Increasing Pulse Repetition Rate Research Articles

Optical Properties of GaAs Nanoparticles in Acetone by Laser Ablation

Gallium arsenide (GaAs) nanoparticles' optical and emission properties can be tuned bychanging their size across the visible spectrum. GaAs nanoparticles' optical characteristics aredegraded by oxidation on their surface. This work investigated the optical constants and the opticalband gap for a GaAs nanoparticle immersed in acetone using the laser ablation into liquids (LAL)technique after being exposed to a Nd: YAG pulsed laser operating at the wavelength (1064nm)10Hz frequency, and 7ns pulse width for a fixed flounce of 1.32 J/cm2, and the time of ablationwas 5 minutes. In order to calculate the optical conductivity (σ), refractive index (n), extinctioncoefficient (k), dielectric constant, absorption coefficient (α), and optical band gap, an opticalinvestigation was carried out utilizing a UV-Visible Spectrophotometer region in the wavelengthrange 300-1200 nm. The band gap energy was determined to be 3.8 eV, which is greater than thebulk Ga energy. The band gap energy of nanoparticles increases with increasing laser pulse energyand decreases with increasing pulse repetition rate. Transmission spectra increased as wavelengthsincreased, while optical absorption coefficients, extinction coefficients, and refractive coefficientsdecreased. The difference in optical constants is explained by defect states and the average bandenergy of the system. The acetone stability test reveals a peak at -0.69 mV, indicating low stabilityin nanoparticles.

Influence of the pulse repetition rate on the acoustic output of ballistic pressure wave devices

Ballistic devices that generate radial pressure waves are used for the treatment of different therapeutic indications. In order to assess the effectiveness of these devices and to interpret and transfer the results of clinical trials, it is important to know their acoustic output. In this paper, two ballistic devices and their reproducibility at different clinically relevant settings were investigated in the same in-vitro test setup. Pressure curves were measured in water at different intensity levels and pulse repetition rates. The sound field parameters (peak pressures, positive pulse intensity integral) were calculated from the pressure curves. Additionally, the surface velocity of the applicator was determined in air using a vibrometer. Both devices show a good pulse-to-pulse reproducibility. While the peak maximum pressure and the positive pulse intensity integral decrease only slightly (pmax up to 12%, PII+ up to 18.8%) comparing 1 Hz and 25 Hz for one device, they drop sharply (pmax up to 68.4%, PII+ up to 90.2%) for the other device comparing 1 Hz to 21 Hz. The same effect was observed in the vibrometer measurements. The results show that with increasing pulse repetition rate the stability of the parameters varies between different devices. Hence, all sound field parameters should be compared before transferring settings from one device to another.

Neural Network-Powered Nonlinear Compensation Framework for High-Speed Continuous Variable Quantum Key Distribution

In this paper, we present a neural network-based framework to compensate for the nonlinear distortion of high-speed continuous variable quantum key distribution (CV-QKD) systems. In practice, the nonlinear impairment of imperfect devices could bring interference to the measured quadrature values and thus compromise the parameter estimation procedure and the secure key rate. Specifically, for a high-speed CV-QKD system, the balanced homodyne detector (BHD) and the following analog-to-digital converter are the primary sources of nonlinear impairment. The nonlinear distortion effect will be more pronounced as the pulse repetition rate increases. We propose an autoencoder-based network to learn the inner patterns of distorted data and then compensate the quadrature measurements in real-time at the receiver’s side. The numerical simulation demonstrates that the excess noise induced by nonlinear impairment is reduced to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-3}$</tex-math></inline-formula> level in the shot noise unit. Consequently, such a software approach has the ability to significantly boost the bandwidth efficiency of the BHD and the secure key bit rate.

Faster Repetition Rate Sharpens the Cortical Representation of Echo Streams in Echolocating Bats.

