Aperture Plane Research Articles

Inexpensive SAR testbed for 3D mmWave imaging applications

AbstractIn this paper, the authors investigate and develop a low‐cost synthetic aperture radar (SAR) testbed and utilise it to capture and reconstruct high‐resolution 3D mmWave SAR images. Despite much attention from both industrial and academic scholars, cost restraints and complexity have hindered the deployment of SAR systems, particularly in the mmWave domain. This paper outlines the major components and systems of the 3D SAR testbed. The authors build a testbed with low‐cost, commercially available hardware components and include open‐source software that is highly reconfigurable and simple to reproduce. The multiple‐input multiple‐output (MIMO) radar sensor uses a frequency‐modulated continuous wave (FMCW) chirp at the V‐band (40–75 GHz) and synthesises a rectilinear two‐dimensional planar aperture. The authors presented several reconstructed 3D images imitating real‐world scenarios using the testbed. A non‐linear sampling technique that has significantly decreased the amount of time required for data acquisition was implemented. In addition, the authors employed multiple image quality metrics to quantify and compare the images produced by the testbed. The testbed can be used to test and validate new techniques and algorithms.

Optical performance analysis of a hybrid parabolic trough collector with photovoltaic slats.

Small-sized parabolic trough collectors are a promising solution for renewable heat supply, meeting the industrial demand for thermal energy up to 250°. In this manuscript, a novel, to our knowledge, optical design hybridizing parabolic trough concentrators with photovoltaic generators is introduced, incorporating actionable photovoltaic slats in the aperture plane. This configuration allows efficient operation under diffuse irradiance and improves electricity production when direct irradiation is insufficient. Optical simulations using OTSunWebApp software demonstrate that the inclusion of photovoltaic slats does not significantly reduce optical efficiency. The hybrid collector allows simultaneous or exclusive production of thermal and photovoltaic energy, adapting to various energy demand conditions.

Phase synthesis method in multi-aperture optical systems based on iterative image processing algorithms

Background. Modern technologies in the field of optics and photonics place high demands on the quality and output power of the radiation source. The fulfillment of these requirements can currently be achieved by creating multi-aperture laser systems with coherent beam addition. The main problem of creating such systems is the development of phase synthesis methods in a multichannel optical system. Aim. In this paper, we consider a phase synchronization method without a reference beam for a coherent addition system with active feedback, which is based on the Gerchberg–Saxton algorithm. This algorithm makes it possible to reconstruct complex field amplitudes in the aperture and focal planes from intensity distributions in these fields. The application of this algorithm for multichannel systems is analyzed and its features, such as the occurrence of stagnation and ambiguity of the solution, are revealed. Methods. Solutions are proposed to eliminate the problem of convergence of the algorithm to side solutions and getting the iterative procedure into the local extremum using global optimization algorithms, methods of reducing the dimension of the problem and the introduction of antisymmetric amplitude modulation. Results. The paper demonstrates the results of phase field reconstruction for a large number of optical sources. For a seven-aperture system, a physical simulation was performed to restore phase information, confirming the results of numerical modeling. Conclusion. The proposed approach is a reasonable alternative to currently existing methods and can be used in the problem of coherent addition in multichannel optical systems.

TE-polarized leaky-wave beam launchers: Generation of Bessel and Bessel–Gauss beams

The generation of focused beams in the millimeter- and submillimeter-wave ranges, with transverse-electric (TE) polarization, is investigated in the radiative near-field region. The desired field distribution is achieved through a leaky-wave beam launcher consisting of a grounded dielectric slab with an annular strip grating on top excited by a circular slot on the bottom ground plane. The latter is fed by a Marié transducer, which converts the input, fundamental TE10 mode of a standard rectangular waveguide into the higher-order TE01 mode propagating in the circular waveguide connected to the device. The generation of TE-polarized diffraction-limited Bessel and Bessel–Gauss distributions is achieved by suitably synthesizing the annular strip grating. Simulated results are in excellent agreement with those predicted by leaky-wave analysis providing a proof-of-concept for the generation of TE-polarized Bessel and Bessel–Gauss beams at 300 GHz with a beam size of 1.7 and 1.9 mm up to the nondiffractive range of about 25 and 15 mm from an aperture plane with radius of 12.75 mm, respectively.

