Mode‐matching analysis for characterization of the sectoral waveguide mode converters

The mode converter, as a passive mode conversion device in transmission lines, is well-investigated and widely implemented in various electromagnetic systems. However, most traditional mode converters can only realize a single conversion mode. Thus, a mode converter achieving multiple controllable output modes is urgently needed. In this paper, a reconfigurable mode converter operating in the microwave range is achieved by embedding a deformable all-dielectric material with quadrilateral shape into a rectangular waveguide based on coupled-mode theory. It can achieve different target modes with controllable output for the same input by exciting the deformable all-dielectric material. The design principle of the mode converter is expounded concretely and simulation is carried out using HFSS software 2022 R2. Experimental results, consisting of the simulation results, demonstrate that the proposed mode converter can achieve various mode conversions with mode purity higher than 95%. This article innovatively applies deformable materials to waveguide mode conversion, expanding the application of deformable memory materials in electromagnetic devices.

To accurately measure the performance of dielectric loaded circuits for gyrotron traveling wave tubes, a novel topological structure to generate the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> or TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> (superscript represents circular waveguide) mode is proposed and verified in this paper. The topology is constituted by an overmoded TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">□</sup> to TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">□</sup> (superscript represents rectangular waveguide) mode converter with multiple matching steps and a taper from TE20 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">□</sup> to TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> or TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> mode. This topology allows the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> or TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> mode to be excited by changing the taper dimensions. Simulation results reveal that the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sup> mode converters, respectively, have a 1-dB bandwidth of 12.8 and 9.3 GHz with an excellent performance. Converters for the back-to-back millimeter-wave measurements were fabricated with computerized numerical control and wire cutting techniques. Vector network analyzer measurements are in good agreement with the simulations.

A novel mode converter that can transform coaxial transverse electromagnetic (TEM) mode into circularly polarized TE11 (CPT) mode was developed and experimentally studied with high-power microwave (HPM). The mode conversion is realized by two steps. First, the TEM mode is converted into linearly polarized TE11 (LPT) mode by the webbed-shaped balun structure. Second, the LPT mode is transformed into the CPT mode with the insertion of circular-arc diaphragms in a circular waveguide. In this paper, the principle of a mode converter working at 1.57 GHz is demonstrated, as well as the experimental results. The experimental and simulation results are in good agreement. At the central frequency, the conversion efficiency is more than 99.5%, the measured axial ratio approximates to 1.39 dB, and the power handling capacity is in excess of 1.5 GW. These measured results meet the demand for narrow-band HPM source application.

Small circular waveguide mode converters used in the 28 GHz, 200 kW ECH system for the Hitachi tokamak HT-1 have been developed. In order to convert from the TE02 to the TE11 mode, TE0202-to-TE01 and TE01-to-TE11 mode converters are used. Both converters use periodic waveguide wall perturbations, the case of the former, circularly symmetric perturbations, and in the case of the latter, curvature perturbations. The lengths of the mode converters were successfully shortened with high mode conversion efficiencies by decreasing the number of periods and then optimizing the periods using quite a small diameter for the converters. The 30ϕ TE02-to-TE01 mode converter is 103 · 34 mm long and its mode conversion efficiency measured by a scalar network analyser is 96% at 28GHz. The TE01-to-TE11 mode converter is 427 · 38 mm long and has an efficiency of 79%, which is 10% lower than the design value. This discrepancy resulted mainly from mode conversion from the TE11 to the TE12 mode, caused by the smaller number of...

In this paper, a novel approach based on the differential evolution algorithm is proposed for the optimal design of high-power millimeter-wave TM/sub 01/-TE/sub 11/ circular waveguide mode converters. The axes of the waveguide mode converters bent in one plane are represented by cubic B-splines, and the coordinates of the B-spline nodes are parameters to be optimized. Numerical optimization results show that the proposed approach can be used to design highly efficient mode converters with very short converter lengths. Furthermore, the input and output of the optimized mode converters can be in the same direction.

