Pulse Shape Variations Research Articles

Dispersion scan frequency resolved optical gating for consistency check of pulse retrieval.

Many methods commonly used to characterize ultrafast laser pulses, such as the frequency-resolved optical gating (FROG) or the dispersion scan (d-scan), face problems when they are used on pulses with a spectrum or phase varying within the laser beam cross section or the acquisition time. The presence of such pulse shape variation leads to discrepancy between the measured FROG trace and its reconstructed counterparts. Nevertheless, it is difficult to reliably discern this shape variation because even the distorted experimental FROG trace can be reasonably reproduced by a realistic pulse shape. In this work, we examine and discern the variation of the pulse shape based on a new method, dispersion-scan FROG (D-FROG), which combines the idea of dispersion scanning with the FROG method. This technique provides a means of careful evaluation of the laser pulse based on a set of FROG traces connected by known dispersion changes. Therefore, this method can disclose seemingly correct pulse retrievals from distorted datasets. The D-FROG method can be used as a simple extension of the FROG technique to provide a consistency check able to identify the shortcomings in the pulse characterization.

Suppression of neutron background using deep neural network and Fourier frequency analysis at the KOTO experiment

We present two analysis techniques for distinguishing background events induced by neutrons from photon signal events in the search for the rare KL0→π0νν̄ decay at the J-PARC KOTO experiment. These techniques employed a deep convolutional neural network and Fourier frequency analysis to discriminate neutrons from photons, based on their variations in cluster shape and pulse shape, in the electromagnetic calorimeter made of undoped CsI. The results effectively suppressed the neutron background by a factor of 5.6×105, while maintaining the efficiency of KL0→π0νν̄ at 70%.

Modeling peripheral arterial and venous pressure signals with integral pulse frequency modulation

This paper proposes a novel mathematical modeling framework for peripheral arterial and peripheral venous blood pressure signals from porcine experiments. Peripheral blood pressure signals can be acquired using regular catheters during standard patient treatment. The minimally invasive nature and ubiquitous availability of catheters render it an ideal candidate for various applications. However, there is no analytical model for peripheral blood pressure signals in the literature. We address this issue by proposing a model for these signals under the integral pulse frequency modulation (IPFM) framework. The model incorporates the impacts of physiological phenomena, such as the heartbeat pulse shape variation, heart rate variability, respiratory rate, etc. The model parameters are obtained by applying the IPFM model to experimental data collected from four pigs under different anesthetic dosages. The proposed model can fit the experimental data with Pearson correlation coefficients greater than 0.99 and 0.90 for arterial and venous blood pressure signals, respectively. The performance of model-synthesized data on the classification of two different anesthesia is comparable with experimental data. Parameters like pulse shape and duration can also work as distinguishable features under different anesthesia. We also proposed a way to distinguish respiratory-induced heart rate variability from other causes. Increasing doses of vasodilating anesthesia is similar to going from dehydration to hydration. Thus the results obtained in this study can be extended in distinguishing hydrated and dehydrated subjects. This model can be extended to similar biomedical signals like photoplethysmography, cerebral blood flow velocity, and Doppler waveforms.

Pulse-to-pulse field characterization at x-ray free-electron lasers using a speckle-correlation scattering matrix

In coherent diffraction experiments, knowledge of the incident field is important to extract pure sample-induced data from measured diffraction patterns. However, this becomes challenging in x-ray free-electron lasers (XFELs), as incident fields vary from pulse to pulse due to the stochastic nature of self-amplified stimulated emission. Despite various proposed field retrieval methods, single-shot characterization of each pulse remains elusive. To address this, we propose the speckle-correlation scattering matrix as a solution. Our method directly reconstructs the complex field without sample constraints or multiple acquisitions by introducing a designed diffuser before the detector. We demonstrate the robustness of our approach through successful field reconstructions in various experimental configurations. Based on the retrieved field results, pulse-to-pulse variations in pulse intensity, position, illumination angle, and shape were analyzed. We believe our method can readily serve as an on-field and real-time pulse diagnostic tool at XFELs and improve the overall quality of all experiments performed at XFELs.