Far-field Pattern Analysis Based on Piecewise Aperture Field Integration for Leighton Chajnantor Telescope Antenna under Gravity and Wind Load

The Caltech Submillimeter Observatory telescope will be relocated from Mauna Kea, Hawaii to Chajnantor Plateau, Chile, refurbished there and renamed the Leighton Chajnantor Telescope (LCT). At the new site, the open-air environment, as well as the high altitude, will amplify the influence of gravity and wind load on structural deformation, thus affecting the antenna’s radiation characteristics, which can be characterized by a far-field pattern. To quickly and accurately compensate for the effect of reflector structural deformation on the far-field pattern of LCT’s antenna, we propose a far-field pattern quick calculation method (FPQCM) to analyze the antenna’s radiation characteristics under gravity and various wind loads, which combines the optical path difference calculation method on the aperture plane, the piecewise Zernike polynomial function, and the piecewise aperture field integration method. Through extensive experimental comparisons with the numerical method based on physical optics and physical theory of diffraction, it is validated that the proposed FPQCM significantly reduces the time by more than 100 times for acquiring the far-field patterns while maintaining the accuracy of the main figure of merits at the highest frequency (i.e., 856 GHz). In addition, we analyzed the far-field pattern of the antenna at different zenith angles, gravity, and winds of different strengths and directions by FPQCM, which provided guidance for performing more precise control through an active surface optimization system after LCT resumes operation, thus facilitating the acquisition of higher surface accuracy and better observation effect.

Delineation of the impact on temporal behaviors of off-axis photoemission in an ultrafast electron microscope

Efforts to push the spatiotemporal imaging-resolution limits of femtosecond laser-driven ultrafast electron microscopes (UEMs) to the combined angstrom–fs range will benefit from stable sources capable of generating high bunch charges. Recent demonstrations of unconventional off-axis photoemitting geometries are promising, but connections to the observed onset of structural dynamics are yet to be established. Here we use the in-situ photoexcitation of coherent phonons to quantify the relative time-of-flight (r-TOF) of photoelectron packets generated from the Ni Wehnelt aperture and from a Ta cathode set-back from the aperture plane. We further support the UEM experiments with particle-tracing simulations of the precise electron-gun architecture and photoemitting geometries. In this way, we measure discernible shifts in electron-packet TOF of tens of picoseconds for the two photoemitting surfaces. These shifts arise from the impact that the Wehnelt-aperture off-axis orientation has on the electron-momentum distribution, which modifies both the collection efficiency and the temporal-packet distribution relative to on-axis emission. Future needs are identified; we expect this and other developments in UEM electron-gun configuration to expand the range of material phenomena that can be directly imaged on scales commensurate with fundamental structural dynamics.

Laboratory demonstration of image plane self-calibration in interferometry

We demonstrate the shape-orientation-size conservation principle for a three-element interferometer using aperture plane masking at the ALBA visible synchrotron radiation light source. We then use these data to demonstrate image plane self-calibration.

A rigorous theory on electromagnetic diffraction by a planar aperture in a perfectly conducting screen

In this paper, we revisit the classic problem of diffraction of electromagnetic waves by an aperture in a perfectly conducting plane. We formulate the diffraction problem using a boundary integral equation that is defined on the aperture using Dyadic Green’s function. This integral equation turns out to align with the one derived by Bethe using fictitious magnetic charges and currents. We then investigate the boundary integral equation using a saddle point formulation and establish the well-posedness of the boundary integral equation, including the existence and uniqueness of the solution in an appropriately defined Sobolev space.

