Physical Aperture Research Articles

Enhanced dual-mode imaging: Superior photoacoustic and ultrasound endoscopy in live pigs using a transparent ultrasound transducer.

Dual-mode photoacoustic/ultrasound endoscopy (ePAUS) is a promising tool for preclinical and clinical interventions. To be clinically useful, ePAUS must deliver high-performance ultrasound imaging comparable to commercial systems and maintain high photoacoustic imaging performance at long working distances. This requires a transducer with an intact physical aperture and coaxial alignment of acoustic and optical beams within the probe, a challenging integration task. We present a high-performance ePAUS probe with a miniaturized, optically transparent ultrasonic transducer (TUT) called ePAUS-TUT. The 1.8-mm-diameter probe, fitting into standard endoscopic channels, aligns acoustic and optical beams efficiently, achieving commercial-level ultrasound and high-resolution photoacoustic imaging over long distances. These imaging capabilities were validated through in vivo imaging of a rat's rectum and a pig's esophagus. The ePAUS-TUT system substantially enhances feasibility and potential for clinical applications.

Fast high voltage pulser for the southern advanced photon source

The Southern Advanced Photon Source (SAPS) is a planned fourth-generation synchrotron radiation facility featuring a low-emittance storage ring. For the H-7BA lattice design, the dynamic aperture is significantly smaller than the physical aperture, necessitating the use of a strip-line kicker based on an on-axis injection scheme. To minimize interference from the pulser on adjacent beams, a nanosecond pulser with stringent rise and fall time requirements is essential. A prototype pulser, based on a semiconductor opening switch, was developed to meet these demands. A nonlinear transmission line loaded with ferrite was utilized for pulse shaping, yielding favorable results. The prototype generated a pulse at a 50 Ω load with a rise time (10%–90%) of 2.4 ns, a fall time (90%–10%) of 3.9 ns, a pulse width (10%–10%) of 7.8 ns, and an amplitude of 19.8 kV. This paper presents the detailed design, optimization, and performance evaluation of the pulser.

DOA Estimation Based on Virtual Array Aperture Expansion Using Covariance Fitting Criterion

Providing higher precision Direction of Arrival (DOA) estimation has become a hot topic in the field of array signal processing for parameter estimation in recent years. However, when the physical aperture of the actual array is small, its aperture limitation means that even with super-resolution estimation algorithms, the achievable estimation precision is limited. This paper takes a novel approach by constructing an optimization algorithm using the covariance fitting criterion based on the array output’s covariance matrix to fit and obtain the covariance matrix of a large aperture virtual array, thereby providing high-precision angular resolution through virtual aperture expansion. The covariance fitting expansion analysis and discussion are unfolded for both uniform linear arrays (ULAs) and sparse linear arrays (SLAs) under four different scenarios. Theoretical analysis and simulation experiments demonstrate that these methods can enhance the effective performance of angle estimation, especially in low signal-to-noise ratios (SNRs) and at small angular intervals by fitting virtual extended aperture data.

High intensity beam dynamics assessment and challenges for HL-LHC

The High Luminosity (HL-LHC) project aims to increase the integrated luminosity of CERN's Large Hadron Collider (LHC) by an order of magnitude compared to its initial design. This requires a large increase in bunch intensity and beam brightness compared to the first three LHC runs, and hence poses serious collective-effects challenges, related in particular to electron cloud, instabilities from beam-coupling impedance, and beam-beam effects. Here, we present the associated constraints and the mitigation measures proposed to achieve the baseline performance of the upgraded LHC machine. We also discuss the interplay of these mitigation measures with other aspects of the accelerator, such as optics, physical and dynamic apertures, the collimation system, and crab cavities. Additional potential sources of intensity limitations are also briefly discussed.

Optimising laser-induced phased-arrays for defect detection in continuous inspections

This paper proposes a novel optimisation approach for laser induced phased-arrays for detection purposes in large area inspection. The optimisation aims to maximise the detection capability of a phased-array while minimising the inspection time. This is done by using a simulation ray-tracing model that evaluates multiple candidate configurations. The metrics that measure the detection performance for a target defect are the receiver operating characteristic curve and the signal-to-noise ratio. The inspection time is indirectly computed through the number of measurement points per unit length in a continuous inspection. This depends on the physical aperture and pitch of the phased-array and they are the quantities optimised. The optimisation and modelling approach are validated in an experimental case study for a side-drilled hole as target defect. Further recommendations about directional defects (e.g. cracks at different angles) are provided.

