Pulse Reflection Research Articles

The reflection of a planar impulsive shock wave at a liquid–gas interface

The reflection of a shock pulse at a liquid–gas interface occurs in many applications, from lithotripsy to underwater explosions and additive manufacturing. In linear theory, reflection and transmission at an interface depend only on the impedance difference, but this does not hold for a nonlinear pulse. This work develops an analytical framework for computing the reflection and transmission coefficients for an impulsive shock wave at a liquid–gas interface. The problem is treated analytically by considering idealised pulses and solving a series of consecutive Riemann problems. These correspond to the initial interaction with the interface and important subsequent wave interactions that enable a complete description of the process to be obtained. Comparisons with numerical and existing analytical approaches are made for the case of a water–air interface. In the acoustic limit, the method produces results identical to those of linear acoustic theory. As the pulse strength increases, the proposed method agrees well with numerical simulation results, whereas existing analytical methods that consider only the interface fail. We detail how a reflecting pulse can put water into tension without any incident negative pressure. It is further shown that the magnitude of the reflection coefficient decreases with increasing incident shock pressure, and the reflected pulse widens. Reflections of pulses with positive and negative pressures temporarily create negative pressure regions with greater magnitude than the incident pulse. Finally, we consider non-idealised waves. Comparisons with simulations show that the reflection characteristics can be explained qualitatively using the analytical method, and the reflection coefficients are predicted accurately.

Optimal radar ranging pulse to resolve two reflectors

Previous work established fundamental bounds on subwavelength resolution for the radar range resolution problem, called superradar []. In this work, we identify the optimal waveforms for distinguishing the range resolution between two reflectors of identical strength, leveraging results in quantum metrology. We discuss both the unnormalized optimal waveform as well as the best square-integrable pulse and their variants. Using orthogonal function theory, we give an explicit algorithm to optimize the wave pulse in finite time to have the best performance. We also explore range resolution estimation with unnormalized waveforms with multiparameter methods to also independently estimate loss and time of arrival. These results are consistent with the earlier single parameter approach of range resolution only and give deeper insight into the ranging estimation problem. Experimental results are presented using radio pulse reflections inside coaxial cables, showing robust range resolution smaller than a tenth of the inverse bandlimit, with uncertainties close to the derived Cramér-Rao bound. Published by the American Physical Society 2024

Temporal reflection from short pump pulses inside a dispersive nonlinear medium: the impact of pump parameters

We have studied, through a series of experiments and numerical simulations, how temporal reflection from an intense pump pulse inside a photonic crystal fiber is affected by parameters of the pump pulse used to form a moving high-index boundary. We used femtosecond pump pulses, which slow down inside the fiber as their spectrum red-shifts because of intrapulse Raman scattering. Temporal reflection of probe pulses occurs from such decelerating pump pulses. We changed the width and chirp of our pump pulses with a 4f pulse shaper capable of providing both spectral filtering and frequency chirping. We found that temporal refection exhibited novel features, to our knowledge, when pump pulses were made wider or chirped. In both cases, two or more reflected pulses were produced at different wavelengths in a specific range of the initial pump-probe delays. Numerical simulations reveal that the origin of such novel features is related to the complex nonlinear evolution of pump pulses inside optical fibers.

Reflection and Transmission of Airy Pulse from Controllable Periodic Temporal Boundary

AbstractThe interaction between two Airy pulses propagating at different wavelengths is numerically investigated. The periodically varying peak intensity of the soliton that emerges from stronger Airy pulse (pump pulse) leads to the formation of periodic temporal boundary. The relatively weaker Airy pulse (probe pulse) on interaction with this boundary gets partially reflected as well as transmitted. As a result, the probe pulse spectrum splits into two parts‐ the reflected pulse spectrum undergoes redshift while transmitted pulse exhibits blueshift. The probe pulse witnesses maximum reflection when point of interaction lies on the intensity maxima of the emergent soliton from pump Airy pulse. On the other hand, maximum transmission occurs when probe Airy pulse interacts at the intensity minima of the soliton. The reflection and transmission processes can be manipulated by tuning the time delay between pump and probe Airy pulses. In the case of a sufficiently intense pump pulse, the temporal boundary mimics the artificial optical event horizon, and the weak probe Airy pulse is completely reflected. This phenomenon is equivalent to the temporal version of total internal reflection. The results of the study hold potential applications in optical manipulation and temporal waveguiding.

