Interferometric Imaging Research Articles

Simultaneous Widefield Voltage and Dye-Free Optical Mapping Quantifies Electromechanical Waves in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes.

Coupled electromechanical waves define a heart's function in health and diseases. Optical mapping of electrical waves using fluorescent labels offers mechanistic insights into cardiac conduction abnormalities. Dye-free/label-free mapping of mechanical waves presents an attractive non-invasive alternative. In this study, we developed a simultaneous widefield voltage and interferometric dye-free optical imaging methodology that was used as follows: (1) to validate dye-free optical mapping for quantification of cardiac wave properties in human iPSC-cardiomyocytes (CMs); (2) to demonstrate low-cost optical mapping of electromechanical waves in hiPSC-CMs using recent near-infrared (NIR) voltage sensors and orders of magnitude cheaper miniature industrial CMOS cameras; (3) to uncover previously underexplored frequency- and space-varying parameters of cardiac electromechanical waves in hiPSC-CMs. We find similarity in the frequency-dependent responses of electrical (NIR fluorescence-imaged) and mechanical (dye-free-imaged) waves, with the latter being more sensitive to faster rates and showing steeper restitution and earlier appearance of wavefront tortuosity. During regular pacing, the dye-free-imaged conduction velocity and electrical wave velocity are correlated; both modalities are sensitive to pharmacological uncoupling and dependent on gap-junctional protein (connexins) determinants of wave propagation. We uncover the strong frequency dependence of the electromechanical delay (EMD) locally and globally in hiPSC-CMs on a rigid substrate. The presented framework and results offer new means to track the functional responses of hiPSC-CMs inexpensively and non-invasively for counteracting heart disease and aiding cardiotoxicity testing and drug development.

Periodic Distributions and Ultrafast Dynamics of Hot Electrons in Plasmonic Resonators.

Understanding and managing hot electrons in metals are of fundamental and practical interest in plasmonic studies and applications. A major challenge for the development of hot electron devices requires the efficient and controllable generation of long-lived hot electrons so that they can be harnessed effectively before relaxation. Here, we report the ultrafast spatiotemporal evolution of hot electrons in plasmonic resonators. Using femtosecond-resolution interferometric imaging, we show the unique periodic distributions of hot electrons due to standing plasmonic waves. In particular, this distribution can be flexibly tuned by the size, shape, and dimension of the resonator. We also demonstrate that the hot electron lifetimes are substantially prolonged at hot spots. This appealing effect is interpreted as a result of the locally concentrated energy density at the antinodes in standing hot electron waves. These results could be useful to control the distributions and lifetimes of hot electrons in plasmonic devices for targeted optoelectronic applications.

QuantifyingP-wave secondary microseisms events: a comparison of observed and modelled backprojection

SUMMARYSecondary microseisms are caused by nonlinear interactions between ocean waves of approximately equal wavelengths and opposite propagation directions. This seismic forcing is evaluated using ocean sea-state hindcast data and further modulated by the bathymetric effect. The numerical ocean model provides a global activity representation of the secondary microseisms, from which we isolate major events. We backprojected teleseismic P-wave propagation into the Earth's mantle to validate these events as effective seismic sources. The ocean model provides spectral amplitude information for modelling microseisms generated seismic wavefield. A comparison of the backprojection for P and PP phases from observed and synthetic microseisms forcing indicates high reliability in the ocean model, at least for major sources. A combination of P and PP phases detected across a global network of stations enables global ocean coverage. We improve backprojection images even further by introducing a two-step stacking for the P phase to address the problem of unbalanced station distribution. Thresholds of microseisms events forces valuable for seismic imaging are determined by comparing backprojections and ocean models for the years 2015 and 2020. Finally, we extracted a catalogue of microseisms events every 3-hr from 1994 to 2020 from the ocean hindcast data set. This catalogue is an intriguing resource for future applications of interferometric imaging at large scale.

