Pulse Reflection Research Articles

Refraction of light by light in vacuum

In very intense electromagnetic fields, the vacuum refractive index is expected to be modified due to nonlinear quantum electrodynamics (QED) properties. Several experimental tests using high intensity lasers have been proposed to observe electromagnetic nonlinearities in vacuum, such as the diffraction or the reflection of intense laser pulses. We propose a new approach which consists in observing the refraction, i.e. the rotation of the waveplanes of a probe laser pulse crossing a transverse vacuum index gradient. The latter is produced by the interaction of two very intense and ultra short laser pulses, used as pump pulses. At the maximum of the index gradient, the refraction angle of the probe pulse is estimated to be 0.2 × (w 0/10 μm)-3 × I/1J prad, where I is the total energy of the two pump pulses and w 0 is the minimum waist (fwhm) at the interaction area. Assuming the most intense laser pulses attainable by the LASERIX facility (I = 25 J, 30 fs fwhm duration, 800 nm central wavelength) and assuming a minimum waist of w = 10 μm (fwhm) (corresponding to an intensity of the order of 1021 W/cm2), the expected maximum refraction angle is about 5 prad. An experimental setup, using a Sagnac interferometer, is proposed to perform this measurement.

Wearable Photoplethysmographic Sensor based on Different LED Light Intensities

Wearable photoplethysmographic sensors in the form of wristbands and watches have become increasingly popular in mobile healthcare. We propose a technique to cancel motion artifacts by using two reflective pulse signals from a single green LED, switched by a microcontroller between high and low light intensities. We accurately estimate motion artifacts by applying differential measurement and common-mode rejection. The performance was assessed for four different motion artifacts: artificial white noise, artificial color noise, and vertical/horizontal random movements of a hand. Under each different condition, we accurately cancelled out motion artifacts.

基于数学形态学的直流接地极引线C型行波故障测距

本文在介绍C型行波测距原理和数学形态学理论的基础上，根据脉冲信号在接地极线路上的传播特性，提出了一种基于数学形态学的C型行波故障测距方法，该方法首先对脉冲信号在线路上的传播过程进行模量分离，再根据形态学算法进行滤波，并利用梯度变换分离出正反向行波浪涌，不仅具有一定的抗干扰性，而且能准确地识别第一个反射脉冲到达测量点的时刻。通过PSCAD仿真验证和MATLAB图形处理表明，该方法是可行的，且有利于提高高压直流接地极引线故障测距的可靠性和准确性。 On the basis of introducing the principle of C type traveling wave fault location and mathematical morphology theory, this paper proposes a C type traveling wave fault location method based on mathematical morphology according to the propagation characteristics of the pulse signal in the grounding line. Firstly, this method carries on modulus separation of propagation process of the pulse signal on line, then, it uses the morphological algorithm for wave filtering, and the forward and backward waves are separated by the gradient transform. This method not only has certain anti-interference ability, but also can accurately distinguish the moment of the first reflection pulse reaching to the measuring point. Results show that the method is not only feasible but also helpful to improve the reliability and accuracy of the fault location for high voltage DC grounding wire through simulation of PSCAD and graphics processing of MATLAB.

General principles for reflection of laser pulse from a relativistic uniformly-moving mirror

Abstract We investigate the reflection of laser pulse from a uniformly moving mirror at relativistic speeds in the general case of inclined mirrors. We use geometrical optics to derive general relations for length, frequency, wavelength and electric field of the pulse after reflection from inclined mirror in the lab frame. In addition, the pulse geometric deformation from reflection is investigated and simulated in two dimensions.

