Low-loss Structure Research Articles

A compact broadband circularly polarized monopulse slot array antenna based on gap waveguide technology

ABSTRACT This paper presents a circularly polarized (CP) monopulse slot array antenna based on gap waveguide (GWG) technology. The proposed monopulse antenna is composed of a 4 × 4 array antenna fed by the monopulse comparator network in a compact three-layer structure. To achieve a broad axial ratio (AR) bandwidth of right-handed circular polarization (RHCP) in the sum radiation pattern, a septum polarizer is used in the radiating aperture. Based on this approach, horizontal transition structures are used to excite the radiating array from the WR-28 waveguide to the GWG, which helps to reduce the reflection loss and expand the impedance bandwidth. Experimental results show that the impedance bandwidth is 30.5 GHz to 40 GHz, which is 27.1%. The 3 dB AR bandwidth is 31%. The measured null depth is −40 dB, and the maximum measured gain in the sum pattern is 22 dBi at 34 GHz, while the antenna efficiency is greater than 86%. The advantages of low loss, broad bandwidth, and compact structure make the proposed antenna a competitive candidate for millimeter-wave (mmWave) monopulse direction-finding applications.

Compact Substrate Integrated Waveguide Cubical Dielectric Resonator Antenna for fifth-generation applications

A comprehensive analysis of a wideband Cubical Dielectric Resonator Antenna is presented, specifically designed for 5G-NR wireless communication applications, focusing on the N257, and N258 frequency bands. This antenna exhibits symmetrical radiation patterns, a wider bandwidth, improved efficiency, and substantial gain characteristics. The design employs a low-loss substrate-integrated waveguide structure to enhance gain while reducing cross-polarization effects. This is accomplished through an innovative approach of establishing boundary conditions using copper wire stitching for vias, superseding the conventional soldering iron method, which guarantees minimal radiation leakage, compact dimensions, and reduced losses. The perforated dielectric resonator antenna possesses higher mode excitations, and implementing metallic strips on two resonator surfaces facilitates the simultaneous excitation of fundamental and higher-order modes, thereby yielding a broadband response. The overall dimensions of the antenna are 15.29 × 8.42 × 3.8 mm3(1.3×0.7×0.3)λ03, which facilitates a measured impedance bandwidth of 8.2 GHz, spanning from 24.4 to 32.6 GHz (28.7%), accompanied by a high gain of 8.1 dBi and efficiency levels reaching up to 94%. The prototype is constructed utilizing the standard printed circuit board technique, and simulation outcomes are derived using the High-Frequency Structure Simulator (HFSS).

Optimization of Grating Coupler over Single-Mode Silicon-on-Insulator Waveguide to Reach < 1 dB Loss through Deep-Learning-Based Inverse Design

Grating couplers are essential components in silicon photonics that facilitate the coupling of light between waveguides and fibers. Optimization of the grating couplers to reach <1 dB loss when coupling to single-mode fibers (SMFs) has been reported in the literature, but this was based on silicon-on-insulator (SOI) waveguides supporting multi-modes. In this paper, using a deep-learning model combined with an inverse-design process, we achieve <1 dB losses for grating couplers implemented over single-mode SOI waveguides, i.e., a maximum efficiency of 80.5% (−0.94 dB) for gratings constrained with e-beam (EB) lithography critical dimension (CD), and a maximum efficiency of 77.9% (−1.09 dB) for gratings constrained with deep ultraviolet (DUV) lithography CD. To verify these results, we apply covariance matrix adaptation evolution strategy (CMA-ES) and find that while CMA-ES yields slightly better results, i.e., 82.7% (−0.83 dB) and 78.9% (−1.03 dB) considering e-beam and DUV, respectively, the spatial structures generated by CMA-ES are nearly identical to the spatial structures generated by the deep-learning model combined with the inverse-design process. This suggests that our approach can achieve a representative low-loss structure, and may be used to improve the performance of other types of nanophotonic devices in the future.