Ultrasonic Non-Destructive Testing and Evaluation of Stainless-Steel Resistance Spot Welding Based on Spiral C-Scan Technique.

In order to achieve the non-destructive testing and quality evaluation of stainless-steel resistance spot welding (RSW) joints, a portable ultrasonic spiral C-scan testing instrument was developed based on the principle of ultrasonic pulse reflection. A mathematical model for the quality evaluation of RSW joints was established, and the centroid of the ultrasonic C-scan image in the nugget zone of the RSW was determined based on the principle of static moment. The longest and shortest axes passing through the centroid in the image were extracted, and the ratio of the longest axis to the shortest axis (RLS) factor and the average of axis (AOA) factor were calculated, respectively, to evaluate the quality of the joint. To study the effectiveness of the detection results, tensile tests, and stereo analysis were conducted on the solder joints after sampling. The results indicate that this detection method can realize online detection and significantly improve the detection efficiency; the detection value of internal defect size is close to the true value with an error of 0.1 mm; the combination of RLS and AOA factors can be used to evaluate the mechanical properties of RSW joints. This technology can be used to solve the NDT, evaluate problems of RSW joints, and realize engineering applications.

Coherent manipulation of bright and dark solitons of reflection and transmission pulses through sodium atomic medium

Abstract The coherent manipulation and control of bright and dark solitons through sodium atomic medium have been investigated in this manuscript. Dark soliton is reported for reflection and bright soliton is reported for transmission pulses with variation in position and driving field parameters through sodium atomic medium. Further the transmission pulse is periodic dark and bright solitonic behaviors and reflection pulse is periodic bright solitonic behavior with variation in the incident angle and Rabi frequency of the control field. Elliptical dark and bright solitons as well as breather types solitons are also investigated for reflection and transmission pulses. The dark soliton in reflection is due to slow light propagation and bright soliton is obtained due to fast light propagation of transmission through the medium. The modified results of the dark and bright solitons are useful for telecommunication and ultra-fast signal routing system.

Absorption of ultrashort laser pulses in medium with account for propagation effects

The absorption of ultrashort laser pulses in an optically dense medium is theoretically studied based on the total absorption coefficient over the entire duration of the pulse, taking into account the effects of propagation and reflection of pulse from vacuum/medium interface. The approach is the generalization of our previous work (available at: www.nobelprize.org/prizes/physics/2023/summary), which traced a ‘bridge’ between the micro- and macro-description of radiation processes in a medium induced by electromagnetic pulses of arbitrary duration. The expression for the total absorption coefficient is obtained in terms the complex refractive index of the medium. Various limits of this expression are considered. Numerical calculations of the dependence of the total absorption coefficient on the parameters of the laser pulses and the thickness of the medium layer were carried out and the results obtained were discussed.

Correction of microwave pulse reflection by digital filters in superconducting quantum circuits

Abstract Reducing the control error is vital for high-fidelity digital and analog quantum operations. In superconducting circuits, one disagreeable error arises from the reflection of microwave signals due to impedance mismatch in the control chain. Here, we demonstrate a reflection cancelation method when considering that there are two reflection nodes on the control line. We propose to generate the pre-distortion pulse by passing the envelopes of the microwave signal through digital filters, which enables real-time reflection correction when integrated into the field-programmable gate array (FPGA). We achieve a reduction of single-qubit gate infidelity from 0.67% to 0.11% after eliminating microwave reflection. Real-time correction of microwave reflection paves the way for precise control and manipulation of the qubit state and would ultimately enhance the performance of algorithms and simulations executed on quantum processors.