DOA estimation based on sparse Bayesian learning with moving synthetic virtual array

AbstractIn scenarios with constrained physical aperture sizes, aiming to enhance the resolution and accuracy of Direction of Arrival (DOA) estimation, this paper proposes a novel approach that integrates a moving synthetic virtual array with Sparse Bayesian Learning (SBL) for DOA estimation. Initially, a virtual array is constructed based on the motion characteristics of the target. Subsequently, the SBL method is employed to estimate the DOA of the target. Simulation experiments validate the effectiveness of this approach, demonstrating comparable DOA estimation performance to synthetic aperture methods with larger aperture sizes, even in situations with limited aperture expansion. Furthermore, under constant virtual aperture expansion, this method surpasses non‐SBL methods regarding DOA resolution.

CoIR: Compressive Implicit Radar.

Using millimeter wave (mmWave) signals for imaging has an important advantage in that they can penetrate through poor environmental conditions such as fog, dust, and smoke that severely degrade optical-based imaging systems. However, mmWave radars, contrary to cameras and LiDARs, suffer from low angular resolution because of small physical apertures and conventional signal processing techniques. Sparse radar imaging, on the other hand, can increase the aperture size while minimizing the power consumption and read out bandwidth. This paper presents CoIR, an analysis by synthesis method that leverages the implicit neural network bias in convolutional decoders and compressed sensing to perform high accuracy sparse radar imaging. The proposed system is data set-agnostic and does not require any auxiliary sensors for training or testing. We introduce a sparse array design that allows for a 5.5× reduction in the number of antenna elements needed compared to conventional MIMO array designs. We demonstrate our system's improved imaging performance over standard mmWave radars and other competitive untrained methods on both simulated and experimental mmWave radar data.

Optimization of the Physical Aperture of the Parabolic Reflector Antenna

Optimization of the Physical Aperture of the Parabolic Reflector Antenna

High-resolution photogrammetry to measure physical aperture of two separated rock fracture surfaces

Photogrammetry, reconstructing three-dimensional (3D) models from overlapping two-dimensional (2D) photos, finds application in rock mechanics and rock engineering to extract geometrical details of reconstructed objects, for example rock fractures. Fracture properties are important for determining the mechanical stability, permeability, strength, and shear behavior of the rock mass. Photogrammetry can be used to reconstruct detailed 3D models of two separated rock fracture surfaces to characterize fracture roughness and physical aperture, which controls the fluid flow, hydromechanical and shear behavior of the rock mass. This research aimed to determine the optimal number of scale bars required to produce high-precision 3D models of a fracture surface. A workflow has been developed to define the physical aperture of a fracture using photogrammetry. Three blocks of Kuru granite (25 cm × 25 cm × 10 cm) with an artificially induced fracture, were investigated. For scaling 3D models, 321 markers were used as ground control points (GCPs) with predefined distances on each block. When the samples were well-matched in their original positions, the entire block was photographed. Coordinate data of the GCPs were extracted from the 3D model of the blocks. Each half was surveyed separately and georeferenced by GCPs and merged into the same coordinate system. Two fracture surfaces were extracted from the 3D models and the vertical distance between the two surfaces was digitally calculated as physical aperture. Accuracy assessment of the photogrammetric reconstruction showed a 20–30 μm digital control distance accuracy when compared to known distances defined between markers. To attain this accuracy, the study found that at least 200 scale bars were required. Furthermore, photogrammetry was employed to measure changes in aperture under normal stresses. The results obtained from this approach were found to be in good agreement with those obtained using linear variable displacement transducers (LVDTs), with differences ranging from 1 μm to 8 μm.

A Feasibility Study for a Hand-Held Acoustic Imaging Camera

Acoustic imaging systems construct spatial maps of sound sources and have potential in various applications, but large, cumbersome form factors limit their adoption. This paper investigates methodologies to miniaturize acoustic camera systems for improved mobility. Our approach optimizes planar microphone array design to achieve directional sensing capabilities on significantly reduced footprints compared to benchmarks. The current prototype utilizes a 128−microphone, 50 × 50 cm2 array with beamforming algorithms to visualize acoustic fields in real time but its stationary bulk hampers portability. We propose minimizing the physical aperture by carefully selecting microphone positions and quantities with tailored spatial filter synthesis. This irregular array geometry concentrates sensitivity toward target directions while avoiding aliasing artefacts. Simulations demonstrate a 32−element, ≈20 × 20 cm2 array optimized this way can outperform the previous array in directivity and noise suppression in a sub-range of frequencies below 4 kHz, supporting a 4× surface factor reduction with acceptable trade-offs. Ongoing work involves building and testing miniature arrays to validate performance predictions and address hardware challenges. The improved mobility of compact acoustic cameras could expand applications in car monitoring, urban noise mapping and other industrial fields limited by current large systems.

