Near-field Measurements Research Articles

Measuring near-field wave elevation with monocular vision using self-supervised learning techniques

Ocean waves are among the most influential factors affecting marine mechanical systems. The existing methods for measuring wide-area wave-field data have limitations and require high maintenance costs. Therefore, it is crucial to develop a reliable and efficient field measurement method for wide-area wave fields. This study proposes a monocular vision-based approach for wave field measurement, where the depth of each pixel is equalised to summation of the depth of the mean water surface and the depth variation induced by wave elevation. A deep network was trained using a self-supervised algorithm to approximate the depth variation induced by wave elevation. A loss of representative features was proposed to improve the performance of deep networks. The images of an open-source dataset were used to train and validate the proposed model. By using the proposed method, the images were utilised to reconstruct the three-dimensional wave field of the test set. The time history and wave spectrum of the wave fields were compared with those calculated by existing method and measured by wave gauge. The influence of the training set on the measurement results was also investigated. The results indicate that the proposed method is capable of performing effective near-field wave measurements by monocular images, effectively reducing the device requirements, and increasing update frequency for ocean wave measurement.

Measuring Velocity and Discharge of High Turbidity Rivers Using an Improved Near-Field Remote-Sensing Measurement System

Large-scale particle image velocimetry (LSPIV) is a computer vision-based technique renowned for its precise and efficient measurement of river surface velocity. However, a crucial prerequisite for utilizing LSPIV involves camera calibration. Conventional techniques rely on ground control points, thus restricting their scope of application. This study introduced a near-field remote-sensing measurement system based on LSPIV, capable of accurately measuring river surface velocity sans reliance on ground control points. The system acquires gravity-acceleration data using a triaxial accelerometer and converts this data into a camera pose, thereby facilitating swift camera calibration. This study validates the system through method verification and field measurements. The method verification results indicate that the system’s method for retroactively deriving ground control-point coordinates achieves an accuracy exceeding 90%. Then, field measurements were performed five times to assess the surface velocity of the Datong River. These measured results were analyzed and compared with data collected from the radar wave velocity meter (RWCM) and the LS1206B velocity meter. Finally, a comprehensive sensitivity analysis of each parameter was conducted to identify those significantly impacting the river’s surface velocity. The findings revealed that this system achieved an accuracy exceeding 92% for all river surface velocities measured.

Design of Orbital Angular Momentum Antenna Array for Generating High-Order OAM Modes

Orbital angular momentum (OAM) modes can offer high density and high-capacity communication. The traditional phased array antenna can only produce a limited number of OAM beam modes l, usually less than half of the number of array elements (N): −N/2 < lmax < N/2. An OAM antenna array for generating high-order OAM modes is proposed in this letter. The proposed antenna array consists of a ring patch antenna that can generate vortex waves with OAM mode l = 1 or −1 and a phase-shifting feeding network. By adding different feed excitation signals to each element, the generated beam carries a higher-order mode: l = N or −N, breaking the previous limitations. Near-field measurements were conducted on antenna arrays composed of 3, 4, and 5 elements, revealing a high degree of correspondence between their phase distribution and radiation patterns with numerical simulation results. This alignment further substantiates the practical efficacy of this approach in significantly enhancing the generation of high-order OAM modes within antenna arrays. This advancement improves component utilization efficiency, reduces system complexity, and meets the high demands for spectral resources and channel capacity in future communication applications.

Underwater radiated noise from a submerged cylinder: measurements in far and near field conditions

Measurements of radiated pressure from a cylinder hull immersed in water are of interest for naval research. Generally, the far-field pressure pattern in a wide frequency range is required. We compare the direct measurement of the far-field underwater noise generated by a vibrating cylinder submerged in a large dock to the far-field prediction obtained via near-field measurements of the same structure in a reverberant tank. Both measurements require advanced signal processing to minimize the impacts of undesired factors such as low signal-to-noise ratio or with disturbing echos. The results associated with both environments are confronted and provide general guidance to evaluate the radiated sound power.

