Variety Of Pulse Shapes Research Articles

Comparison of parameters of the electric strength of air in a surface antenna during the radiation of single microwave pulses of various shapes

When substantiating the requirements for the radiation parameters of high-power microwave generators it becomes necessary to assess their maximum permissible values, conditioned to air breakdown in the radiating surface antenna. To predict the parameters of the dielectric strength of air, a theory based on a model of microwave pulses with a rectangular envelope is widely used. However, in practice, there are cases when the shape of the pulse envelope of the generator differs significantly from the rectangular. The aim of this work is to evaluate and compare the amplitude and energy parameters of the electric strength of air in the surface antenna of a high-power relativistic microwave generator when pulses are emitted with envelopes of various shapes. Pulses with rectangular, trapezoidal, parabolic and Gaussian envelopes were selected as the initial ones. The breakdown parameters were found based on the criterion of pulsed breakdown of gases and the regularity connecting the electron density with the pulse amplitude. The problem is solved numerically. In the range of pulse durations typical for high-power relativistic microwave generators, the dependences of the breakdown field and the maximum allowable energy on the pulse duration under normal atmospheric conditions are determined. The regularities connecting the maximum permissible energy and the breakdown field for the considered pulses are determined. With the same breakdown fields, a pulse with a Gaussian envelope has the highest maximum permissible energy, and a pulse with a rectangular envelope has the lowest.

Dynamic Boundary Layer Simulation of Pulsed CO2 Electrolysis on a Copper Catalyst

Author(s): Bui, JC; Kim, C; Weber, AZ; Bell, AT | Abstract: Pulsed electrolysis has been demonstrated to improve the faradaic efficiency (FE) to C2+ products during the electrochemical reduction of CO2 over a Cu catalyst, but the nature of this enhancement is poorly understood. Herein, we developed a time-dependent continuum model of pulsed CO2 electrolysis on Cu in 0.1 M CsHCO3 that faithfully represents the experimentally observed effects of pulsed electrolysis. This work shows that pulsing results in dynamic changes in the pH and CO2 concentration near the Cu surface, which lead to an enhanced C2+ FE as a consequence of repeatedly accessing a transient state of heightened pH and CO2 concentration at high cathodic overpotential. Using these insights, a variety of pulse shapes were explored to establish operating conditions that maximize the rate of C2+ product formation and minimize the rates of H2 and C1 product formation.

О распространении видеоимпульсных сигналов в диссипативных средах

The results of the numerical solution of the problem of propagation of an electromagnetic video pulse signal of nanosecond duration in a dissipative medium are presented for use in the deep georadar data interpretation. Based on the developed scheme for integrating Maxwell's equations, the results of numerical modeling of the propagation of electromagnetic pulses of various shapes without approximations, usually used in the traditional solution of GPR problems, including the separation of temporal and spatial variables of the electromagnetic field and neglect of the rapid field change in comparison with the processes occurring in the medium, are obtained. The influence of the pulse shape on the shape of the electromagnetic echo signal is considered for various models of the medium close to real conditions, including those with a gradient change in electrical conductivity and permittivity. The influence of the distribution of electrical conductivity of the medium is studied.

Energy density and spectrum of single-cycle and sub-cycle electromagnetic pulses

Based on the exact solution of the Maxwell equations in the form of a collapsing spherical vector wave specified by an arbitrary function of time, we have calculated the maximum energy density that can be achieved when focusing extremely short pulses of various shapes. It is shown that our earlier formula expressing the maximum energy density in terms of the spectrum parameters for Gaussian quasi-monochromatic pulses is approximately (with an accuracy of ±40 %) valid for the main types of extremely short pulses.

Search for anomalies in pulse flows of acoustic and electromagnetic emissions

Timely warning of disasters caused by earthquakes ensures life safety. Therefore, the search for markers of pre-seismic events preceding earthquakes remains an important research task. The article presents experimental methods for assessing seismic activity in the Kamchatka region based on the results of processing and analysis of geoacoustic and electromagnetic emission signals. The research is aimed at detecting anomalies in quantitative and qualitative indicators that characterize the pulse streams of acoustic emission of near-surface rocks and electromagnetic emission in the surface layer of the atmosphere. Signal processing and analysis are carried out using special algorithms that take into account the structural features of the variety of pulse shapes and their distribution over time.

Systematic study of the influence of coherent phonon wave packets on the lasing properties of a quantum dot ensemble

Coherent phonons can greatly vary light–matter interaction in semiconductor nanostructures placed inside an optical resonator on a picosecond time scale. For an ensemble of quantum dots (QDs) as active laser medium, phonons are able to induce a large enhancement or attenuation of the emission intensity, as has been recently demonstrated. The physics of this coupled phonon–exciton–light system consists of various effects, which in the experiment typically cannot be clearly separated, in particular, due to the complicated sample structure a rather complex strain pulse impinges on the QD ensemble. Here we present a comprehensive theoretical study how the laser emission is affected by phonon pulses of various shapes as well as by ensembles with different spectral distributions of the QDs. This gives insight into the fundamental interaction dynamics of the coupled phonon–exciton–light system, while it allows us to clearly discriminate between two prominent effects: the adiabatic shifting of the ensemble and the shaking effect. This paves the way to a tailored laser emission controlled by phonons.