Time-walk and jitter correction in SNSPDs at high count rates

Superconducting nanowire single-photon detectors (SNSPDs) are a leading detector type for time correlated single photon counting, especially in the near-infrared. When operated at high count rates, SNSPDs exhibit increased timing jitter caused by internal device properties and features of the RF amplification chain. Variations in RF pulse height and shape lead to variations in the latency of timing measurements. To compensate for this, we demonstrate a calibration method that correlates delays in detection events with the time elapsed between pulses. The increase in jitter at high rates can be largely canceled in software by applying corrections derived from the calibration process. We demonstrate our method with a single-pixel tungsten silicide SNSPD and show it decreases high count rate jitter. The technique is especially effective at removing a long tail that appears in the instrument response function at high count rates. At a count rate of 11.4 MCounts/s, we reduce the full width at 1% maximum level (FW1%M) by 45%. The method, therefore, enables certain quantum communication protocols that are rate-limited by the FW1%M metric to operate almost twice as fast.

Ultrafast spin transport and control of spin current pulse shape in metallic multilayers

We study the emission of femtosecond spin current pulses at Fe/Au interfaces in well-defined epitaxial Fe/Au(001) and Fe/Au/Fe(001) structures. Using the exceptional sensitivity of optical second harmonic generation to spin currents and transient spin density at interfaces, we perform direct measurements of the ultrafast spin transport. To link the optical data to microscopic material parameters, we use a simple model of electron emission from ultrathin Fe layers, which ignores the diffusive transport in Fe, and develop a description of superdiffusive spin transport in Au. This allows us to extract the electron velocity and scattering rates in Au, estimate the dynamics of secondary carrier population in Fe, and retrieve the spin current pulse shape from the experimental data. We study the variation of spin current pulse shape with increasing thickness of the Fe emitter layer. The observed dependency is well explained by including the diffusive spin transport in Fe. This example shows the potential of our approach to develop spin current emitters delivering desired pulse shapes.

Single-pulse Variability in Gamma-Ray Pulsars

The Fermi Large Area Telescope receives ≪1 photon per rotation from any γ-ray pulsar. However, out of the billions of monitored rotations of the bright pulsars Vela (PSR J0835−4510) and Geminga (PSR J0633+1746), a few thousand have ≥2 pulsed photons. These rare pairs encode information about the variability of pulse amplitude and shape. We have cataloged such pairs and find the observed number to be in good agreement with simple Poisson statistics, limiting any amplitude variations to <19% (Vela) and <22% (Geminga) at 2σ confidence. Using an array of basis functions to model pulse-shape variability, the observed pulse phase distribution of the pairs limits the scale of pulse-shape variations of Vela to <13%, while for Geminga we find a hint of ∼20% single-pulse-shape variability most associated with the pulse peaks. If variations last longer than a single rotation, more pairs can be collected, and we have calculated upper limits on amplitude and shape variations for assumed coherence times up to 100 rotations, finding limits of ∼1% (amplitude) and ∼3% (shape) for both pulsars. Because a large volume of the pulsar magnetosphere contributes to γ-ray pulse production, we conclude that the magnetospheres of these two energetic pulsars are stable over one rotation and very stable on longer timescales. All other γ-ray pulsars are too faint for similar analyses. These results provide useful constraints on rapidly improving simulations of pulsar magnetospheres, which have revealed a variety of large-scale instabilities in the thin equatorial current sheets where the bulk of GeV γ-ray emission is thought to originate.