A mode conversion from the TE 10 in WR3 rectangular waveguide to the TE 53 in an oversized circular waveguide is required to perform the cold test of a Bragg resonator for a high power 250 GHz Cyclotron Auto-Resonance Maser (CARM) under development at the Frascati ENEA research centre. High operating frequency and oversized cylindrical cavity (exceeding any previous CARM experiment) make the design particularly challenging. An initial transition, aimed at transforming the TE 10 in WR3 rectangular waveguide into the TE 11 , in a circular waveguide, is performed using a linear taper, a well-assessed device. Then a serpentine mode converter in circular waveguide with average radius of 1.48 mm and appropriate length has been adopted for the conversion into the TE 01 mode. The subsequent transducers, for not-rotating (TE 0m ) and rotating (TE 53 ) modes, are provided by means of rippled wall mode converters with small periodic perturbations of waveguide radius. The ripple period corresponds approximately to the beat wavelength (X b = 2π/(β m − β n )) with β m and β n the wavenumbers of the two interacting modes. For not-rotating modes an efficient conversion has been obtained by cascading single step TE 01 to TE 02 , TE 02 to TE 03 and TE 03 to TE 04 mode converters. For each of them, profiles, radius and number of periods have been optimized in order to obtain low levels of unwanted spurious modes, low reflection and sufficient bandwidth. As the modes to convert (TE 04 to TE 53 ) present similar wavenumbers, the most appropriate method for the conversion consists in a three-periods rippled-wall mode converter made by a helically corrugated waveguide with an average radius of 4 mm and five azimuthal variations. The longitudinal profile of the ripples has been properly smoothed to avoid steps between the circular waveguide and the corrugated converter. In this paper the conversion chain will be described and the simulation results (S-parameters, efficiency and electric field) of each single converter will be reported. Finally some considerations about the high precision micromachining technique to be used, such as copper electroforming and Computer Numerically Controlled (CNC) milling, for the construction of these high frequency components, will be presented.

In this paper, a compact, high efficiency TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> to TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode converter is presented. In the proposed structure, dielectric material is loaded below a tapered centre conductor, in lower section of a circular waveguide. The input TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode is initially transformed to TEM mode by the tapered centre conductor. The loaded dielectric material is designed to provide the desired phase shift for the mode conversion. Different dielectric materials are analyzed for loading to achieve high conversion efficiency and wide bandwidth. The proposed mode converter is fabricated with PTFE as dielectric for mode conversion and the results are verified through far field radiation pattern measurements. The mode conversion efficiency is more than 90% for 2.95-3.60 GHz band. Due to compactness and high conversion efficiency for wide bandwidth, it is suitable for S-band high power microwave (HPM) applications.

In this paper, full wave analysis of substrate integrated waveguide is presented based on method of moment in combination with mode matching technique. For this purpose, input and output via pairs are terminated with two waveguide sections. Waveguide width, as an optimization parameter, is optimized to obtain the minimum reflection from the substrate integrated waveguide. Field distribution in the structure, currents on the vias and dispersion characteristic of structure are investigated. Our results are compared with other numerical techniques. Good agreement between results confirms the accuracy of the proposed method.

Dissipative solitons (DSs), which are pulses with high pulse energy and a broad rectangular output spectrum, have several applications. In a fiber system, the generation of higher-order DS requires a mode converter with a high and flat conversion efficiency that can cover the working waveband. The requirements on high and flat conversion efficiency as well as broadband working waveband is a great challenge for a traditional mode converter. This study presents an L-band mode-locked fiber laser with a 3-dB bandwidth of more than 47 nm and a first-order cylindrical vector (CV) mode DS output based on an ultra-broadband long-period fiber grating (LPFG). The fabricated mode converter is a novel cascading LPFG with an ultra-broadband working bandwidth and flat conversion efficiency which could meet the demand of DS mode conversion. The 15-dB bandwidth is 149.76 nm, from 1515.36 nm to 1665.12 nm. Furthermore, the conversion efficiency in the range of the output spectrum is 98.60 ± 0.23%. The single-, double-, and triple-soliton output characteristics and their real-time spectra are observed using time-stretched dispersion Fourier transformation (TS-DFT) technology. Experimental results demonstrate that the first-order CV mode DS laser can maintain its dynamics after mode conversion under the accuracy of detection system, implying that the LPFG works only as a mode converter. The broadband DS laser with CV mode output in the L-band has been presented for the first time in study, to the best of our knowledge. This study is also the first to detect real-time characteristics for a higher-order mode-locked fiber laser using the TS-DFT technology. The proposed type of LPFG is s good candidate for generating higher-order mode lasers with an all-fiber structure.