Soliton Content of Fiber-Optic Light Pulses

This is a review of fiber-optic soliton propagation and of methods to determine the soliton content in a pulse, group of pulses or a similar structure. Of central importance is the nonlinear Schrödinger equation, an integrable equation that possesses soliton solutions, among others. Several extensions and generalizations of this equation are customary to better approximate real-world systems, but this comes at the expense of losing integrability. Depending on the experimental situation under discussion, a variety of pulse shapes or pulse groups can arise. In each case, the structure will contain one or several solitons plus small amplitude radiation. Direct scattering transform, also known as nonlinear Fourier transform, serves to quantify the soliton content in a given pulse structure, but it relies on integrability. Soliton radiation beat analysis does not suffer from this restriction, but has other limitations. The relative advantages and disadvantages of the methods are compared.

Selection of the heating method of semiconductor devices during their testing in a high-conductivity state

Possible heating methods of power semiconductor devices during their testing in a high-conductivity state are discussed. It is shown that the diffusion capacitance of the p–n junction has a significant effect on measurements of device parameters. The effect of the diffusion capacitance on the results of testing of power semiconductor devices at various shapes of heating-current pulses was investigated. Conclusions on the possibility of using current pulses of various shapes for testing power semiconductor devices in the highconductivity state are drawn.

Energy Model of Neuron Activation.

On the basis of the neurophysiological strength-duration (amplitude-duration) curve of neuron activation (which relates the threshold amplitude of a rectangular current pulse of neuron activation to the pulse duration), as well as with the use of activation energy constraint (the threshold curve corresponds to the energy threshold of neuron activation by a rectangular current pulse), an energy model of neuron activation by a single current pulse has been constructed. The constructed model of activation, which determines its spectral properties, is a bandpass filter. Under the condition of minimum-phase feature of the neuron activation model, on the basis of Hilbert transform, the possibilities of phase-frequency response calculation from its amplitude-frequency response have been considered. Approximation to the amplitude-frequency response by the response of the Butterworth filter of the first order, as well as obtaining the pulse response corresponding to this approximation, give us the possibility of analyzing the efficiency of activating current pulses of various shapes, including analysis in accordance with the energy constraint.

Combined weak-current discharge in a copper-vapor laser

We have considered the application of a new method of pumping of active media on metal vapors by a combined weak-current discharge. A distinguishing feature of a weak-current discharge compared to the method for the traditional pumping of self-contained lasers is the regime of lower energy input to the discharge. Using this regime, it is possible to realize a pulsed-periodic form of the discharge with laser pulses of various shapes and durations at low current amplitudes (several amperes). Additional pulsed-periodic discharge is used to heat the active zone.

Emulation of spike-timing dependent plasticity in nano-scale phase change memory

The spike-timing dependent plasticity (STDP) of biological synapses, which is known to be a function of the formulated Hebbian learning rule of human cognition, learning and memory abilities, was emulated with two-phase change memory (2-PCM) cells built with 39nm technology. For this, we designed a novel time-modulated voltage (TMV) scheme for changing the conductance of 2-PCM cells, that could produce both long-term potentiation (LTP) and long-term depression (LTD) by applying variable (decreasing/increasing) pulse voltages according to the sign and magnitude in time interval between pre- and post-spikes. Since such schemes can be easily modified to have a variety of pulse shapes and time intervals between pulses, it is expected to be a proper scheme for designing diverse synaptic connection abilities. In addition, the small form factor and low energy consumption of 2-PCM make them comparable to biological synapses, which makes phase change memory a promising candidate for electronic synapses in large-scale neuromorphic system applications.

Photodecay of a Negative Ion Exposed to Ultrashort Electromagnetic Pulses of Different Shapes

Dependences of photodecay of a negative ion exposed to ultrashort electromagnetic pulses on pulse duration and its carrier frequency (for a Gaussian envelope) are theoretically investigated, and the energy spectrum of emitted photoelectrons is analyzed. Calculation is performed within the framework of the perturbation theory for ultrashort pulses of various shapes.

A versatile high voltage nano- and sub-nanosecond pulse generator

High-voltage (HV) ultrashort pulse technologies have found many applications in areas such as medicine, biology, food processing, environmental science and defense. Some applications are dependent on the pulse parameters such as duration, amplitude, shape, as well as the number of pulses applied and the frequency rate. Generators that can produce powerful electrical pulses with adjustable duration, amplitude and shape are convenient but still unusual. In this paper, we present and characterize a robust and versatile generator built on the frozen wave generator concept using photoconductive semiconductor switches. The results show that the generator can produce pulses of various shapes, peak-to-peak amplitudes up to 13.1 kV, maximum amplitudes of 6.9 kV and with durations in the nanosecond and subnanosecond range. Intense subnanosecond pulses have recently gained attention due to their potential ability to reach cells interior.