Single Pulse Variability in Gamma-ray Pulsars

The Fermi Large Area Telescope receives $\ll$1 photon per rotation from any $\gamma$-ray pulsar. However, out of the billions of monitored rotations of the bright pulsars Vela (PSR~J0835$-$4510) and Geminga (PSR~J0633$+$1746), a few thousand have $\geq$2 pulsed photons. These rare pairs encode information about the variability of pulse amplitude and shape. We have cataloged such pairs and find the observed number to be in good agreement with simple Poisson statistics, limiting any amplitude variations to $<$19% (Vela) and $<$22% (Geminga) at 2$\sigma$ confidence. Using an array of basis functions to model pulse shape variability, the observed pulse phase distribution of the pairs limits the scale of pulse shape variations of Vela to $<$13% while for Geminga we find a hint of $\sim$20% single-pulse shape variability most associated with the pulse peaks. If variations last longer than a single rotation, more pairs can be collected, and we have calculated upper limits on amplitude and shape variations for assumed coherence times up to 100 rotations, finding limits of $\sim$1% (amplitude) and $\sim$3% (shape) for both pulsars. Because a large volume of the pulsar magnetosphere contributes to $\gamma$-ray pulse production, we conclude that the magnetospheres of these two energetic pulsars are stable over one rotation and very stable on longer time scales. All other $\gamma$-ray pulsars are too faint for similar analyses. These results provide useful constraints on rapidly-improving simulations of pulsar magnetospheres, which have revealed a variety of large-scale instabilities in the thin equatorial current sheets where the bulk of GeV $\gamma$-ray emission is thought to originate.

Effect of process parameters on pulsed laser welding of AA5083 alloy using response surface methodology and pulse shape variation

Aluminium alloys exhibit eco-friendly aspects related to global environmental issues, such as almost unlimited recyclability. Nevertheless, some intrinsic characteristics are challenges to explore all their benefits. In the welding process, the high thermal conductivity and low melt point require high control of heat input. Alternatively, the pulsed laser mode provides a sharp beam focus with precise control enabling regulating the energy delivered. In this sense, this present work analysed the effect of pulsed laser welding parameters on 3-mm-thick AA5083 aluminium alloy sheets. Trials targeted to develop sound welds with minimum defect and high penetration depth adopting statistical methods. The optimum parameter arrangement was achieved by varying peak power, spot diameter, and pulse duration. Finally, the best parameter combination was applied using different pulse shapes to mitigate crack formation and pores. As a result, the pulse shape with step-down at the end of each pulse generated crack-free spot welds.

Pulse Shaping Strategies for Electroceuticals: A Comprehensive Survey of the Use of Interphase Gaps in Miniature Stimulation Systems.

Interphase gaps (IPGs) are among the most commonly suggested pulse shape variations to try to enhance neural stimulation efficiency by reducing the action potential (AP) suppressing effect of an early anodic hyperpolarization. The majority of published literature on the effect of IPGs is based on investigations of monopolar stimulation configurations. However, many contemporary neuromodulation applications including the emerging field of electroceutical devices operate in a bipolar electrode configuration. We investigated the effect of IPGs and asymmetric biphasic current controlled pulses with reduced anodic amplitude on neural activation in both principal electrode configurations in a rodent in-vivo nerve muscle preparation. In the monopolar electrode configuration, our findings of 10.9 ± 1.5% decreased stimulation amplitude with 200 μs IPGs in biphasic pulses of 40 μs phase width are in agreement with published literature in this configuration. Surprisingly, using the bipolar configuration, opposite effects of IPGs were observed and neural activation required up to 18.6 ± 3.1% (phase width 100 μs, IPG = 1000 μs) higher amplitudes. Electroneurogram recordings of the stimulated nerve revealed temporal differences in AP generation between the monopolar and bipolar configuration. In the bipolar configuration excitation first occurred in response to the middle field transition of biphasic pulses. This is the first study to report consistently increased amplitude requirements with IPGs in bipolar stimulation configurations. Our findings must be taken into consideration when designing stimulation waveforms for neuromodulation devices that operate in a bipolar mode to avoid increased amplitude requirements that result in increased energy consumption.