There is consensus that primary auditory cortex (A1) utilizes a combination of rate codes and temporally precise population codes to represent discreet auditory objects. During the response to auditory streams, forward suppression constrains cortical rate coding strategies, but it may also be well positioned to enhance temporal coding strategies that rely on synchronized firing across neural ensembles. Here, we exploited the rapid temporal dynamics of bat echolocation to investigate how forward suppression modulates the cortical ensemble representation of complex acoustic signals embedded in echo streams. We recorded from auditory cortex of anesthetized free-tailed bats while stimulating the auditory system with naturalistic biosonar pulse-echo sequences covering a range of pulse emission rates. As expected, increasing pulse repetition rate significantly reduced the number of spikes per echo stimulus, but it also increased spike timing precision and doubled the information gain. This increased spike-timing precision translated into more robust inter-neuronal synchronization patterns with >10-dB higher signal-to-noise ratios (SNRs) at the ensemble level. We propose that forward suppression dynamically mediates a trade-off between the sensitive detection of isolated sounds versus precise spatiotemporal encoding of ongoing sound sequences in auditory cortex.

Influence of Waveform of Applied Voltage on H2 Production From Methane Reforming Using Dielectric Barrier Discharge

The influence of waveforms of the applied voltage on hydrogen production from methane reforming using dielectric barrier discharge (DBD) is investigated. Three types of high voltage power generators with different pulse shape with widths of 50 μs-250 ns and voltage rise speeds of 7.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> -5.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> V/s of sinusoidal, bipolar pulses, and a nanosecond pulse voltage are used to control the waveform. The energy efficiency for hydrogen production in the case of nanosecond pulse is approximately 0.9 g/kW·h, much higher than those of sinusoidal and bipolar pulses, with approximately 0.5 g/kW·h. This is due to that the voltage rise speed by the short pulse is much higher than those using the sine wave and the bipolar pulse, and so the discharge by the short pulse is uniformly generated. The hydrogen production increases and the energy efficiency decreases slightly with increasing pulse repetition rate. The hydrogen production efficiency is independent of the gas pressure within the ranges from 0.12 to 0.16 MPa.

The role of pulse repetition rate on picosecond pulsed laser processing of Zn and Zn-coated steel

Zinc and zinc-coated steel is processed with a picosecond laser source at a wavelength of 1030 and 515 nm to investigate the effect of time between consecutive pulses, i.e. pulse repetition rate on the laser ablation efficiency, in terms of maximum achievable depth of the ablated crater, material removal rate and processing quality. With increasing pulse repetition rate up to 40 kHz and number of pulses on the same location, material removal rate increases due to heat accumulation, while the maximum achievable depth decreases due to particle shielding for both zinc and zinc coated steel. It was found that, unlike the zinc-coated sample, both material removal rate and achievable depth is enhanced for bulk zinc at high repetition rates, due to a greater degree of heat accumulation than particle shielding. Using a numerical heat flow model, it is shown that the difference between bulk zinc and zinc-coated steel stems from the steel substrate that effectively acts as a heat sink for the absorbed energy in the zinc coating, inhibiting a higher degree of heat accumulation.

Research on quantitative method of GIS partial discharge

Optical method is a commonly used method for detecting partial discharge in electrical equipment. How to use photometric quantitative detection is one of the most urgent problems to be solved. Based on the in-depth analysis of the physical meaning of the integral value of the PD optical signal, it is concluded that the linear integral value of the signal is linear with the apparent discharge. The experimental results show that the integral value of the PD photometric signal at different polarities and different distances is linear with the apparent discharge. The larger the gap distance between the needle and the plate electrode is, the larger the slope is. The linear relationship between the average primary integrated value of the PD photometric signal and the average apparent discharge is different. With the increase of the applied voltage, the PD discharge intensity and the pulse repetition rate increase, and the phase interval of the PD becomes wider. The actual air gap volume of the PD becomes larger, and the variation interval becomes wider, that is, the apparent discharge amount distribution interval becomes wider, and the maximum value and the minimum value of the distribution interval become larger. Therefore, it is possible to obtain the PD apparent discharge amount by using the photometric signal, and it is effective and feasible to quantitatively detect the internal PD of the GIS by optical method.