Theoretical and Experimental Investigation of N-Bit Reconfigurable Retrodirective Metasurface

The PIN diode-based N-bit reconfigurable retrodirective metasurface (N-bit RRDM) is a next-generation retro-reflector that offers the advantages of effective electronical control of the retro-reflection angle, low loss, and thin planar structure. However, since the unit cell of an N-bit RRDM is controlled by a quantized N-bit phase (360°/2<sup>N</sup>), it encounters operational errors, such as beam gain reduction and spurious beams. This can be a fatal disadvantage in military radar or satellite communication, which requires accurate beam tracking. This paper theoretically analyzes the operation of the N-bit RRDM by utilizing generalized Snell’s law and array factor theory. The analysis results present the design criteria for an N-bit RRDM that eliminates issues related to beam gain reduction and spurious beam errors. Furthermore, to verify the theoretical analysis results, High-Frequency Structure Simulator (HFSS) full-wave simulation and experimentation are conducted using the 1-bit RRDM.

Multiplexed rectangular dielectric gratings with multiple narrow-band refractive index filtering and sensing

We propose a low-loss compound structure consisting of a multiplexed rectangular dielectric grating and a waveguide layer, which can function as multi-band optical filters and sensors in TE and TM polarization by utilizing the resonant mode of the waveguide (WG) and the hybrid SP, respectively. By manipulating the parameters and subsequently constraining the local density of multi-resonant modes to several distinct resonant wavelengths, we propose a novel category of highly sensitive refractive index sensing platforms. Spectral shifts ranging from 110 to 131 nm/RIU with FOM of (22, 26.2)/RIU under TE polarization and 80 to 114 nm/RIU with FOM of (5.7, 8.1)/RIU under TM polarization can be accurately discerned for multiple individual analytes across a broad spectral range. The proposed structures offer enhanced flexibility in the design of structures across a wide spectral range, catering to various potential applications in multi-band optical filters, sensors, and photodetectors.

Transition-free indirect bonding towards 3D multi-layer glass stacking

Optical and photonic devices comprised of fused-silica glass portfolios play an important role in astronomical detections, which requires the bonding interfaces to own robust mechanical properties and less optical loss. Indirect bonding lowers the difficulty of the bonding process than direct bonding by introducing the non-negligible intermediate or transition layers but will inversely diminish the optical properties. In this study, a transition-free indirect bonding (TFB) strategy was developed to realize glass bonding with hard-to-distinguished interfacial boundary. In the process, the oxygen plasma activation was applied to reconstruct the glass surface. Owing to the produced chemical sites with the plasma surface activation, multiple paths could be provided for surface and interface reactions than conventional indirect bonding method hydroxide-catalyzed bonding (HCB). These adequate reactions constructed a whole and uniform Si-O network crossing the bonding interfaces, which was free of transition boundary between the interface and bulk substrates. The uniform and non-transition interface performed higher bonding strength and excellent optical transmittance with less loss than HCB. Based on the TFB strategy, multilayer chips were combined together enabling a 3D multi-layer chip stacking with low transmittance loss and stable mechanical structure for state-of-the-art optical applications in astronomical detection, laser instruments, and optical sensors.

High-solid-content paste for stereolithography self-supporting printing of Luneburg lenses with a high degree of geometric-design freedom

Luneburg lenses prepared by alumina stereolithography, have the advantages of low dielectric loss, corrosion resistance, and flexible structure. However, the supporting structure limits the degree of geometric design freedom. This paper analyzes the effects of modifiers and dispersants on paste properties from the molecular structure, which is different from the traditional process that doesn’t analyze the influence of similar mechanisms and binding site competition while ignoring the correlation. Based on the dispersant’s better flexibility bringing more obvious change in viscosity, we first determine the amount of dispersant and then adapt the amount of modifier to obtain self-supporting pastes of 82 wt % and 24.9 Pa.swithout gradation. The gain and beam deflection angle of Luneburg lenses with high geometric complexity and working bandwidth of 38° formed by stereolithography self-supporting process is matched to the simulation results without manufacturing defects, which provides a theoretical basis for more flexible design and manufacture of Luneburg lenses.