In this paper, a class of nonradiative dielectric (NRD)-guide discontinuities is studied toward the establishment of design rules of NRD-guide circuits and components for millimeter-wave applications. A mode-matching technique with a multimodal transverse resonance condition is formulated to derive a generalized scattering matrix that allows accounting for effects of higher order modes and intermode coupling. Transmission properties of an NRD structure featuring a multilayered dielectric in cross section are presented. Mode conversion and power transfer among principal NRD-guide modes are, in particular, characterized for design consideration of NRD-guide components and circuits. New sets of easy-to-use design curves are introduced, thereby allowing practitioners to choose appropriate dielectric materials and NRD-guide topologies. Equivalent-circuit models are extracted from the generalized S-matrix for some basic and practically useful discontinuities involved in the design of almost every NRD-guide component, which include open ends, junctions, steps, and gaps. Calculated results of the selected structures are found to be in a good agreement with measurements. Dispersion diagrams of periodic NRD structures are also given in this paper.

An analytical method to estimate the scattering of RF power into multiple modes inside a nonuniform waveguide using perturbation techniques is presented. This method is quite general, much faster than numerical methods, and may be applied to a wide variety of nonuniform waveguides. We have used this method to design and analyze a three-section mode transformer that converts a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> circular waveguide mode to a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> rectangular waveguide mode. This mode converter is specially suitable for use in ultra-high-power applications. Due to the complexity of the problem, we have used the symbolic solver Mathematica to produce most of our analytical results. The results match within reasonable accuracy with High Frequency Structure Simulator field solver simulations. We also present the application of this mode converter to variety of ultra-high-power microwave components and experimentally show the performance of these devices.

Step approximation models provide a simple method of studying mode propagation and mode conversion in planar Y-branch optical waveguides. They are, however, only applicable to small branching angles since they neglect the wavefront tilt of the separating waveguides. A phasefront tilt approximation which can be used in conjunction with the local normal mode profile of the five-layer structure is introduced. By rotating the Y-branch about the branching point, a symmetrical branching angle is defined so that the phasefront approximation can be used not only for symmetrical Y-branches, but also for Y-branches which have an asymmetrical branching angle or have a change in branching angle along the longitudinal axis of the Y-branch. >

In this paper, an antenna with 8 GHz (7–15 GHz) bandwidth is designed, simulated, fabricated, and measured. Commonly, for the effective use of electromagnetic sources, mode converters are used to transform donut-shaped patterns to directive patterns. This paper introduces a novel antenna called the pattern director antenna (PDA) that solves most problems associated with the azimuthally symmetric output modes of high-power microwave sources. The PDA accepts directly (without the need for mode conversion) an azimuthally symmetric generated mode of an electromagnetic source and converts it to radiate a directive pattern. For the proof of concept and validation of the design by simulations, the 3D printing technology [using polylactic acid (PLA)] is used to fabricate the PDA and measure its radiation patterns and return loss. The selected material is cheap and also environmentally friendly. The antenna was coated with aluminum to become a conductor. The gain is from 16.8 to 21.8 dB in the frequency range. The S11, main lobe deviation (MLD), and sidelobe level (SLL) are less than −15 dB, 2°, and −7 dB, in all frequency range, respectively. The simulation results are in good agreement with the measurement.

Based on the coupled mode theory, this paper presents the study on the influence of input mode mixture in circular oversized waveguide mode converters. Three kinds of commonly used waveguide mode converters, including the waveguide mode converters with varying wall radius or small axis perturbations, and the waveguide mode converters with bent structures, are taken as the examples. The results show that the spurious input modes do not simply superimpose onto the output modes, and in some cases they may deteriorate the conversion efficiency for the main output mode. Methods for transforming such spurious input mode mixture simultaneously into the main output mode are also presented in this paper.

Gyrotrons are sources of high-power millimeter waves. Current interest in gyrotrons is mainly because they can act as a source of microwave power for electron-cyclotron-resonance heating (ECRH) for fusion experiments. However, the field pattern of these gyrotron modes is unsuitable for long path waveguide transmission or plasma heating. Here, it is desired to have a linearly polarized wave requiring the use of a mode conversion system which consists of many nonuniform waveguide components. Based on the theory of mode coupling, this paper discusses the circular waveguide mode conversion with waveguide axis curved and radius tapered in detail. It also carries out the optimized analysis about the geometry structure of TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0n</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode converter. Adopting different phase rematch techniques, the optimal geometry parameter can be obtained. The mode converter designed in this way owns high conversion efficiency exceeding 98%.