes and a -10-dB reduction in cross-polar levels are demonstrated.     

Efficient nonadiabatic planar waveguide tapers

We report taper designs with high transmission efficiencies and with lengths shorter than those needed for adiabatic operation. The tapering occurs between rectangular optical waveguides with the same vertical silicon-on-insulator layer structure, but with different horizontal widths, namely 0.5 and 2.0 /spl mu/m, and for taper lengths between 0.5 and 3.0 /spl mu/m. After a comparison between two different optimization methods in a two-dimensional calculation scheme, one of these is repeated using three-dimensional calculations. The results show that, also in the length region where conventional linear and parabolic tapers are not yet adiabatic, tapers with a high efficiency can be designed by applying complex taper structures with more degrees of freedom.     

On the role of lateral waves in the radiation from the dielectric wedge

The field on the dielectric wedge is approximated by a plane-wave expansion as in [1]. Contributions from this solution to both the surface field and the radiation field are examined. Finally, an experimental radiation field is compared with the plane-wave solution and with a geometric-optical diffraction field.     

The shape of fiber tapers

A model for the shape of optical fiber tapers, formed by stretching a fiber in a heat source of varying length, is presented. Simple assumptions avoid any need for the techniques of fluid mechanics. It is found that any decreasing shape of taper can be produced. The procedure for calculating the hot-zone length variation required to produce a given shape of taper is described, and is used to indicate how an optical adiabatic taper can be made. A traveling burner tapering system is capable of realizing the model's prediction, and a complete practical procedure for the formation of fiber tapers with any reasonable shape is thus presented     

Millimeter wavelength radiation emission during dielectric aging

The current-voltage characteristic for a “metal — thick insulator — metal” structure has been investigated by applying high voltage impulses on a variety of industrial insulators. It appears that, in the high field regime (～1MV/cm) S-type Negative Differential Conductance may occur, leading to self sustained conductivity oscillations possibly originating in field dependent carrier mobility. The phenomenon is associated with radiation emission during thermalization of hot electrons. Measurements have established the presence of millimeter wavelength radiation during prebreakdown and breakdown stages. The radiation emission intervals within a high-voltage pulse application and response have also been investigated.     

Computation of the dielectric rod waveguide radiation modes

The continuous mode spectrum of a dielectric rod waveguide is analysed in detail. The validity of the orthogonality relations is proved explicitly and the related normalization factors are computed and presented. The results are compared with existent literature and some discrepancies are noted. A full wave approach is pursued without making use of the “weak guidance” approximantion.     

Propagation and radiation characteristics of low-permittivity dielectric cones

An approximate theory is proposed to explain the propagation and radiation characteristics of low-permittivity conical dielectric waveguides of narrow flare angle. Theoretical radiation patterns are predicted and compared with those obtained for corrugated conical horns. The considerable similarity which is predicted is supported by experimental results obtained at 9 GHz using cones made from expanded polystyrene.     

Anomalous radiation loss of curved dielectric ridge waveguides

The radiation loss of curved ridge dielectric waveguides can show an oscillatory dependence on radius for the TM-polarisation. This effect is predicted only by a three-dimensional full vectorial analysis and is confirmed experimentally in S-bend dielectric ridge waveguides     

Reflection and radiation of flanged circular dielectric rod

The reflection coefficients of a circular dielectric rod terminated by a metallic flange are computed by the Hankel transform for the TE01 mode. The field distribution in the half space is determined by taking the inverse Hankel transform of the field components. Therefore the field produced by a circular diathermy applicator can be determined theoretically.     

Input VSWR of profiled E-plane tapers

An efficient method for determining the input VSWR characteristics of waveguide E-plane tapers is presented. The taper profile is approximated by radial waveguide sections and the junction wave transfer matrices are derived applying field matching. The input reflection coefficient is easily determined using these matrices. Results computed using the present method are compared with some measured and calculated results in the literature and very good agreement is obtained.     

Experimental demonstration of evanescent coupling from optical fibre tapers to photonic crystal waveguides

Experimental results demonstrating nearly complete mode-selective evanescent coupling to a photonic crystal waveguide from an optical fibre taper are presented. Codirectional coupling with 98% maximum power transfer to a photonic crystal waveguide of length 65 /spl mu/m and with a coupling bandwidth of 20 nm is realised.