Pulse Burst Research Articles

Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses.

Laser processing with ultra-short double pulses has gained attraction since the beginning of the 2000s. In the last decade, pulse bursts consisting of multiple pulses with a delay of several 10 ns and less found their way into the area of micromachining of metals, opening up completely new process regimes and allowing an increase in the structuring rates and surface quality of machined samples. Several physical effects such as shielding or re-deposition of material have led to a new understanding of the related machining strategies and processing regimes. Results of both experimental and numerical investigations are placed into context for different time scales during laser processing. This review is dedicated to the fundamental physical phenomena taking place during burst processing and their respective effects on machining results of metals in the ultra-short pulse regime for delays ranging from several 100 fs to several microseconds. Furthermore, technical applications based on these effects are reviewed.

Inception Voltage of Burst Pulses, Onset Streamers, and Positive Glow in Short Rod-to-Plane Gaps

The charge simulation technique (CST) was applied for computing the electric field in long space-charge-free rod-to-plane gaps with G (=gap-spacing to rod-radius) ratio in the range of 50-500. In this article, a new formation of CST is proposed for computing the electric field in short positively stressed rod-to-plane gaps characterized by G in the range of 2-160 and gap spacing up to 8 cm, provided that the gap can accommodate space charges. This is a prerequisite for computing the inception voltage of different corona modes, including burst pulses (BP), onset streamers (OS), and positive glow (PG). The criteria developed before for computing the inception voltage of BP and OS are refined for short rod-to-plane gaps. However, the inception criterion for PG cannot be extended to short gaps. A method is proposed for calculating the inception voltage of PG in short rod-to-plane gaps in the air at room temperature. The initiatory electrons for burst avalanches are produced by photoionization inside the ionization zone rather than by detachment of negative ions as experienced in long gaps. The resulting BP merges in time to form the steady glow at the rod surface in conformity with Loeb's postulate. The computed inception voltage of BP, OS, and PG agreed reasonably with those measured experimentally for short rod-to-plane gaps.

Silicon Wafer Etching Rate Characteristics with Burst Width Using 150 kHz Band High-Power Burst Inductively Coupled Plasma

The high-speed etching of a silicon wafer was experimentally investigated, focusing on the duty factor of 150 kHz band high-power burst inductively coupled plasma. The pulse burst width was varied in the range of 400–1000 µs and the repetition rate was set to 10 Hz. A mixture of argon (Ar) and carbon tetrafluoride (CF4) gas was used as the etching gas and injected into the vacuum chamber. The impedance was changed with time, and the coil voltage and current were changed to follow it. During the discharge, about 3 kW of power was applied. The electron temperature and plasma density were measured by the double probe method. The plasma density in the etching region was 1018–1019 m−3. The target current increased with t burst width. The etching rate of Ar discharge at burst width of 1000 µs was 0.005 µm/min. Adding CF4 into Ar, the etching rate became 0.05 µm/min, which was about 10 times higher. The etching rate increased with burst width.

Investigation of supercontinuum generation in photonic crystal fiber using bursts of femtosecond pulses

Results of experimental and numerical investigation of supercontinuum generation in polarization maintaining photonic crystal fiber using a burst of two orthogonal polarization femtosecond pump pulses are presented. The initial pump pulse source was a mode-locked Yb:KGW laser generating 52 nJ energy 90 fs duration pulses at 1033 nm with 76 MHz repetition rate. Analysis of cross-correlation frequency-resolved optical gating traces and numerical simulation results reveal that there are slight changes in supercontinuum generation when temporal delay between pump pulses in the burst is changed. Weak dependence of supercontinuum characteristics on temporal distance between the pump pulses is related to supercontinuum generation dynamics which is analyzed in detail.