Nucleation and growth of Si nanoparticles under different pulse repetition rates without the baffle for nanosecond pulsed laser-ablated deposition

AbstractIn this article, Si nanoparticle (NP) films were prepared by pulsed laser ablation (PLA) in the argon atmosphere of 10 Pa at room temperature under different pulse repetition rates from 1 to 40 Hz without the baffle. Different from the conventional PLA method, the substrates were placed below and parallel to the ablated plume axis. The obtained films containing NPs were characterized by scanning electron microscopy and Raman spectrometer. The experimental results under constant laser fluence demonstrate the strong dependence of the mean size and the area number density of NPs on the repetition rate. Specifically, with the increase of pulse repetition rate, the mean size of the NPs in the film first decreases and reaches its minimum at 20 Hz, and then increases after 20 Hz, and decreases again till 40 Hz. The area number density shows the contrary trend versus mean size. The in situ diagnostic results of Langmuir probe denote the ablated Si ion density increases monotonously with the increase of repetition rate, while the temperature is almost constant. Combining with the nucleation probability, the growth/aggregation duration of NPs in the “nucleation region” and the effect of the baffle, the influence of pulse repetition rate on the formation of NPs is addressed. It is found that the repetition rate impacts the growth modes of NPs (i.e., growth and aggregation). 1–20, 20–30, and 30–40 Hz, respectively, correspond to growth-, aggregation-, and growth-controlled rate ranges without the baffle; however, 1–10, 10–20, and 20–40 Hz, respectively, correspond to growth-controlled, aggregation/growth-coexisted, and aggregation-controlled rate ranges with the baffle.

Experimental research on thermal‐dynamic damage effect of K9 optical lens irradiated by femtosecond laser

AbstractThe photothermal effect and photodynamic effect are the main factors causing the damage of materials when laser irradiates on it. Therefore, the damage effect of laser irradiation on optical lens evaluated by thermal and mechanical effects will contribute to the development of laser processing and laser damage. In order to study the damage effect of femtosecond laser irradiating on typical K9 optical lens, real‐time temperature during irradiation was measured by infrared thermal imaging combined with laser loading system, at the same time, high‐speed camera observed the irradiation process. Then, with the help of known parameters, the stress at the moment of laser loading was calculated. Finally, the optical lens target was placed under a scanning electron microscope for observation. The results show that maximum temperature and peak stress in experiments were lower than the damage threshold of optical lenses. With the increase of pulse repetition rate, the thermal damage has a cumulative effect, while the photo‐induced pressure only increases slightly. It leads to the dominance of mechanical damage in low‐frequency pulse irradiation, whereas in high‐frequency pulse irradiation, the opposite rule occurs. For transparent materials, the laser pulse shows a stronger damage effect, the product spatters in the direction opposite to the incident beam, while the plasma discharge damage product spatters in the opposite direction, so the damage occurs on both front and back surfaces of the material at the same time.

Surface damage and threshold determination of Ge–As–Se glasses in femtosecond pulsed laser micromachining

AbstractGe–As–Se chalcogenide glasses were prepared and the surface damage characteristics under femtosecond laser irradiation were experimentally investigated. Femtosecond laser beams with different power intensities, focusing depths, pulse repetition rates, and pulse numbers were employed to determine the damage thresholds of the samples. The surface morphologies and feature sizes of the damage craters were studied with regard to laser energy density to evaluate the damage circumstances of the glasses. The generation of the surface damage was also recorded by using different numbers of pulses. Results showed that the damage area increased monotonically with the laser power at low energy density and tended to saturation when reaching a critical value. According to the linear circumstances of laser energy density on the damage crater area, the average damage thresholds of the Ge–As–Se glasses with different repetition rates and pulse numbers were obtained. Results showed that the damage threshold decreased with the increase in pulse repetition rate and number of pulse.

Improving Signal and Photobleaching Characteristics of Temporal Focusing Microscopy with the Increase in Pulse Repetition Rate