Compensation of optical nonlinearities in a femtosecond laser system in a broad operation regime

We present here a detailed study of a compact and tunable nonlinear phase pre-compensation method based on a tunable chirped fiber Bragg grating (TCFBG) applied to an ultrafast fiber laser system. Frequency resolved optical gating (FROG) measurements reveal a significant variation in the output pulse shape due to the variation in accumulated nonlinear phase for varying pulse energy and repetition rate (even if the pulse energy is kept constant) of the laser system. The TCFBG, used as a stretcher and a pre-compensating element in this chirped fiber amplification (CPA) system, eliminates the need for additional pulse shaping components which greatly simplifies the system for tunable pre-compensation. We demonstrate an efficient pre-compensation of the accumulated nonlinear phase between 0 and 14 rad resulting in pulses with a constant pulse duration and Strehl ratio over a broad operating regime of the laser with repetition rates ranging from 100 kHz up to 30 MHz and pulse energies spanning from 1 µJ up to 70 µJ.

End-to-end trained encoder–decoder convolutional neural network for fetal electrocardiogram signal denoising

Objective: Non-invasive fetal electrocardiography has the potential to provide vital information for evaluating the health status of the fetus. However, the low signal-to-noise ratio of the fetal electrocardiogram (ECG) impedes the applicability of the method in clinical practice. Quality improvement of the fetal ECG is of great importance for providing accurate information to enable support in medical decision-making. In this paper we propose the use of artificial intelligence for the task of one-channel fetal ECG enhancement as a post-processing step after maternal ECG suppression. Approach: We propose a deep fully convolutional encoder–decoder framework, learning end-to-end mappings from noise-contaminated fetal ECGs to clean ones. Symmetric skip-layer connections are used between corresponding convolutional and transposed convolutional layers to help recover the signal details. Main results: Experiments on synthetic data show an average improvement of 7.5 dB in the signal-to-noise ratio (SNR) for input SNRs in the range of −15 to 15 dB. Application of the method with real signals and subsequent ECG interval analysis demonstrates a root mean square error of 9.9 and 14 ms for the PR and QT intervals, respectively, when compared with simultaneous scalp measurements. The proposed network can achieve substantial noise removal on both synthetic and real data. In cases of highly noise-contaminated signals some morphological features might be unreliably reconstructed. Significance: The presented method has the advantage of preserving individual variations in pulse shape and beat-to-beat intervals. Moreover, no prior knowledge on the power spectra of the noise or the pulse locations is required.

Development of TES Microcalorimeters with Solar-Axion Converter

Several issues in developing transition edge sensor (TES) microcalorimeters for the search of monochromatic solar axions expected at 14.4 keV are studied. The potential problem of developing TES’s of this purpose is in that an axion absorber of $$^{57}$$Fe must be placed in close vicinity of a TES. We estimated the minimum distance to avoid magnetic interference from iron using magnetic FEM simulations, and found it 30 $$\upmu $$m for an iron of 5 $$\upmu $$m thickness. We fabricated a TES with a 10 $$\upmu $$m thick iron membrane separated by 60 $$\upmu $$m. We confirmed the superconducting transition for this TES. However, both the residual normal resistance and the transition temperature was different from those of TES without iron. We also estimated the low-temperature thermal conductivity of an iron membrane by measuring the low-temperature electrical resistance and by applying the Wiedemann–Franz law. We estimated the pulse-shape dependency on the interaction position within the $$^{57}$$Fe converted using thermal FEM simulations. We found that the pulse-shape variations will limit the energy resolution to about 30 eV FWHM.