Interaction of highly focused intense laser radiation with transparent dielectrics: Subwavelength nanogratings formation

Femtosecond laser writing of birefringent subwavelength nanolattices in dielectrics has been studied for almost two decades since it reveals a number of applications for optical memory devices, optical waveguides, microfluidic channels, etc. In this work, a numerical study of the formation of plasma quasiperiodic nanostructures in a fused silica in the propagation direction of a focused laser pulse is carried out. It is shown that the focused beam creates a dense plasma, which provides an effective reflection of the incident laser pulse, leading to the formation of a standing wave. In the bundles of standing wave, an effective ionization emerges, which forms plasma gratings with a subwavelength period. The conducted modeling allows us to determine the conditions under which the proposed regime of material nanostructuring is possible. It is shown that the energy absorbed in plasma gratings will ensure the substance melting, which leads to the formation of “frozen” quasiperiodic nanopatterns.

Optimization of Weak Ultrasonic Defect Signal Detection of Carbon Fiber Composites Based on Double-Sided Pulse Reflection Scanning

Optimization of Weak Ultrasonic Defect Signal Detection of Carbon Fiber Composites Based on Double-Sided Pulse Reflection Scanning

Design and validation of dual-point time-differentiated photoplethysmogram (2PPG) wearable for cuffless blood pressure estimation

Background & objectivesMeasurement of blood pressure (BP) in ambulatory patients is crucial for at high-risk cardiovascular patients. A non-obtrusive, non-occluding device that continuously measures BP via photoplethysmography will enable long-term ambulatory assessment of BP. The aim of this study is to validate the metasense 2PPG cuffless wearable design for continuous BP estimation without ECG. MethodsA customized high-speed electronic optical sensor architecture with laterally spaced reflectance pulse oximetry was designed into a simple unobtrusive low-power wearable in the form of a watch. 78 volunteers with a mean age of 32.72 ± 7.4 years (21 to 64), 51% male, 49% female were recruited with ECG-2PPG signals acquired. The fiducial features of the 2PPG morphologies were then attributed to the estimator. A 9–1 K-fold cross-validation was applied in the ML. ResultsThe correlation for PTT-SBP was 0.971 and for PTT-DBP was 0.954. The mean absolute error was 3.167±1.636 mmHg for SBP and 6.4 ± 3.9 mm Hg for DBP. The ambulatory estimate for SBP and DBP for an individual over 3 days with 8-hour recordings was 0.70–0.81 for SBP and 0.42–0.51 for DBP with a ± 2.65 mmHg for SBP and ±2.02 mmHg for DBP. For SBP, 98% of metasense measurements were within 15 mm Hg and for DBP, 91% of metasense measurements were within 10 mmHg ConclusionsThe metasense device provides continuous, non-invasive BP estimations that are comparable to ambulatory BP meters. The portability and unobtrusiveness of this device, as well as the ability to continuously measure BP could one day enable long-term ambulatory BP measurement for precision cardiovascular therapeutic regimens.

Correlations of light scattering properties in human skin with the person's age assessed using a non-invasive technique.

We analyze the influence of a person's age on the thicknesses and reduced scattering coefficients of the epidermis and dermis in visible part of the spectrum. Their values were assessed using a non-invasive technique which combines pulsed photothermal radiometry and diffuse reflectance spectroscopy with Monte Carlo modeling of light transport in a four-layer model of skin. The analysis is affected by the strong influences of the melanin content on the reduced scattering coefficient of the epidermis, a epi, and blood content in the case of dermis (a der). Separating their contributions reveals a significant decrease of a der with the person's age at an average rate of -0.25 mm-1 per decade, while the contribution of blood in the papillary dermis amounts to 1.0 mm-1%-1. Meanwhile, no influence of the person's age was found on a epi and the thicknesses of the epidermis or dermis.