PMMA based compound kinoform lenses for sub-150 nm hard x-rays focusing

It is the first time to fabricate the polymethyl methacrylate (PMMA) based compound kinoform lenses (CKLs) for nano-focusing of hard x-rays. The PMMA CKLs with a physical aperture of 160 µm and a focal length of 20 mm are designed and fabricated by x-ray LIGA technology at the Beijing Synchrotron Radiation Facility. To improve the stiffness of the PMMA lines, PMMA sheets with high molecular weight are fabricated, and the results revealed that the PMMA lines with higher molecular weight were much stronger. After optimization, CKLs with the narrowest lines of 4 µm width can achieve 110 µm height and 2.99 nm surface roughness without deformation. The CKLs were tested at the Diamond Light Source, which provided a focal width of 122 nm for 12 keV x rays. Though the PMMA has a poor radiation tolerance, it has a better process tolerance, and it can lay a good foundation for the fabrication of SU8 based CKLs by LIGA technology, which has a better radiation tolerance. Compared to the Si CKLs fabricated by dry etching, the PMMA CKLs are higher in depth, smoother at the sidewall, and have better nanofocusing.

Beamspace-based underdetermined DOA estimator via Khatri-Rao subspace for quasi-stationary signals

The Khatri-Rao subspace methods utilize the characteristics of the Khatri-Rao product of the array response to achieve the extension of the physical aperture and further resolve the quasi-stationary signals in the underdetermined cases. However, the estimation performance of the existing Khatri-Rao subspace methods suffers from the input signal-to-noise (SNR). Especially for a low SNR, the estimation accuracy of the local covariance matrix signals decreases, which will further result in the DOA estimation performance degradation. To address these problems, an enhanced Khatri-Rao subspace method based on beamspace processing for the DOA estimation is proposed. The total array is first split into several identical subarrays, and the beamspace transformation for the data received by subarrays is implemented via beamforming. This results in the improvement of the SNR and further improves the noise suppression ability for the low SNR regions. Then, the Khatri-Rao subspace processing is used for the separated beamspace sources to realize the aperture extension. Finally, the DOAs for the separated sources in the beamspace domain can be further obtained when combined with the subspace-based methods, and the undetermined problem can also be achieved by combining the DOAs obtained from different directions in the beamspace domain.

A Compact Axial-Mode Helical Antenna Based on Spoof Surface Plasmon Polaritons

A compact axial-mode helical antenna (AMHA) is presented based on spoof surface plasmon polaritons (SSPPs). The proposed antenna consists of an SSPP waveguide, a matching metal strip, and a reflective metal ground. To achieve miniaturization and good circular polarization (CP) performance, an SSPP waveguide formed by a periodically slotted metal strip wound around a uniform foam cylinder surface is designed as the radiation part of AMHA. Compared with the traditional AMHA using metallic strip line, the SSPP AMHA can reduce the length of metal strip by 20%, while the physical aperture, height, and volume are reduced by 36%, 20%, and 49%, respectively. Experimental results demonstrate that the antenna has good radiation performance in the operation frequency from 3.3 to 4.7 GHz with a gain of 9.6 dBic at the central frequency. In addition, the size of the proposed SSPP AMHA can be further significant reduced if the foam rod in the center of helix is replaced by a high-permittivity dielectric rod. The proposed SSPP AMHA has the advantages of compact size and good CP radiation performance, which may find potential applications in radar and wireless communication systems.

Angular Momentum Multiplexing via a Shared Aperture Patch Antenna

Angular momentum (AM) multiplexing technique provides multiple information channels for electromagnetic waves (EMWs) modulation at the same frequency, improving spectrum efficiency effectively. Among AM multiplexing techniques, the patch antenna has the advantage of minimization, simple feeding circuits, and easy integration. Nevertheless, limited reports provide theoretical guidance for designing AM multiplexing patch antennas. Herein, we propose a scheme to achieve AM multiplexing via a shared-aperture patch antenna. The radiating structure consists of multiple radiating patches sharing a common physical aperture, each of which is responsible for producing different AM-carrying waves. By analyzing the theoretical model built on a dipole representation of the patch antenna, the shapes and the desired current patterns of these patches can be determined according to the AM mode of the expected EMWs. Besides, the sizes and feeding structures of these patches can be studied by investigating their radiating characteristics using characteristic mode (CM) theory. For proof-of-concept, a prototype was designed, fabricated, and measured for the generation of four orthogonal AM-carrying waves coaxially at 5.8GHz. The experimental results agree reasonably well with the simulated ones, demonstrating that the proposed scheme provides an efficient guidance for the design of AM multiplexing patch antennas.