Field maturity detection via interferometric synthetic aperture radar images time-series: a case study for maize crop

ABSTRACT Detecting the field maturity moment for maize (Zea mays L.) crop represents a relevant point to estimate its optimal harvest time. Knowing the optimal harvest time (defined by grain moisture content) at the end of the crop season is a major concern for maize farmers, as it could lead to substantial economic losses if not harvested on time. For this crop, optimal harvest time usually occurs 3–4 weeks after field maturity, depending on weather conditions. Therefore, this study focused on the interferometric coherence time-series analysis at the end of the maize crop season, to indirectly estimate the field maturity. For such purpose, a coherence object-based change detection method using Sentinel-1 SAR images was developed aiming to estimate the potential field maturity time. These estimations were assessed using an independent data set of field maturity dates obtained through field inspection and crop growth modelling. The technique was tested over 52 fields in the northwest region of Kansas, United States, with a detection rate of 80%, and a field maturity estimation error of 10 days (assessed with the root mean square error). The proposed method constitutes a promising approach to estimating the maize field maturity in near-real time, determining the field harvest readiness, and developing a decision support tool to assist farmers in prioritizing the allocation of fields at harvest time.

Kelvin‐Helmholtz Billows in the Troposphere and Lower Stratosphere Detected by the PANSY Radar at Syowa Station in the Antarctic

AbstractWe conducted two 10‐day observational campaigns in 2019 targeting turbulence in the troposphere and lower stratosphere by adopting a frequency domain radar interferometric imaging technique using Program of the Antarctic Syowa radar (PANSY radar) and radiosonde observations obtained at Syowa Station in the Antarctic. Seventy three Kelvin‐Helmholtz (K‐H) billows were detected, and two characteristic cases likely excited by the same cyclone were examined in detail. In the first case with the longest observational duration of ∼6.5 hr, the K‐H billows had a thickness of ∼800 m and a horizontal wavelength of ∼2,500 m. According to a numerical simulation, continuously existing gravity waves associated with the cyclone maintained strong vertical wind shear sufficient to cause the K‐H instability. In the second case with the deepest thickness of ∼1,600 m, the K‐H billows had a horizontal wavelength of ∼4,320 m. Numerical simulation suggested that an enhanced upper‐tropospheric jet associated with a well‐developed synoptic‐scale cyclone caused the K‐H instability. Such background conditions, frequently observed in the Antarctic coastal region, are typical mechanisms for K‐H excitation. Linear stability analysis also indicated that the characteristics of the observed K‐H billows were consistent with the most unstable modes. Furthermore, statistical analysis was performed using data of all 73 observed cases. The characteristics of K‐H billows observed at Syowa Station are similar to those observed over Japan. However, the K‐H billows tend to have longer wave periods over Syowa Station than over Japan, likely due to the weaker tropospheric jet in the Antarctic.

Vertical autofocus for the phase screen in a turbulent ionosphere

The performance of spaceborne synthetic aperture radars (SARs) is affected by the Earth’s ionosphere. In particular, the ionospheric turbulence causes phase perturbations of the SAR signals, which may lead to image distortions. A convenient way to model those phase perturbations is by means of a phase screen. The latter is an infinitesimally thin layer positioned at a certain elevation above the Earth’s surface. The radar signal acquires an instant perturbation once its trajectory intersects the screen. The trajectory is a ray between the antenna and the target, and the magnitude of the perturbation is equal to the screen density at the intersection point. The density is a bivariate function of the coordinates along the screen. The coordinates of a specific intersection point are determined by the ray itself, as well as the screen elevation. Thus, the magnitude of the phase perturbation explicitly depends on the screen elevation. Accordingly, to compensate for the resulting image distortions one should be able to determine the elevation of the screen. In the paper, we develop an algorithm of vertical autofocus that derives this elevation from the received SAR data, given a pair of point scatterers in the target area. The proposed algorithm exploits a modification of the coherent interferometric imaging that was previously employed to reduce the effect of phase errors due to the trajectory uncertainty. In our analysis, we highlight the differences between this case and transionospheric propagation.