Research on detection of switch rail defects based on pulse reflection method of the dominant mode

Guided wave structural health monitoring offers a promising way for continuous surveillance and identification of structural damage. However, due to the variable cross-section properties of the switch rail in the longitudinal direction, there are different modes present on different cross-sections, so the guided wave signals excited are very complex. The traditional baseline subtraction method can be used to extract defect echo features, but it is prone to false positives due to the influence of external factors such as field temperature change and environmental noise. This paper proposes a switch rail defect detection method based on the pulse reflection method of the dominant mode. Initially, eight typical cross-sections are chosen based on the cross-sectional dimensions of the switch rail. Semi-analytical finite element(SAFE) method is utilized to solve the dispersion curves and wave structure matrix of guided waves in the switch rail. According to the 3σ rule, the dominant mode whose node amplitude is much higher than other modes is selected, and the excitation signal is applied at the maximum value of the mode, which successfully excites the dominant mode. Through simulation and experimental validation, the reflected waves of the dominant mode at the defect are observable and visible, and the pulse reflection method can be used to detect the defects at the switch rail foot with a positioning error of less than 10 cm. The pulse reflection method of the dominant mode proposed in this paper, which has high detection efficiency and is not affected by the ambient temperature, provides a new idea and way to detect defects of the switch rail based on the ultrasonic guided wave method.

Strong multiperiod forerunning light jet within a double-prism gap: a counterintuitive manifestation of causality

We report on the effect of a strong and protracted advanced response in pulse transmission and reflection in a double-prism scheme. In distinction to the well-known activity on superluminal-like tunneling of an electromagnetic pulse through a gap of a double prism, we consider an optical pulse refracting to a gap and sliding therein. The formation of a multiperiod light jet running within the gap well before the incident pulse is shown with account of normal material dispersion and excitation of leaky modes in the gap. Conditions for the paradoxical appearance of the forerunning jet are revealed to have a geometrical nature with a specific relation between phase and group velocities involved. This deduction assigns the stated effect to the counterintuitive manifestation of causality with no reference to superluminal propagation.

ON THE FEATURES OF REFLECTION OF SHEAR ELASTIC WAVE PULSES FROM A VERTICAL CRACK

A computer simulation of the characteristics of pulse signals of shear waves reflected from the edge of a vertical crack coming to the surface of the control object and a bottom crack using radiated pulses of various lengths with narrow, medium width and wide spectra was carried out. Within the limits of changing the calculation parameters, a feature of pulse reflection is established – the ratio of the maxima of the envelope pulses when reflected from a crack coming to the surface of the OK and the bottom crack is approximately 3.40 (10.6 dB), which coincides with the corresponding ratio of the maxima of the spectra. Based on the results of calculations of pulse shapes with frequency spectrum of different widths when waves are reflected from a vertical crack, the recommendation is justified not to use long pulses with narrow spectra with reduced beam resolution in the time of flight diffraction method.

The 1952 Ph.D. dissertation of T.F.W. Embleton on nonlinear acoustic propagation and reflection

In his doctoral research Tony Embleton studied nonlinear acoustics experimentally using techniques available in the Physics Department of Imperial College, University of London. Originally developed by V. Timbrell, the interferometric equipment was substantially improved by Tony Embleton, enabling his Ph.D. findings. Studying propagation in a tube and reflection by both closed and open ends, Tony Embleton was able to substantially confirm the theory of the time, developed by Stokes, Earnshaw, Rankine, Rayleigh, and Taylor, many pieces confirmed for the first time. He also found discrepancies regarding the attenuation coefficients regarding the first and second half cycle of each pulse as well as a shortening of a pulse at reflection from a closed end. These findings are still worthy of further investigation. The results of reflection of pulses from the open end of a tube, forming a pressure release boundary condition, still are very interesting to the present author. A pulse propagating toward the open end steepens, but the reflected wave “unsteepens” which is a unique nonlinear acoustic phenomenon. [Work supported by the Penn State College of Engineering and its United Technologies Corporation Professorship.]

