Sub-centimeter Spatial Resolution Research Articles

Enabling long-term distributed OFDR monitoring by exploiting the persistency of the Rayleigh signature

Rayleigh-based distributed fiber optic sensing offers unmatched spatial resolution and accuracy among the different fiber optic techniques. With its sub-centimeter spatial resolution and microstrain accuracy over tens of meters, this technology is ideal for precise monitoring in geotechnical and civil engineering applications.This technique utilizes Rayleigh scattering to track environmental effects on light propagation, and the measurement is done through the spectral correlation analysis of the so-called Rayleigh signature, which is specific to each fiber and can be accounted for to track variation in the light propagation due to the environment. A first measurement of the Rayleigh signature is kept at the beginning of the monitoring campaign, and it is then used as a reference to determine the change in the strain or temperature field affecting the fiber. Still, measurements in installations where fibers were employed in harsh conditions have been shown only in quite favorable conditions, i.e., by using the same specific device and set up over a short time.In this work, we investigate the enduring presence of Rayleigh’s signature in optical sensing fibers installed in challenging environments. In one site, the fibers were cast in the concrete of a bridge foundation pile, subject to an unremitting contraction over the years; in another site, the fibers were integrated within soil anchors subject to the action of an active landslide, which has imparted thousands of strain with marked localized strain peaks. Our study demonstrates that measurements obtained from optical fibers used in adverse conditions continue to exhibit a Rayleigh signature correlating to the reference one, even after a period exceeding five years since the initial measurements were taken and using different interrogators and setups.

Physically based illumination correction for sub-centimeter spatial resolution hyperspectral data

Vegetation biophysical- and chemical traits, defined on the basis of leaf area, can be retrieved from their spectral reflectance. Ultra high resolution hyperspectral images, such as ones collected from drones, allows measuring the spectra of individual leaves. The reflectance signal of such data is calibrated with respect to the top-of-canopy (TOC) irradiance, as the local illumination conditions on leaf surfaces are largely unknown and can vary significantly from the TOC conditions. We developed an inversion algorithm that uses the PROSPECT leaf radiative transfer model and the theory of spectral invariants to retrieve the actual leaf reflectance from TOC-calibrated hyperspectral images. Compared with more traditional canopy reflectance models, this approach accounts for the spatial variation in leaf-level irradiance visible in sub-centimeter-resolution images and is computationally more efficient. We used simulated and measured leaf and canopy reflectance data to validate the approach and found the retrieved leaf reflectances to match closely the actual reflectances (relative RMSD was 12% for simulated data on the average and below 10% for measured data). The proposed method provides an efficient approach for illumination correction, enabling reliable, physically based applications for monitoring vegetation biochemical and biophysical properties from ultra-high-resolution spectral imagery.

Faraday-effect polarimetry for current profile measurement in the tokamak plasma edge.

Toroidal current profile measurements in the tokamak plasma edge are critical for fusion plasma physics research and model validation. A three-wave Faraday-effect polarimeter-interferometer with a sub-centimeter spatial resolution is proposed on the DIII-D tokamak to determine the edge current profile via Abel inversion. By using probe beams with 316 µm wavelength, a low-field-side, vertical-view, single-pass optical layout covering the plasma edge region (R = 2.15-2.27m) is assessed. Measurements with no greater than 0.1° polarimetric systematic uncertainty, no greater than 0.01° polarimetric root-mean-square noise (1kHz bandwidth), and a 0.8cm radial chord spacing are considered feasible based on the achieved performance of existing systems using similar wavelengths on fusion devices. Synthetic diagnostic calculations taking various factors into account, such as diagnostic uncertainty and quality of magnetic flux surfaces, find that the edge current profile can be determined with up to 0.12 MA/m2 uncertainty, or about 10% of the peak current density in the pedestal of an investigated high-confinement plasma.