Frequency spectra of various events pertinent to lightning cloud flashes obtained from wavelet transform technique and ratified by narrow band measurement technique

In the present study, frequency spectra of the electric field corresponding to the various cloud events, such as, initial breakdown process, regular pulse bursts, chaotic pulse trains and recoil streamers have been analysed. For the purpose, electric field radiated by cloud flashes were obtained simultaneously by a wide bandwidth antenna system and two narrow bandwidth antenna systems tuned at 3 MHz and 30 MHz. The frequency spectra of the broad band electric field signatures were obtained by using the wavelet transform technique and were compared with the magnitudes of the narrow band signals at the given central frequencies. To the best of our knowledge, it is the first study in which frequency spectra is obtained by transforming the time domain signal using wavelet transform technique and ratified by narrow bandwidth system for the cloud flashes. Fifteen cloud flashes pertinent to the Swedish thunderstorms were selected for the purpose. It is found that, the cloud flashes radiate at frequencies as low as 3 kHz to as high as a few tens of Mega Hertz (MHz). Electric field radiation corresponding to the initial breakdown process were found to radiate in the frequency range of 50 kHz to 5 MHz on the average, maximum energy is being radiated in the frequency range of 500 kHz to 5 MHz. Similarly, the final stage corresponding to the regular pulse bursts was found to radiate in the frequency range of 50 kHz to 5 MHz and that corresponding to the chaotic pulse trains was found to be in the range of 100 kHz to 5 MHz. Whereas, the very narrow pulses at the final stage, that can be termed as pulses corresponding recoil streamers (or Q-streamers) were found to radiate in the frequency range of 50 kHz to well above 10 MHz. Q-streamers can be considered as the strongest source of very high frequency and is justified by the simultaneous measurement of the electric fields at very high frequency (30 MHz) narrow bandwidth system. Rectification of the biasness of the conventional wavelet power spectrum has also been performed, however, no significant change in the spectrum was observed. Therefore, this study provides a strong basis for applying the wavelet transform technique without employing large number of narrowband system to acquire frequency domain information of lightning phenomena.

Burst pulses for positive corona discharges in atmospheric air: the collective movement of charged species

Positive corona burst pulses are an unstable pulse mode. They appear in a small range of the onset stage, and their current pulses result from the collective movement of charged species. This paper focused on the connections between these pulses and the collective movement of charged species. The movement of species is divided into four parts with respect to time: the (1) initial growth of species, (2) formation and development of the streamer region and negative ion sheath, (3) dead time (the time interval between the pulses), and (4) rapid re-growth of species. The movement of the species in the four parts and the correspondence with the current pulse were analyzed. The numerical results indicated the following: the rapid rising of the species matched the rising edge of the pulses, the streamer region, and negative ion sheath appeared in the falling edge of the primary pulse, and the rapid re-growth of species matched the re-ignition of the pulses. The results were in qualitative agreement with deductions and experimental observations in the literature.

Process monitoring based on plasma emission for power-modulated glass welding with bursts of subpicosecond laser pulses.

Defects and process irregularities influence the bonding strength and thus the stability and lifetime of welded glass components. The present paper proposes to monitor the laser-based glass welding process by means of a single photodetector that records the radiation emitted from the laser-induced plasma. It is shown that the plasma emission provides information about irregularities of the welded seam height, gap bridging, process interruptions, and the position of the seam. The method is suitable for different welded glass types.

Single-cell electroporation with high-frequency nanosecond pulse bursts: Simulation considering the irreversible electroporation effect and experimental validation

To study the electroporation characteristics of cells under high-frequency nanosecond pulse bursts (HFnsPBs), the original electroporation mathematical model was improved. By setting a threshold value for irreversible electroporation (IRE) and considering the effect of an electric field on the surface tension of a cell membrane, a mathematical model of electroporation considering the effect of IRE is proposed for the first time. A typical two-dimensional cell system was discretized into nodes using MATLAB, and a mesh transport network method (MTNM) model was established for simulation. The dynamic processes of single-cell electroporation and molecular transport under the application of 50 unipolar HFnsPBs with field intensities of 9 kV cm−1 and different frequencies (10 kHz, 100 kHz and 500 kHz) to the target system was simulated with a 300 s simulation time. The IRE characteristics and molecular transport were evaluated. In addition, a PI fluorescent dye assay was designed to verify the correctness of the model by providing time-domain and spatial results that were compared with the simulation results. The simulation achieved IRE and demonstrated the cumulative effects of multipulse bursts and intraburst frequency on irreversible pores. The model can also reflect the cumulative effect of multipulse bursts on reversible pores by introducing an assumption of stable reversible pores. The experimental results agreed qualitatively with the simulation results. A relative calibration of the fluorescence data gave time-domain molecular transport results that were quantitatively similar to the simulation results. This article reveals the cell electroporation characteristics under HFnsPBs from a mechanism perspective and has important guidance for fields involving the IRE of cells.