Nanoscale dark-field imaging in full-field transmission X-ray microscopy

The dark-field signal uncovers details beyond conventional X-ray attenuation contrast, which is especially valuable for material sciences. In particular, dark-field techniques are able to reveal structures beyond the spatial resolution of a setup. However, its implementation is limited to the micrometer regime. Therefore, we propose a technique to extend full-field transmission X-ray microscopy by the dark-field signal. The proposed method is based on a well-defined illumination of a beam-shaping condenser, which allows to block the bright field by motorized apertures in the back focal plane of the objective lens. This method offers a simple implementation and enables rapid modality changes while maintaining short scan times, making dark-field imaging widely available at the nanometer scale.

A Review of the Antenna Field Regions

We review the field regions and their boundaries around an electromagnetic source. We consider the cases of sources whose dimensions are comparable or larger than the wavelength, of planar sources/apertures, and of sources whose dimensions are small with respect to the wavelength and the criteria involving the strength of the reactive components of the electromagnetic field with respect to the radiative ones. The Fraunhofer and the Fresnel Regions are detailed, along with references to the paraxial approximation for planar apertures. The near-field and intermediate regions are also discussed. We review the standard boundaries between the regions. However, the standard boundaries are not clearly marked, nor are the regions uniquely defined. Accordingly, we also discuss different criteria that have been proposed during the years, which depend on the application and typically rely on numerical arguments, but are not necessarily universally accepted.

Tunable liquid-based lenses for ultrasonic beamforming

Quasi-planar stepped lenses (QSL’s) based on phasing have proven to perform closely to conventional refractive lenses for the generation of both focused and limited diffraction beams. However, since QSLs are passive and made from solid materials, each must be designed with a specific frequency and beam type in mind. In this presentation, we report on a tunable lens with a planar aperture that employs liquid channels to perform phase-based beamforming. The desired phase patterns are produced using specific speed-of-sound profiles and can be flexibly configured for a range of frequencies and beam types without modification. In this talk, we report on the results of both experimental and numerical results for this class of lenses and discuss their potential applications.

A Quick Calculation Method for Radiation Pattern of Submillimeter Telescope with Deformation and Displacement

Radiation pattern captures the electromagnetic performance of reflector antennas, which is significantly affected by the deformation of the primary reflector due to gravity and the displacement of the secondary reflector. During the design process of large reflector antennas, a substantial amount of time is often dedicated to iteratively adjusting structural parameters and validating electromagnetic performance. To improve the efficiency of the design process, we first propose an approximate calculation method of optical path difference (OPD) for the deformation of the primary reflector under gravity and the displacement of the secondary reflector. Then an OPD fitting function based on the modified Zernike polynomials is proposed to capture the phase difference of radiation over the aperture plane, based on which the radiation pattern will be obtained quickly by the aperture field integration method. Numerical experiments demonstrate the effectiveness of the proposed quick calculation method for analyzing the radiation pattern of a 10.4 m submillimeter telescope antenna at its highest operating frequency of 856 GHz. In comparison with the numerical simulation method based on GRASP (which is an antenna electromagnetic analysis tool combining physical optics (PO) and physical theory of diffraction (PTD)), the quick calculation method reduces the time for radiation pattern analysis from more than one hour to less than two minutes. Furthermore, the quick calculation method exhibits excellent accuracy for the figure of merit (FOM) of the radiation pattern. Therefore, the proposed quick calculation method can obtain the radiation pattern with high speed and accuracy. Compared to the time-consuming numerical simulation method (PO and PTD), it can be employed for quick analysis of the radiation pattern for the lateral displacement of the secondary reflector and the deformation of the primary reflector under gravity in the design process of a reflector antenna.