A Directional Gamma-Ray Spectrometer With Microcontroller-Embedded Machine Learning

The enhancement of a compact gamma-ray detection module for spectroscopy and imaging with machine learning for directional sensitivity is presented. In particular this development is targeted towards drone-based localization of radioactive sources in the environment. The unit is composed of a cylindrical monolithic scintillator crystal (3” <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 3” LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> +Sr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> )), read by an array of 144 solid-state SiPM detectors whose signals are conditioned by an integrated front-end. In addition to state-of-the-art energy resolution (2.6% at 662keV) and sub-centimeter spatial resolution in the reconstruction of the photon interaction point projected on the base, this portable unit enables the 2D angular localization of gamma sources on a plane parallel to the detectors array as a function of the reconstructed interaction point distribution, thanks to a decision tree. The classifier is compared with other techniques (k-NN, PCA) and optimized with 1000 splits. It runs in a 32-bit ARM micro-controller for real-time operation with a processing time of 2.75 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> per event, compatible with high gamma-ray counting rate (100kcps) operation. Despite the absence of a collimator, classification is correct within ±30° for a single photon. Angular resolution of 0.5° and accuracy better than 2° are experimentally demonstrated (along with 0.8W power consumption and 3kg weight), showing potential for identification of sources in the field.

Volume IV. The DUNE far detector single-phase technology

The preponderance of matter over antimatter in the early universe, the dynamics of the supernovae that produced the heavy elements necessary for life, and whether protons eventually decay—these mysteries at the forefront of particle physics and astrophysics are key to understanding the early evolution of our universe, its current state, and its eventual fate. DUNE is an international world-class experiment dedicated to addressing these questions as it searches for leptonic charge-parity symmetry violation, stands ready to capture supernova neutrino bursts, and seeks to observe nucleon decay as a signature of a grand unified theory underlying the standard model. Central to achieving DUNE's physics program is a far detector that combines the many tens-of-kiloton fiducial mass necessary for rare event searches with sub-centimeter spatial resolution in its ability to image those events, allowing identification of the physics signatures among the numerous backgrounds. In the single-phase liquid argon time-projection chamber (LArTPC) technology, ionization charges drift horizontally in the liquid argon under the influence of an electric field towards a vertical anode, where they are read out with fine granularity. A photon detection system supplements the TPC, directly enhancing physics capabilities for all three DUNE physics drivers and opening up prospects for further physics explorations. The DUNE far detector technical design report (TDR) describes the DUNE physics program and the technical designs of the single- and dual-phase DUNE liquid argon TPC far detector modules. Volume IV presents an overview of the basic operating principles of a single-phase LArTPC, followed by a description of the DUNE implementation. Each of the subsystems is described in detail, connecting the high-level design requirements and decisions to the overriding physics goals of DUNE.

Monitoring the vegetation vigor in heterogeneous citrus and olive orchards. A multiscale object-based approach to extract trees’ crowns from UAV multispectral imagery