Relativistic Time-of-Arrival Measurements: Predictions, Post-Selection and Causality Problems

We analyze time-of-arrival probability distributions for relativistic particles in the context of quantum field theory (QFT). We show that QFT leads to a unique prediction, modulo post-selection that incorporates properties of the apparatus into the initial state. We also show that an experimental distinction of different probability assignments is possible especially in near-field measurements. We also analyze causality in relativistic measurements. We consider a quantum state obtained by a spacetime-localized operation on the vacuum, and we show that detection probabilities are typically characterized by small transient non-causal terms. We explain that these terms originate from Feynman propagation of the initial operation, because the Feynman propagator does not vanish outside the light cone. We discuss possible ways to restore causality, and we argue that this may not be possible in measurement models that involve switching the field–apparatus coupling on and off.

Off-axis limiting photometric distance of Lambertians and narrow beams

The near-field - far-field disparity of light sources leads to discussion within the lighting community. In the far-field, light sources are approximated by a point source with a luminous intensity distribution from which other photometric quantities can be computed. In the near-field, light sources must be considered as extended sources and no closed-loop analytical solutions can be found for the illuminance in off-axis directions. The illuminance allows to compute the apparent intensity used for the assessment of the limiting photometric distance (LPD), that is, the threshold between near- and far-field regions. Numerically, the illuminance can be determined through a discretisation of the luminous surface. This approach is verified for on- and off-axis directions through direct comparison with ray tracing simulations based on near-field goniophotometric measurements. A good match is observed for Lambertian and narrow beam light sources. Using the numerical approach, the LPD in all directions is assessed. For light sources of which the luminous intensity strictly decreases while moving away from the optical axis, the LPD decreases as well until a minimal value is reached. After this, the LPD increases again, in certain scenarios up to values resulting in the invalidity of the inverse square law.

Scanning near-field optical microscopy measurements and simulations of regularly arranged silver nanoparticles

Silver nanoparticles on a glass substrate are experimentally investigated by aperture scanning near-field optical microscopy (a-SNOM). To understand the experimental results, finite-element-method simulations are performed building a theoretical model of the a-SNOM geometry. We systematically vary parameters like aperture size, aluminum-coating thickness, tip cone angle, and tip-surface distance and discuss their influence on the near-field enhancement. All these investigations are performed comparatively for constant-height and constant-gap scanning modes. In the end, we establish a reliable and stable optical model for simulating a-SNOM measurements, which is capable of reproducing trends observed in experimental data.

Hyperbolic Polaritonic Rulers Based on van der Waals α-MoO3 Waveguides and Resonators.

Low-dimensional, strongly anisotropic nanomaterials can support hyperbolic phonon polaritons, which feature strong light-matter interactions that can enhance their capabilities in sensing and metrology tasks. In this work, we report hyperbolic polaritonic rulers, based on microscale α-phase molybdenum trioxide (α-MoO3) waveguides and resonators suspended over an ultraflat gold substrate, which exhibit near-field polaritonic characteristics that are exceptionally sensitive to device geometry. Using scanning near-field optical microscopy, we show that these systems support strongly confined image polariton modes that exhibit ideal antisymmetric gap polariton dispersion, which is highly sensitive to air gap dimensions and can be described and predicted using a simple analytic model. Dielectric constants used for modeling are accurately extracted using near-field optical measurements of α-MoO3 waveguides in contact with the gold substrate. We also find that for nanoscale resonators supporting in-plane Fabry-Perot modes, the mode order strongly depends on the air gap dimension in a manner that enables a simple readout of the gap dimension with nanometer precision.

Near-field modeling of the head-related transfer function by using the dummy head sound source

Various approaches based on direct measurement have been used to obtain the precise headrelated transfer function (HRTF). However, several problems persist, such as scattering of sound source and distance effect, which are related to the measurement configuration. In this work, the modified dummy head having two sound sources located at the ear positions is applied as the sound source reciprocally to overcome the trouble. By using the near-field acoustic holography, the sound sources located in the ear holes are reconstructed from the measured data in the near-field hologram. One can predict the acoustic transfer function between the ear holes and the field positions at any point using the recovered source information described in spherical harmonics. The obtained transfer function is valid for predicting the binaural response from any sound source position. This configuration is possible to measure the near-field response with the relatively small scattering effect because of the low Helmholtz number of microphones. The far-field reconstruction of HRTF is also possible with the near-field measurement data using acoustical holography. The proposed method is demonstrated with the modified B&K HATS system, in which the microphones in the ear holes are replaced by pipe sources connected to the loudspeaker.