Damage, waveguide, and electrical properties in (La, Sr)(Al, Ta)O3 single crystal irradiated with carbon ions

The (La, Sr)(Al, Ta)O3 crystal was irradiated via 20.0 MeV C ion with fluence of 1.0×1015ions/cm2. The Rutherford backscattering (RBS)/channeling spectra, the hardness and elastic modulus as continuous functions of the depth, and X-ray diffraction (XRD) are used to analyze the irradiation damage, hardness, and structural changes in the near-surface area of samples. Prism coupling and end-face coupling methods were used to study the changes of optical waveguide properties under different annealing conditions. Considering the potential applications of low-loss waveguide structure in photoelectric sensors, electrical properties of (La, Sr)(Al, Ta)O3 samples were studied as an important detection indicator of sensors.

Low-loss and high-sensitivity surface plasmon resonance sensor composed of microstructured optical fibers for wide visible-to-infrared and refractive index ranges

ObjectiveOptical fiber sensing technology develops rapidly and is widely used. In order to design a SPR sensor with better performance and easy production, a new type D-MOF sensor with low loss, high sensitivity and simple structure is proposed. MethodsThe full vector finite element method (FEM) was used to optimize the structure parameters by using the multi-physics analysis software COMSOL, and the performance of the structure was systematically analyzed. ResultsThe sensor operational wavelength spans the visible and infrared range (430 nm to 7200 nm). The refractive index detection range of 1.00–1.42, the maximum wavelength sensitivity and the optimal resolution are 60,000 nm/RIU and 1.67 × 10−6 RIU−1, respectively, and the maximum limiting loss peak is 64.99 dB/cm. In addition, MOF-SPR sensors have large tolerance and low sensitivity of structural parameters in actual manufacturing. It has great application potential in biosensing and other fields.

Large permittivity, low loss and defect structure in (Nb, Zn) co-doped SnO2 ceramics studied through a combined experimental and DFT calculational method

Dielectric ceramics with high permittivity and low loss are widely used in electronic components and devices. In this study, (Nb, Zn) co-doped NbxZnySn1-x-yO2 with different doping levels and Nb/Zn ratios was designed to tune the defect structure toward the optimal dielectric performance. The lattice parameters firstly increased from x = y = 0.01 to 0.03 and then decreased, while the oxygen vacancy concentration decreased with doping. The co-doped sample with x = y = 0.02 exhibits stable permittivity up to 800 with an ultra-low loss tanδ ∼0.03 at 40 Hz. DFT calculation showed that the oxygen vacancy was formed with single-Zn doping and co-doping at low doping level, while the hole was generated at higher doping level. The achieved large permittivity and low loss of the sample are related to both Electron-Pinned Defect Dipoles (EPDD) and Hole-Pinned Defect Dipoles (HPDD) effects in the lattice, which was determined by the relative positions of donor and acceptor dopants.

Analysis, Reduction, and Utilization of Loss in Reconfigurable Spoof Surface Plasmon Polaritons

Reconfigurability of dispersion properties of spoof surface plasmon polaritons (SSPPs) have been realized by loading tunable elements such as varactors and p-i-n diodes. However, the parasitic resistance of tunable elements will lead to transmission loss of SSPPs, which still lacks investigation. In this article, we propose to comprehensively answer three critical questions in this area: 1) how to analyze the loss of reconfigurable SSPPs; 2) how to reduce the loss of reconfigurable SSPPs; and 3) how to utilize the loss of reconfigurable SSPPs. For the first question, the complex characteristic impedance of reconfigurable SSPPs is theoretically obtained, which indicates the mechanism of the enhanced loss in reconfigurable SSPPs. For the second question, a low-loss reconfigurable SSPP structure loading parallel varactors is proposed to reduce and reconfigure the loss of reconfigurable SSPPs. For the third question, a multifunctional 2-unit antenna array is designed and tested using a feeding network composed of the loss-reconfigurable SSPPs. Experimental results demonstrate that two functions of sum-difference beam forming and amplitude modulation are achieved by the single sample. Thus the technique of loss reconfiguration in SSPPs may find wide applications in efficiency-sensitive circuits and smart microwave systems in the future.

