Interferometric Technique Research Articles

Three-dimensional full-field velocity measurements in shock compression experiments using stereo digital image correlation.

Shock compression plate impact experiments conventionally rely on point-wise velocimetry measurements based on laser-based interferometric techniques. This study presents an experimental methodology to measure the free surface full-field particle velocity in shock compression experiments using high-speed imaging and three-dimensional (3D) digital image correlation (DIC). The experimental setup has a temporal resolution of 100ns with a spatial resolution varying from 90 to 200μm/pixel. Experiments were conducted under three different plate impact configurations to measure spatially resolved free surface velocity and validate the experimental technique. First, a normal impact experiment was conducted on polycarbonate to measure the macroscopic full-field normal free surface velocity. Second, an isentropic compression experiment on Y-cut quartz-tungsten carbide assembly is performed to measure the particle velocity for experiments involving ramp compression waves. To explore the capability of the technique in multiaxial loading conditions, a pressure shear plate impact experiment was conducted to measure both the normal and transverse free surface velocities under combined normal and shear loading. The velocities measured in the experiments using digital image correlation are validated against previous data obtained from laser interferometry. Numerical simulations were also performed using established material models to compare and validate the experimental velocity profiles for these different impact configurations. The novel ability of the employed experimental setup to measure full-field free surface velocities with high spatial resolutions in shock compression experiments is demonstrated for the first time in this work.

Feasibility study of an enhanced heterodyne dispersion interferometer

Interferometry is the commonly exploited technique for electron density measurements in magnetically confined fusion plasma experiments. Reliable electron density measurements are fundamental both for machine protection and for plasma physics understanding. In the last years, attention was drawn on the dispersion interferometer concept, because of its robustness and simplicity. Nevertheless, the heterodyne version of this configuration, which has several advantages over the homodyne scheme, loses one of the main benefits of the dispersion interferometer technique, that is its inherent insensitivity to vibration errors. In this paper, two methods are proposed and theoretically investigated to reduce the vibration noise in the electron density measurements performed with heterodyne dispersion interferometers.

Real-Time Ellipsometric Surface Plasmon Resonance Sensor Using Polarization Camera May Provide the Ultimate Detection Limit.

Ellipsometric Surface Plasmon Resonance (SPR) sensors are known for their relatively simple optical configuration compared to interferometric and optical heterodyne phase interrogation techniques. However, most of the previously explored ellipsometric SPR sensors based on intensity measurements are limited by their real-time applications because phase or polarization shifts are conducted serially. Here we present an ellipsometric SPR sensor based on a Kretschmann-Raether (KR) diverging beam configuration and a pixelated microgrid polarization camera. The proposed methodology has the advantage of real-time and higher precision sensing applications. The short-term stability of the measurement using the ellipsometric parameters tanψ and cos(Δ) is found to be superior over direct SPR or intensity measurements, particularly with fluctuating sources such as laser diodes. Refractive index and dynamic change measurements in real-time are presented together with Bovine Serum Albumin (BSA)-anti-BSA antibody binding to demonstrate the potential of the developed sensor for biological sensing applications with a resolution of sub-nM and down to pM with additional optimization. The analysis shows that this approach may provide the ultimate detection limit for SPR sensors.

PREDICTION OF DEFORMATION CAUSED BY LANDSLIDES BASED ON GRAPH CONVOLUTION NETWORKS ALGORITHM AND DINSAR TECHNIQUE

Abstract. Around the world, the occurrence of landslides has become one of the greatest threats to human life, property, infrastructure, and natural environments. Despite extensive research and discussions on the spatiotemporal dependence of landslide displacements, there is still a lack of understanding concerning the factors that appear to control displacement distribution in landslides because of their significant variations. This paper implements a Graph Convolutional Network (GCN) to predict displacement following the Moio della Civitella landslide in southern Italy and identify factors that may affect the distribution of movement following the landslide. An interferometric technique, known as permanent scatter interferometry (PSI), has been developed based on Synthetic Aperture Radar (SAR) satellite imagery to derive permanent scatter points that can be used to represent the deformation of landslides. This study utilized the GCN regression model applied to PSs points and data reflecting geological and geomorphological factors to extract the interdependency between paired data points, resulting in an adjacency matrix of the interval [0, 0,8). The proposed model outperforms conventional machine learning and deep learning algorithms such as linear regression (LR), K-nearest neighbors (KNN), Support vector regression (SVR), Decision tree, lasso, and artificial neural network (ANN). The absolute error between the actual and predicted deformation is used to evaluate the proposed model, which is less than 2 millimeters for most test set points.

