Interferometry Mode Research Articles

Experimental Demonstrations of Alignment and Mode Matching in Optical Cavities with Higher-Order Hermite-Gauss Modes.

Higher-order spatial laser modes have recently been investigated as candidates for reducing test-mass thermal noise in ground-based gravitational-wave detectors such as advanced LIGO. In particular, higher-order Hermite-Gauss (HG) modes have gained attention within the community for their more robust behaviors against random test-mass surface deformations and stronger sensing and control capacities. Inthis Letter we offer experimental investigations on various aspects of HG mode interferometry. We have generated purified HG modes up to the twelfth order HG_{6,6} mode, with a power conversion efficiency of 38.8% and 27.7% for the HG_{3,3} and HG_{6,6} modes respectively. We demonstrate for the first time the misalignment and mode mismatch-induced power coupling loss measurements for HG modes up to the HG_{6,6}. We report an excellent agreement with the extended numerical power loss factors that in the "small power loss" region converge to 2n+1 or n^{2}+n+1 for a misaligned or mode mismatched HG_{n,n} mode. We also demonstrate the wavefront sensing (WFS) signal measurement for HG modes up to the HG_{6,6}. The measurement result is accurately in accordance with theoretical WFS gain β_{n,n-1}sqrt[n]+β_{n,n+1}sqrt[n+1] for an HG_{n,n} mode, with β_{n,n-1} being the beat coefficient of the adjacent HG_{n,n} and HG_{n-1,n} modes on a split photodetector.

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. IV. NIRISS Aperture Masking Interferometry Performance and Lessons Learned

We present a performance analysis for the aperture masking interferometry (AMI) mode on board the James Webb Space Telescope Near Infrared Imager and Slitless Spectrograph (JWST/NIRISS). Thanks to self-calibrating observables, AMI accesses inner working angles down to and even within the classical diffraction limit. The scientific potential of this mode has recently been demonstrated by the Early Release Science (ERS) 1386 program with a deep search for close-in companions in the HIP 65426 exoplanetary system. As part of ERS 1386, we use the same data set to explore the random, static, and calibration errors of NIRISS AMI observables. We compare the observed noise properties and achievable contrast to theoretical predictions. We explore possible sources of calibration errors and show that differences in charge migration between the observations of HIP 65426 and point-spread function calibration stars can account for the achieved contrast curves. Lastly, we use self-calibration tests to demonstrate that with adequate calibration NIRISS F380M AMI can reach contrast levels of ∼9–10 mag at ≳λ/D. These tests lead us to observation planning recommendations and strongly motivate future studies aimed at producing sophisticated calibration strategies taking these systematic effects into account. This will unlock the unprecedented capabilities of JWST/NIRISS AMI, with sensitivity to significantly colder, lower-mass exoplanets than lower-contrast ground-based AMI setups, at orbital separations inaccessible to JWST coronagraphy.

Optimization of the Vertical Wavenumber for PolInSAR Inversion Performance Based on Numerical CRLB Analysis

A number of advanced SAR missions have been planned to launch, which can operate in fully polarimetric SAR interferometry mode to acquire structural parameters of global forests. Before the PolInSAR mission, the system configuration of vertical wavenumber kz must be carefully designed because it has a significant impact on the inversion performance. To minimize the estimation error of forest height caused by the system error from the future PolInSAR campaigns, it is valuable for us to optimize the vertical wavenumber. To quantitatively investigate the impact of kz on PolInSAR inversion performance, this paper proposes the optimization of kz based on the Cramér–Rao Lower Bound (CRLB) analysis. Extensive numerical CRLB simulations have been conducted to analyze the impact of several parameters, including extinction level, incident angle, and system decorrelation, etc., on the optimum kz. Finally, by minimizing the simulated CRLB, the numerical optimum kz maps are provided for the system engineers to easily design the system parameters.

Detection of seismic anisotropy from seismic data recorded at SMNH01 station of KiK-net using seismic interferometry and empirical mode decomposition