Compact Microstrip Patch Filtenna Array Based on Reusable High-Order SIW Cavity

In this letter, a compact microstrip patch (MP) filtenna array designed by reusing high-order substrate integrated waveguide (SIW) cavity is investigated. The SIW cavity operating at TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">201</sub> mode not only acts as the first-stage resonator of the filter, but also serves for the four-way dividing realized by the mode power allocation. In addition, by appropriately controlling the coupling coefficient of the output window of the SIW cavity, the radiating 2 × 2 MP array can become the second-stage resonator of the filter. Finally, benefiting from the reuse of the employed high-order SIW cavity, the proposed filtenna array exhibits good performance such as low loss, high efficiency and compact structure. For demonstration, a filtenna array prototype centered at 13.6 GHz is fabricated and tested, showing 13 dBi peak gain and 90% peak efficiency. Good agreement is obtained between simulated and measured results.

LssDet: A Lightweight Deep Learning Detector for SAR Ship Detection in High-Resolution SAR Images

Synthetic aperture radar (SAR) ship detection has been the focus of many previous studies. Traditional SAR ship detectors face challenges in complex environments due to the limitations of manual feature extraction. With the rise of deep learning (DL) techniques, SAR ship detection based on convolutional neural networks (CNNs) has achieved significant achievements. However, research on CNN-based SAR ship detection has mainly focused on improving detection accuracy, and relatively little research has been conducted on reducing computational complexity. Therefore, this paper proposes a lightweight detector, LssDet, for SAR ship detection. LssDet uses Shufflenet v2, YOLOX PAFPN and YOLOX Decopuled Head as the baseline networks, improving based on the cross sidelobe attention (CSAT) module, the lightweight path aggregation feature pyramid network (L-PAFPN) module and the Focus module. Specifically, the CSAT module is an attention mechanism that enhances the model’s attention to the cross sidelobe region and models the long-range dependence between the channel and spatial information. The L-PAFPN module is a lightweight feature fusion network that achieves excellent performance with little computational effort and a low parametric count. The Focus module is a low-loss feature extraction structure. Experiments showed that on the Sar ship detection dataset(SSDD), LssDet’s computational cost was 2.60 GFlops, the model’s volume was 2.25 M and AP@[0.5:0.95] was 68.1%. On the Large-scale SAR ship detection dataset-v1.0 (LS-SSDD-v1.0), LssDet’s computational cost was 4.49 GFlops, the model’s volume was 2.25 M and AP@[0.5:0.95] was 27.8%. Compared to the baseline network, LssDet had a 3.6% improvement in AP@[0.5:0.95] on the SSDD, and LssDet had a 1.5% improvement in AP@[0.5:0.95] on the LS-SSDD-v1.0. At the same time, LssDet reduced Floating-point operations per second (Flops) by 7.1% and Paraments (Params) by 23.2%. Extensive experiments showed that LssDet achieves excellent detection results with minimal computational complexity. Furthermore, we investigated the effectiveness of the proposed module through ablation experiments.

