Microwave and Optical Technology Letters最新文献

This work investigates flat-band slow-light performance in photonic crystal waveguides featuring ring-shaped holes. Using Finite Element Method based numerical optimization, we show that precise tuning of the inner radii and spatial arrangement of the first two rows of holes strongly governs the dispersion profile. For the first time, a combined optimization strategy employing independent tuning of both ring dimensions and bi-directional positional shifts in the first two rows is demonstrated to enhance slow-light performance in RPCWs. The optimized designs achieve ultra-low dispersion across 16–31 nm bandwidths, with group indices of 11.2–14.85. A best-performing structure delivers a normalized delay–bandwidth product of 0.264 and group velocity dispersion ≈10⁵ps²/km, representing a clear improvement over previously reported ring-hole PCW designs. These results demonstrate a robust pathway for engineering high-performance slow-light waveguides suitable for compact delay lines and enhanced nonlinear photonic devices.

In this paper, a balanced nonreciprocal linear-phase bandpass filter (BPF) based on time-modulated resonators is reported for the first time. A linear-phase response is achieved by integrating group delay equalization circuits adjacent to both the balanced input and output ports of a balanced nonreciprocal BPF, implemented on a microstrip circuit. Compared to conventional passive BPFs, the balanced nonreciprocal BPFs based on time-modulated resonators exhibits different group delay characteristics. This work introduces an innovative group delay equalization circuit, specifically designed for nonreciprocal BPFs based on time-modulated resonators. By utilizing resonators with different resonant frequencies under common-mode and differential-mode excitation, the balanced circuit provides effective common-mode suppression, which enhances immunity to electromagnetic interference and crosstalk compared to single-ended circuits. A microstrip prototype was fabricated for the proof-of-concept demonstration, and the measurements show good agreement with simulations.

In this paper, we proposed a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor system for detecting dissolved carbon monoxide (CO) in transformer oil. The sensor system was designed to be highly sensitive, compact, and cost-effective. An on-beam configured acoustic micro-resonator (AmR) coupled with a quartz tuning fork functions as an acoustic transducer to enhance photoacoustic signals. This sensor system achieved a minimum detection limit (MDL) of 0.37 ppm for CO based on the absorption line with a wavelength of 2333.72 nm. This value is three orders of magnitude lower than the safety threshold specified in relevant standards for dissolved gas detection in transformer oil. The proposed method offered high sensitivity, low cost, a compact form factor, and ease of integration, offering a novel technical solution for dissolved gas detection in transformer oil.

The Born approximation (BA) and Rytov approximation (RA) are common in solving electromagnetic inverse scattering problems (ISPs). However, purely applying these approximations only yields rough inversion results. To this end, a noniterative combined modified method using the real part (CMM-R) is presented to extend the limitation in accuracy and effectiveness encountered by traditional Born/Rytov-based approaches. Based on the connection between modified RA (MRA) and modified BA (MBA), the complementary strengths of MRA and MBA are integrated. Specifically, these two modified inversion methods are implemented by employing the dominant current, whereas only the real part of MRA is used to mitigate phase wrapping issues. The simulation and experimental results demonstrate that the proposed CMM-R achieves higher accuracy than traditional modified Born/Rytov-based methods for imaging mild scatterers. As a noniterative method, CMM-R provides a robust initial guess for further iterative algorithms.

This paper presents a compact ultra-wideband (UWB) power divider that uses microstrip lines to achieve stable performance over a wide frequency range. The circuit is designed for wideband operation as well as being easily fabricated using standard FR-4 substrate. Simulation results show that a bandwidth of 5.38 GHz is achieved in the frequency range of 1.6–7.0 GHz and the minimum S₁₁ value is –31.78 dB. The measured transmission coefficients (S₂₁ and S₃₁) are in close agreement with the simulated data, both around –3.56 dB, confirming consistent power splitting. The proposed design has been optimized through a detailed parametric study and demonstrates improved isolation and reduced reflection while maintaining simple rectangular geometry. Compared with previous multilayer or high-cost laminate designs, the proposed FR-4-based divider achieves 125% fractional bandwidth and high fabrication tolerance while maintaining a simple geometry suitable for wide-band microwave and communication systems.

This work proposes an improved shooting and bouncing ray (SBR) method for calculating the near-field scattering of electrically large objects. In near-field calculations, there is a divergence problem in the ray tube of the SBR method, which exacerbates the splitting of the ray tube. The traditional near-field SBR method suffers from accuracy loss, as the problem of ray tubes aperture divergence is only solved in first-order scattering calculations and is ignored in higher-order scattering calculations. This problem can be solved by dividing the ray tubes into denser ones, but this will reduce computational efficiency of the SBR method. This work proposes an adaptive tube subdivision method to control the ray tube aperture size and ensure ray tracing accuracy, which can effectively solve the problem of computational accuracy loss caused by tube divergence in near-field calculations, while taking into account the computational efficiency. Several examples are designed to verify the effectiveness of the method.

This paper presents a comprehensive study on the gain enhancement and higher Isolation of a 28 GHz MIMO antenna system for 5 G communications using Frequency Selective Surfaces (FSS). The antenna under study is designed with dimensions of 2.86λ × 2.86λ × 0.047λ; the ground patches have a dimension of 7.6 × 11 mm², the substrate is a Rogers RT885 (εr = 2.2, height = 0.51 mm), and operating at 28 GHz. A 4-element MIMO antenna array is formed by orthogonally placed elements in order to optimise spatial diversity. To improve isolation between the antenna elements, a mutual coupling reduction technique is employed. Furthermore, a 36-element FSS array is strategically placed above the MIMO antenna to significantly boost the gain. The initial gain performance without the FSS is 4.2 dBi at 28 GHz, by placing the FSS surface 7 mm above the MIMO antenna, this increases the gain to 8.4 dBi, resulting in a gain enhancement of approximately 3.6 dBi. Simulation results, obtained using CST Microwave Studio, demonstrate notable improvements in the antenna gain. The integration of FSS proved to be an effective solution for meeting the stringent performance requirements of 5 G communication systems, particularly in high-frequency mmWave applications. The present work aims to enhance gain, get high efficiency, improve the isolation between the ports, good value of ECC.

In this letter, a filtering antipodal Vivaldi antenna (AVA) integrating spoof surface plasmon polaritons (SSPPs) and directors, featuring controllable passband characteristics and high gain, is proposed. In the antenna configuration, a set of slots etched along the outer edges of the radiation arms improves the low-frequency gain, while antenna-terminal-loaded semi-annular directors further enhance the overall gain. The filtering characteristics can be conveniently tuned by adjusting the dimensions of the branched circular metallic patch and the SSPPs structure. A prototype operating from 7.44 to 11.76 GHz was fabricated and experimentally characterized to validate the design. Measurement results demonstrate an impedance bandwidth of 45%, stable in-band gain with a peak value of 12.14 dBi, and inherent advantages of wide bandwidth and low profile. Compared with conventional AVAs, the proposed design exhibits improved radiation characteristics, achieving both higher gain and superior filtering performance.