Pulse Reflection Research Articles

Short surface waves in the Canadian Arctic in 2007 and 2008

[1] We have collected time series data of short oceanic waves as a part of the International Polar Year (IPY) 2007–2008. Using a shipboard laser wave slope (LAWAS) system operating at 900 nm, we have obtained wave slopes measurements up to 60 rad m−1 wave number. We have compared our in situ wave slopes with collocated and concurrent high-resolution upwind Normalized Radar Cross Sections (NRCS) collected by QuikSCAT. The LAWAS measured wave slope spectra were consistent with local wind speeds and QuikSCAT measured NRCS. Our measured short wave mean slopes indicate their enhancement by long waves (0–1 rad m−1) at small values of long-wave slope. Concurrent with wave slope measurements, the strength of the reflected LAWAS light beam was analyzed in terms of the light attenuation coefficient at 900 nm. We have observed a correlation between surface elevation and light attenuation. The mechanism of wave modulated beam attenuation was found to be related to the instantaneous long wave skewness.

Puistu esimese rinde võrastiku alguse kõrguse hindamine lennukilidari mõõdistusandmete järgi / A simple model to estimate forest canopy base height from airborne lidar data

Abstract Airborne laser scanner (ALS) measurements from two test sites in Estonia were used to estimate forest canopy-base height (HVL). The ALS data was collected by Estonian Land Board using Leica ALS50-II scanner. The HVL was estimated by using mode value and standard deviation of the ALS pulse reflection position height distribution. The pulse reflections which had height less than 0.5 m over the estimated digital terrain model were excluded from the analysis. In situ measurements of canopy base height (HVA) were carried out in 20 mesotrophic Norway spruce and silver birch forest stands in Järvselja and in 45, mostly Scots pine dominant, mesotrophic forest stands in Aegviidu. Determination coefficients of linear regression between HVL and HVA for both test sites were over 0.8 and the residual standard errors of the models were less than two meters. The influence of forest understory vegetation to the estimation of HVL was tested by excluding the near-to-ground vegetation reflections which had height less than 1.5 m. The test results revealed no significant impact of forest understory to the HVL models. The cross validation showed that the HVL models were independent of test sites and tree species composition. The Järvselja data based HVL model had 1.3 m negative bias if applied to Aegviidu forests and the Aegviidu data based HVL model had 1.4 m positive bias if applied to Järvselja forests. In the Aegviidu test site, difference of HVL models of coniferous and deciduous stands was tested and the difference was found not to be significant

A comparative study on reflection of nanosecond Nd-YAG laser pulses in ablation of metals in air and in vacuum

A comparative study on reflection of nanosecond Nd-YAG laser pulses in ablation of aluminum in air and in vacuum under the same other experimental conditions is performed. We find that, hemispherical total reflectivity of aluminum undergoes a sharp drop at the plasma formation threshold both in the air and in vacuum. The initial large value (0.8) of aluminum reflectivity decreases to a level of about 0.14 and 0.24 for ablation in the air and in vacuum, respectively. These decreased reflectivity values remain virtually unchanged with further increasing laser fluence. The reflectivity drop in the air is observed to be sharper than in vacuum. Our study indicates that the reflectivity drop is predominantly caused by absorption of the laser light in plasma. Nano/micro-structural defects present on practical sample surfaces play the important role in the plasma formation, especially for the ablation in the air, where the plasma formation threshold is found to be by a factor of 3 smaller than in vacuum.

Generation of electron nanobunches and short-wavelength radiation upon reflection of a relativistic-intensity laser pulse from a finite-size target

We have proposed an efficient scheme of generation of short dense electron bunches during the interaction at large angles of incidence of a laser pulse with a thin transversally semibounded laser target. Streams of bunches can be used to simultaneously and independently generate pulsed X-ray radiation as fast electrons hit secondary targets. Dependences of bunch parameters (the number of particles in the bunch and the bunch energy and thickness) on the angle of incidence and laser intensity have been obtained. It has been shown that, upon reflection from the target, the relativistic-intensity laser wave is efficiently converted (the energy-conversion factor reaches ∼20%) into a sequence of electromagnetic tens-of-nanometer-long atto pulses, which follow one after another in the period of the initial laser wave. We have investigated how the parameters of the atto pulse depend on the angle of incidence and the laser intensity. We have shown that atto pulses are generated most efficiently at large angles of incidence (≥50°) of the laser pulse on the target.