Wide-field temporal focused (WF-TeFo) two-photon microscopy allows for the simultaneous imaging of a large planar area, with a potential order of magnitude enhancement in the speed of volumetric imaging. To date, low repetition rate laser sources with over half a millijoule per pulse have been required in order to provide the high peak power densities for effective two-photon excitation over the large area. However, this configuration suffers from reduced signal intensity due to the low repetition rate, saturation effects due to increased excitation fluences, as well as faster photobleaching of the fluorescence probe. In contrast, with the recent advent of high repetition rate, high pulse energy laser systems could potentially provide the advantages of high repetition rate systems that are seen in traditional two-photon microscopes, while minimizing the negatives of high fluences in WF-TeFo setups to date. Here, we use a 100 microjoule/high repetition rate (50–100 kHz) laser system to investigate the performance of a WF-TeFo two-photon microscope. While using micro-beads as a sample, we demonstrate a proportionate increase in signal intensity with repetition rate, at no added cost in photobleaching. By decreasing pulse intensity, via a corresponding increase in repetition rate to maintain fluorescence signal intensity, we find that the photobleaching rate is reduced by ~98.4%. We then image live C. elegans at a high repetition rate for 25 min. as a proof-of-principle. Lastly, we identify the steady state temperature increase as the limiting process in further increasing the repetition rate, and we estimate that repetition rate in the range between 0.5 and 5 MHz is ideal for live imaging with a simple theoretical model. With new generation low-cost fiber laser systems offering high pulse energy/high repetition rates in what is essentially a turn-key solution, we anticipate increased adoption of this microscopy technique by the neuroscience community.

Gain-guided soliton: Scaling repetition rate of passively modelocked Yb-doped fiber lasers to 12.5 GHz.

Fundamental repetition rates of 3.1 GHz, 7.0 GHz, and 12.5 GHz in passively modelocked Yb-doped fiber lasers are demonstrated. To the best of our knowledge, the fundamental repetition rate of 12.5 GHz is the highest value for 1.0 μm mode-locked fiber lasers. The mode-locked oscillator has a peak wavelength of 1047.5 nm and a pulse duration of 1.9 ps. The repetition rate signal has a signal-to-noise ratio of 57 dB. The peak wavelength of mode-locked spectra gradually makes a blue-shift and the modelocked threshold power increases with an increase in pulse repetition rate. Furthermore, in contrast to most of the all-normal-dispersion mode-locked fiber lasers, the present linear resonator (e.g., length < 1 cm) allows the buildup of gain-guided soliton without any filter effect. To unveil the underlying pulse shaping mechanism, a combined model comprising dynamic rate equations and the generalized nonlinear Schrödinger equation is established. Surprisingly, an essential gain filtering effect, which is contributed by a 26-nm gain bandwidth, is revealed and can verify the gain-guided pulse dynamics. Moreover, the pulse build-up in temporal and frequency domain, like spectral evolution and gain bandwidths, is numerically carried out in detail.

Volume added surface barrier discharge plasma excited by bipolar nanosecond pulse power in atmospheric air: optical emission spectra influenced by gap distance

In this paper, volume barrier discharge with different gap distances is added on the discharge border of high-voltage electrode of annular surface barrier discharge for generating volume added surface barrier discharge (V-SBD) excited by bipolar nanosecond high-voltage pulse power in atmospheric air. The excited V-SBDs consist of surface barrier discharge (d = 0 mm) and volume added surface barrier discharges (d = 2 mm and 3 mm). The optical emission spectra are recorded for calculating emission intensities of N2 (C3Πu → B3Πg) and (B2 → X2 ), and simulating rotational and vibrational temperatures. The influences of gap distance of V-SBD on emission intensity and plasma temperature are also investigated and analyzed. The results show that d = 0 mm structure can excite the largest emission intensity of N2 (C3Πu → B3Πg), while the existence of volume barrier discharge can delay the occurrence of the peak value of the emission intensity ratio of (B2 → X2 )/N2 (C3Πu → B3Πg) during the rising period of the applied voltage pulse and weaken it during the end period. The increasing factor of emission intensity is effected by the pulse repetition rate. The d = 3 mm structure has the highest threshold voltage while it can maintain more emission intensity of N2 (C3Πu → B3Πg) than that of d = 2 mm structure. The structure of d = 2 mm can maintain more increasing factor than that of the d = 3 mm structure with varying pulse repetition rate. Besides, the rotational temperatures of three V-SBD structures are slightly affected when the gap distance and pulse repetition rate vary. The vibrational temperatures have decaying tendencies of all three structures with the increasing pulse repetition rate.