Bianisotropic Metasurface for Spatial Separation of Multiple Harmonic Waves

An approach to tuning harmonic responses of surface impedance is introduced to spatially separate fundamental, third-harmonic, and fifth-harmonic waves by focusing on the physical implementation perspective. The concept of the bianisotropic metasurface is adopted to refract the wave toward different angles by assigning identical phase gradients at fundamental and third-harmonic frequencies. On the other hand, high-sheet impedance is realized by maintaining the in-phase condition at the fifth harmonic to bypass the incident wave without refraction. The target properties of each unit cell are implemented using I-shaped patterns based on a three-stacked-layer topology. Different sizes and intervals are applied for the I-shaped patterns, and only their horizontal lengths are tuned to achieve the desired sheet impedances. The feasibility of the proposed approach is validated for the fundamental frequency of 10 GHz and two of odd harmonics through far-and near-field measurements. The results confirm that the fabricated metasurface separates the superimposed harmonic waves toward 44 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , and 0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> with the measured refraction efficiencies of 65.6%, 67.2%, and 74% at 10, 30, and 50 GHz, respectively.

Modeling of experimentally observed topological defects inside bulk polycrystals

A rigorous methodology is developed for computing elastic fields generated by experimentally observed defect structures within grains in a polycrystal that has undergone tensile extension. An example application is made using a near-field high energy x-ray diffraction microscope measurement of a zirconium sample that underwent tensile extension from an initially well-annealed state. (Sub)grain boundary features are identified with apparent disclination line defects in them. The elastic fields of these features identified from the experiment are calculated.

Deeply subwavelength integrated excitonic van der Waals nanophotonics

The wave nature of light sets a fundamental diffraction limit that challenges confinement and control of light in nanoscale structures with dimensions significantly smaller than the wavelength. Here, we study light–matter interaction in van der Waals MoS2 nanophotonic devices. We show that light can be coupled and guided in structures with dimensions as small as ≃λ/16 (∼60nm at 1000 nm excitation wavelength), while offering unprecedented optical field confinement. This deep subwavelength optical field confinement is achieved by exploiting strong lightwave dispersion in MoS2. We further study the performance of a range of nanophotonic integrated devices via far- and near-field measurements. Our near-field measurements reveal detailed imaging of excitation, evolution, and guidance of fields in nanostructured MoS2, whereas our far-field study examines light excitation and coupling to highly confined integrated photonics. Nanophotonics at a fraction of a wavelength demonstrated here could dramatically reduce the size of integrated photonic devices and opto-electronic circuits with potential applications in optical information science and engineering.

Shift-variant deconvolution beamforming using Implicit Neural Representation for near-field acoustic image measurement

The conventional beamforming (CBF) output can be expressed as a convolution of the source distribution and the array dependent point spread function (PSF), which is defined as the response of the beamformer to a point source. The resolution and accuracy of sound source localization can be improved by deconvolution of CBF. The shift-invariant PSF assumption is only a good approximation when the source region is small compared to the distance between the array and the source. However, the PSF of the near-field underwater acoustic map measurement is shift-variant, whose mismatch with the beam could lead to performance degradation. In this paper, we proposed an optimized deconvolution method by using the neural network called implicit neural representation (INR) to solve the beam mismatch problem, due to its strong performance in learning and reconstructing spatial scenes. Rather than recompute the PSF for each pixel in the scene, INR predicts the complex-valued weight matrix in two-dimensional space that encapsulates both the spatially varying properties of the PSF and the scatter distribution of the scene. Numerical simulations and experimental results verify the effectiveness of our method. And this work can be broadly applied to arrays with shift-variant PSFs.

Method of acoustic separation on the surface of cylindrical baffle

This paper suggests a statistically optimal near-field acoustic holography method based on the combination of plane waves and cylindrical waves, which is used for underwater cylindrical baffles. The method addresses the issue that the extraction of scattered acoustic waves from the surface of underwater cylindrical baffles requires too many measurement points in practical applications: separation of incident and dispersed sound fields and near-field measurements of sound fields at baffle surfaces.The underwater detection sound source is situated in the scatterer's far-field region, so in the actual application scenario for the surface acoustic scattering separation of the cylindrical baffle, the plane wave expansion is used to represent the surface incident acoustic wave and the cylindrical wave expansion is used to describe the scattered acoustic wave on the baffle surface. As a result, fewer expansion orders are required to describe the entire sound field on the baffle's surface. It is verified by simulation that the method is valid and efficient for near-field total sound field measurement and dispersed sound field separation of roughly cylindrical targets under underwater far-field incidence situations because the measurement holographic surface of the surface permits unlimited geometries.