This study uses deconvolution of earthquakes registered at the SMNH01 station of KiK-net, which is located in Japan, to identify the presence of: (a) anisotropic layers and (b) waves which could have been induced by motion of fluid, crack propagation and local fault activation. The SMNH01 station has a borehole which is equipped with triaxial accelerometers installed at surface and at 100 m of depth. By deconvolving seismic events registered at surface and at 100 m of depth, anisotropic layers are identified. To interpret interferograms, the following tools are utilized: (a) a one dimensional (1D) isotropic layered medium, (b) empirical mode decomposition and (c) an orthorhombic model. From the empirical mode decomposition, genuine resonances or tremors are identified along directions that are subparallel to the horizontal stress orientations that seem to bisect an effective orthogonal-conjugate fracture set. The strong changes in the amplitude of the main descending deconvolved wave could be mainly due to the combined influence of: motion of fluids through fractures on the spatial scale of meters, recurrent seismic events, evolving anisotropy, dilation and temperature variations. This wave travels from the surface of the earth to the receiver located at 100 m of depth. The detected anisotropy could be associated with the presence of layers constituted by naturally fractured basalt, which are situated beneath the soil. Such anisotropic layers could have orthorhombic symmetry or lower. This study has implications in the exploration and production of natural resources.

The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope. IV. Aperture Masking Interferometry

The James Webb Space Telescope’s Near Infrared Imager and Slitless Spectrograph (JWST-NIRISS) flies a 7-hole non-redundant mask (NRM), the first such interferometer in space, operating at 3–5 μm wavelengths, and a bright limit of ≃4 mag in W2. We describe the NIRISS Aperture Masking Interferometry (AMI) mode to help potential observers understand its underlying principles, present some sample science cases, explain its operational observing strategies, indicate how AMI proposals can be developed with data simulations, and how AMI data can be analyzed. We also present key results from commissioning AMI. Since the allied Kernel Phase Imaging (KPI) technique benefits from AMI operational strategies, we also cover NIRISS KPI methods and analysis techniques, including a new user-friendly KPI pipeline. The NIRISS KPI bright limit is ≃8 W2 (4.6 μm) magnitudes. AMI NRM and KPI achieve an inner working angle of ∼70 mas, which is well inside the ∼400 mas NIRCam inner working angle for its circular occulter coronagraphs at comparable wavelengths.

Detection characteristics and features of signal processing of aerial objects on images of spacborne synthetic aperture radars

The article considers the possibilities of detecting airborne moving targets by space radars with aperture synthesis using interferometry mode along the path. The method of calculating the signal-to-noise ratio, the characteristics of detecting aerial targets in the mode of selection of moving targets is described. Based on calculations, it is shown that the dynamics of the underlying surface caused by fluctuations in grass, foliage or an agitated sea surface significantly reduces the ability to detect aerial objects by space radars with aperture synthesis in the selection of moving targets mode. It is shown by the examples of existing space radars with aperture synthesis that when using the interferometry method along the path, at least 2 interferometric bases and refocusing on the marks of bright moving targets in the mode of selection of moving targets channel, the effectiveness of the detection procedure can be sufficient for practical applications. However, the region of small radial velocity values remains a blind zone for any multi-base circuit configurations and can only be reduced by increasing the energy potential of the space radars with aperture synthesis.

Detecting planetary mass companions near the water frost-line using JWST interferometry

ABSTRACT JWST promises to be the most versatile infrared observatory for the next two decades. The Near Infrared and Slitless Spectrograph (NIRISS) instrument, when used in the Aperture Masking Interferometry (AMI) mode, will provide an unparalleled combination of angular resolution and sensitivity compared to any existing observatory at mid-infrared wavelengths. Using simulated observations in conjunction with evolutionary models, we present the capability of this mode to image planetary mass companions around nearby stars at small orbital separations near the circumstellar water frost-line for members of the young, kinematic moving groups β Pictoris, TW Hydrae, as well as the Taurus–Auriga association. We show that for appropriately chosen stars, JWST/NIRISS operating in the AMI mode can image sub-Jupiter companions near the water frost-lines with ∼68 per cent confidence. Among these, M-type stars are the most promising. We also show that this JWST mode will improve the minimum inner working angle by as much as ∼50 per cent in most cases when compared to the survey results from the best ground-based exoplanet direct imaging facilities (e.g. VLT/SPHERE). We also discuss how the NIRISS/AMI mode will be especially powerful for the mid-infrared characterization of the numerous exoplanets expected to be revealed by Gaia. When combined with dynamical masses from Gaia, such measurements will provide a much more robust characterization of the initial entropies of these young planets, thereby placing powerful constraints on their early thermal histories.