Multifrequency Black Hole Imaging for the Next-generation Event Horizon Telescope

The Event Horizon Telescope (EHT) has produced images of the plasma flow around the supermassive black holes in Sgr A* and M87* with a resolution comparable to the projected size of their event horizons. Observations with the next-generation Event Horizon Telescope (ngEHT) will have significantly improved Fourier plane coverage and will be conducted at multiple frequency bands (86, 230, and 345 GHz), each with a wide bandwidth. At these frequencies, both Sgr A* and M87* transition from optically thin to optically thick. Resolved spectral index maps in the near-horizon and jet-launching regions of these supermassive black hole sources can constrain properties of the emitting plasma that are degenerate in single-frequency images. In addition, combining information from data obtained at multiple frequencies is a powerful tool for interferometric image reconstruction, since gaps in spatial scales in single-frequency observations can be filled in with information from other frequencies. Here we present a new method of simultaneously reconstructing interferometric images at multiple frequencies along with their spectral index maps. The method is based on existing regularized maximum likelihood (RML) methods commonly used for EHT imaging and is implemented in the eht-imaging Python software library. We show results of this method on simulated ngEHT data sets as well as on real data from the Very Long Baseline Array and Atacama Large Millimeter/submillimeter Array. These examples demonstrate that simultaneous RML multifrequency image reconstruction produces higher-quality and more scientifically useful results than is possible from combining independent image reconstructions at each frequency.

Radio and mid-infrared properties of the blazar J1419–0838

The radio quasar NVSS J141922−083830 (J1419−0838) was initially classified as an uncategorised blazar-type object, following its detection in the gamma -ray band with the Fermi space telescope. Later, using multi-waveband observations and modeling, its was found to be a flat-spectrum radio quasar (FSRQ). However, its radio emission has never been discussed in depth in the literature. Here we present a detailed analysis on the radio properties of J1419−0838 using archival interferometric imaging data at pc and kpc scales. We conclude that the flux density variations, the flat radio spectrum, the compact nature of the quasar structure at all scales, and the relativistic Doppler enhancement of the radio emission all support the previous classification as an FSRQ. We also investigated the short- and long-term mid-infrared (MIR) light curve of the quasar based on observations by the Wide-field Infrared Survey Explorer satellite, and found that there is significant variability on time-scales of days as well as years. Comparison of the MIR light curve to the times of previously reported gamma -ray and optical flares shows no clear correlation between the events at different wavebands.

Centimeter-wave Free-space Neural Time-of-Flight Imaging

Depth sensors have emerged as a cornerstone sensor modality with diverse applications in personal hand-held devices, robotics, scientific imaging, autonomous vehicles, and more. In particular, correlation Time-of-Flight (ToF) sensors have found widespread adoption for meter-scale indoor applications such as object tracking and pose estimation. While they offer high depth resolution at competitive costs, the precision of these indirect ToF sensors is fundamentally limited by their modulation contrast, which is in turn limited by the effects of photo-conversion noise. In contrast, optical interferometric methods can leverage short illumination modulation wavelengths to achieve depth precision three orders of magnitude greater than ToF, but typically find their range is restricted to the sub-centimeter. In this work, we merge concepts from both correlation ToF design and interferometric imaging; a step towards bridging the gap between these methods. We propose a computational ToF imaging method that optically computes the GHz ToF correlation signal in free space before photo-conversion. To acquire a depth map, we scan a scene point-wise and computationally unwrap the collected correlation measurements. Specifically, we repurpose electro-optical modulators used in optical communication for ToF imaging with centimeter-wave signals, and achieve all-optical correlation at 7.15 GHz and 14.32 GHz modulation frequencies. While GHz modulation frequencies increase depth precision, these high modulation rates also pose a technical challenge. They result in dozens of wraps per meter which cannot be estimated robustly by existing phase unwrapping methods. We tackle this problem with a proposed segmentation-inspired phase unwrapping network , which exploits the correlation of adjacent GHz phase measurements to classify regions into their respective wrap counts. We validate this method in simulation and experimentally, and demonstrate precise depth sensing using centimeter wave modulation that is robust to surface texture and ambient light. Compared to existing analog demodulation methods, the proposed system outperforms all of them across all tested scenarios. CCS Concepts: • Computing methodologies ;