Precision agriculture (PA) constitutes one of the most critical sectors of remote sensing applications that allow obtaining spatial segmentation and within-field variability information from field crops. In the last decade, an increasing source of information is provided by unmanned aerial vehicle (UAVs) platforms, mainly equipped with optical multispectral cameras, to map, monitor, and analyze, temporal and spatial variations of vegetation using ad hoc spectral vegetation indices (VIs). Considering the centimeter or sub-centimeter spatial resolution of UAV imagery, the geographic object-based image analysis (GEOBIA) approach, is becoming prevalent in UAV remote sensing applications. In the present paper, we propose a quick and reliable semi-automatic workflow implemented to process multispectral UAV imagery and aimed at the detection and extraction of olive and citrus trees’ crowns to obtain vigor maps in the framework of PA. We focused our attention on the choice of GEOBIA data input and parameters, taking into consideration its replicability and reliability in the case of heterogeneous tree orchards. The heterogeneity concerns the different tree plantation distances and composition, different crop management (irrigation, pruning, weeding), and different tree age, height, and crown diameters. The proposed GEOBIA workflow was implemented in the eCognition Developer 9.5, coupling the use of multispectral and topographic information surveyed using the Tetracam µ-MCA06 snap multispectral camera at 4 cm of ground sample distance (GSD). Three different study sites in heterogeneous citrus (Bergamot and Clementine) and olive orchards located in the Calabria region (Italy) were provided. Multiresolution segmentation was implemented using spectral and topographic band layers and optimized by applying a trial-and-error approach. The classification step was implemented as process-tree and based on a rule set algorithm, therefore easily adaptable and replicable to other datasets. Decision variables for image classification were spectral vegetation indices (NDVI, SAVI, CVI) and topographic layers (DSM and CHM). Vigor maps were based on NDVI and NDRE and allowed to highlight those areas with low vegetative vigor. The accuracy assessment was based on a per-pixel approach and computed through the F-score (F). The obtained results are promising, considering that the resulting accuracy was high, with F-score ranging from 0.85 to 0.91 for olive and bergamot, respectively. Our proposed workflow, which has proved effective in datasets of different complexity, finds its strong point is the speed of execution and on its repeatability to other different crops with few adjustments. It appears worth of interest to highlights that it requests a working day of two good skilled operators in geomatics and computer image processing, from the on-field data collection to the obtaining of vigor maps.

Monitoring temperature and vibration in a long weak grating array with short-pulse generation using a compact gain-switching laser diode module

Quasi-distributed temperature sensing and single point vibration sensing were performed. Ultrashort pulses generated by a gain-switching laser were used to interrogate a fiber Bragg gratings (FBG) array sensor. Temperature changes were measured down to 1°C with sub-centimeter spatial resolution. The advantages of our fast interrogation setup were exploited, as the higher frequency limit of a dynamic measure that can be sensed is limited by the time needed to generate the optical pulse and to acquire the data from the sensor. The experimental approach described in this paper can sense mechanical vibrations up to a frequency of 245 kHz and a strain resolution as low as 1.2 µɛ.

V-band nanosecond-scale pulse reflectometer diagnostic in the TCV tokamak.

This article describes the realization of a novel approach to short pulse (∼1 ns) reflectometry (SPR) recently implemented in the tokamak configuration variable tokamak. Taking advantage of a fast arbitrary waveform generator and vector-network-analyzer extension modules, the design offers flexibility regarding pulse output frequency, duration, and repetition rate. Such flexibility allows the instrument to overcome traditional SPR spatial sampling limitations while reducing hardware complexity. In order to measure the group-delay of nanosecond-scale pulses, both traditional analog and novel digital sampling techniques have been explored. A group-delay range resolution of 17 ps (2.6 mm) in average over the V-band has been achieved with both timing techniques against a waveguide mirror featuring 10 dB power fluctuations. Direct pulse sampling during L-mode plasmas shows that reflected pulse widths increase only by 4% in average. However, pulse width dispersion does occur in L-mode plasmas and leads to an increase in the group-delay uncertainty up to 40 ps (6 mm). Raw histograms of group-delay data show interesting qualitative changes from the L mode to the H-mode. Frequency spectra of group-delay data allow the identification of macroscopic density fluctuations as well as edge quasicoherent modes during edge-localized mode-free H-modes. Finally, fast changes to the density profile have been measured with microsecond time resolution and subcentimeter spatial resolution in both O and X-mode polarizations.

Fast and high spatial resolution distributed optical fiber sensor

Distributed optical fiber sensors (DOFS) based on stimulated Brillouin scattering (SBS) are increasingly gaining popularity due to their vast practical applications. There have been a lot of Brillouin-based distributed fiber sensing methods proposed so far but they suffer from a trade-off between spatial resolution, dynamic range and measurement time. In this paper, we propose a scanning-free Brillouin optical time domain analyzer (BOTDA) based on multiple short pulse stimulated Brillouin scattering. The accumulation of acoustic phonons which are stimulated due to the passage of successive pico-second pulses along the sensing fiber causes the sub-centimeter spatial resolution and kilometer dynamic range can be achieved simultaneously in our proposed technique called multiple short pulse BOTDA (MSP-BOTDA). Moreover, since the effective Brillouin gain bandwidth of pump pulse train is more than the natural SBS bandwidth, a wider range of temperature/strain changes can be detectable by MSP-BOTDA technique compared to conventional BOTDA method.