Time series phase unwrapping algorithm using LP-norm optimization compressive sensing

Time series phase unwrapping is an essential step in the time series interferometric synthetic aperture radar technique for deformation monitoring. However, traditional unwrapping algorithms are prone to unwrapping errors in the case of intense noise and steep deformation gradients, which directly affect the accuracy of the deformation information interpretation. To address this problem, a time series phase unwrapping algorithm using LP-norm optimization compressive sensing is proposed in this paper. This algorithm first transforms the minimizing L0-norm describing compressive sensing technique into a nonconvex problem with weaker constraints, that is, minimizing the LP-norm. Then, an optimized compressed sensing model was proposed by combining the constraint criterion of the phase triplet closure in the temporal domain and the solution strategy of the iterative reweighted least squares method. Improved temporal dimensional phase unwrapping and unwrapping error correction algorithms were developed based on this optimized model. Finally, the proposed algorithm was perfected by combining the above improved methods and the pseudo-three-dimensional phase unwrapping framework. Simulated and Sentinel-1A real datasets verify that the proposed algorithm has better noise robustness, unwrapping stability, and efficiency than traditional algorithms, particularly in the case of a steep deformation gradient and intense noise. The temporal coherence estimated by the proposed algorithm for the real dataset is generally greater than 0.99, which is far superior to traditional algorithms. Furthermore, the proposed algorithm facilitates high-precision interpretation of deformation information. The cross-validation results show that the deformation results obtained by traditional algorithms are underestimated by at least 73 mm/a and 68 mm at the maximum subsidence than the DSInSAR. However, the deformation result obtained by the proposed algorithm is highly consistent with that of the DSInSAR.

Accurately measuring phase profiles of structured light in holographic optical tweezers

Accurate phase measurement of structured light is highly desirable in holographic optical tweezers, since quality of phase profiles have a strong impact on performance of manipulating particles. Here, we report a novel interferometric phase measurement technique for accurately measuring phase profile of structured light. The related experiments are performed, in which the measured phase profiles agree well with the designed phase profiles. It reveals that the phase profiles of structured light can be accurately obtained. Importantly, the fast switch between phase measurement and particle manipulation in the experiment can be easily implemented, which are strongly preferable in holographic optical tweezers. This study offers a promising way for accurately obtaining phase profiles of structure light, which is very helpful for particle manipulation.

Study and Realization of Environmental Health Diagnosis by Using Nanomaterial Based Fiber Optic Sensor–A Review

In these days, the significance of fiber optic sensors has exceptionally increased due to technological advances in modern society. Nowadays, researchers are attracted toward the development of a sensor that possesses the integration of both fiber optic sensors and nanocomposite thin film to improve the sensor’s performance. Thin-film nanomaterial based optical fiber sensors have great potential applications, namely, gas sensing, chemical sensing, biomolecules, food plant, industrial hazards, safety issues, drug monitoring, and heavy metal detections. There are various techniques, namely, evanescent field absorption spectroscopy, fiber Bragg grating (FBG), long-period grating (LPG), ringdown spectroscopy, surface plasmon resonance (SPR), Mach–Zehnder interferometer (MZI) techniques, and so on, which are used for the development of fiber optic sensors that are majorly emphasized in the current article. Numerous scientific researchers have demonstrated that functionalized fiber optic sensors are found more beneficial in terms of improvement of the sensing performance, such as sensitivity, repeatability, response time, specificity, recovery time, and repeatability. Henceforth, in this review article, we have focused on these requisite sensing parameters in detail. Furthermore, to keep human beings healthy, it is our responsibility to develop a sensor that can monitor the optimum toxic concentrations accurately. Thus, the monitoring of industrial environmental pollutants, such as NH3, ethylenediamine, alcohol, and acetone, is immensely important to save human lives on Earth. In the report, all the details of the target analytes, such as optimum concentrations, adverse effects on human beings, and monitoring techniques, are discussed.