Innovative design and optimization of a two-dimensional deployable nine-grid planar antenna mechanism with a flat reflection surface

With the development of the aerospace industry, space missions are becoming more complicated and diversified, and there is a demand for antenna mechanisms with a larger physical aperture. In this paper, a planar deployable mechanism is proposed, which can form a flat reflection surface with a small gap between plates. To this end, a novel large-scale two-dimensional deployable nine-grid planar antenna mechanism is designed. First, two antenna folding schemes and four supporting mechanism schemes are proposed. Through comparison analysis, the antenna configuration scheme with the best comprehensive performance is selected. A kinematic model of the deployable mechanism is established, and its kinematic characteristics are analyzed. Then, the correctness of the kinematic model is verified by comparing the analytical and simulation results of the kinematic model. Subsequently, a finite element model of the antenna is developed. Based on the response surface method, the structural parameters of the support rods of the antenna are optimized, and a set of optimized solutions with lightweight and high fundamental frequency characteristics are obtained. Finally, a prototype of the proposed nine-grid planar antenna is fabricated. The feasibility of the deployment principle and the rationality of the designed mechanism are verified by deployment experiments.

Symmetric Extended Nested Array for Passive Localization of Mixture Near- and Far-Field Sources

Nowadays, symmetric nonuniform linear arrays (SNLAs) have been considered for mixture near-field (NF) and far-field (FF) source localization since it can provide larger physical array aperture and higher degree-of-freedom (DOF) than uniform linear arrays (ULAs). In this communication, a symmetric extended nested array (SENA) is proposed on the basis of compressed symmetric nested array (CSNA). The closed-form expression for the SENA and its difference coarray (DCA) is presented in detail. For the sake of comparison to the existing SNLAs with sensors, the longer consecutive DCA lags and larger physical array aperture are obtained with optimal design to achieve better estimation results. Numerical results demonstrate that utilizing SS-MUSIC and MUSIC algorithms, the SENA outperforms exiting SNLAs in terms of the DOA and range estimate accuracy for mixture field sources.

Distributed Antenna in Drone Swarms: A Feasibility Study

Unmanned aerial vehicles offer a versatile platform for the realization of phased array antenna systems, enabling multiple antenna elements to be distributed spatially in an agile, flexible, and cost-effective manner. Deploying individual antenna elements on single drones and using a swarm of such drones to create an antenna array has the potential to be a disruptive technology. Antenna directivity is limited by the physical aperture size as compared to the wavelength of the radiation being transmitted/received, with electrically larger antennas giving a higher directivity at the cost of an increased size and weight. The authors presented a brief feasibility study using a simple mathematical model implemented in software to explore the predicted performance of the novel UAV deployed antenna array, the limitations of such a system, and the potential applications where such a capability would be beneficial. The authors concluded that it is possible to achieve a suitably coherent superposition of electromagnetic radiation at frequencies of ~1 GHz and lower with current global positioning technologies which offer centimeter scale positioning accuracy and with current drone positioning systems used to control drone swarms.

Rectifying unexpected injection issues due to an elliptically polarizing undulator

Top-up operation requires high efficiency injection with insertion devices at small gaps. We present a case study of one elliptically polarizing undulator at the Canadian Light Source that unexpectedly began having a detrimental impact on injection efficiency after operating for many years without issue. By performing beam-based measurements in the storage ring and magnetic measurements in our magnetic measurement laboratory, we built a model that provided an explanation and a remedy. An unrelated change in the vertical limiting physical aperture coupled to an octupole resonance caused a loss of dynamic aperture that explained the decreased injection efficiency. The remedy was to change the on-axis tunes of the storage ring, avoiding the resonance. We hope this detailed and thorough case study may help other laboratories to maintain high injection efficiency while requiring a minimum of restrictions on insertion device operating ranges.

Integrated sensing and communication in 6G: a prototype of high resolution multichannel THz sensing on portable device

6G is believed to go beyond communication and provide integrated sensing and computing capabilities for a vision of Connected Intelligence with everything connected, everything sensed, and everything intelligent. Integrated sensing and communication will play a vital role for the fusion of physical and cyber worlds. The exploration of higher frequency bands, larger bandwidth, and more advanced large antenna technologies is paving the way towards the goal. In particular, the study of THz opens the possibility to have high resolution sensing and imaging capability on a communication mobile device. In this paper, we take a step along this direction and justify such possibility by building a THz sensing prototype with millimeter level imaging resolution while considering the physical aperture constraint of typical mobile device.

A New Sparse Optimal Array Design Based on Extended Nested Model for High-Resolution DOA Estimation

In this paper, we propose a new strategy with increased element spacings placed on two-side subarrays and increase the physical sparse sensors array aperture. In order to ensure the continuity of difference co-array, four conditions are proposed. Based on these conditions, a new array structure named TS2-ENA is derived. Compared with other nested-like arrays with a fixed number of elements, our proposed structure possesses a higher degree of freedom, closed-form expression and outstanding performance. Simulation results show the superiority of the new configuration with the spacial smoothing MUSIC algorithm conducted.