Incorporation of a spatial source distribution and a spatial sensor sensitivity in a laser ultrasound propagation model using a streamlined Huygens’ principle

The near-field, surface-displacement waveforms in plates are modeled using interwoven concepts of Green’s function formalism and streamlined Huygens’ principle. Green’s functions resemble the building blocks of the sought displacement waveform, superimposed and weighted according to the simplified distribution. The approach incorporates an arbitrary circular spatial source distribution and an arbitrary circular spatial sensitivity in the area probed by the sensor. The displacement histories for uniform, Gaussian and annular normal-force source distributions and the uniform spatial sensor sensitivity are calculated, and the corresponding weight distributions are compared. To demonstrate the applicability of the developed scheme, measurements of laser ultrasound induced solely by the radiation pressure are compared with the calculated waveforms. The ultrasound is induced by laser pulse reflection from the mirror-surface of a glass plate. The measurements show excellent agreement not only with respect to various wave-arrivals but also in the shape of each arrival. Their shape depends on the beam profile of the excitation laser pulse and its corresponding spatial normal-force distribution.

Effect of acute nitrate ingestion on central hemodynamic load in hypoxia

Acute hypoxia results in local vasodilation that may temporarily unload the left ventricle (LV) through nitric oxide (NO)-mediated mechanisms. Whether increasing NO levels augments LV unloading and improves ventricular-vascular coupling in hypoxia remains unknown. PurposeInvestigate the effect of acute nitrate ingestion on central hemodynamic load in hypoxia. Methods20 Healthy men (23 ± 3 yrs, BMI 24.6 ± 2.8 kg m−2) consumed 70 mL of either a) 0.45 g nitrate (NIT) or b) an inert placebo (PLA) prior to 105 min of normobaric hypoxia (11.6 ± 0.1%) in this randomized, double-blind, crossover-design study. Wave reflection index (RIX; ratio of forward to reflected wave pressure), augmentation index (AIX75) and pulse wave velocity were calculated as measures of wave reflection magnitude and aortic stiffness, respectively, from the aortic blood pressure (BP) waveform. LV wasted pressure effort (WPE) was calculated as an index of LV work due to wave reflections. Subendocardial viability ratio (SEVR) was assessed a measure of myocardial O2 supply/demand ratio. ResultsAortic diastolic BP decreased in hypoxia compared to normoxia (p < 0.05). Aortic RIX, AIX75, and LV WPE significantly decreased in hypoxia compared to normoxia (p < 0.05). Aortic systolic BP, SEVR, and PWV were unaffected by hypoxia (p > 0.05). Compared to placebo, nitrate ingestion did not significantly alter central hemodynamics in hypoxia (p > 0.05). ConclusionsAcute hypoxic exposure unloads the LV (WPE, AIX75, and RIX) without disturbing myocardial O2 supply-demand ratio (SEVR). Reductions in LV work with hypoxia are likely due to reductions in pressure from wave reflections as hypoxia had negligible effects on aortic stiffness. Nitrate ingestion did not affect the central hemodynamic response to acute systemic hypoxia.

Large depth exploration using pulsed radar

SUMMARY We present an overview of the Adrok radar scanning technology and describe experimental results that suggest that ground penetrating radar can be utilized to much greater depths in selected environments than commonly assumed. High frequencies were found to penetrate very little, but the low frequency component had very low losses. Results were analysed to estimate the skin depth and interpreted in terms of a constitutive model incorporating Maxwell’s equations with conductivity and polarization losses. To explain these results we hypothesize that moisture penetrates limestone only relatively superficially and once the outer wet layer is penetrated the conductivity and therefore the losses are greatly reduced. In a second experiment we successfully detected the reflection of the radar pulse from a body of water through 350m of rock. A numerical simulation of the model confirmed that these results do not contradict theoretical expectations for dry limestone.

Functional status of microcirculatory-tissue systems during the cold pressor test

Using non-invasive optical methods of laser Doppler flowmetry, tissue reflectance oximetry and pulse oximetry, we investigated the dynamics of parameter changes of microcirculatory-tissue systems (MTS) when using cold pressor test (CPT) on 32 conventionally healthy volunteers. According to differences in the recovery rate of oxygen consumption during CPT, volunteers can be conditionally divided into 2 groups: 1--displaying normal physiology and 2--with a tendency to angiospasm and lack of functional recovery of the MTS. To identify possible causes of angiospasm, we performed a detailed analysis of blood flow oscillations. This analysis revealed that the cause of angiospasm may be a violation of the tone-shaping myogenic mechanisms regulating resistive microvessels, disturbances to the microcirculation of stagnant blood, or a combination of both of these factors. Use of the CPT for functional assessment of MTS allows us not only to estimate the reserve capabilities of the MTS, but also to identify--at a pre-clinical stage-propensity to angiospasm, which has practical value in the clinic.