Probing of crack breathing by pulsed laser-generated acoustic waves

Experimental results on all-optical monitoring of the nonlinear motion of a surface-breaking crack are reported. Crack closing is induced by quasi-continuous laser heating, while Rayleigh surface acoustic pulses and bulk longitudinal surface skimming acoustic pulses are also generated and detected by lasers. By exploiting the strong dependence of the acoustic pulses reflection and transmission efficiency on the state—open or closed—of the contacts between the crack faces, the parametric modulation of ultrasonic pulses is achieved. It is observed that bulk acoustic waves, skimming along the surface can be more sensitive to crack motion than Rayleigh surface waves. It has been found that crack closure by thermo-elastic stresses modifies the propagation paths of the acoustic rays from the point source to the point receiver. Consequently, the arrival times of the acoustic waves contain information on the state of crack closure induced by a particular intensity of laser heating. An important dependence of the detected signals on the initial width/state of the crack and on the presence of necks in the crack opening profile is revealed. It is demonstrated that the mode conversion of the skimming longitudinal bulk waves incident on the crack into the transmitted Rayleigh waves is very sensitive to imperfectness of crack closure. The proposed interpretation of the experimental observations is supported by atomic force microscopy measurements.

Analysis of Northern bottlenose whale pulses and associated reflections recorded from the Gully Marine Protected Area

The Gully Marine Protected Area (MPA) is a large submarine canyon at the edge of the Scotian Shelf, south of Nova Scotia. A resident population of northern bottlenose whales are known to occur in the Gully throughout the year, and the canyon provides important foraging grounds for the population. Bottom-mounted Autonomous Multichannel Acoustic Recorders (AMAR) were deployed in the Gully for ten days in in March 2010 (sampling rate = 375 ksps) and two days in October 2011 (sampling rate = 128 ksps). Bisonar pulses produced northern bottlenose whales (likely used to echolocate prey) were recorded consistently throughout these AMAR deployments. The swept FM characteristics of the northern bottlenose whale pulses recorded were consistent over both years, and both data sets contained clear pulse reflections from bottom clutter or prey targets. In this paper, we provide a description of the northern bottlenose whale pulses recorded in the Gully and make recommendations on short-time Fourier transform parameters...

Influence of test method used to evaluate resistance of C‐Mn steel to hydrogen induced cracking

AbstractThe presented paper deals with a comparison of test methods for evaluating the resistance of API grade X65 steel to hydrogen induced cracking (HIC). It concerned particularly the ultrasonic immersion method using an AGFA non‐destructive testing device by Krautkrämer and metallographic evaluation of the results of ultrasonic tests according to the directive NACE TM 0284. Comparison of test methods was made on the basis of established parameters of cracks, i. e. crack length ratio (CLR), crack thickness ratio (CTR) and crack sensitivity ratio (CSR). The determination of parameters in case of ultrasonic examination was performed using the pulse reflection method. Metallographic evaluations were performed always at hundred‐fold magnification on a light microscope ZEISS NEOPHOT 32. The obtained results showed that all the test methods used had very small differences between the investigated parameters.

지면 투과 레이더(GPR) 기반의 지뢰 탐지 시스템을 위한 표적 후보 검출 기법

최근 금속을 포함하여 다양한 재질의 지뢰를 탐지할 수 있는 지면 투과 레이더 (Ground Penetrating Radar, GPR)가 지뢰 탐지 분야에서 주목 받고 있으며 지면 투과 레이더를 이용한 지뢰 탐지 기술이 활발하게 연구되고 있다. 본 논문에서는 1차원 지면 투과 레이더 배열을 이용하여 지뢰가 존재할 가능성이 높은 후보 지점을 검출하는 방법을 제안하였다. 제안 기법은 먼저 지면 투과 레이더에 수신되는 신호 세기의 비선형적 감쇠 특성을 반영하여, 지연 시간에 따른 지면 투과 레이더 응답 신호의 세기를 효과적으로 나타낼 수 있는 회귀 모델을 추정하였다. 이를 토대로 지면 투과 레이더 응답 신호를 분석해보면, 토양 신호들은 주로 회귀 모델의 근처에 밀집되어 있고, 지뢰일 가능성이 있는 후보 신호들은 회귀 모델로부터 멀리 떨어져서 분포하는 특성을 보인다. 이러한 사실을 기반으로, 지면 투과 레이더의 측정값과 회귀 모델의 예측 값에 대한 예측 오차를 계산하여 토양 신호로부터 지뢰일 가능성이 있는 후보 신호들을 분리하였다. 후보 신호에는 잡음과 표적 신호가 섞여 있으므로 표적 신호가 갖는 고유한 기하학적 특성을 이용하여 최종적으로 표적 신호를 검출하였다. 성능 검증을 위해 실내 환경에서 획득한 지면 투과 레이더 데이터를 이용하여 실험을 수행한 결과, 제안하는 전처리 기법의 지뢰 탐지율 대비 오경보율이 낮은 것을 확인할 수 있었다.