Estimating the statistical uncertainty due to spatially correlated noise in interferometric images

Interferometers (e.g., ALMA and NOEMA) allow us to obtain the detailed brightness distribution of astronomical sources in three dimensions (R.A., Dec., and frequency). However, the spatial correlation of the noise makes it difficult to evaluate the statistical uncertainty of the measured quantities and the statistical significance of the results obtained. The noise correlation properties in the interferometric image are fully characterized and easily measured by the noise autocorrelation function (ACF). We present the method for (1) estimating the statistical uncertainty due to the correlated noise in the spatially integrated flux and spectra directly, (2) simulating the correlated noise to perform a Monte Carlo simulation in image analyses, and (3) constructing the covariance matrix and chi-square χ2 distribution to be used when fitting a model to an image with spatially correlated noise, based on the measured noise ACF. We demonstrate example applications to scientific data showing that ignoring noise correlation can lead to significant underestimation of statistical uncertainty of the results and false detections/interpretations.

A combination interferometric and morphological image processing approach to rapid quality assessment of additively manufactured cellular truss core components

Advanced manufacturing (AM) processes such as laser powder bed fusion (LPBF) are increasingly capable of fabricating components with useful and unprecedented mechanical properties by incorporating complex internal bracing structures. From the standpoint of quality control and assessment, however, internally complex assemblies present significant build-verification challenges. Here we propose a hybrid approach to the inspection involving the application of computer-aided speckle interferometry (CASI) and morphological image processing as a rapid, inexpensive, and facile method for AM quality control. The described methodology has low capital equipment costs, is full-field and non-contact, can be used in an industrial setting, and has very low requirements in terms of operator training and expertise. Consisting primarily of the combination of image processing software with a simple optical system of variable sensitivity, the method is shown to be effective for inspection of a titanium honeycomb component subjected to differential pressure. Results are compared to those achieved with computed tomography (CT), immersion ultrasound testing (UT), and optical holographic interferometry. Lastly, we propose several possible processing strategies for automated quality assessment based on this powerful hybrid approach.

Improving the upper atmospheric temperature accuracy of the ground-based instrument by eliminating noise ways.

In view of the special properties of the upper atmosphere at the altitude of 80-120km, a ground-based passive remote sensing instrument ground-based airglow volume emission rate and temperature imaging interferometer (GBAVTII) is built to detect the atmospheric temperature used O 2(0-1) spectral line of night airglow at the altitude of 94km. In the process of photographing the upper atmosphere airglow with the GBAVTII, the stray light (white noise) such as moonlight, city lights, and starlight will be affected. In this paper, the theoretical expression of denoising is derived based on the rotational line temperature measurement of diatomic O 2(0-1) airglow. Through a slight adjustment of different parameters in the forward equation of the GBAVTII and noise reduction in laboratory flat-field fine calibration, and other denoising methods in the GBAVTII image processing process, the maximum accuracy of the GBAVTII detection of the upper atmospheric temperature is enhanced to 2.4K. Also, the minimum error of the GBAVTII detecting data with the satellite instrument sounding of atmosphere using broadband emission radiometry is 0.4K. Thus, the absolute accuracy of the GBAVTII in detecting the upper atmospheric temperature can be improved to ±(0.4-2.4)K through the theory and method studied in this paper.

Imaging: the next generation

Abstract Sue Bowler talks to the editors of a RASTI Special Issue about interferometric imaging and the computational challenges needed for the next wave of radio astronomy instruments

Comparison of TIDI Line of Sight Winds With ICON-MIGHTI Measurements.

The Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite has been making observations of the mesosphere and lower thermosphere (MLT) region for two decades. The TIMED Doppler Interferometer (TIDI) measures the neutral winds using four orthogonal telescopes. In this study, the line of sight (LOS) winds from individual telescopes are compared to the measurements from the Ionospheric Connection Explorer's (ICON's) Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument from 90 to 100km altitude during 2020. With the MIGHTI vector winds projected onto the LOS direction of each TIDI telescope, coincidences of the two data sets are found. The four telescopes perform differently and the performance depends on the satellite configuration and local solar zenith angle. Measurements from the coldside telescopes, Telescope 1 (Tel1) and Telescope 2 (Tel2), are better correlated with the MIGHTI winds in general with Tel2 having higher correlation coefficients across all conditions. The performance of Tel1 is comparable to that of Tel2 during backward flight while showing systematic errors larger than the average wind speeds during forward flight. The warmside LOS winds from Telescope 3 (Tel3) and Telescope 4 (Tel4) vary widely in magnitude, especially on the nightside. Compared with MIGHTI winds, the Tel4 measurements have the weakest correlation, while the Tel3 performance is comparable to that of the coldside telescopes during the ascending phase but deteriorates during the descending phase. Based on the TIDI/MIGHTI comparisons, figures of merit are generated to quantify the quality of measurements from individual telescopes in different configurations.

Characteristics of recoil leaders as observed by LOFAR

Lightning dart leaders and recoil leaders are current pulses that propagate along previously established lightning channels reionizing the channel. They are poorly understood since it is not known how a previously ionized channel can undergo dielectric breakdown multiple times. In this work we investigate five recoil leaders on two different channels using lightning observations of the LOFAR radio telescope in vhf (where we observe 30--80 MHz) using our TRI-D interferometric imaging algorithm. We show that while recoil leaders do slow down over time on average, they can also clearly accelerate. In addition, the vhf power emitted by the recoil leader is closely correlated with speed. We also investigate the vhf-emitting width of the recoil leaders and show that it is thinner than our meter-scale resolution. This shows that recoil leaders have significant streamer activity in their channel core or at most very inner region of the corona sheath, and that there is either very little, or weakly vhf-emitting, streamer activity in the majority of the corona sheath. Finally, we show that recoil leaders can have small microsecond-scale fluctuations in emitted vhf power that can occur in the same spot across subsequent recoil leaders on the same channel, demonstrating that recoil leader propagation is affected by small-scale channel geometry.

Highly-Sensitive, Label-Free Detection of Microorganisms and Viruses via Interferometric Reflectance Imaging Sensor

Pathogenic microorganisms and viruses can easily transfer from one host to another and cause disease in humans. The determination of these pathogens in a time- and cost-effective way is an extreme challenge for researchers. Rapid and label-free detection of pathogenic microorganisms and viruses is critical in ensuring rapid and appropriate treatment. Sensor technologies have shown considerable advancements in viral diagnostics, demonstrating their great potential for being fast and sensitive detection platforms. In this review, we present a summary of the use of an interferometric reflectance imaging sensor (IRIS) for the detection of microorganisms. We highlight low magnification modality of IRIS as an ensemble biomolecular mass measurement technique and high magnification modality for the digital detection of individual nanoparticles and viruses. We discuss the two different modalities of IRIS and their applications in the sensitive detection of microorganisms and viruses.