A comparison study of the pile-up correction algorithms

The pulse pile-up is a long-standing problem which deteriorates the energy, timing and spatial resolution of the radiation detection systems. Several algorithms have been developed for recovering the true pulse information using the available digital processing platforms. In this study, four well-established pile-up correction methods are numerically compared: single event reconstruction, template matching, maximum likelihood estimation and high-yield pile-up event recovery. Different pile-up waveforms are simulated by using the extracted parameters from the experimental data of a NaI(Tl) scintillation detector. The experimental pulses are taken from the charge sensitive preamplifier output, and therefore the methods are compared on the timescale of the RC constant of the preamplifier. Under such circumstance, the methods are compared with respect to their sensitivity to noise, variation of pulse shape, sampling rate and probable errors in time pick-off. In addition, their performance is assessed by calculating the experimental energy spectra of 137Cs at different high-count-rates. The results of the present study provide the benefits and limitations of each method, which can be used to select the best pile-up correction strategy for each specific application.

Rectify the effect of measurement in survival probability of the ground vibrational state in diatomic molecule

We have proposed a methodology that uses secondary electromagnetic field to overcome the effect of measurement on quantum dynamics. Our aim is also to find out some characteristics of the used secondary (recovery) field. We have applied the methodology to reduce the quantum Zeno effect (ZE), a consequence of repeated measurements in survival probability of ground vibrational state. As the model systems, we choose HBr[Formula: see text] and HI. The study is done with variation in frequency, pulse shape and other parameters of the secondary field. In all cases, suitable secondary field for which the Zeno effect is minimum is found near 0 [Formula: see text] 1 vibrational transition frequency. Suitable time gap between the measurement and the application of secondary field depends on the shape of the secondary field. When the secondary field is optimized, the recovery is more than 90% which almost nullify the Zeno effect.

Pulse pileup analysis for a double-sided silicon strip detector using variable pulse shapes.

Due to pulse pileup, photon counting detectors (PCDs) suffer from count loss and energy distortion when operating in high count rate environments. In this paper, we studied the pulse pileup of a double-sided silicon strip detector (DSSSD) to evaluate its potential application in a mammography system. We analyzed the pulse pileup using pulses of varied shapes, where the shape of the pulse depends on the location of photon interaction within the detector. To obtain the shaped pulses, first, transient currents for photons interacting at different locations were simulated using a Technology Computer-Aided Design (TCAD) software. Next, the currents were shaped by a CR-RC2 shaping circuit, calculated using Simulink. After obtaining these pulses, both the different orders of pileup and the energy spectrum were calculated by taking into account the following two factors: 1) spatial distribution of photon interactions within the detector, and 2) time interval distribution between successive photons under a given photon flux. We found that for a DSSSD with thickness of 300 μm, pitch of 25 μm and strip length of 1 cm, under a bias voltage of 50 V, the variable pulse shape model predicts the fraction free of pileup can be > 90 % under a photon flux of 3.75 Mcps/mm2. The double-sided silicon-strip detector is a promising candidate for digital mammography applications.

A comparison between single and double cable neuron models applicable to deep brain stimulation

Computational models for activation assessment in deep brain stimulation (DBS) are commonly based on neuronal cable equations. The aim was to systematically compare the activation distance between a single cable model implemented in MATLAB, and a well-established double cable model implemented in NEURON. Both models have previously been used for DBS studies. The field distributions generated from a point source and a 3389 DBS lead were applied to the neuron models as input stimuli. Simulations (n = 670) were performed with intersecting axon diameters (D) between the models (2.0, 3.0, 5.7, 7.3, 8.7, 10.0 μm), variation in pulse shape and amplitude settings (0 to 5 in increments of 0.5 mA or V) with the single cable model as reference. Both models responded linearly to change of input (point source: 0.93 < R2 < 0.99, DBS source: R2 > 0.98), but with a systematic extended activation distance for the single cable model. The difference for a point source ranged from −0.2 mm (D = 2.0 μm) to −1.1 mm (D = 5.7 μm). For the DBS lead a D = 3.2 μm agreed with the commonly used double cable simulations D =5.7 μm in voltage mode. Possible reasons for the deviation at larger axons are the internodal length, the ion channel selection and physiological data behind the models. The single cable model covers a continuous range of small axon diameters and calculated the internodal length for each iteration, whereas the double cable models uses fixed defined axon diameters and tabulated data for the internodal length. Despite different implementations and model complexities, both models present similar sensitivity to pulse shape, amplitude and axon diameter. With awareness of the strength and weakness both models can be used to extract activation distance used to relate a specific electric field isolevel and thus estimate the volume of tissue activated in DBS simulation studies.