On-chip spatiotemporal optical vortex generation using an integrated metal–dielectric resonator

We theoretically demonstrate the possibility of generating a spatiotemporal optical vortex (STOV) beam in a dielectric slab waveguide. The STOV is generated upon reflection of a spatiotemporal optical pulse from an integrated metal–dielectric structure consisting of metal strips “buried” in the waveguide. For describing the interaction of the incident pulse with the integrated structure, we derive its “vectorial” spatiotemporal transfer function (TF) describing the transformation of the electromagnetic field components of the incident pulse. We show that if the TF of the structure corresponds to the TF of a spatiotemporal differentiator with a π/2 phase difference between the terms describing temporal and spatial differentiation, then the envelope of the reflected pulse will contain an STOV in all nonzero components of the electromagnetic field. The obtained theoretical results are in good agreement with the results of rigorous numerical simulation of the STOV generation using a three-strip metal–dielectric integrated structure. We believe that the presented results pave the way for the research and application of STOV beams in the on-chip geometry.

Application of Machine Learning Algorithm for Defect Analysis in Semiconductors Using High Resolved Scanning Acoustic Microscopy

Scanning Acoustic Microscopy is a dedicated powerful technology in NDT to analyze and characterize bonded interfaces for different technologies. As the complexity of the semiconductor package grows, even seemingly insignificant faults become quite important. Main applications include detection of voids, inclusions and delaminated areas in bonding interfaces and thickness variations in layer structures. The SAM operates with the pulse reflection method. The acoustic lens transforms high frequency electromagnetic vibrations that are transmitted inside the lens as a planar parallel wave field. Through the coupling medium, which is water, the cavity concentrates the sound field on the sample. The sound pulses reflected from the sample are gathered up by the acoustic lens and are converted into electromagnetic pulses by the transducer. These pulses are reconstructed into images with defined gray values. Pixels with certain gray levels are used to depict pulses.New developments in SAM combine high throughput inspection and high resolution down to the sub-micrometer range.The wafer bow is quickly eliminated by altering the array transducer focus point throughout the entire scanning procedure. New techniques for SAM's bonded interface protection keep the wafer almost completely dry throughout inspection. In order to discover and categorize failures with the least amount of data, research focuses on deep learning networks and scanning acoustic microscopes for automated failure assessment. High speed automated wafer-level inspections were made possible by the development of image and signal-based machine learning techniques. These operations can be carried out at both the wafer level and the die level without the need for sample preparation.The automatic handling, scanning, drying, and separation of good wafers from defected wafers based on acoustic data made possible by technological developments in SAM systems. Despite the high sensitivity, minute variations in the reflected signal that would point to a flaw can be lost during image reconstruction. More emphasis is being placed on creating AI-based algorithms for highly accurate automated signal interpretation for failure detection as a result of recent breakthroughs in AI (Artificial Intelligence). Since it requires less human work, automated failure analysis is becoming more and more popular for the detection and qualification of wafer bond integrity. This new technique trains the network for automated fault prediction using raw A-Scan data. In the first phase, die level prediction is reviewed and in the later phase, a wafer-level high volume inspection is performed. Combining SAM with an A-Scan-based Deep Learning approach primarily benefits non-destructive analysis with less labelling effort and training time. With minimal human effort, this process flow is perfect for automated high-volume inspection.SAM also uses image based machine learning approach for automated defect analysis. The U-net machine-learning algorithm serves as the foundation for the integrated image analysis program. Deep Learning technologies is integrated into a new modular image analysis (MIA) architecture with built-up integrated pipeline for Label Studio Software-based Deep Learning activities (labelling, training, analysis). The built-up pipeline is used to identify, train, and test various classes of defect in a bonded wafer. This method employs model confidence to manually mark marginal and defective predictions.High resolution and high throughput analysis for the defect characterization and defect analysis of various types of bonded wafers is improved in scanning acoustic microscopes using the array transducers, signal processing methods like SAFT, Wavelet filtering techniques, Artificial intelligence methods based on both image and signal. With all these advanced technologies, Scanning Acoustic Microscopy is capable of inspection of wafer bond integrity, TSV characterization, 3D package evaluation, Wafer Level Chip Scale Packaging (WL-CSP) and Crack inspection. For the metrology of mass production, from initial research and development, our equipment services and software algorithms assist manufacturers of nano electronics in better defect analysis and material characterization. Next Generation transducers were designed and developed in particular for WL-CSP applications.