Thermal conductivity of titanium aluminum silicon nitride coatings deposited by lateral rotating cathode arc

A series of physical vapour deposition titanium aluminum silicon nitride nanocomposite coating with a different (Al+Si)/Ti atomic ratio, with a thickness of around 2.5μm were deposited on stainless steel substrate by a lateral rotating cathode arc process in a flowing nitrogen atmosphere. The composition and microstructure of the as-deposited coatings were analyzed by energy dispersive X-ray spectroscopy, and X-ray diffraction, and cross-sectional scanning electron microscopy observation. The titanium nitride (TiN) coating shows a clear columnar structure with a predominant (111) preferential orientation. With the incorporation of Al and Si, the crystallite size in the coatings decreased gradually, and the columnar structure and (111) preferred orientation disappeared. Thermal conductivity of the as-deposited coating samples at room temperature was measured by using pulsed photothermal reflectance technique. Thermal conductivity of the pure TiN coating is about 11.9W/mK. With increasing the (Al+Si)/Ti atomic ratio, the coatings' thermal conductivity decreased monotonously. This reduction of thermal conductivity could be ascribed to the variation of coatings' microstructure, including the decrease of grain size and the resultant increase of grain boundaries, the disruption of columnar structure, and the reduced preferential orientation.

Evaluation of geo-mechanical properties of very weak and weak rock materials by using non-destructive techniques: Ultrasonic pulse velocity measurements and reflectance spectroscopy

The main purpose of this study is to evaluate the geo-mechanical properties of very weak and weak rock materials by using ultrasonic pulse velocity measurements and considering specifically detailed mineralogical compositions. For the purpose, P-wave velocity (Vp) measurements of 66 core samples of the sedimentary rocks including claystones and mudstones collected from Firuzkoy area of Istanbul (Turkey) were carried out. Unconfined Compressive Strength (UCS) tests were then conducted. The axial deformations recorded during the UCS tests were also evaluated and the elastic moduli of the rock materials (Ei) were calculated. Statistical relations were investigated between the values of P-wave velocity measurements and the UCS and Ei. In order to evaluate the effects of detailed mineralogical compositions on the prediction performances of the empirical equations Vp-UCS and Vp-Ei, the reflectance spectroscopy was introduced. 1035 spectral measurements were taken from smooth and fresh surfaces of the failed core samples. Different genetic rock types were defined according to crystal field effect and charge transfer absorptions of transition elements, and water and OH vibrational spectral diagnostics. The statistical relations were reinvestigated, and the individual empirical equations were reproduced for each genetic rock type. The mean values of the multiple coefficients of correlations (R) were obtained to be 0.904 and 0.916 for the equations of Vp-UCS and Vp-Ei. Considering the maximum values of R, the increment rates for the values of the explained variances reach up to 14.3% and 13.5% for the equations, respectively. Additionally, according to the results of the analysis of variance (ANOVA) evaluations, the empirical equations of Vp-UCS and Vp-Ei reproduced for each genetic rock type were found to be statistically significant at the significance level of 0.05. As a consequence, the non-destructive techniques Vp measurements and reflectance spectroscopy could be efficiently used together for the evaluation of the geo-mechanical properties of very weak and weak rock materials in particular.