Intra-cavity measurement concept of dispersion properties with a tunable fiber-integrated laser

The dispersion properties of fibers depict a key characteristic to model the propagation of ultra-short pulses in waveguides. In the following, a new method is presented to directly measure the dispersion properties of fibers and optical components in the time domain. The analysis is based on pulse shape variations along the tuning range of a theta cavity fiber laser (TCFL) depending on the adjusted repetition rate. The automated measurement procedure, evaluating pulse symmetry, achieves a temporal sensitivity below 5 ps surpassing the resolution of the acquisition electronics. Exemplarily, two samples of Nufern PM980-XP fiber are investigated with an Yb-doped tunable TCFL retrieving the mean dispersion parameter by comparative measurements. The obtained results are compared to a reference method based on spectral interferometry. With deviations in between either approach of and , respectively, the results agree well within the measurement errors of the TCFL, verifying the presented concept. Due to the pulse formation process extending over multiple round trips, this approach achieves an enhanced sensitivity compared to competing direct temporal methods. Together with an alignment free operation, the fiber-integrated TCFL depicts a simple and robust concept showing potential in specific measurement scenarios such as in quality management.

Extremely low frequency electromagnetic field reduces oxidative stress during the rehabilitation of post-acute stroke patients.

One of the therapeutic methods used in stroke rehabilitation is magnetotherapy using extremely low frequency and variable pulse shape electromagnetic field (ELF-EMF). The aim of our study was to investigate the effect of magnetotherapy on the condition of postacute stroke patients, as measured by plasma oxidative stress markers and clinical parameters which show the progress of rehabilitation. The selected 57 post-stroke patients were divided into 2 groups, those with ELFEMF therapy and those without. The level of oxidative stress in the plasma was estimated by typical markers: thiobarbituric acid reactive substances (TBARS), thiol groups, and carbonyl groups. The effect of ELF-EMF on the course of the patients' rehabilitation following ischemic stroke was evaluated with the use of scales of physical activity and mental state: Activities of Daily Living (ADL), Mini-Mental State Examination (MMSE) and Geriatric Depression Scale (GDS). Our comparative analysis showed that all parameters of oxidative stress are significantly reduced during rehabilitation using ELF-EMF, compared to the control group rehabilitated only by kinesiotherapy. We also recorded much higher therapeutic benefits using magnetotherapy, which revealed a significant improvement of clinimetric parameters. The ELF-EMF therapy meaningfully improves the overall condition of patients through a decrease of oxidative stress markers and it significantly affects the psychophysical abilities of patients after stroke. The change in carbonyl group level correlates with the change in the degree of physical and mental disability; therefore, it could be a marker for the effectiveness of rehabilitation.

Acoustic signal investigation in the ice melting under high-power laser irradiation at a wavelength of 2940 nm

We have measured acoustic signals emerging in the melting of a thin (about 2 μm) ice layer, which is sandwiched between two substrates, under high-power laser irradiation at a wavelength of 2940 nm. A negative-pressure acoustic signal arising from the difference in the specific volumes of water and ice was found to occur in the ice melting. The amplitude of this signal was comparable to the amplitude of the positive-pressure signal produced under laser irradiation of the same layer of water. The shapes of acoustic signals were measured for different energy densities of the incident radiation. The variation of negative-pressure pulse shape with increasing radiation energy density was interpreted as a continuity break of the medium. The temperature and pressure in the layer were estimated in the heating of water and the melting of ice.