Low Repetition Rate Research Articles

TipScreener: A Framework for Mining Tips for Online Review Readers

User-generated content explodes in popularity daily on e-commerce platforms. It is crucial for platform manipulators to sort out online reviews with repeatedly expressed opinions and a large number of irrelevant topics in order to reduce the information processing burden on review readers. This study proposes a framework named TipScreener that generates a set of useful sentences that cover all of the information of features of a business. Called tips in this work, the sentences are selected from the reviews in their original, unaltered form. Firstly, we identify information tokens of the business. Second, we filter review sentences that contain no tokens and remove duplicates. We then use a convolutional neural network to filter uninformative sentences. Next, we find the tip set with the smallest cardinality that contains all off the tokens, taking opinion words into account. The sentences of the tip set contain a full range of information and have a very low repetition rate. Our work contributes to the work of online review organizing. Review operators of e-commerce platforms can adopt tips generated by TipScreener to facilitate decision makings of review readers. The convolutional neural network that classifies sentences into two classes also enriches deep learning studies on text classification.

Localized nitrogen-vacancy centers generated by low-repetition rate fs-laser pulses

Among hundreds of impurities and defects in diamond, the nitrogen-vacancy (NV) center is one of the most interesting to be used as a platform for quantum technologies and nanosensing. Traditionally, synthetic diamond is irradiated with high-energy electrons or nitrogen ions to generate these color-centers. For precise positioning of the NV centers, fs-laser irradiation has been proposed as an alternative approach to produce spatially localized NV centers in diamond. However, most of the studies reported so far used high-repetition rate fs-laser systems. Here, we studied the influence of the irradiation conditions on the generation of NV$^-$. Specifically, we varied pulse fluence, laser focusing, and the number of pulses upon irradiation with 150 fs pulses at 775 nm from a Ti:sapphire laser amplifier operating at 1 kHz repetition rate. Optically Detected Magnetic Resonance (ODMR) was used to investigate the produced NV centers, revealing a sizeable zero-field splitting in the spectra and indicating the conditions in which the lattice strain produced in the ablation process may be deleterious for quantum information applications.

Commissioning results from the high-repetition rate nanosecond-kilojoule laser beamline at the extreme light infrastructure

High-power laser–matter interaction and the related high-energy density physics (HEDP) are two important topics in modern physics. However, for decades, a very restrictive limitation prevents researchers from studying these topics efficiently: nanosecond-kilojoule class lasers are typically characterized by a low repetition rate (1 shot h−1) that restricts the data acquisition and limits the available statistics for analysis. Here, we describe the first results obtained using the high-repetition rate nanosecond-kilojoule (L4n laser beamline) experimental platform commissioned at the Extreme Light Infrastructure—Beamlines. We proved the capability to deliver hundreds of joules shots every three min with a very good repeatability. Using high-resolution x-ray spectroscopy, we studied highly resolved spectra of H-like and He-like Cl lines emitted from polyvinyl chloride targets and demonstrated that the plasma parameters (electron temperature and density) derived from the measured spectra are consistent with hydro-simulations. These results demonstrate the fulfillment of the designed platform performance which should become one of the most important assets for the HEDP community in the following decades.

High current short pulse driver using a high power diode laser for optoacoustic biomedical imaging techniques.

Optoacoustic biomedical imaging combines the high spatial resolution of the ultrasound imaging with the specificity of the optical absorption spectroscopy techniques. It is being used in various scenarios such as anatomical, functional and molecular imaging. Typically light sources for this imaging technique is based on solid state lasers since they can produce high energy short optical pulses. However, they are bulky, expensive and the imaging speed is limited because their low pulse repetition rate. High power diode lasers (HPDLs) are a promising alternative for imaging small volume absorbers as they are compact, affordable and allow high repetition rates. However, HPDLs provide relative low peak optical power compared to solid state lasers. Therefore, imaging systems based on diode lasers require much longer pulse duration resulting in lower in-depth resolution and optoacoustic conversion efficiency. HPDLs need dedicated fast electronics to generate short optical pulses. In this work, we have designed, built and test a pulsed diode laser driver based on RF power MOSFETs, specifically considering the optimization of the current pulse in order to maximize the optical peak power, achieving current pulses of more than 900 A with a duration of 50 ns. We have studied the operation of a low cost HPDL out of the manufacturers datasheet ratings without noticeable degradation at high current (> 250 A) and short pulse duration (< 60 ns). We have obtained an optical peak power of 750 W and a energy per pulse of 31.2 µJ at 40 ns optical pulse duration. The optoacoustic images obtained in this operation regime shown a clear enhancement respect to the ones obtained in standard operation of the HPDL.