Flow and acoustic fields investigation of noise reduction by micro vortex generators in supersonic nozzles

The role of micro-vortex generators (MVGs) in supersonic jet noise reduction is investigated. Studies are performed to understand the noise reduction mechanisms of these devices. MVGs are implemented on a scale model representative of GE-F404 nozzles. Configurations consisting of single and dual arrays of MVGs were tested for pressure ratios relevant to takeoff operating conditions. A combination of laboratory measurements and large-eddy simulations are used. Particle image velocimetry reveals that MVGs placed inside the nozzle form oblique shocks that interact with the jet plume's shock cell structure and greatly modify the shock-cell structure issued from the nozzle throat. This modification can weaken the jet plume shock-cell structure and even reduce the jet core velocities. Convective velocities estimated from the high-speed Schlieren imaging showed a significant reduction in magnitude caused by MVGs. When compared to the baseline design, the MVG nozzles showed noise reduction up to 10 dB in the upstream direction and near 5 dB in the peak downstream radiation angle at cold jet conditions. Near-field acoustic measurements showed a significant reduction of low-frequency turbulent mixing noise. Scaling analysis showed that MVGs are capable of delivering the same noise reduction performance across different model scales.

Accurate and efficient planar near-field measurements: a new perspective from electromagnetic information theory

Accurate and efficient planar near-field measurements: a new perspective from electromagnetic information theory

A Linear-Scan-Based Wideband Single-Cut Near-Field RCS Measurement Technique With Wide Effective Angle Coverage

A linear-scan-based wideband near-field radar cross section (RCS) measurement technique for achieving wide effective-angle-coverage is proposed in this letter. Both the data preprocessing and linear near-field to far-field transformation (LNF2FFT) techniques are used to ensure the accurate measurement of wideband far-field RCS in a wide azimuthal angle range. With the linear-scan-based near-field sampling strategy, the object under test (OUT) keeps stationary during the measurement process, and the equipment of turntable is not required. Moreover, based on the 2D plane wave expansion, LNF2FFT is proposed to translate the linearly sampled near-field data into far-field RCS pattern. An adaptive time gating strategy is also proposed to deal with the varied background property in liner samplings. Serval important practical factors relating to the measurement accuracy, such as phase center calibration, background subtraction and calibration, are also discussed in detail. Two illustrative test examples are presented, in which RCS measurement with wide effective-angle-coverage of ±35° and the average wideband measurement errors less than 0.4 dB are achieved in 1 m limited measurement range.

Phaseless Spherical Near-Field Antenna Measurements With Reduced Samplings

Phaseless Spherical Near-Field Antenna Measurements With Reduced Samplings

Fault Diagnosis of Realistic Arrays From a Reduced Number of Phaseless Near-Field Measurements

Fault Diagnosis of Realistic Arrays From a Reduced Number of Phaseless Near-Field Measurements

Near-Field Gain Expression for Tapered Circular Aperture Antennas

Near-field electromagnetic wave, which is radiated by tapered circular aperture antennas, has found more engineering applications in many diverse fields. Through the simulation of a reflector antenna whose efficiency is up to 51.4%, it can be found that the aperture distribution cannot be regarded as uniform distribution but as tapered distribution. Thus, expressions for near fields of uniform circular apertures cannot be applied to tapered circular aperture antennas in practical engineering. To alleviate such problem, near-field calculation formulas are proposed based on the Sommerfeld integrals with steepest descents technique, which ensures the accurate and efficient analysis of the near field of the tapered circular aperture antenna. And near-field characteristics of tapered circular aperture antennas are displayed. The maximum difference of normalized near-field amplitudes between tapered and uniform distribution is 0.35V/m. Three-dimensional near-field gain of circular aperture antennas is developed from the near-field radiation pattern. The proposed expression extends the range of calculation from the axial to more directions compared with the previous expression. Also, this expression is valid in both the near field and the far field. Using full-wave simulations, results obtained from the proposed expression are compared with the simulated results. And the deviation in near-field gain is less than 0.3dB, which indicates the convenience and effectiveness of our expression. The proposed near-field gain expression can be applicable to the near-field measurement and design of millimeter or terahertz radar systems that work in the near-field region.