Control over the performance characteristics of a passively mode-locked erbium-doped fibre ring laser

We report an all-fibre ultrashort pulse erbium-doped ring laser passively mode-locked by single-wall carbon nanotubes dispersed in carboxymethylcellulose-based polymer films. Owing to intracavity dispersion management and controlled absorption in the polymer films, the laser is capable of generating both femto- and picosecond pulses of various shapes in the spectral range . We have demonstrated and investigated the generation of almost transform-limited, inversely modified solitons at a high normal cavity dispersion.

Conditions for the translational vibratory motion of small objects under the action of pulses of various shapes

It is theoretically shown that the translational wave-induced movement of a small (compared to the wavelength) object is possible even when the time-average force on it is zero. In this case, the object moves, oscillating in the forward and backward directions. The predominant motion of the object in a given direction is found to require the optimum choice of a carrier-wave phase with respect to the leading edge of the wave pulses. A substantial feature of this wave transport mechanism is the possibility of inversion of the object motion direction by merely changing the phase shift and retaining the previous direction of wave propagation. The transport of an object under the action of pulses with various envelope shapes is studied. The undesirable backward motion of the object with respect to the main forward direction is found to decrease for an exponential envelope; when this envelope is optimized, the backward motion is completely eliminated.

Control of the temporal profile of the local electromagnetic field near metallic nanostructures

We study control of the temporal profile of the local electric field in the vicinity of a small doped semiconductor or metal nanostructure. Unlike in the case of control in a gas or liquid phase, the collective response of electrons in the nanostructure may significantly enhance different frequency components of the external field. This enhancement strongly depends on the geometry of the nanostructure and can substantially modify the temporal profile of the local field. The changes in the amplitude and phase of the local field are studied using linear response theory within the random phase approximation. The inverse problem of finding the external electromagnetic field to generate an arbitrary target temporal profile of the local field, including the time-dependent polarization of the field, is considered and solved. We systematically study the pulse enhancement and shape distortion effects for a set of control pulses of various shapes.

Theory for direct frequency-comb spectroscopy

Direct frequency-comb spectroscopy is a technique that employs a train of well-stabilized ultrashort pulses to study the spectral properties of atomic or molecular systems. In this way, it opens the possibility of incorporating various coherent-control techniques for such spectral investigations. Here we introduce a theory for the interaction of a multilevel atom with such pulse trains, which is general enough to take into account an arbitrarily shaped frequency comb. We illustrate its application by studying the interaction of $^{87}\text{R}\text{b}$ atoms with trains of pulses of various shapes, resonant with the $5S\text{\ensuremath{-}}5D$ two-photon transition of rubidium. More specifically, we treat the interaction with hyperbolic-secant pulses, chirped pulses, and 0-$\ensuremath{\pi}$ pulses, respectively. The theory is designed to work at an arbitrary perturbation order. For the results presented here, we mostly used a 12th-order perturbation series at the pulse's electric field. Due to the large number of levels involved, such modeling may be quite complex computationally, and an important point of the present work is then to introduce the required numerical approach to treat this problem efficiently.

Generation of flat-top picosecond pulses by coherent pulse stacking in a multicrystal birefringent filter

This paper deals with the pulse-shaping properties of birefringent filters that feature an optical layout similar to a Solc fan filter. A simple computational model is given that explains the pulse-shaping process in the fan filter in two steps: First, the input pulse is split into several mutually delayed replicas due to the birefringence of the crystals. Second, these replicas interfere at the output polarizer of the filter and form the shaped output pulse. Fine-tuning of the phases of the replicas of the input pulse is permitted by tuning the temperature of the crystals. A birefringent pulse shaper containing ten birefringent crystals was investigated experimentally. The shape of the output pulses was measured by means of a special cross-correlation technique. Although a variety of pulse shapes can be generated with the described filter, it is particularly well suited for generation of flattop pulses featuring a 20-ps-long plateau and rising and falling edges shorter than 2 ps.

Positron Emission Tomography Applied to Fluidization Engineering

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Two-dimensional spectral shearing interferometry resolved in time for ultrashort optical pulse characterization

We present a new scheme for coherent measurement of ultrashort optical pulses that relies on spectral shearing interferometry combined with all-optical time gating. The spectral phase is encoded along a temporal coordinate resulting in a two-dimensional (2D) self-referenced and self-calibrating set of data. The method works without time delay between the two interfering signals. The pulse amplitude and phase are reconstructed with a high signal-to-noise ratio using a direct algorithm that applies a Fourier transform (FT) to the 2D recorded interference pattern. Characterization of low-energy, high-repetition-rate femtosecond pulses of various shapes and validation of a single-shot operation are reported.