A non-axisymmetric temperature field reconstruction method based on the interferometric fringe schlieren method

The typical non-axisymmetric temperature field distribution in the tail nozzle outlet area of the combustion chamber of an aircraft engine can reflect the operating state of the engine. As an effective non-contact method for measuring a non-axisymmetric temperature field, the traditional background-oriented schlieren (BOS) method has the problem that the extraction accuracy of light deflection is not high. In this paper, interference fringes are introduced into the schlieren measurement, and the highly sensitive fringe offset replaces the traditional background speckle offset for measurement, which realizes accurate reconstruction of the non-axisymmetric temperature field. Firstly, through the interferometric fringe schlieren (IFS) method comprising multiple sets of double-slit interferometric systems and imaging sensors, the offset of the interference fringe after passing through the non-axisymmetric temperature field is measured by image processing. The spatial refractive index field is then reconstructed by the filtered back-projection algorithm under the sector model proposed in this paper and the spatial temperature field is obtained. The experimental results show that the temperature field reconstruction error is smaller for the IFS method than the BOS method, the measurement area is accurate from 0.5 mm for the BOS method to 0.2 mm for the IFS method and the temperature error range is reduced from 50 K for the BOS method to 25 K for the IFS method.

Learned interferometric imaging for the SPIDER instrument

Abstract The Segmented Planar Imaging Detector for Electro-Optical Reconnaissance (SPIDER) is an optical interferometric imaging device that aims to offer an alternative to the large space telescope designs of today with reduced size, weight, and power consumption. This is achieved through interferometric imaging. State-of-the-art methods for reconstructing images from interferometric measurements adopt proximal optimization techniques, which are computationally expensive and require handcrafted priors. In this work, we present two data-driven approaches for reconstructing images from measurements made by the SPIDER instrument. These approaches use deep learning to learn prior information from training data, increasing the reconstruction quality, and significantly reducing the computation time required to recover images by orders of magnitude. Reconstruction time is reduced to ∼10 ms, opening up the possibility of real-time imaging with SPIDER for the first time. Furthermore, we show that these methods can also be applied in domains where training data are scarce, such as astronomical imaging, by leveraging transfer learning from domains where plenty of training data are available.

Super-Resolution Multicomponent Joint-Interferometric Fabry–Perot-Based Technique

We propose a new spectral super-resolution technique combined with a Fabry–Perot interferometer (FPI) and an interferometric hyperspectral imager. To overcome the limitation of the maximal optical path difference (OPD) on the spectral resolution, the object spectrum is periodically modulated based on the FPI, and an optical Fourier transform of the modulated spectrum information is performed using a double-beam interferometer to obtain an interferogram. Drawing on the concept of nonlinear structured light microscopy, the displacement of the high-frequency interference information in the interferogram after adding the FPI is analyzed to restore the high-frequency interference information and improve the spectral resolution. The optical system has a compact structure with little impact on complexity, spectral range, or luminous flux. Our simulation results show that this method can realize multicomponent joint-interference imaging to obtain spectral super-resolution information. The effects of the FPI’s reflectance and interval are analyzed, and the reflectance needs to be within 20~80% and the interval must be as close as possible to the maximum optical range of the interferometer. Compared with previous, related innovations, this innovation has the advantages of higher system stability, higher data utilization, and better suitability for interferometric imaging spectrometers.

Remarkable changes in F region winds and plasma drifts during the stratospheric QBO disruption of 2019–2020

Analysis of the thermospheric wind and plasma drift data acquired respectively by the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) and Ion Velocity Meter (IVM) onboard Ionospheric Connection Explorer (ICON) satellite reveals remarkable changes during the disruption of the stratospheric quasi-biennial oscillation (QBO) due to high planetary wave activity in December 2019-January 2020. The thermospheric winds, in particular, are found to be unusually equatorward over the dip equator and northern geomagnetic low latitudes. Moreover, the plasma drifts are upward over the dip equator during midnight hours . Besides, the plasma drift exhibits a clear semi-diurnal variation. It is suggested to be due to the decrease in the diurnal tidal amplitude as a result of the non-linear interaction between the diurnal tide and the planetary waves. The relative increase of semi-diurnal tide may have imposed semi-diurnal variation in the meridional winds which could turn equatorward around midnight hours. The equatorward winds can increase the growth rate of the Rayleigh-Taylor instability to generate delayed F-region irregularities around midnight hours.