Active Suppression of Piezoelectric Ringing in Pockels Cells for Laser Cavity Application

Pockels cells used as electro-optical modulators in high-power high-repetition lasers suffer from piezoelectric ringing phenomenon due to piezoelectric properties of the crystals. A new method for active suppression of the piezoelectric ringing in Pockels cells is proposed in this work. It is based on symmetric control of Pockels cell using burst of short positive and negative voltage pulses with the same amplitude instead of a single long pulse for light polarization modulation. Rising and falling edges of pulses of the burst induce symmetrical acoustic waves of the opposite phase and cancel the piezoelectric ringing of the crystal. A new high voltage driver capable of generating positive and negative pulses of tens of nanoseconds of 3 kV magnitude was developed for this purpose. The amplitude of laser beam intensity pulsations caused by the piezoelectric ringing can be reduced up to five times when active suppression method is used for the deuterated potassium dihydrogen phosphate (DKDP) Pockels cell. Such crystals like DKDP, LiNbO3, and LiTaO3 may benefit from the proposed method and find new use in lasers of high repetition rate where piezoelectric ringing is a major limiting factor.

Influence of Nanosecond Pulse Bursts at High Repetition Rates on Ablation Process

Influence of Nanosecond Pulse Bursts at High Repetition Rates on Ablation Process

Long‐term passive acoustics to assess spatial and temporal vocalization patterns of Atlantic common bottlenose dolphins (Tursiops truncatus) in the May River estuary, South Carolina

AbstractPassive acoustics has been used extensively to study bottlenose dolphins; yet very few studies have examined the spatial, temporal, and environmental influences on vocalization types (echolocation, burst pulse sounds, and whistles), and few are long‐term and provide high temporal resolution over multiple years. We used data from 2013 to 2018 to establish baseline acoustic patterns for bottlenose dolphins in the May River estuary, South Carolina. We deployed acoustic recorders at six stations during 2013–2014 and three stations during 2015–2018, with locations spanning the entire estuary (headwaters to the mouth). We discovered that acoustic detection of dolphins varied not only spatially, but also yearly, monthly, and tidally. Higher numbers of echolocation bouts, burst pulse sounds, and whistles were detected at the mouth as compared to the headwaters. At the mouth, vocalization detections were greatest in fall and winter for multiple years, and echolocation detection was greatest during falling and low tides. This study provides an example of another tool, long‐term passive acoustics monitoring, to better understand spatial and temporal distribution of dolphins in a typical salt marsh estuary, that can be applied to other ecosystems throughout the southeastern United States and globally.

Investigating the influence of pulse duration in burst mode ablation of stainless steel

Using ultrashort laser pulses in a megahertz burst to create a smoothing effect in the ablation of stainless steel for various industrial applications.

Burst Pulses for Positive Corona Discharge in an Air Atmosphere: A Simulation Study

Positive corona discharge is a diverse, uncertain phenomenon. Burst pulses are a type of preonset pulses occurring in the onset stage of positive coronas. In this study, we simulated successive burst pulses using a fluid numerical model coupled with Poisson and Helmholtz equations in a 1-D axisymmetric model. The typical burst pulse train—a primary pulse followed by several secondary pulses and its repetition—was uncovered in simulation. Our modeling results closely agree with experimental results in the literature. We investigated the current waveform, electric field, species distribution, photoionization of the second burst pulse train and discussed reignition, rise, and amplitude characteristics of the burst pulses. According to our results, the second burst pulse train has the characteristics similar to the first one studied by Morrow (2002). In addition, the proper amplitude of the second primary pulse is governed by the density of net positive ionic charges required to distort the electric field (to a level where more collisional ionization cannot be supported). Our study supplemented the findings of Morrow (2002) and elucidated the mechanisms and characteristics of positive corona burst pulses.