Probing the New Generation Geodetic Radio Telescopes for FRB Observations

We investigate the possibility of fast radio burst (FRB) observation on new special geodetic 13[Formula: see text]m wideband radio telescopes in Very Large Base Interferometry (VLBI) and single-dish modes. The Crab Pulsar Giant Pulses (GPs) were used as a reference source of FRB. In order to verify our results we use joint simultaneous observations with 32[Formula: see text]m legacy and 13[Formula: see text]m new VLBI Global Observing System (VGOS) radio telescopes of the Quasar VLBI network in the L and S/X frequency bands. The special observation session contains two 10-min scans of Crab Pulsar. We successfully detect a total of 712 pulses with flux density [Formula: see text][Formula: see text]Jy at 1653.5[Formula: see text]MHz on 32-m antenna and 262 pulses with flux density [Formula: see text][Formula: see text]Jy at 2406.3[Formula: see text]MHz on 13-m antenna in Badary observatory. The VLBI fringes were obtained for approximately every third pulse. The energy distribution of detected pulses has a break in the power law fit at [Formula: see text] and [Formula: see text]-values are [Formula: see text]0.2 and [Formula: see text]1.8 for lower and higher energies. A GP with a fluence of the order of 190[Formula: see text][Formula: see text] in L-band was detected and 48[Formula: see text][Formula: see text] in S-band. We found that the peak coincidence of right and left circular polarizations of dedispersed data on a single antenna can provide a maximum detection rate with fixed false probability. It was shown that the best GP detection rate on 13[Formula: see text]m VGOS radio telescope was achieved at Badary observatory with 13 pulses per minute in the S-band.

A refocusing iterative optimization method based on the quad-beam mode for accurate estimation of the azimuth velocity of slow-moving targets using SAR

ABSTRACT SAR slow-moving target detection and velocity estimation in ultra-high speed platform and strong clutter are a serious problem. The existing slow-moving target velocity estimation methods including fractional Fourier transform (FrFT), bidirectional imaging mode (DiBi) and the dual-beam interferometry mode (DBI) cannot suppress the clutter, which reduces the accuracy of velocity estimation. To solve the problem, we propose an accurate azimuth velocity estimating method refocusing iterative optimization (RIO) based on the quad-beam mode. Firstly, we use the back projection (BP) algorithm for the quad-beam imaging, and perform accurate registration and clutter suppression for the two forward (fore-) and two afterward (aft-) beams respectively. After that, the azimuth offset in fore- and aft-beam images is utilized to achieve the coarse azimuth velocity estimation. Then we put forward the RIO algorithm to compensate the error in velocity estimation. Through iterative search and compensation the azimuth velocity, the azimuth offset is gradually reduced to disappear, thus a more accurate estimation of azimuth velocity is realized. The experiments validate the accuracy of azimuth velocity estimation of the proposed method and the error is less than 0.01 m , which is much better than the other three typical methods (FrFT, DiBi, DBI).

Estimating instantaneous sea-ice dynamics from space using the bi-static radar measurements of Earth Explorer 10 candidate Harmony

Abstract. This article describes the observation techniques and suggests processing methods to estimate dynamical sea-ice parameters from data of the Earth Explorer 10 candidate Harmony. The two Harmony satellites will fly in a reconfigurable formation with Sentinel-1D. Both will be equipped with a multi-angle thermal infrared sensor and a passive radar receiver, which receives the reflected Sentinel-1D signals using two antennas. During the lifetime of the mission, two different formations will be flown. In the stereo formation, the Harmony satellites will fly approximately 300 km in front and behind Sentinel-1, which allows for the estimation of instantaneous sea-ice drift vectors. We demonstrate that the addition of instantaneous sea-ice drift estimates on top of the daily integrated values from feature tracking have benefits in terms of interpretation, sampling and resolution. The wide-swath instantaneous drift observations of Harmony also help to put high-temporal-resolution instantaneous buoy observations into a spatial context. Additionally, it allows for the extraction of deformation parameters, such as shear and divergence. As a result, Harmony's data will help to improve sea-ice statistics and parametrizations to constrain sea-ice models. In the cross-track interferometry (XTI) mode, Harmony's satellites will fly in close formation with an XTI baseline to be able to estimate surface elevations. This will allow for improved estimates of sea-ice volume and also enables the retrieval of full, two-dimensional swell-wave spectra in sea-ice-covered regions without any gaps. In stereo formation, the line-of-sight diversity allows the inference of swell properties in both directions using traditional velocity bunching approaches. In XTI mode, Harmony's phase differences are only sensitive to the ground-range direction swell. To fully recover two-dimensional swell-wave spectra, a synergy between XTI height spectra and intensity spectra is required. If selected, the Harmony mission will be launched in 2028.