Learning-based surface deformation recovery for large radio telescope antennas

Abstract The surface deformation of the main reflector in a large radio telescope is closely related to its working efficiency, which is important for some astronomical science studies. Here, we present a deep learning-based surface deformation recovery framework using non-interferometric intensity measurements as input. The recurrent convolutional neural network (RCNN) is developed to establish the inverse mapping relationship between the surface deformation of the main reflector and the intensity images at the aperture plane and at a near-field plane. Meanwhile, a physical forward propagation model is adopted to generate a large amount of data for pre-training in a computationally efficient manner. Then, the inverse mapping relationship is adjusted and improved by transfer learning using experimental data, which achieves a 15-fold reduction in the number of training image sets required, which is helpful to facilitate the practical application of deep learning in this field. In addition, the RCNN model can be trained as a denoiser, and it is robust to the axial positioning error of the measuring points. It is also promising to extend this method to the study of adaptive optics.

Optical and thermal investigation of hyperbolic cavity receiver with secondary reflector for solar parabolic dish collector

A hyperbolic-shaped cavity-type receiver with a second stage reflection for a 40 m2 solar parabolic dish concentrator (PDC) is presented in this paper,along with optical and thermal modeling. The receiver geometry and the secondary reflector complete the hyperbolic geometrical shape of the present receiver. This study aims to estimate heat losses owing to natural convection and radiation under influential parameters associated with solar parabolic dish system. Optical modeling has been performed with verified model using ASAP® 2013 V1R1. The optical simulations have been carried out for varying receiver height (h1), height of the secondary (h2) and diameter-to-total height ratios (d/h) of 0.5, 1, and 1.5 while keeping the diameter of the plane aperture of the receiver (d) constant. The optimum receiver geometry has been selected based on the performance in terms of optical efficiency. The maximum efficiency found is 91.72% for h1 = 0.75 m and d/h = 0.5 for the mounting distance of 4.21 m. The optimized shape is further studied to determine how the receiver absorptivity and the secondary reflector's reflectivity affect the optical performance. The absorptivity and reflectivity range from 0.86 to 0.94. Obtained optimum receiver geometry from the optical investigation is further analyzed to estimate the heat transfer rate. The thermal modeling is performed for 0°, 30°, 60°, and 90° orientations of the receiver in ANSYS Fluent 2020 R1. A comprehensive comparison among the available models has been reported and validated with well-established experimental data. This work has developed free convective Nusselt number correlation. It can be a valuable addition to the existing literature.

Structural Design of Planar Synthetic Aperture Radar (SAR) Antenna for Microsatellites

This paper presents the structural design of a planar synthetic aperture radar (SAR) antenna applied to a microsatellite. For micro-satellite applications, the SAR antenna structure must be lightweight, flat, and designed to withstand the launch environment. To satisfy these conditions, our novel antenna structure was designed using aluminium (AL) alloy. Structural analysis was performed for quasi-static load, random vibration, and shock load to verify its robustness in the launch environment, and the results are presented here.

Multi-objective optimization of a solar photovoltaic thermal water collector using Grey Wolf Optimizer

Solar photovoltaic thermal (PV-T) collectors offer enhanced overall efficiency owing to reduced heat losses at the aperture plane and the potential for increased irradiance per unit area in comparison to larger prototypes. In spite of its promise, investigations into optimizing PV-T collector performance remain limited. This paper introduces a novel approach: the multi-objective optimization of a hybrid PV-T water system with a channel-type absorber design, utilizing the Grey Wolf Optimizer. The proposed approach employs regression models to depict key performance metrics—average thermal and electrical efficiencies. Optimal parameters, including rate of flow of cooling fluid, temperature at inlet, and slope of the solar panel, are determined and reported. In this paper, Grey Wolf Optimization algorithm, has been used to systematically explore the interaction of various PV-T parameter values. Optimal values, in line with the defined objective function(s) were obtained. This approach enabled the researchers to concurrently ascertain the comprehensive performance of the collector. Remarkably, the optimization process revealed a unique insight. Despite the inherently conflicting nature of thermal and electrical efficiencies as observed in single-objective optimization outcomes, the multi-objective MOGWO approach unveiled a solution where compromise was attained.