Study on Low-Loss and High-Energy Density Coil Structure of a Wireless Power Transmission System Using High Temperature Superconducting Coils for Railway Vehicle

We have been investigating a wireless power transmission (WPT) system using a high-temperature superconducting (HTS) coil for a railway vehicle. In the previous research, we investigated the conceptual design of the WPT system using the HTS coils for the railway vehicle. However, in order to suppress the AC loss per HTS coil below the cooling power of a cryocooler, it was necessary to install multiple HTS coils in parallel on the vehicle and ground sides. Therefore, we investigated the low-loss and high-energy density coil structure in the WPT system for the railway vehicle. In this study, we analyzed the AC loss and the coupling coefficient of various HTS coil structures using the narrow REBaCuO (RE: Rare Earth elements, REBCO) wires in parallel by finite element method (FEM) calculation, and evaluated the power transmission characteristics of the WPT system using the HTS coil structures using the narrow REBCO wires in parallel for the railway vehicle. As a result, we found that the AC loss of the HTS coil structure using the narrow REBCO wires in parallel was lower than that using the wide REBCO wire. The coupling coefficient between coils using a single pancake coil structure was higher than that using a double pancake coil structure. Also, the receiving power density of the WPT system was improved by the HTS coil structures using the narrow REBCO wires. Particularly, the receiving power density of the single pancake coil using the twisted narrow wire in parallel in the coil radial direction increased to about 1.5 times that of the single pancake coil using the wide wire.

Varactor-Based Phase Shifters Operating in Differential Pairs for Beam-Steerable Antennas

A concept of varactor-based phase shifter operating in differential pair for beam-steerable antennas is presented in this article. The basic phase-tuning unit consists of a microstrip transmission line and several shorted-stub-terminated varactors loaded on its edge. A pair of these phase-tuning units working in differential mode can achieve a full 360° phase shift, which allows a low loss and simple structure. An accurate equivalent circuit model for this phase shifter is developed for fast calculation allowing the design and optimization of the structure with a genetic algorithm (GA). A pair of phase-tuning units, each including three stub-loaded varactors, is identified as a practical solution to produce a continuously tunable phase difference nearly covering 0°–360°, with two symmetrical 11°-wide blind zones and a maximum insertion loss of 2.04 dB. A 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times4$ </tex-math></inline-formula> beam-steerable dielectric resonator antenna (DRA) array is then designed, fabricated, and tested to demonstrate the practicality of the proposed phase shifter. The measured results illustrate that the array is able to continuously scan from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 45^\circ $ </tex-math></inline-formula> to 45°, with a tunable operating frequency band from 4.75 to 5.25 GHz. All these findings suggest that the proposed phase shifter concept is promising and practical to realize beam-scanning antennas.

Low-Loss Reconfigurable Power Divider With Arbitrary Operating Channels Using Switchable K Inverters for Antenna Feeding Network Applications

In this article, a four-way low-loss reconfigurable power divider with arbitrary operating channels is designed with a simple circuit configuration. A reconfigurable transmission line (RTL) is proposed, which can be switched between two different characteristic impedances with constant electric length. When setting the constant electric length as 90°, the RTLs are used as switchable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> inverters to construct a four-way reconfigurable power divider. Then, by controlling the switchable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> inverters, the power divider can be fully reconfigured in one-channel (State 1), two-channel (State 2), three-channel (State 3), four-channel (State 4), and all-channel-OFF (State 5) operations. Without using extra impedance matching networks, good port matching is achieved in the states with arbitrary transmission channels. Thus, this simple circuit configuration can be applied in the feeding network of antenna systems to achieve flexibly reconfigurable coverages. For verification, a circuit is designed and fabricated. Good agreement between the simulated and measured results is observed. In comparison with reported multiway reconfigurable power dividers, the proposed one exhibits low insertion loss, wide bandwidth, and simple structure.

Properties of Nitrous Oxide and Helium mixtures for space nuclear recompression Brayton cycle