Rapid characterization of nano-scale structures in large-scale ultra-precision surfaces

Characterizations of nano-scale structures using conventional stylus profilometer and optical microscopy are respectively restricted by tip radius and diffraction limit. Atomic force microscope (AFM) can measure smaller structures but it is difficult and time-consuming to find them in large-scale ultra-precision surfaces. In this paper, a new profilometer for rapid positioning and measurement of nano-scale structures in large-scale ultra-precision surfaces is proposed. A technique based on white light interference is integrated into an AFM to detect probe deflection. The adapted profilometer has two measurement modes: a white light interferometry mode used to find an area of interest, an AFM mode for high-resolution measurement. Using this profilometer, the time of positioning an area of interest can be significantly reduced and the measurement efficiency is thus improved. Furthermore, a tracking measurement method is proposed to improve the measurement range of the AFM mode, which can enhance the measuring ability of the profilometer for complex large-scale surfaces. Experiments demonstrate that the profilometer can quickly positioning nano-scale structures in large-scale surfaces and achieve three-dimensional measurement with nanometer resolution.

Observing flare stars below 100 MHz with the LWA

ABSTRACT We observed the flare stars AD Leonis, Wolf 424, EQ Pegasi, EV Lacertae, and UV Ceti for nearly 135 h. These stars were observed between 63 and 83 MHz using the interferometry mode of the Long Wavelength Array. Given that emission from flare stars is typically circularly polarized, we used the condition that any significant detection present in Stokes I must also be present in Stokes V at the same time in order for us to consider it a possible flare. Following this, we made one marginal flare detection for the star EQ Pegasi. This flare had a flux density of 5.91 Jy in Stokes I and 5.13 Jy in Stokes V, corresponding to a brightness temperature 1.75 × 1016(r/r*)−2 K.

The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science Case and Survey Design

The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution (≈2.″5), sensitivity (a 1σ goal of 70 μJy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2–4 GHz). The first observations began in 2017 September, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hr of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (decl. > −40°), a total of 33 885 deg. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an “on the fly” interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations.

A High-Accuracy Synchronization Phase-Compensation Method Based on Kalman Filter for Bistatic Synthetic Aperture Radar

Phase synchronization is one of the key issues that must be addressed for the bistatic synthetic aperture radar (BiSAR) system. LuTan-1 (LT-1) is an innovative spaceborne BiSAR mission based on the use of two radar satellites operating in the L-band to generate the global digital terrain models in the bistatic interferometry mode. An advanced synchronization scheme is used for the LT-1 system. The synchronization pulses are exchanged immediately after the ending time of the radar echo-receiving window and before the starting time of the next pulse-repetition interval. Therefore, it cannot interrupt the normal SAR data acquisition, further improving the synchronization accuracy and avoiding the data missing effect. In this letter, a robust phase-error estimation and compensation method is proposed to improve the accuracy of the synchronization by using the Kalman filter. The test data acquired from the ground validation system of the LT-1 synchronization module are used to demonstrate the feasibility of the proposed scheme. The results validate the effectiveness of the proposed scheme and prove the promise for its future application in LT-1.

Calibration of ALMA as a Phased Array. ALMA Observations During the 2017 VLBI Campaign

We present a detailed description of the special procedures for calibration and quality assurance of Atacama Large Millimeter/submillimeter Array (ALMA) observations in Very Long Baseline Interferometry (VLBI) mode. These procedures are required to turn the phased ALMA array into a fully calibrated VLBI station. As an illustration of these methodologies, we present full-polarization observations carried out with ALMA as a phased array at 3 mm (Band 3) and 1.3 mm (Band 6) as part of Cycle-4. These are the first VLBI science observations conducted with ALMA and were obtained during a 2017 VLBI campaign in concert with other telescopes worldwide as part of the Global mm-VLBI Array (GMVA, April 1–3) and the Event Horizon Telescope (EHT, April 5–11) in ALMA Bands 3 and 6, respectively.

Active SU(1,1) atom interferometry

Active interferometers use amplifying elements for beam splitting and recombination. We experimentally implement such a device by using spin exchange in a Bose–Einstein condensate. The two interferometry modes are initially empty spin states that get spontaneously populated in the process of parametric amplification. This nonlinear mechanism scatters atoms into both modes in a pairwise fashion and generates a non-classical state. Finally, a matched second period of spin exchange is performed that nonlinearly amplifies the output signal and maps the phase onto readily detectable first moments. Depending on the accumulated phase this nonlinear readout can reverse the initial dynamics and deamplify the entangled state back to empty spin states. This sequence is described in the framework of SU(1,1) mode transformations and compared to the SU(2) angular momentum description of passive interferometers.