Lorentz near-field electron ptychography

Over the past few years, electron ptychography has drawn considerable attention for its ability to recover high contrast and ultra-high resolution images without the need for high quality electron optics. In this Letter, we focus on electron ptychography's other potential benefits: quantitatively mapping phase variations resulting from magnetic and electric fields over extended fields of view. To this end, we propose an implementation of near-field ptychography that employs an amplitude mask located in the electron microscope's condenser aperture plane. We demonstrate the capabilities of our method by imaging a magnetic Permalloy sample and compare our results with those of off-axis electron holography.

A closed-form expression of electric polarizability for elliptical apertures excited by nonuniform electric field

It is well known that electric field penetration through a small aperture in a perfectly conducting plane behaviors like an electric dipole. When excited by uniform electric field, the dipole moment has analytical solution for elliptical apertures and numerical/experimental values for other shapes. In this paper, for the case of nonuniform field excitation, based on the electrostatic reciprocity theorem, the closed-form expression of electric dipole moment is derived for elliptical apertures. This expression is in integral form, and its integrand is the product of the nonuniform electric field distribution and the potential distribution when excited by uniform field in the aperture plane. Taking a point charge located on the aperture plane as an example of nonuniform field excitation, the dipole moment is calculated using the proposed closed-form expression. The validation is verified by comparisons with exact integral solution for circular apertures and with finite element simulations for elliptical apertures.

Angle-resolving spectral ellipsometry using structured light for direct measurement ofellipsometric parameters.

We propose a compact angle-resolving spectral ellipsometry. Using the structured light generated from a digital micro-mirror device (DMD), what we believe to be a novel pattern is illuminated to the back focal plane of the high numerical aperture (NA) objective lens. As a result, ellipsometric parameters with fine resolution of both the wavelength and incidence angle domain can be directly measured. The incidence angle can be resolved by resolution under 1° ranging from 35° to 59° by the radius of the projected images. A spectrometer as a detector enables acquisition by the resolution of 0.7nm from 410 to 700nm, and the fiber reduces measurement spot size to a single micrometer. Additionally, the measurement process does not require any rotating optical components or moving parts, needing only digital modification of the projected image. This simplifies the sequences and reduces the measurement time. The 2D (angle of incidence and spectral domain) ellipsometric parameter plane measured by the proposed method was used to measure the thickness of various samples. The measurement result was verified in comparison with a commercial ellipsometer. The accuracy and precision of the result show that the proposed method is capable of precise measurement of thin films.

Study of Potential Application Air Curtains in Livestock Premises at Cattle Management Farms

Recommendations on the selection of air curtains and the calculation of their parameters for livestock premises in cattle management farms are made. The air curtain functioning principle is analyzed from the air jet theory point of view. The block diagram and modular design of air curtains with a variable air jet direction vector and with controlled slit width are designed. Laboratory tests of the newly designed air curtain structure are performed in accordance with the microclimate requirements for the cattle management farm premises. Based on the experimental results, the major air curtain parameters are calculated for the range from 10° to 60° of angle α between the direction of the air jet outward from the air curtain slit and aperture plane, and for the air curtain slit width b0 in the range from 0.05 m to 0.15 m with the account of the wind speed Vw variations. Calculated values for amounts of energy that have to be consumed to ensure the required air jet velocity, in the output from the air curtain, and those for the quantity of thermal energy required to heat the air supplied to the air curtain, depending on the angle α and on the slit width b0, can be helpful for selecting the power capacity of both the air curtain fan and electric heater. A block diagram of the air curtain control for cattle management farm premises is designed, enabling automatic control of the airflow rate, the angle of the air jet output from the air curtain slit, and the temperature of the heated air supplied to the air curtain, considering particular climate conditions. According to the preliminary estimate, applications of the newly designed air curtain will make it possible to reduce the energy consumed to maintain the required microclimate conditions in cattle management premises by 10% to 15% in the cold period.