Propagation Through a Twisted Medium

An explicit solution is obtained for propagation through a uniform twisted anisotropic medium subject to the conditions that propagation is along the twist axis, and that the structure is fine. The propagation constants are altered and coupling exists between the propagation modes. A parameter is defined which indicates the tendency of the radiation to adhere to the structure of the medium. The effects at boundary discontinuities are discussed, and tapers to an isotropic medium are dealt with. The particular application of the theory to cases of polarization conversion, circular to pIane, and plane to plane are discussed.     

Propagation in Twisted Square Waveguide

The problem of propagation of TE modes in twisted rectangular waveguides has been solved except for the case where one of the propagating modes becomes degenerate. The purpose of this paper is to show how to obtain a solution for degenerate modes in a twisted rectangular waveguide, with emphasis on the particular case of the square waveguide, for which the lowest order mode is degerate. It is shown that the propagation constant for the twisted square waveguide can be expressed as an asymptotic series, the first term being the propagation constant for a straight square waveguide and the first order correction term being of order 1/ L where L is the distance in which the guide makes one full rotation. The propagation constant for a nondegenerate mode in a twisted rectangular waveguide, on the other hand, can be expressed in a similar manner except that the first-order correction term is of order 1/ L/sup2/. Some comments are offered on the nature of the transition when the propagating mode is almost degenerate.     

AGSM光束的聚焦

基于有扭曲的各向异性高斯-谢尔模型(AGSM)光束二阶矩矩阵的传输公式,研究了AGSM光束通过透镜的聚焦。作了详细的数值计算,说明聚焦AGSM光束的传输特性,包括光束参数和横向光强的变化,特别是扭曲项和空间相关长度对聚焦特性的影响。     

Sensitivity of Crosstalk in Twisted-Pair Circuits to Line Twist

The prediction of crosstalk induced in an unbalanced twisted pair by an adjacent transmission line is investigated. The induced voltage at one end of the twisted pair is shown, experimentally, to be extremely sensitive to twist for low-impedance loads and relatively insensitive to twist for high-impedance loads. Ex- perimental results are given which show that a slight rotation of one end of the twisted pair ( no more than 1800) can cause a change of as much as 35 dB in the induced voltage. The consequence of these results is that, in a practical situation, one would therefore not be able to predict this crosstalk for low-impedance loads with any con- sistency. A simple model explaining these results is given.     

Deterministic Fabrication of Twisted Van Der Waals Structures

In 2D van der Waals (vdW) materials, rotational misalignment by a twist angle between adjacent layers can significantly affect their properties, referred as twistronics. Accurate tuning of twist angle of vdW materials in a controllable and efficient way is highly required. Here a thin-film assisted transfer (TAT) technique that can controllably fabricate vdW structures with deterministic twist angles (accuracy of 0.37°) is developed. The transparent and ultrathin film as transfer medium ensures the visible and intact manipulation of monolayer building blocks, thus constructing vdW architectures with multi-materials and designated position. Multilayer homo-/hetero-architectures in the geometry of series connection, spiral structure and chiral symmetry at deterministic twist angles are for the first time demonstrated. They can be located on various substrates for both structural analysis and device application. The capability of fabricating twisted homo-/hetero- vdW structures in a deterministic, high-throughput manner will serve as a powerful tool for twistronics.     

Polarization Transformation in Twisted Anisotropic Media

Polarization transformation of plane waves propagating in twisted anisotropic media is studied theoretically and numerically. It is shown that rotation of linear polarization is effected by such a medium when the anisotropy is of the order of 2 to 1 and twist rates commensurate with the relative value of the dielectric constants of the medium are used (less than 15/spl deg///spl lambda//sub 0/ for low dielectric constants and up to 90/spl deg///spl lambda//sub 0/ for dielectrics in the vicinity of 1000).     

Machined waveguide twist

A solution to the problem of obtaining a 90° twist in a machined-aluminium block waveguide is presented. At the waveguide input, half the waveguide wide-dimension is machined into one aluminum block the mirror image being machined into a second block. The technique involves gradually reducing the width of the input waveguide in one block, and simultaneously increasing the depth of the waveguide in the opposite block until the output waveguide depth is equal to the input waveguide width. Care must be taken to maintain a constant waveguide cutoff frequency through the length of the twisted section so that a good match is obtained. Two twists made using this technique in WR-22 waveguide are shown to give satisfactory results     

Control and elimination of disclinations in twisted nematic liquid-crystal displays

A method for controlling and eliminating reverse-twist and reverse-tilt disclinations in twisted nematic cells without using cholesterics has been developed both theoretically and experimentally. Pretilt angle and tilting direction as well as twist angle and sense have been correlated with the appearance of disclinations. It is shown that for practical twisted nematic cells having a special sense of pretilt, the disclination free cells are realized by adopting the pretilt angle of 1°-5° and the twist angle of 80°-85°.     