Ship Detection in Dual-Channel ATI-SAR Based on the Notch Filter

Synthetic aperture radar (SAR) in along-track interferometry (ATI) mode has been extensively applied in velocity measurements of ocean currents and ship detection. A notch filter was recently proposed and was demonstrated to be a promising tool for ship detection exploiting quad- or dual-polarization SAR information. In this paper, the investigation of the notch filter is extended to the dual-channel ATI-SAR mechanism for ship detection on the ocean surface. First, a theoretical proof that the interferometric magnitude performs better in ship detection than single-channel amplitude/intensity is given based on the signal-clutter-ratio improvement, which validates the advantages of interferometric SAR against conventional single-channel SAR for the purpose of ship observations on the ocean surface. Second, based on the proof, the version of the notch filter that is suitable for the ATI-SAR, that is, the interferometric notch filter (INF), is proposed. We then analyze the statistical models of the INF distance, which can facilitate the adaptive realization of INF for ship detection in ATI-SAR. Finally, the experimental results for dual-channel ATI-SAR data measured by NASA/JPL L-band AIRSAR verify the accuracy of the theoretical analysis and effectiveness of the INF.

A Portable FMCW Interferometry Radar With Programmable Low-IF Architecture for Localization, ISAR Imaging, and Vital Sign Tracking

This paper presents a portable radar system for short-range localization, inverse synthetic aperture radar imaging, and vital sign tracking. The proposed sensor incorporates frequency-modulated continuous-wave (FMCW) and interferometry (Doppler) modes, which enable this radar system to obtain both absolute range information and tiny vital signs (i.e., respiration and heartbeat) of human targets. These two different operation modes can be switched through an on-board microcontroller. To simplify the system, the proposed radar utilizes the audio card of a laptop to sample the baseband signal. The FMCW mode of the radar uses operational-amplifier-based circuits to generate an analog sawtooth signal and a reference pulse sequence (RPS). The RPS is locked to the sawtooth signal to obtain coherence for the radar system. For the interferometry mode, a low-intermediate-frequency modulation method is implemented to avoid the slow vital signs from being distorted by the high-pass filter of the audio card. Several experiments were carried out to reveal the capability and distinct operational features of the proposed portable hybrid radar. The experiments also showed that the system can easily detect glass, which is usually difficult to identify for optical-based sensors. In addition, 2-D scanning in a complex environment revealed that the proposed radar was able to differentiate human targets from other objects. Moreover, ISAR images were used to isolate moving human targets from surrounding clutter. Finally, the proposed radar also demonstrated its ability to accurately measure vital signs when a human subject sits still.

High sensitivity far infrared laser diagnostics for the C-2U advanced beam-driven field-reversed configuration plasmas.

A high sensitivity multi-channel far infrared laser diagnostics with switchable interferometry and polarimetry operation modes for the advanced neutral beam-driven C-2U field-reversed configuration (FRC) plasmas is described. The interferometer achieved superior resolution of 1 × 1016 m-2 at >1.5 MHz bandwidth, illustrated by measurement of small amplitude high frequency fluctuations. The polarimetry achieved 0.04° instrument resolution and 0.1° actual resolution in the challenging high density gradient environment with >0.5 MHz bandwidth, making it suitable for weak internal magnetic field measurements in the C-2U plasmas, where the maximum Faraday rotation angle is less than 1°. The polarimetry resolution data is analyzed, and high resolution Faraday rotation data in C-2U is presented together with direct evidences of field reversal in FRC magnetic structure obtained for the first time by a non-perturbative method.

Calibration of mixed-polarization interferometric observations

Heterodyne receivers register the sky signal on either a circular polarization basis (where it is split into left-hand and right-hand circular polarization) or a linear polarization basis (where it is split into horizontal and vertical linear polarization). We study the problem of interferometric observations performed with telescopes that observe on different polarization bases, hence producing visibilities that we call "mixed basis" (i.e., linear in one telescope and circular in the other). We present novel algorithms for the proper calibration and treatment of such interferometric observations and test our algorithms with both simulations and real data. The use of our algorithms will be important for the optimum calibration of forthcoming observations with the Atacama Large mm/submm Array (ALMA) in very-long-baseline interferometry (VLBI) mode. Our algorithms will also allow us to optimally calibrate future VLBI observations at very high data rates (i.e., wide bandwidths), where linear-polarization feeds will be preferable at some stations, to overcome the polarimetric limitations due to the use of quarter-wave plates.