Effects of pulse rise time and repetition frequency on nanosecond pulsed plasma ignition for combustion

Transient plasma ignition (TPI) employs highly non-equilibrium plasmas driven by nanosecond high-voltage pulses for combustion ignition. The effects of rise time and pulse repetition frequency (PRF) on combustion ignition are evaluated when transient plasmas, powered by 10-ns, kilovolt pulses, are employed to initiate combustion of lean, stoichiometric, or rich methane-dry air mixtures at atmospheric pressure. It was found that the plasmas driven by high voltage pulses with a faster rise time and PRF between 6–10 kHz enhanced combustion with higher peak pressure and/or shorter ignition delay. A mode transition in the plasma, corresponding to the combustion initiation, was observed based on voltage and current waveforms of four consecutive pulsed plasmas driven by a pulse burst at kilohertz PRFs. Although benefits in combustion ignition from a shorter rise time and optimal PRF were observed for all three equivalence mixture ratios, the highest peak pressure and shortest ignition delay were associated with the stoichiometric mixtures. In addition, the gas temperature of the transient plasma was measured by determining the rotational temperature of the 2nd positive system of nitrogen to be ∼1000 K for the first pulse and increased up to 2000 K after the application of the 4th pulse in a four-pulse burst TPI.

Characterisation and classification method of burst pulses produced by Guiana dolphins (Sotalia guianensis)

ABSTRACT Although burst pulses are the least studied sounds produced by odontocetes, their characterisation is essential to describe the acoustic repertoire of a species. This study aimed to characterise burst pulses produced by Guiana dolphins and propose a visual method to classify these sounds into structural categories. Data collection occurred during boat surveys in south-eastern Brazil. Five acoustic parameters were extracted: number of pulses, duration, interclick interval, click repetition rate, and minimum frequency. All sounds were visually classified into four categories and used in a discriminant function analysis. In addition, a K-means cluster was used to compare the visual classification with this unsupervised grouping method and confirm the representativeness of each category. According to the discriminant analysis, most sounds were correctly classified (77%) into the four categories. Both methods grouped burst pulses in similar patterns, confirming the use of the categories. Click repetition rate was the most important parameter to separate categories. The variation in this parameter could be related to different behaviours as already suggested for other species. Thus, each category could have a different function in Guiana dolphin communication. The visual classification method is efficient and could be used for other species that produce pulsed sounds.

Laser micromachining of steel and copper using femtosecond laser pulses in GHz burst mode

Investigations on the ablation of steel and copper with femtosecond laser pulses using single pulses and bursts of infinite sequence of pulses with 1 GHz intraburst repetition rate are presented in this paper. The ablation efficiency is compared to the case of single femtosecond laser pulses. Based on the literature a model of the ablation process is presented explaining the behavior of a pulse burst and the good qualitative agreement of the numerical and experimental ablation efficiencies is obtained.

A Theoretical Argument for Extended Interpulse Delays in Therapeutic High-Frequency Irreversible Electroporation Treatments.