Experimental setup for evaluation of cavitation effects in ESWL

Abstract Cavitation is a major fracture mechanism in extracorporeal shock wave lithotripsy (ESWL). However, it can cause tissue trauma and its effects on kidney stones and surrounding tissue are not fully understood. Therefore experimental setups enabling systematic parameter studies are crucial. We developed and evaluated a testing rig comprising three measuring methods in order to examine this mechanism. Our initial evaluation of this setup based on standard components showed promising results. Primary cavitation was displayed by high-speed photography 195 μs after the shock front had passed the focal zone. The effect of different pulse repetition rates (30, 60, 90, 120 SW/min) on the extension of the cavitation area was determined. The lifetime of secondary cavitation was analysed by B-mode ultrasound imaging. In a post processing progress the images showing bubbles were compared to a reference picture for both types of cavitation and the number of pixels that changed colour was counted. Furthermore stone comminution at different pulse repetition rates (30, 60, 90, 120 SW/min) was investigated by fixed-dose fragmentation. We observed an inverse correlation of cavitation and fragmentation. As the pulse repetition rate increases, the area of primary cavitation grows whereas the fragmentation efficiency decreases. B-mode imaging showed that secondary cavitation bubbles persisted between the shocks and can serve as nuclei. The higher the pulse repetition rate is, the more of these nuclei remain and thus facilitate formation of primary cavitation. The experimental setup provides reproducible results regarding the development of primary and secondary cavitation on the one hand and the fragmentation of phantom stones on the other hand. Therefore it can be utilized to further investigate the effect of different boundary conditions and shock wave parameters on cavitation and stone comminution. The impact of different focal sound fields is subject of ongoing research.

Quality Evaluation of Micro Drilled Hole Using Pulsed Nd:YAG Laser: a Case Study on AISI 316

Laser drilling is increasingly becoming a suitable choice for performing micro-drilling on advanced engineering and bio-compatible materials. In laser drilling operation, two types of major defects like spatter deposition and taper are generally encountered. In order to reduce the drilling defects, understanding the influence of process parameters on quality of hole is highly desirable. The influence of process parameters such as laser energy, pulse repetition rate, pulse width and gas pressure on the quality characteristics viz. circularity at entry and exit, taper and spatter area is studied in this work using Taguchi’s L27 orthogonal array. Analysis of variance (ANOVA) indicates that pulse width is most influencing parameter having percentage contribution of 43.981, 51.93, 88.28 and 39.94 for circularity at entry, circularity at exit, taper and spatter area respectively. It is observed that spatter area and heat affected zone (HAZ) increases with increase in laser energy and pulse width. This is due to the fact that heat input increases causing more material to melt but sufficient time is not allowed to completely flush away the molten material. As a result, heat is not dissipated adequately resulting in spatter formation and heat affected zone. The analysis also reveals that increase in pulse repetition rate and gas pressure leads to decrease in spatter area. Increase in pulse repetition rate causes increase in laser power leading to formation of laser supported absorption (LSA) wave which may cause the blockage of input energy. Hence, material ejection becomes less and scope for formation of spatter turns out to be less. Micrographs of drilled surface are analyzed using scanning electron microscope (SEM). The micrograph study suggests that increase in laser energy leads to increase in HAZ and circularity. The study also reveals that increase in pulse repetition rate leads to increase in burr formation in the hole periphery. Empirical models have been proposed to predict the quality characteristics of drilled hole. Mean relative error within 10% between the predicted value from empirical relations and experimental value for quality characteristics of hole indicates the validity of the proposed models. In order to obtain optimum parametric setting for all the quality characteristics, a well-known evolutionary approach known as genetic algorithm (GA) is used. The suggested best parametric conditions are definitely useful for the practitioners to avoid trial-and-error process of setting the process parameters for obtaining laser drilled holes of desired quality.

Nanosecond laser ablation of Al-Si coating on boron steel

Al-Si coating is routinely used for hot-press-forming (HPF) steel to prevent oxidation and decarburization under high-temperature processing conditions. However, the Al-Si coating is not suitable for subsequent welding process and has to be removed before laser welding. In this work, the ablation effect of nanosecond pulse laser parameters (i.e., pulse energy, pulse repetition rate, and pulse number) on Al-Si coating was studied. Experimental results show that for the 100ns pulse duration laser ablation, the complete removal of Al-Si coating requires the pulse repetition rate to be larger than 5kHz and a higher pulse repetition rate needs a smaller pulse number. Despite a statistical ablation threshold in long pulse regime, the calculated ablation threshold of fully removed Al-Si coating was obtained at the laser fluence ~4.4J/cm2 at the pulse repetition rate 25kHz. Due to the accumulation effects, increased pulse number and pulse repetition rate will reduce the ablation threshold. The evolution of ablation depth was simulated using Hirschegg model and a gradual increase and then constant ablation depth was found as the pulse number and pulse repetition rate increase.