Ballistic performance of spaced multi-ply soft fabrics: Experimental and numerical investigation

It has been reported that the ply gap influences the ballistic resistance of spaced multi-ply fabric systems, but its working mechanism was not well-understood. This paper reports the experimental and numerical approaches and results of an investigation on the mechanisms that enable the improved ballistic performance of spaced multi-ply systems. Penetration tests were performed over a range of impact velocities ranging from 200 m/s to 400 m/s. The results confirmed that the ply gap is beneficial to the energy absorption capability of the systems. This is because the front plies tend to absorb more energy when they are not immediately constrained by the rear plies. During a ballistic event, the gap relieves the reflection of the compressive pulse, prolonging the projectile engagement time with the front plies; on the other hand, the rear plies become increasingly less active in dissipating energy as the gap increases. When the gap is sufficiently widened to avoid any interference between the plies before the failure of the front ply, the responses of the whole system no longer vary. It was also found that the ballistic performance of the spaced systems is influenced by ply thickness, impact velocity, and the stacking order of the ply gap.

Prospective clinical validation of the Empatica EmbracePlus wristband as a reflective pulse oximeter.

Respiratory diseases such as chronic obstructive pulmonary disease, obstructive sleep apnea syndrome, and COVID-19 may cause a decrease in arterial oxygen saturation (SaO2). The continuous monitoring of oxygen levels may be beneficial for the early detection of hypoxemia and timely intervention. Wearable non-invasive pulse oximetry devices measuring peripheral oxygen saturation (SpO2) have been garnering increasing popularity. However, there is still a strong need for extended and robust clinical validation of such devices, especially to address topical concerns about disparities in performances across racial groups. This prospective clinical validation aimed to assess the accuracy of the reflective pulse oximeter function of the EmbracePlus wristband during a controlled hypoxia study in accordance with the ISO 80601-2-61:2017 standard and the Food & Drug Administration (FDA) guidance. Healthy adult participants were recruited in a controlled desaturation protocol to reproduce mild, moderate, and severe hypoxic conditions with SaO2 ranging from 100% to 70% (ClinicalTrials.gov registration #NCT04964609). The SpO2 level was estimated with an EmbracePlus device placed on the participant's wrist and the reference SaO2 was obtained from blood samples analyzed with a multiwavelength co-oximeter. The controlled hypoxia study yielded 373 conclusive measurements on 15 subjects, including 30% of participants with dark skin pigmentation (V-VI on the Fitzpatrick scale). The accuracy root mean square (Arms) error was found to be 2.4%, within the 3.5% limit recommended by the FDA. A strong positive correlation between the wristband SpO2 and the reference SaO2 was observed (r = 0.96, P < 0.001), and a good concordance was found with Bland-Altman analysis (bias, 0.05%; standard deviation, 1.66; lower limit, -4.7%; and upper limit, 4.8%). Moreover, acceptable accuracy was observed when stratifying data points by skin pigmentation (Arms 2.2% in Fitzpatrick V-VI, 2.5% in Fitzpatrick I-IV), and sex (Arms 1.9% in females, and 2.9% in males). This study demonstrates that the EmbracePlus wristband could be used to assess SpO2 with clinically acceptable accuracy under no-motion and high perfusion conditions for individuals of different ethnicities across the claimed range. This study paves the way for further accuracy evaluations on unhealthy subjects and during prolonged use in ambulatory settings.