Unambiguous 2-D Angle-of-Arrival Estimation Receiver Based on a 16-Port Interferometer

Multiport interferometer (MPI) techniques are getting more and more attention especially for passive receiver beyond 24 GHz. However, multiport interferometers with more than six ports are rarely been studied. And the existed MPI techniques only can achieve 2-D angle-of-arrival (AOA) estimation in a narrow range of angle, which means they have ambiguous problem. In this paper, an unambiguous 2-D AOA estimation receiver (Rx) based on a 16-port interferometer is proposed and demonstrated. It has three main components: eight antennas, a 16-port interferometer, and a signal processor. The 16-port interferometer with 8 input ports and 8 output ports is the key part and the main point of this paper. The eight input signals are superposed on eight output port signals in the interferometer. First, a 16-port interferometer and the corresponding AOA estimation algorithm are proposed. The algorithm transposes the phase differences between the antenna elements into the power relationship of the output signals. Then, the Rx error models are established to obtain the systematic errors. After that, a method for reducing angle ambiguity is introduced. The method allows for the distance between the antenna elements to be larger than λ/2, and the angle detection range is theoretically –90° to 90°. An Rx operating at 31 GHz was fabricated. The measurements demonstrate that the system operates as expected, with AOAs ranging from –40° to 40° and a mean squared error of approximately 1°.

Determination of phase change correction on gauge block measurement in two different interferometric measurement systems

Phase change correction (PCC) is an important correction value of the end effect in an optical interferometry system. Normally, this value is used to compensate for gauge block measurement by an optical interferometry system based on ISO 3650:1998. Two different interferometric measurement systems in terms of fringe fraction measurement were performed to determine the phase change correction by a five-stacking method. These results are used to determine the length measurement of gauge blocks in an optical interferometer technique and consequently, to evaluate the uncertainty of gauge blocks measurement. The preliminary results for steel gauge block are shown that the value of phase change correction in a phase shift gauge block interferometer (PSGBI) system and a standard uncertainty are 35.2 nm and 5.8 nm, respectively. In contrast, the values from an average slits gauge block interferometer (ASGBI) system and a standard uncertainty are 66.0 nm and 6.0 nm, respectively. We found that the phase correction from the PSGBI system is lower than ASGBI about 0.53 - 0.56 times because the different of wave front correction in two interferometric systems. However, the lengths of gauge blocks of all materials measured by the two systems were consistent as assessed by En number. According to the study, we can conclude that phase change correction is based on the characteristics of each GBI system, surface texture characteristic in term of wringing condition and material types of gauge block and optical plates such as the fringe fraction measurement technique, and wave front error compensation. Consequently, measurements that require a high accuracy should determine the phase change correction before each measurement due to this value is not interchangeable.

An Unsupervised CNN-Based Multichannel Interferometric Phase Denoising Method Applied to TomoSAR Imaging

Tomographic synthetic aperture radar (TomoSAR) is an advanced SAR interferometric technique to retrieve 3D spatial information. However, the standard deviation in the reconstructed elevation could be high due to the noise in the interferometric phases, which makes the denoising filter crucial before tomographic reconstruction. In this paper, we propose an unsupervised multichannel SAR interferometric phase denoising method based on the Convolution Neural Network (CNN). It utilizes the Weighted Least-Squares (WLS) regularization combining with the covariance of multichannel interferometric phases to minimize the standard deviation of phase noise, which leads to the accurate and complete TomoSAR reconstruction. This network is trained by real SAR images and the results of both simulated and real observations verify the effectiveness of our proposed method.