Localized Residual Heat and Formation of Nonbranched Positive Streamer With Highly Repetitive Streamer Discharge

The mechanism of a mode change from a branch-type to a nonbranch positive streamer with increase in pulse repetition rate was investigated from the point of view of background gas heating. In this experiment, a repetitive streamer was generated between a needle and a plane electrode. The repetition rate of impulse positive streamer discharge was changed from 500 to 10 500 pulse/s. The branching of a streamer decreased with the increase in pulse repetition rate and the streamer shape completely changed to a nonbranched type at 10 000 pulse/s. The gas temperature in the repetitive streamer was quantitatively measured at different pulse repetition rates by a spectroscopic method using N2 second positive system band (0, 0). In addition, the gas density change between the electrodes due to gas heating caused by the repetitive streamers was observed using the laser schlieren method. As a result, it was confirmed that the gas temperature in repetitive streamers increased with the increase in pulse repetition rate. The gas temperature in a streamer increased up to 420 K by the accumulation of background gas heating at 10 000 pulse/s. Furthermore, residual gas heat in the shape of a single filament due to a highly repetitive streamer and its dissipation process were clearly visualized.

The influence of laser pulse repetition rate on the intensity of spectral lines in femtosecond laser-induced breakdown spectroscopy of a liquid

The influence of pulse repetition rate of a femtosecond Ti:Sa laser (800 nm, 45 fs, 0.82 mJ) on intensity of spectral lines at optical breakdown on the surface of CaCl2 water solution is experimentally determined. It is demonstrated that, with increasing pulse repetition rate from 20 to 1000 Hz, the intensity of Ca II (393.3 nm) emission line reduces by 44%, while the plasma continuum remains unchanged within measurement error.

The effects of pulse repetition rate on the structural, optical, and electrical properties of CIGS films grown by pulsed laser deposition

Well crystallized CuIn1−xGaxSe2 (CIGS, x=0.22–0.26) films were deposited at the temperatures below 300°C by pulsed laser deposition. Increasing pulse repetition rate from 5Hz to 1kHz significantly increased the crystallinity of films. All the films were Cu rich and those deposited with 5Hz showed higher Ga deficiency, but in those deposited with 1kHz the loss of Se was higher. The growth mechanisms under the low and high repetition rates as well as their effects on the grain size and composition were discussed in detail. All the films had p-type conductivity. The resistivity of 1kHz films was two times higher but the carrier concentration was four times lower than the 5Hz films. I–V characteristics were measured on the n-ZnO/p-CIGS heterojunctions. The junction made with 1kHz film had a single ideality factor of 1.3 up to the high current limit, which is often seen in the high efficiency CIGS solar cells, whereas the I–V curve of the junction made with 5Hz film consisted of two sections described by large ideality factors of 3.0 and 4.8, respectively, indicating that there are two recombination mechanisms each of which dominates at a different current region.

Effect of Head-To-Head Addition on Vinyl Acetate Propagation Kinetics in Radical Polymerization

The radical propagation kinetics of vinyl acetate (VAc) has been studied by pulsed-laser polymerization coupled with analysis of the resulting polymer molar mass distributions. The significant increase in the apparent propagation rate coefficient observed with increasing pulse repetition rate is explained by the influence of the head-to-head defects formed during chain growth. Simulations that include head-to-head monomer addition are combined with an analysis of the experimental results to estimate the subsequent rate coefficients for reaction of the resultant radical as well as the propagation rate coefficient of normal head-to-tail addition. An analytical expression is derived for an averaged rate coefficient, kpav, for the system, with the best-fit Arrhenius parameters of ln(A/L mol–1 s–1) = 16.56 ± 0.35 and E/R= 2508 ± 108 K.