Analysis of Rapidly Twisted Hollow Waveguides

The propagation characteristics of twisted hollow waveguides are considered, and various analysis methods are proposed. It is shown that a twisted hollow waveguide can support waves that travel at a speed slower than the speed of light c. These modes are of particular interest, as slow wave structures have many potential applications in accelerators and electron traveling wave tubes. Since there is no exact closed form solution for the electromagnetic fields within a twisted waveguide or cavity, several previously proposed approximate methods are examined. It is found that the existing perturbation theory methods yield adequate results for slowly twisted structures; however, our efforts here are geared toward analyzing rapidly twisted structures using newly developed finite-difference methods. To validate the results of the theory and simulations, rapidly twisted cavity prototypes have been designed, fabricated, and measured. These measurement results are compared to simulated results, and very good agreement has been demonstrated.     

新型长周期光纤光栅的扭曲特性研究

发现高频CO2激光脉冲写入的长周期光纤光栅（LPFG）具有独特的扭曲特性：1）扭曲特性具有明硅的扭曲方向相关性——顺时针扭曲时谐振波长发生“红”移，逆时针扭曲时谐振波长发生“蓝”移，无论顺时针还是逆时针扭曲损耗峰幅值都近似线性减小；2）若被扭曲的光纤比被扭曲的长周期光纤光栅长得多，则扭曲时长周期光纤光栅谐振波长和幅值的变化出现周期性的起伏。利用扭曲光栅引起椭圆双折射进而导致偏振态变化的相关理论合理解释了这些独特的扭曲特性。     

Response of a Twisted Pair Excited by the Nonuniform Electromagnetic Fields of Nearby Loops

This paper derives equations for the noise induced in a twisted pair when the pair is embedded in a nonuniform electromagnetic field of a small current loop. The derived equations are applied to calculate the noise induced in a typical 135-? twisted pair. In the frequency range of 10 kHz-100 MHz, the noise induced in this twisted pair is shown to be less by about 70-135 dB when compared to a similar parallel-conductor line without twist that is embedded in the same field. Included is a simplified method of applying the derived equations to the problem of finding the response of the twisted pair to distinct uncorrelated noise sources, assuming that the sources can be approximated by small current loops.     

Low-loss twists in oversized rectangular waveguide

The unwanted mode conversion from twists in overmoded waveguides is calculated from numerical integration of the coupled-mode equations, considering simultaneous coupling of the five lowest-order modes coupled in a twist. Twists with tapered or linearly varying rates of twist are shown to be superior in medium- or broad-band applications to those with uniform twist rate. Measurements consistent with these theoretical calculations are discussed for uniform twists in WR90 waveguide at 60 GHz and for an electroformed twist having a linearly tapered rate of twist in WR187 waveguide from 15.7 to 17.7 GHz. The coupling coefficients needed in the calculations are derived and are compared with the results of other work, including a modal expansion of the dominant mode in twisted waveguide. The work also considers the transmission through an oversized waveguide with a mode converter generating a trapped unwanted mode, and the result for the dependence of the resonance depth on the mode conversion and the attenuation of the trapped mode is found     

Polarisation maintenance in circularly birefringent fibres

The feasibility of twisted circularly birefringent polarisation maintaining fibres is investigated. It is shown that the twist rate required to provide immunity from external effects is excessively large.     

Dispersion Characteristics of Twisted Rectangular Waveguides

Based on the expressions for the dominant hybrid-mode fields in twisted rectangular waveguides, dispersion formulas with two perturbational factors have been derived theoretically. These factors correspond to the two effects of the twist on the dominant wave propagation, respectively. The one expresses a shift in the cutoff frequency while the other expresses the effect of elongation in the transmission path. A set of 20-cm-long waveguides twisted uniformly by various multiples of 90° has been manufactured by the method of electroforming. The resonant frequencies of the respective waveguides have been measured as a transmission cavity in the 10-GHz band to obtain dispersion relations. Experimental results are found to be in good agreement with the theoretically derived formulas. The results of Lewin's theory are also compared with the present ones.     