Increased aortic wave reflection and smaller pulse pressure amplification in smokers and passive smokers confirmed by urinary cotinine levels: The Nagahama Study

Central blood pressure (cSBP) is suggested to be a better predictor of cardiovascular risk than brachial BP. Although brachial BP levels among smokers have been reported to be the same or somewhat lower than those in nonsmokers, it is suggested that smoking might have a substantial impact on cSBP. We conducted a cross-sectional study to clarify the association of smoking habit with arterial tone and cSBP in a general population of 8557 participants using urinary cotinine levels as an objective marker of smoking intensity. Absolute pressure of the late systolic peak (SBP2) was obtained by calibrating the radial waveform with brachial systolic BP (bSBP) and considered to be the cSBP. Confounding factor-adjusted mean pulse pressure amplification (PPa = bSBP-cSBP) was significantly smaller in habitual smokers (current, 9.3 ± 0.15; past, 10.2 ± 0.13; never, 10.6 ± 0.10 mmHg; p<0.001). Further, among smokers, PPa was linearly decreased with increasing urinary cotinine quartile (Q1, 10.9 ± 0.38; Q2, 10.9 ± 0.39; Q3, 10.4 ± 0.39; Q4, 9.7 ± 0.41 mmHg; p = 0.020). Multiple linear regression analysis identified both smoking habit (p = 0.003) and urinary cotinine levels (p = 0.008) as independent determinants of PPa. Urinary cotinine was also detected in a small fraction of never smokers (1.8%). These passive smokers showed a smaller PPa (passive smoker, 9.4 ± 0.4; never smoker, 10.4 ± 0.12 mmHg, p = 0.020) but not bSBP (122.7 ± 0.6, 123.1 ± 0.2 mmHg, p = 0.474). Not only habitual smoking but also passive smoking had harmful effects on AIx and central BP. Our results strongly emphasize the importance of avoiding passive smoking to the prevention of cardiovascular risks of which the subject is likely unaware.

Generation of rectangular optical waves by relativistic clipping

Theoretical results are reported for the reflection and transmission of few-cycle laser pulses on a very thin conducting layer, which may represent the surface current density of the massless relativistic charges of graphene. It is shown that the pulse may undergo violent distortions to the extent that the scattered radiation contains rectangular trains, which are approximate physical realizations of Rademacher functions in the optical or terahertz regime.

Transmission of pulsatility in the microcirculation: Where and how damping occurs?

BackgroundPulsatility of blood flow plays a critical role in vascular regulation. It is prominent in arterioles but attenuated in venules, due to damping in microvascular beds. However, the damping mechanisms of flow pulsatility in microcirculation remain unclear.MethodsTo explore the damping process, we developed a mathematical model describing pulse propagation and reflection through vascular network and applied it to artificial test networks with defined properties and a mesenteric experimental network (546 segments). Pulse damping was quantified by percent change in pulsatility index (PI, pressure amplitude/mean pressure).ResultsIn passage through experimental and test networks, PI is reduced by about 30%. Possible damping mechanisms were investigated by changing modeling parameters and properties of test networks. PI damping was not affected by eliminating viscous properties of vascular wall. Upon eliminating wave reflection related to output boundaries, bifurcations, and to vascular resistance, PI damping was reduced to 27, 18, and 2%, respectively.ConclusionDamping related to vascular resistance dominates in the microcirculation and is mostly related to wave reflection. (supported by National Nature Science Foundation of China, # 81271662).

Characterization of periodic multilayer mirrors optimized for single attosecond pulse reflection

Periodic Mo/Si multilayer mirrors with high peak reflectivity and broad reflection band for reflecting single attosecond pulse are designed, by optimizing the thickness ratio between the absorber layer and the bilayer. The reflective characteristics of periodic Mo/Si multilayer mirrors with different thickness ratio are analyzed and compared both in frequency domain and in time domain, for picking out optimized thickness ratio to achieve high-efficiency attosecond pulse reflection. According to our simulation, periodic quarter-wavelength Mo/Si multilayer mirrors with different layers all show better pulse reflection capability for incident Gaussian pulses than multilayers with other thickness ratio. When studying the spectral and the temporal responses of periodic multilayer mirrors, the transfer-matrix method and fast Fourier transform have been used in our simulation.

Recognition Study of TCM pulse based on different Classification Methods

Pulse diagnosis is one of important diagnosis methods in Traditional Chinese Medicine (TCM). Recognition of TCM pulse has received more and more attention in recent years. Extracting proper features is crucial for satisfactory classification. While most of previous methods for feature extraction of TCM pulse have no specific correlation with the mechanism of TCM pulse, a hemodynamics method is used to calculate the pulse waveform velocity (PWV) and pulse reflection factor(Rf), which reflects the principle of TCM pulse diagnosis. Then the Fisher linear discriminant (FLD) and the K-Nearest Neighbor (KNN) algorithm are employed to classify the data and double cross-validation method is used for accuracy assessment. The average recognition rate was 80.10% by using FLD classifier and it was 87.13% by using KNN classifier. It is concluded that the PWV and Rf may be used as the features for the classification of TCM pulses.