Space nuclear reactors are the research foundation of space nuclear power and nuclear propulsion. Thermoelectric conversion efficiency and mass of high-power nuclear reactors have always been essential factors that restrict aerospace design. Supercritical nitrous oxide (S-N2O) Brayton cycle is becoming hot research due to its high-power conversion efficiency, low energy loss, compact and simple system structure, making it widely used in space nuclear reactor application. The recompression cycle is proposed to improve the thermal efficiency of the S-N2O cycle and effectively weaken the “pinch point” phenomenon that may occur in the regenerators. The thermodynamic of the S-N2O Brayton cycle has been studied but has little research on the nitrous oxide (N2O) and helium (He) mixtures as the working medium for the Brayton cycle. In this paper, physical properties were studied on the mixture of supercritical nitrous oxide and helium (S-N2O+He) as the working medium of the space power system based on a recompression Brayton cycle. Consider the thermodynamic properties of the mixture at seven different temperature nodes, analyze the variation trend of compressibility factor, specific heat, specific heat ratio, thermal conductivity, and dynamic viscosity with temperature, respectively. Finally, determined the mixing ratio of the working medium at the maximum thermoelectric conversion efficiency of the cycle and estimated the mass and specific mass of the Brayton rotating unit.

Experimental demonstration of ultrathin broken-symmetry metasurfaces with controllably sharp resonant response

Symmetry-protected resonances can be made to couple with free space by introducing a small degree of geometric asymmetry, leading to controllably sharp spectral response. Here, we experimentally demonstrate a broken-symmetry metasurface for the technologically important low millimeter wave spectrum. The proposed metasurface is fabricated on an ultrathin polyimide substrate, resulting in a low loss and flexible structure. Measurements inside an anechoic chamber experimentally verify the theoretically predicted sharp spectral features corresponding to quality factors of several hundreds. The demonstrated sharp response is also observed with the complementary structure, which responds to the orthogonal linear polarization (Babinet's principle). The designed metasurfaces can be exploited in diverse applications favored by a controllably sharp spectral response, e.g., filtering, sensing, switching, and nonlinear applications, in either reflection or transmission mode operation. More generally, the demonstrated fabrication process provides a generic platform for low-cost, large-scale engineering of metasurfaces with minimal substrate-induced effects.

Design Flexibility of a Modular Low-Loss High-Frequency Inductor Structure

Miniaturization and efficiency of power electronics are limited by magnetic components, which are difficult to scale to small size and high frequency (HF). Inductor structures using field shaping, quasi-distributed gaps, and modular construction can achieve low loss at HF (3-30 MHz) without litz wire. For widespread adoption though, these structures must be shown to remain effective across a wide design range and be economical to manufacture. This article investigates the design flexibility of one such previously proposed inductor structure with a modified pot core and demonstrates that this structure can provide excellent performance for a wide range of inductance and power handling requirements using only a few sets of manufactured core pieces. The core pieces used in the modified pot core structure can be scaled by 4× in volume, compared to roughly 2× for conventional core families, and still achieve high performance over a wide design space. Moreover, this approach can achieve about half the loss of conventional designs at HF and, unlike conventional core sets, can provide a range of low-loss form factors with a single family of components. The proposed inductor structure and design approaches, thus, offer new opportunities in the practical production of low-loss HF inductors.

A Compact Active Ka-Band Filtenna for CubeSats

This letter describes the design of a right-hand circularly polarized planar filtenna (filter + antenna) that can reconfigure its operation between two center frequencies in the Ka-band. The filtenna structure is composed of a four-element array radiator and a reconfigurable bandpass filter, occupying an area of 31 × 14 mm. The circularly polarized array elements are fed by a sequential power divider for an enhanced polarization purity. The filter is based on a ring resonator loaded with a variable open stub. The length of the stub extends through a p-i-n diode switch, resulting in a change of the loaded reactance into the ring, and consequently tuning the filter's operating center frequency from 30.3 to 29 GHz. The proposed low-loss filtenna structure exhibits a reconfigurable fractional bandwidth up to 7.74%, a peak realized gain that can reach up to 11.5 dBic, an axial ratio of less than 1 dB, and a sidelobe level that can go as low as −12 dB. The prototype filtenna is fabricated and tested, where the measured results verify the simulated data. A figure of merit is proposed to highlight the superior performance of the presented Ka-band structure in terms of the polarization, size, reconfiguration ability, and radiation characteristics as proven by the comparison with other works in the literature.