Improved contrast uniformity in twisted nematic liquid-crystal electro-optic display devices

It has already been shown that one source of the patched appearance in twisted nematic displays is reverse twist; we now demonstrate the existence of another. Both types of patches have been eliminated by the use of cholesteric additives in combination with particular types of surface alignment.     

Van der Waals Nanowires with Continuously Variable Interlayer Twist and Twist Homojunctions

Moiré patterns at van der Waals interfaces between twisted 2D crystals give rise to distinct optoelectronic excitations, as well as, narrowly dispersive bands responsible for correlated electron phenomena. Contrasting with the conventional, mechanically stacked planar twist moirés, recent work shows twisted van der Waals interfaces spontaneously formed in nanowires of layered crystals, where Eshelby twist due to axial screw dislocations stabilizes a chiral structure with small interlayer rotation. Here, the realization of tunable twist in germanium(II) sulfide (GeS) van der Waals nanowires is reported. Tapered nanowires host continuously variable interlayer twist. Homojunctions between dislocated (chiral) and defect-free (achiral) segments are obtained by triggering the emission of axial dislocations during growth. Measurements across such junctions, implemented here using local absorption and luminescence spectroscopy, provide a convenient tool for detecting twist effects. The results identify a versatile system for 3D twistronics, probing moiré physics, and for realizing moiré architectures without equivalent in planar systems.     

Absorption mode for e.l.f. electromagnetic propagation in the Earth-crust waveguide

Using a highly idealised 3-layer model of the Earth's crust, it is shown that the modes of propagation fall into two classes. The conventional type leads to attenuation rates greater than the intrinsic attenuation rate for plane waves in the medium of lowest conductivity. The other type of solution leads to attenuation rates that are anomalously low. It is shown that this mode of propagation corresponds to a condition of complete absorption of an inhomogeneous plane wave incident from below the waveguide. To excite this absorption mode, we would need an aperture of infinite extent!     

Electron trajectories in twisted electrostatic deflection yokes

A mathematical analysis is carried out for electron motion in a cylindrical electrostatic deflection yoke which is twisted around its axis and immersed in a uniform axial magnetic field to form a twisted focus projection and scanning (FPS) deflection system. The yoke has an arbitrary twist angle, and is a generalization of yokes with π/2 rad of twist recently used in FPS vidicons. New modes of operation are found which eliminate shading caused by off-normal beam landing at the target, while providing improved deflection sensitivity, increased resolution, and reduced power consumption in the focusing solenoid as compared with the untrusted shading-free mode previously reported by Schlesinger, Wagner, and Saldi [1]-[3]. The theoretical predictions are supported by data from experimental devices.     

Step-Twist Waveguide Components

The step twist is a number of adjoining sections of straight rectangular waveguide, twisted about their common axis at their junction faces. The technique of designing the step twist resides in the proportioning of the section dimensions and the angles at the twist faces. The resulting design is much shorter than the usual twisted waveguide; it offers further advantages in ease of specifying shape and dimensions, and in their reproducibility in construction. A series of fixed 90-degree step twists has been designed for the rated 40 per cent frequency bandwidths in the standard waveguide sizes for 1 to 40 kmc. The total angle is divided equally among seven faces spaced about 1/8 wavelength in the guide. Each step twist is matched within 0.3 db swr with plain flanges or 0.5 db swr with choke flanges at both ends. Rotary step twists for operation at all angles out to +-90 degrees have been designed for the same bandwidths. The total angle is divided unequally among four faces (choke flanges) spaced about 1/4 wavelength in the guide. The entire unit at maximum rotation is matched within 1.2 db swr with choke flanges at both ends; the matching is closer at lesser rotation.     

一种新的测量扭曲向列相液晶盒盒厚和扭曲角的Stokes矢量法

提出了一种新的测量扭曲向列相液晶盒盒厚和扭曲角的Stokes矢量法。根据Jones矩阵推导出了Stokes矢量与液晶盒参数的关系,其原理是通过确定经过液晶盒之后偏振光的偏振特性来求解液晶盒盒厚和扭曲角。该方法在测量一个Stokes矢量时,在液晶盒后依次放两种波片,目的是改变经过液晶盒后的光的偏振态,再置一偏振分光棱镜分开水平Ip和垂直Is线偏振分量,测得两组光强,根据公式算得该Stokes矢量,从而求解液晶盒盒厚以及扭曲角。实验中我们用该方法测量了不同盒厚、扭曲角度的TN液晶盒。实验结果证明该方法可行,结果准确,并且可以测量盒厚较小的液晶盒。最后,我们理论分析了实验误差。