Pulsed heterodyne interferometry for nonlinear SOI waveguide characterization

Silicon waveguides are a promising candidate for integrated nonlinear optics applications. Nonlinear coefficients of Silicon on Insulator (SOI) waveguides have been previously measured using techniques such as Z-scan, D-scan, Four Wave Mixing (FWM) and Self-phase modulation. However, they have several drawbacks such as they operate at high power or are cumbersome to setup and require multiple measurements to determine all the coefficients. In this work, we develop a direct and single measurement technique to characterize the nonlinear processes in SOI waveguides. This is achieved by employing a heterodyne interferometric technique to accurately measure minute nonlinear response. The measured nonlinear amplitude and phase shifts are fit to extract third-order nonlinear coefficients of Two-photon absorption, Kerr nonlinear index, Free carrier absorption and Free carrier dispersion. The obtained coefficients for SOI waveguides are close to that found in literature measured using the above-mentioned techniques. The advantages of this method include easy interpretation of the output signal and relatively low power of operation. It is especially advantageous for studying materials such as Phase Change Materials (PCM) in which phase changes occur dynamically. This aspect is quite promising for characterizing emerging materials for integrated photonics applications.

Virtual Receiver Matrix and Combinatory Analog Operations for Future Multifunction Reconfigurable Sensing and Communication Wireless Systems

This article proposes and presents a topological receiver architecture, named virtual receiver matrix (VRM), suitable for future smart multifunction wireless systems. This concept is devised and benefits from using combinatory analog operations with multiple distributed units in a receiver matrix or array. This mechanism of receiving data through spatially “floating” distributed virtual receiver channels offers an unprecedented solution of providing unparalleled degrees of freedom to implement multiple functions such as data reception, angle-of-arrival (AoA) detection, radar, and imaging operations among many others in a single receiver architecture. Interestingly, the total number of possible virtual receivers from different combinations of unit cells in a matrix is also significantly increased compared with a conventional “fixed” receiver array. Each virtual receiver, made of a combinatory set, depends on the characteristics of incoming signals and their illumination angle. A mathematical model is established and investigated for the design of unit cells. Although a prototype of choice is studied and designed for the fifth-generation (5G) wireless systems, it is anticipated that the VRM concept is applicable to the sixth-generation (6G) and future wireless systems for enhancing their functionality, capacity, agility, and speed. In this article, a multiport interferometric technique is used for each unit cell of VRM as a proof-of-concept; however, any other receiver type that uses the phase difference of incoming signals can be deployed as the unit cell for the realization of the VRM concept. First, the experimental results of the fabricated proof-of-concept prototype for various modulation schemes including QPSK, 16-quadrature amplitude modulation (QAM), and 32-QAM are shown with data rates up to 1 MS/s, having a maximum error vector magnitude (EVM) of 9%. Finally, a systematic scheme for 2-D AoA detection using a special combination of unit cells is proposed and demonstrated through the proposed VRM.

First demonstration of a fiber optic bolometer on a tokamak plasma (invited).

A fiber optic bolometer (FOB) was demonstrated observing a fusion plasma for the first time at the DIII-D tokamak. A FOB uses a fiber optics-based interferometric technique that is designed to have a high sensitivity to temperature changes [75 mK/(W/m2) responsivity in high vacuum with 0.38 mK noise level] with a negligible susceptibility to electromagnetic interference (EMI) that can be problematic for resistive bolometers in a tokamak environment. A single-channel test apparatus was installed on DIII-D consisting of a measurement FOB and shielded reference FOB. The single-channel FOB showed a negligible increase in the noise level during typical plasma operations (0.39 mK) compared to the benchtop results (0.38 mK), confirming an insignificant EMI impact to the FOB. Comparisons to DIII-D resistive bolometers showed good agreement with the single-channel FOB, indicating that the FOB is comparable to a resistive bolometer when the impulse calibration is applied. The noise-equivalent power density of the calibrated FOB during a plasma operation was 0.55 W/m2 with an average sampling time of 20ms. The major potential effect of ionizing radiation on the FOB would be the radiation-induced attenuation, which can be efficiently compensated for by adjusting the probing light power.