We sought to determine whether modifications to the delays within H-FIRE bursts could yield a more desirable clinical outcome in terms of ablation volume versus extent of tissue excitation. We used a modified spatially extended nonlinear node (SENN) nerve fiber model to evaluate excitation thresholds for H-FIRE bursts with varying delays. We then calculated non-thermal tissue ablation, thermal damage, and excitation in a clinically relevant numerical model. Excitation thresholds were maximized by shortening d1, and extension of d2 up to 1,000 μs increased excitation thresholds by at least 60% versus symmetric bursts. In the ablation model, long interpulse delays lowered the effective frequency of burst waveforms, modulating field redistribution and reducing heat production. Finally, we demonstrate mathematically that variable delays allow for increased voltages and larger ablations with similar extents of excitation as symmetric waveforms. Interphase and interpulse delays play a significant role in outcomes resulting from H-FIRE treatment. Waveforms with short interphase delays (d1) and extended interpulse delays (d2) may improve therapeutic efficacy of H-FIRE as it emerges as a clinical tissue ablation modality.

Focus on photonics, spectroscopy, and spectrometry

Focus on photonics, spectroscopy, and spectrometry

Design and Implementation of a Re-Configurable Versatile Direct Digital Synthesis-Based Pulse Generator

Direct digital synthesis (DDS) technology has replaced many traditional analog circuit functions. The frequency, amplitude, and phase of a waveform generated using DDS can be precisely controlled. It is possible to achieve remarkably high-frequency resolution, fast frequency switching, and modulation can be easily introduced in a DDS generator. However, pulse waveform generated using DDS requires re-loading of the pulse waveform in the waveform memory to vary the pulse parameters, which causes phase discontinuity, requires prohibitively large memory to generate pulses with narrow widths and long periods, and suffers from trigger uncertainty of up to 1 clock period when the pulse waveform is triggered by an external event. Moreover, it is not possible to modulate the width and delay of the pulse stored in the waveform memory. This article describes a fully functional, low complexity, low cost, memory less, parallel, and DDS-based pulse waveform generator implemented in a low-cost field programmable gate arrays (FPGA) where pulse parameters such as amplitude, period, width, rise time, fall time, and delay can all be varied without any glitches or phase discontinuity, where the amplitude, frequency and phase, as well as period, width and delay of the pulse can be modulated by an internally generated or external modulating signal and where it is possible to trigger the generation of the pulse or burst of pulses by an external event where the time between the trigger event and the pulse output is fixed.

Incoherent and coherent accumulation of the spectra of a burst of LFM-pulses over the intervals of Doppler ambiguity

In advanced portable radars using the principles of MIMO, obtaining a high resolution and a sufficient S/N ratio is important due to the use of a reduced number of widely directed transmitting and receiving antenna elements, for example, sub lattices with low directional coefficients. In such radars, probing signals in the form of burst of broadband LFM pulses are applicable. When processing these signals reflected from moving targets, two methods are fundamentally possible: classical one in the time domain with matched filtering of the burst of LFM-pulses and subsequent inter-period accumulation, and the spectral one based on preliminary calculation of the full spectrum of the pulses burst with subsequent accumulation over the frequency intervals of Doppler ambiguity. In this case, there are two options for the accumulation of spectra: 1) incoherent and 2) coherent, and their efficiency for a burst of LFM-pulses has not been studied before. In this work on the simulation model of the processing system in the spectral domain the output S/N ratios for these variants of the spectral processing method for a burst of LFM-pulses are determined and compared. In addition, the issues of presenting the results of spectral accumulation in the coordinates «Doppler frequency – delay» plane were worked out. The following results are obtained for a specific variant of the burst signal: the efficiency of coherent spectrum accumulation under the influence of noise exceeds the efficiency of incoherent by a value of about 12 dB; in the case of incoherent accumulation of spectrum modules in the case of a group targets, the accumulated amplitude spectrum, with a sufficient duration of the pulses burst, contains all the Doppler-resoluted target marks; in the case of coherent accumulation of complex spectra of group targets with different Doppler frequencies and ranges, the targets can be resolved based on the analysis of the module of the accumulated complex spectrum in the «Doppler frequency – range» plane; 4) with incoherent and coherent of spectral accumulation over ambiguous Doppler intervals, a single FFT of a LFM-pulses burst is required. The remaining operations are the algebraic summation of the spectral samples over the ambiguity intervals and taking the modulus from the accumulated complex spectrum.