Nonlinear reflection of a nanosecond laser pulse from thin aluminum film in the temperature range 2–14 kK

This letter aims to experimentally demonstrate the possibility of measuring the temporal dependencies of the surface temperature of an aluminum film confined by a transparent dielectric in the range below and above the critical temperature of aluminum (from 2 kK to 14 kK). Such temperatures are achieved under the action of a powerful linearly-polarized laser pulse of one nanosecond in duration onto the film’s surface. To find the temporal dependencies of the temperature of the aluminum film the nonlinear reflection coefficient of its irradiated surface is measured to determine the radiation of a Q-switched Nd:YAG laser at the fundamental wavelength.

Initial Examination of AltiKa's Individual Echoes

The AltiKa altimeter records the reflection of Ka-band radar pulses from the Earth's surface, with the commonly used waveform product involving the summation of 96 returns to provide average echoes at 40 Hz. Occasionally there are one-second recordings of the complex individual echoes (IEs), which facilitate the evaluation of on-board processing and offer the potential for new processing strategies. Our investigation of these IEs over the ocean confirms the on-board operations, whilst noting that data quantization limits the accuracy in the thermal noise region. By constructing average waveforms from 32 IEs at a time, and applying an innovative subwaveform retracker, we demonstrate that accurate height and wave height information can be retrieved from very short sections of data. Early exploration of the complex echoes reveals structure in the phase information similar to that noted for Envisat's IEs.

Relativistic Doppler reflection as a probe for the initial relaxation of a non-equilibrium electron-hole plasma in silicon

This paper reviews the status of investigations of the relativistic Doppler reflection of a broadband terahertz pulse at a counter-propagating plasma front of photo-excited charge carriers in undoped silicon. When a THz pulse with 20-THz bandwidth impinges onto a moving plasma front with a carrier density in the range of 1019 per cm3, one observes a spectral up-shift, which is, however, much less pronounced than expected from simulations assuming a Drude plasma characterized by a single carrier relaxation time τ of the order of 15-100 fs. Qualitative agreement between simulations and experiments can be achieved if τ is chosen to be less than 5 fs. In order to explore carrier relaxation in more detail, optical-pump/THz-probe experiments in the conventional co-propagation geometry were performed. If the pump-probe delay is long enough for monitoring of the equilibrium value of the scattering time, τ ranges from 200 fs at low carrier density to 20 fs in the 1019-cm-3 density range. For small (sub-picosecond) pump-probe delay, the data reveal a significantly faster scattering, which slows down during energy relaxation of the charge carriers.

ALR - Laser altimeter for the ASTER deep space mission. Simulated operation above a surface with crater

To assist in the investigation of the triple asteroid system 2001-SN263, the deep space mission ASTER will carry onboard a laser altimeter. The instrument was named ALR and its development is now in progress. In order to help in the instrument design, with a view to the creation of software to control the instrument, a package of computer programs was produced to simulate the operation of a pulsed laser altimeter with operating principle based on the measurement of the time of flight of the travelling pulse. This software Simulator was called ALR_Sim, and the results obtained with its use represent what should be expected as return signal when laser pulses are fired toward a target, reflect on it and return to be detected by the instrument. The program was successfully tested with regard to some of the most common situations expected. It constitutes now the main workbench dedicated to the creation and testing of control software to embark in the ALR. In addition, the Simulator constitutes also an important tool to assist the creation of software to be used on Earth, in the processing and analysis of the data received from the instrument. This work presents the results obtained in the special case which involves the modeling of a surface with crater, along with the simulation of the instrument operation above this type of terrain. This study points out that the comparison of the wave form obtained as return signal after reflection of the laser pulse on the surface of the crater with the expected return signal in the case of a flat and homogeneous surface is a useful method that can be applied for terrain details extraction.