On the elastic anatomy of heterogeneous fractures in rock

This study examines the feasibility of the full-field ultrasonic characterization of fractures in rock. To this end, a slab-like specimen of granite is subjected to in-plane, O(104 Hz) excitation while monitoring the induced 2D wavefield by a Scanning Laser Doppler Vibrometer (SLDV) with sub-centimeter spatial resolution. Upon suitable filtering and interpolation, the observed wavefield is verified to conform with the plane-stress approximation and used to: (i) compute the maps of elastic modulus in the specimen (before and after fracturing) via a rudimentary application of the principle of elastography; (ii) reconstruct the fracture geometry; (iii) expose the fracture's primal (traction-displacement jump) contact behavior, and (iv) identify its profiles of shear and normal specific stiffness. Through the use of full-field ultrasonic data, the approach provides an unobscured, high-resolution insight into the fracture's contact behavior, foreshadowing in-depth laboratory exploration of interdependencies between the fracture geometry, aperture, interphase properties, and its seismic characteristics.

A simulation study of the spent nuclear fuel cask condition evaluation using high energy X-ray computed tomography

The dry cask storage time could be much longer (>60 years) for the spent nuclear fuels before final disposition in the United States. The casks have multiple internal components that are designed to provide structural integrity during storage and, in some cases, during transportation. The long-term internal stability of the internals and spent fuel and their cladding is important to maintain the sub-criticality of the fissile materials in the casks. In this study, we propose to develop a high-energy X-ray CT system to examine the integrity of casks and their inner components. With careful system design and advanced image reconstruction techniques, we estimate that a sub-centimeter spatial resolution can be achieved through the simulation study. A complete three-dimension “anatomical” picture of the cask and its inner components can be obtained with a reasonable scanning time.

Application of a high resolution distributed temperature sensor in a physical model reproducing subsurface water flow

A distributed temperature optical fiber sensor system with a sub-centimeter spatial resolution has been incorporated in a sand-box model at the aim of investigating the variations induced by internal erosion on the temperature distribution in a dike. In particular, the laboratory investigation aims at studying the spatial distribution of the temperature variations occurring in the surroundings of an erosion channel (pipe). The calibration of the setup consisted in measuring the thermal response of an intact sample to a horizontal flow, with the inflowing water maintained at a constant temperature higher than the room temperature. No erosion occurred in the calibration test. The results of the calibration are presented in this paper and show that with the sensing system adopted temperature mapping in a soil sample can be obtained with such a richness of detail which is not comparable with that achieved adopting a system of pointwise sensors.

Review of SPECT collimator selection, optimization, and fabrication for clinical and preclinical imaging.

In single photon emission computed tomography, the choice of the collimator has a major impact on the sensitivity and resolution of the system. Traditional parallel-hole and fan-beam collimators used in clinical practice, for example, have a relatively poor sensitivity and subcentimeter spatial resolution, while in small-animal imaging, pinhole collimators are used to obtain submillimeter resolution and multiple pinholes are often combined to increase sensitivity. This paper reviews methods for production, sensitivity maximization, and task-based optimization of collimation for both clinical and preclinical imaging applications. New opportunities for improved collimation are now arising primarily because of (i) new collimator-production techniques and (ii) detectors with improved intrinsic spatial resolution that have recently become available. These new technologies are expected to impact the design of collimators in the future. The authors also discuss concepts like septal penetration, high-resolution applications, multiplexing, sampling completeness, and adaptive systems, and the authors conclude with an example of an optimization study for a parallel-hole, fan-beam, cone-beam, and multiple-pinhole collimator for different applications.