Raspberry Pi Reflector (RPR): A Low‐Cost Water‐Level Monitoring System Based on GNSS Interferometric Reflectometry

AbstractAlthough reflectometry is not the primary application of Global Positioning System (GPS) and similar Global Navigation Satellite Systems (GNSS), fast‐growing GNSS tracking networks has led to the emergence of GNSS interferometric reflectometry technique for monitoring surface changes such as water level. However, scientific‐grade or geodetic GNSS instruments are expensive, which is a limiting factor for their prompt and more widespread deployment as a dedicated environmental sensor. We present a prototype called Raspberry Pi Reflector (RPR) that includes a low‐cost and low‐maintenance single‐frequency GPS module and a navigation antenna connected to an inexpensive Raspberry Pi microcomputer. A unit has been successfully operating for almost 2 years since March 2020 in Wesel (Germany) next to the Rhine river. Sub‐daily and daily water levels are retrieved using spectral analysis of reflection data. The river level measurements from RPR are compared with a co‐located river gauge. We find an Root‐Mean‐Square Error (RMSE) of 7.6 cm in sub‐daily estimates and 6 cm in daily means of river level. In August 2021, we changed the antenna orientation from upright to sideways facing the river. The RMSE reduced to 3 cm (sub‐daily) and 1.5 cm (daily) with the new orientation. While satellite radar altimetry techniques have been utilized to monitor water levels with global coverage, their measurements are associated with moderate uncertainties and temporal resolution. Therefore, such low‐cost and high‐precision instruments can be paired with satellite data for calibrating, validating and modeling purposes. These instruments are financially (<US$ 150 as of November 2021) and technically accessible worldwide.

Multiple-input multiple-output radar, ground-based MIMO SAR for ground deformation monitoring

ABSTRACT This study focuses on investigating the capabilities of a Multiple-input multiple-output RADAR. A Radar interferometer, based on an electronically scanned array in MIMO configuration (MIMO‐SAR), has been assessed for operational use in monitoring phenomena of geological interest, such as landslides unstable slopes. The system applies the very well‐known and proven Ground-Based Interferometric technique. It guarantees a very short refreshing time compared to traditional systems based on the mechanical movement of the radar transceiver on a rail or the mechanical steering of a real antenna. The system can monitor several phenomena having deformation rates too high to be correctly retrieved by traditional systems currently in use. Implementing a prototype termed MELISSA allowed the testing technique’s performances in two real case studies: a landslide and an unstable volcanic flank. The experimental results were compared with LISA, a well-known Ground-Based Interferometric Synthetic Aperture Radar (GBInSAR) interferometer. MELISSA allows for obtaining an excellent accuracy, better than 0.01 mm. The range and angular resolution are on the same order of magnitude as those obtained through LISA. However, the refreshing rate obtained from MELISSA, 0.01 s, guarantees a strong coherence even in challenging environmental scenarios as a flank of an active volcano.

Improvement in measuring losses by interferometric technique for glass waveguides produced by femtosecond laser writing

This study describes a straightforward and non-destructive interferometric technique for measuring the propagation loss in single-mode optical glass waveguides by exploiting a Fabry–Perot cavity effect. The experiment involves coupling a powerful tunable laser source to a single-mode waveguide that is being tested, then using a detector to analyze the transmission fringes of that source after coating the glass end facets with highly reflective coating. Our first experiment, which used a 22.4-mm-long fabricated waveguide on Non-Iox Gorilla glass to demonstrate the technique, resulted in a propagation loss of 0.49 dB/cm at a transmitting wavelength of about 1575 nm, which is completely consistent with the value obtained by a conventional characterization method. The measurement procedure is also independent from the coupling losses, making the characterization easier and reliable after the manufacturing just with a reversible deposition process on the sample under testing.