Markers of arterial stiffness in peripheral arterial disease.

Increased arterial stiffness results from reduced elasticity of the arterial wall and is an independent predictor for cardiovascular risk. The gold standard for assessment of arterial stiffness is the carotid-femoral pulse wave velocity. Other parameters such as central aortic pulse pressure and aortic augmentation index are indirect, surrogate markers of arterial stiffness, but provide additional information on the characteristics of wave reflection. Peripheral arterial disease (PAD) is characterised by its association with systolic hypertension, increased arterial stiffness, disturbed wave reflexion and prognosis depending on ankle-brachial pressure index. This review summarises the physiology of pulse wave propagation and reflection and its changes due to aging and atherosclerosis. We discuss different non-invasive assessment techniques and highlight the importance of the understanding of arterial pulse wave analysis for each vascular specialist and primary care physician alike in the context of PAD.

Electronic State-Resolved Electron-Phonon Coupling in an Organic Charge Transfer Material from Broadband Quantum Beat Spectroscopy.

The coupling of electron and lattice phonon motion plays a fundamental role in the properties of functional organic charge-transfer materials. In this Letter we extend the use of ultrafast vibrational quantum beat spectroscopy to directly elucidate electron-phonon coupling in an organic charge-transfer material. As a case study, we compare the oscillatory components of the transient reflection (TR) of a broadband probe pulse from single crystals of quinhydrone, a 1:1 cocrystal of hydroquinone and p-benzoquinone, after exciting nonresonant impulsive stimulated Raman scattering and resonant electronic transitions using ultrafast pulses. Spontaneous resonance Raman spectra confirm the assignment of these oscillations as coherent lattice phonon excitations. Fourier transforms of the vibrational quantum beats in our broadband TR measurements allow construction of spectra that we show report the ability of these phonons to directly modulate the electronic structure of quinhydrone. These results demonstrate how coherent ultrafast processes can characterize the complex interplay of charge transfer and lattice motion in materials of fundamental relevance to chemistry, materials sciences, and condensed matter physics.

A Prototype of Reflection Pulse Oximeter Designed for Mobile Healthcare.

This paper introduces a pulse oximeter prototype designed for mobile healthcare. In this prototype, a reflection pulse oximeter is embedded into the back cover of a smart handheld device to offer the convenient measurement of both heart rate (HR) and SpO2 (estimation of arterial oxygen saturation) for home or mobile applications. Novel and miniaturized circuit modules including a chopper network and a filtering amplifier were designed to overcome the influence of ambient light and interferences that are caused by embedding the sensor into a flat cover. A method based on adaptive trough detection for improved HR and SpO2 estimation is proposed with appropriate simplification for its implementation on mobile devices. A fast and effective photoplethysmogram validation scheme is also proposed. Clinical experiments have been carried out to calibrate and test our oximeter. Our prototype oximeter can achieve comparable performance to a clinical oximeter with no significant difference revealed by paired t -tests ( p = 0.182 for SpO2 measurement and p = 0.496 for HR measurement). The design of this pulse oximeter will facilitate fast and convenient measurement of SpO2 for mobile healthcare.

Simulation study on the effect of tissue geometries to fluence composition for non-invasive fetal pulse oximetry.