Chalcogenide fiber-based distributed temperature sensor with sub-centimeter spatial resolution and enhanced accuracy

We demonstrate a sub-centimeter spatial resolution fiber-based distributed temperature sensor with enhanced measurement accuracy and reduced acquisition time. Our approach employs time domain analysis of backscattered Stokes and anti-Stokes photons generated via spontaneous Raman scattering in a chalcogenide (ChG) As2S3 fiber for temperature monitoring. The sensor performance is significantly improved by exploiting the high Raman coefficient and increased refractive index of the ChG fiber. We achieve a temperature uncertainty of ± 0.65 °C for a short measurement time of only 5 seconds; whilst the detection uncertainty is less than ± 0.2 °C for a longer integration time of 2 minutes. We also investigate the optimum Stokes and anti-Stokes bands for optimal sensing performance. Our theoretical analysis shows that a small detuning frequency regime from a pump is more suitable for rapid measurements while a large detuning regime provides higher temperature resolution.

Sub-Centimeter Spatial Resolution in Distributed Fiber Sensing Based on Dynamic Brillouin Grating in Optical Fibers

Optical fiber sensors based on stimulated Brillouin scattering in optical fibers have now clearly demonstrated their excellent capability for long-range distributed strain and temperature measurements. The fiber is used as sensing element and a value for temperature and/or strain can be obtained from any point along the fiber. While the spatial resolution of classical configurations is practically limited to 1 meter by the phonon lifetime, novel approaches have been demonstrated these past years that can overcome this limit. In this paper, this could be achieved by two physical processes: prior activation of a steady acoustic wave through the classical Brillouin interaction between two Brillouin pumps, and interrogation by Bragg reflection on the acoustic wave using a distinct ultra-short pulse in a highly birefringent fiber. We could achieve a spatial resolution below one centimeter, while preserving the full accuracy on the determination of temperature and strain.

Magnetic Reconnection Triggering Magnetohydrodynamic Instabilities during a Sawtooth Crash in a Tokamak Plasma

Thomson scattering measurements with subcentimeter spatial resolution have been made during a sawtooth crash in a Mega Ampere Spherical Tokamak fusion plasma. The unparalleled resolution of the temperature profile has shed new light on the mechanisms that underlie the sawtooth. As magnetic reconnection occurs, the temperature gradient at the island boundary increases. The increased local temperature gradient is sufficient to make the helical core unstable to ideal magnetohydrodynamic instabilities, thought to be responsible for the rapidity of the collapse.

Dynamic brain imaging: Event-related optical signal (EROS) measures of the time course and localization of cognitive-related activity

This paper describes the concept of dynamic brain imaging and introduces a new methodology, the event-related optical signal, or EROS. Dynamic brain imaging allows one to study noninvasively the time course of activity in specific brain areas. Brain imaging data can contribute to the analysis of the subcomponents of the human information processing system and of their interactions. Several brain imaging methods provide data that possess spatial and temporal resolution at various degrees and can be used for this purpose. In this paper, we focus on the EROS method, which yields data with millisecond temporal resolution and subcentimeter spatial resolution. We describe the fundamentals of this method and report several examples of the types of data that can be obtained with it. Finally, we discuss the possibility of combining different imaging methods, as well as the advantages and limitations that can be expected in this process.

Resolution limits for optical transillumination of abnormalities deeply embedded in tissues

Random walk theory is used to calculate the line spread function (LSF) of photons as they cross the midplane of a slab of finite thickness. The relationship between the LSF and the photon transit time in transillumination time-resolved experiments is investigated. It is found that the LSF is approximately Gaussian distributed, with a standard deviation, sigma, which can be used as a criterion of the spatial resolution of the imaging system. Results are substantiated by comparison with actual data in the literature. Any given resolution can be improved by reducing the excess transit time delta t, but heterogeneity of the scattering medium and low levels of detected light enormously complicate the achievement of subcentimeter spatial resolution. The latter point is discussed by using optical parameters of breast tissues for visible and near-infrared radiation (NIR) light.