Two-photon amplitude interferometry for precision astrometry

Improved quantum sensing of photons from astronomical objects could provide high resolution observations in the optical benefiting numerous fields, including general relativity, dark matter studies, and cosmology. It has been recently proposed that stations in optical interferometers would not require a phase-stable optical link if instead sources of quantum-mechanically entangled pairs could be provided to them, potentially enabling hitherto prohibitively long baselines. A new refinement of this idea is developed, in which two photons from different sources are interfered at two separate and decoupled stations, requiring only a slow classical information link between them. We rigorously calculate the observables and contrast this new interferometric technique with the Hanbury Brown & Twiss intensity interferometry. We argue this technique could allow robust high-precision measurements of the relative astrometry of the two sources. A basic calculation suggests that angular precision on the order of $10$~microarcsecond in the relative opening angle could be achieved in a single night's observation of two bright stars.

Ground Surface Displacement After a Forest Fire Near Mayya, Eastern Siberia, Using InSAR: Observation and Implication for Geophysical Modeling

AbstractForest fires significantly impact permafrost degradation in the subarctic regions. However, interannual and seasonal variations in surface deformation due to permafrost thawing in burned areas were poorly understood. Measuring the ground surface displacement in fire scars helps us understand the freeze‐thaw dynamics of near‐surface ground and predict the future state, particularly in ice‐rich permafrost regions. This study used the L‐ and C‐band interferometric synthetic aperture radar technique to reveal interannual subsidence and seasonal thaw settlement/frost heave in a fire scar near Mayya, Sakha Republic in Eastern Siberia burned in 2013. We found that the cumulative subsidence was up to 7 cm between 2014 and 2020, most of which had occurred by 2016. The magnitude of seasonal thaw settlement and frost heave varied each year from 2017 to 2020 after the fire, but the interannual change in frost heave corresponded to the temporal variation in precipitation during the thawing season from 2017 to 2020. This suggests that the precipitation amount during the thawing season is related to the magnitude of segregation‐ice formation in the sediments, which determines the frost heave amount. The observed seasonal displacements could not be quantitatively explained by models inferred from the Stefan's equation and volume changes associated with ice‐water phase change. This implies that other models associated with segregated ice (ice lens) formation/thaw are required to explain the observed seasonal displacement.

Revising digital elevation models generation from Sentinel-1 synthetic aperture radar and interferometric technique in densely-vegetated humid tropical environment

Generation of digital elevation models (DEM) over densely vegetated humid tropics using Sentinel-1 data and Interferometry synthetic aperture radar (InSAR) technique is revised based on recommended optimum perpendicular baselines, and accuracy assessment based on coherence cells is introduced. Six pairs of Sentinel-1 data within the optimum perpendicular baseline of 150 − 400 m and different temporal baselines acquired between 2015 and 2021 were processed using Sentinel Application Platform. Results revealed: that due to the extreme difficulty of achieving good coherence, interferograms are characterized by noise, generated DEMs are dominated by artefacts, higher elevation accuracy can be achieved within coherence cells, and within the recommended optimum perpendicular baseline range, elevation accuracy within coherence cells is mainly determined by the perpendicular baseline compared to temporal baseline. Therefore, this study can provide practical guide for InSAR generated DEMs evaluation in densely vegetated environments, and remote sensing method for regional-scale measurement and mapping of spots heights.

Resonant two-photon ionization of helium atoms studied by attosecond interferometry

We study resonant two-photon ionization of helium atoms via the 1s3p, 1s4p and 1s5p1P1 states using the 15th harmonic of a titanium-sapphire laser for the excitation and a weak fraction of the laser field for the ionization. The phase of the photoelectron wavepackets is measured by an attosecond interferometric technique, using the 17th harmonic. We perform experiments with angular resolution using a velocity map imaging spectrometer and with high energy resolution using a magnetic bottle electron spectrometer. Our results are compared to calculations using the two-photon random phase approximation with exchange to account for electron correlation effects. We give an interpretation for the multiple π-rad phase jumps observed, both at and away from resonance, as well as their dependence on the emission angle.