Thermal melting and ablation of silicon by femtosecond laser radiation

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by timeresolved optical reflection microscopy. This dynamics is revealed by monitoring picosecond and (sub)nanosecond oscillations of probe pulse reflection, which is modulated by picosecond acoustic reverberations in the dynamically growing surface melt subjected to ablation and having another acoustic impedance, and by optical interference between the probe pulse replicas reflected by the spalled layer surface and the layer retained on the target surface. The acoustic reverberation periods change during the growth and ablation of the surface melt film, which makes it possible to quantitatively estimate the contributions of these processes to the thermal dynamics of the material surface. The results on the thermal dynamics of laser excitation are supported by dynamic measurements of the ablation parameters using noncontact ultrasonic diagnostics, scanning electron microscopy, atomic force microscopy, and optical interference microscopy of the modified regions appearing on the silicon surface after ablation.

Resonant diffraction gratings for differentiation of optical signals in reflection and transmission

The propagation of an optical pulse through a diffraction grating with a resonance at the pulse’s central frequency is considered. It is demonstrated that afore-mentioned gratings can perform time-domain differentiation of optical pulses in reflection and transmission. The correlation between the amplitude of the transformed pulse and the differentiation quality is investigated. A resonant grating intended for differentiation of picosecond pulses in reflection and transmission is computed.

Improving attosecond pulse reflection by large angle incidence for a periodic multilayer mirror in the extreme ultraviolet region

The improvement of attosecond pulse reflection by large angle incidence for a periodic multilayer mirror in the extreme ultraviolet region has been discussed. Numerical simulations of both spectral and temporal reflection characteristics of periodic multilayer mirrors under various incident angles have been analyzed and compared. It was found that the periodic multilayer mirror under a larger incidence angle can provide not only higher integrated reflectivity but also a broader reflection band with negligible dispersion, making it possible to obtain better a reflected pulse that has a higher pulse reflection efficiency and shorter pulse duration for attosecond pulse reflection. In addition, by increasing the incident angle, the promotion of attosecond pulse reflection capability has been proven for periodic multilayer mirrors with arbitrary layers.

Nonlinear effects manifested in infrasonic signals in the region of a geometric shadow

Nonlinear effects manifested in infrasonic signals passing through different atmospheric heights and recorded in the region of a geometric shadow have been studied. The source of infrasound was a surface explosion equivalent to 20–70 t of TNT. The frequencies of the spectral maxima of infrasonic signals, which correspond to the reflections of acoustic pulses from atmospheric inhomogeneities at different heights within the stratosphere-mesosphere-lower thermosphere layer, were calculated using the nonlinear-theory method. A satisfactory agreement between experimental and calculated data was obtained.

Spring sea-ice evolution in Rijpfjorden (80° N), Svalbard, from in situ measurements and ice mass-balance buoy (IMB) data

AbstractRijpfjorden (808 N, 22° E) is a high-Arctic fjord on Nordaustlandet in the Svalbard archipelago. To monitor the thermodynamic change of sea ice in spring, an ice mass-balance buoy (IMB) was deployed for 2.5 months (10 April–26 June 2011), with accompanying in situ measurements, sea-ice sampling on three occasions and ice-core analysis. Uncertainties and sources of error in in situ measurements and IMB data are discussed. The in situ measurements, ice-core analysis and IMB data together depict the development of snow and ice in spring. Snow and ice thickness exhibited large spatial and temporal variability. After relatively stable conditions with only little change in ice thickness and accumulation of snow, a layer of superimposed ice ∼0.06 m thick formed at the snow-ice interface due to refreezing of snow meltwater in late spring. Ice thickness (except for growth of superimposed ice) did not change significantly based on in situ observations. In contrast, the under-ice sonar data from the IMB show reflections from a layer deeper than the underside of the ice during the melting phase. This can be explained as a reflection of the sonar pulses from an interface between a freshwater layer under the ice and more saline water below, or as a false-bottom formation.