Transabdominal fetal pulse oximetry is a method to estimate the state of oxygenation of a fetus in-utero, utilizing the principle of reflection pulse oximetry. The extraction of fetal related information from a mixed fetal-maternal signal is elementary. Minimizing the ratio of purely maternal components of the signal at the detector side obviously facilitates signal separation. In this paper we analyze the influence of tissue geometries to the fluence composition at the surface of the abdomen. Monte-Carlo method is used to compute photon propagation in spherical layered tissue models. Spatial fluence distributions at the surface of the models are visualized and discussed. Our results show the characteristic effects of the distance between the fetus and the surface and the radius of the abdomen to the fluence composition at the detector. Further, the simulations indicate suitable source-detector configurations considering various anatomical conditions. We conclude that an adoption of the source-detector configuration to the individual tissue geometry at hand is necessary to achieve a proper signal composition and quality. Utilizing simulations for sensor design enhances the understanding of photon distributions in complex tissue geometries and supports a successful implementation of transabdominal fetal pulse oximetry.

Recording system and data fusion algorithm for enhancing the estimation of the respiratory rate from photoplethysmogram.

The respiratory rate is a vital parameter that can provide valuable information about the health condition of a patient. The extraction of respiratory information from photoplethysmographic signal (PPG) was actually encouraged by the reported results, our main goal being to obtain accurate respiratory rate estimation from the PPG signal. We developed a fusion algorithm that identifies the best derived respiratory signals, from which is possible to extract the respiratory rate; based on these, a global respiratory rate is computed using the proposed fusion algorithm. The algorithm is qualitatively tested on real PPG signals recorded by an acquisition system we implemented, using a reflection pulse oximeter sensor. Its performance is also statistically evaluated using benchmark dataset publically available from CapnoBase.Org.

Time-dependent Bragg diffraction and short-pulse reflection by one-dimensional photonic crystals

The time-dependence of the Bragg diffraction by one-dimensional photonic crystals and its influence on the short pulse reflection are studied in the framework of the coupled-wave theory. The indicial response of the photonic crystal is calculated and it appears that it presents a time-delay effect with a transient time conditioned by the extinction length. A numerical simulation is presented for a Bragg mirror in the x-ray domain and a pulse envelope modelled by a sine-squared shape. The potential consequences of the time-delay effect in time-dependent optics of short-pulses are emphasized.

Controlling of group velocity via terahertz signal radiation in a defect medium doped by four-level InGaN/GaN quantum dot nanostructure

In this paper, we propose a novel scheme for controlling the group velocity of transmitted and reflected pulse from defect medium doped with four-level InGaN/GaN quantum dot nanostructure. Quantum dot nanostructure is designed numerically by Schrödinger and Poisson equations which solve self consistently. By size control of quantum dot and external voltage, one can design a four-level quantum dot with appropriate energy levels which can be suitable for controlling the group velocity of pulse transmission and reflection from defect slab with terahertz signal field. It is found that in the presence and absence of terahertz signal field the behaviors of transmission and reflection pulses are completely different. Moreover, it is shown that for strong terahertz signal field, by changing the thickness of the slab, simultaneous peak and dip for transmission and reflection pulse are obtained.

Interrogation of Alkaline AA Batteries Using in Operando Electrochemical ­Acoustic Time-of­-Flight Analysis and in Operando energy Dispersive X-Ray Diffraction

Recently, in operando Electrochemical ­Acoustic Time-of­-Flight (EAToF) analysis has been shown to provide information about the state of charge (SoC) and state of health (SoH) of a number of battery geometries and chemistries using ultrasonic interrogation of the density and modulus shifts that occur during charge and discharge. The technique has thus far offered a general understanding of these batteries, but has not been used for in depth analysis of individual battery chemistries. In this work, we performed EAToF measurements on commercially available alkaline Zn/MnO2 AA batteries of a number of different brands under discharge and charge conditions. The data from this method were compared to in situ energy-dispersive x-ray diffraction (EDXRD) data from comparable alkaline AA batteries. Changes in the waveforms in the ultrasonic time-of-flight measurements were shown to correlate with phase transitions in the MnO2 cathode and densification and dehydration of the Zn anode, as shown by peak evolution in the EDXRD data. The mechanisms by which the phase and morphology transitions can change the transmission and reflection of an ultrasonic pulse and the utility of EAToF analysis in determining material differences between different brands of alkaline cells are discussed.