Spatiotemporal singular value decomposition for denoising in photoacoustic imaging with a low-energy excitation light source.

Photoacoustic (PA) imaging is an emerging hybrid imaging modality that combines rich optical spectroscopic contrast and high ultrasonic resolution, and thus holds tremendous promise for a wide range of pre-clinical and clinical applications. Compact and affordable light sources such as light-emitting diodes (LEDs) and laser diodes (LDs) are promising alternatives to bulky and expensive solid-state laser systems that are commonly used as PA light sources. These could accelerate the clinical translation of PA technology. However, PA signals generated with these light sources are readily degraded by noise due to the low optical fluence, leading to decreased signal-to-noise ratio (SNR) in PA images. In this work, a spatiotemporal singular value decomposition (SVD) based PA denoising method was investigated for these light sources that usually have low fluence and high repetition rates. The proposed method leverages both spatial and temporal correlations between radiofrequency (RF) data frames. Validation was performed on simulations and in vivo PA data acquired from human fingers (2D) and forearm (3D) using a LED-based system. Spatiotemporal SVD greatly enhanced the PA signals of blood vessels corrupted by noise while preserving a high temporal resolution to slow motions, improving the SNR of in vivo PA images by 90.3%, 56.0%, and 187.4% compared to single frame-based wavelet denoising, averaging across 200 frames, and single frame without denoising, respectively. With a fast processing time of SVD (∼50 µs per frame), the proposed method is well suited to PA imaging systems with low-energy excitation light sources for real-time in vivo applications.

High-speed photothermally triggered bidirectional current switching in VO2/AlN/Si-based heterostructure device

The bidirectional current switching (BCS) of a two-terminal device based on a vanadium dioxide (VO2) thin film can be realized by harnessing the photothermally induced phase transition of VO2, triggered by laser pulses. Although this photothermally controlled BCS can achieve a large on-state current and high on/off current ratio compared to those obtained by other implementation approaches, a low repetition rate (< 10 Hz) and slow response time (tens of milliseconds) still restrict its practical use. To tackle these issues, we analyzed the fundamental cause of the performance limit of the BCS through simulations using COMSOL Multiphysics and found that the thermal conductivity of the substrate and the critical temperature of the VO2 film were mainly responsible for the switching speed of the BCS. On the basis of this analysis, we fabricated a VO2-based heterostructure device on a silicon substrate with a buffer layer of aluminum nitride. We measured the transient responses of the device current switched by directly irradiating the device with near-infrared laser pulses under various conditions. The highest-ever repetition rate (100 Hz) could be attained in a 50 mA BCS with 5 ms laser pulses. The response time was also remarkably improved to < 1 ms.

Structure-terahertz property relationship and femtosecond laser irradiation effects in chalcogenide glasses

We report structure-terahertz (THz) property relationship for various non-oxide chalcogenide glasses including unary (vitreous selenium (Se)), binary (arsenic sulfide (As-S), arsenic selenide (As-Se), and germanium selenide (Ge-Se)), and ternary (germanium arsenic selenide (Ge-As-Se)), systems along with commercially available AMTIR-1, IRG 22, and IRG 24 Ge-As-Se glasses. This comprehensive study is the first of its kind to combine Raman spectroscopy to examine structural units, connectivity, and glass network and terahertz time-domain spectroscopy (THz-TDS) to record the THz refractive index, n(THz), across a broad THz bandwidth. THz-TDS was carried out at Alfred University (AU) and National Institute of Standards and Technology (NIST), ultimately providing confidence in n(THz) values measured at AU. Vitreous Se, <r> = 2.0, record the minimum THz refractive index value of all Se-containing glasses. As-S and As-Se binary glasses have the highest measurable THz refractive index value at <r> = 2.4. Ge-Se binary glasses measure increased THz refractive index as <r> increases, with the maximum at <r> = 2.8. Ternary Ge-As-Se glasses record the maximum THz refractive index value at <r> = 2.5 for Ge10As30Se60. Low-repetition rate femtosecond laser irradiation (≈1 KHz, ≈40 fs, and ≈70 mW) was used to modify As-S and As-Se glass systems, where Raman and THz-TDS were used to observe minimal structural and THz refractive index values changes, respectively. Long-wave infrared (LWIR) (e.g., 10 μm)-THz (e.g., 1.0 THz) refractive index correlation is presented for all binary and ternary studied chalcogenide glasses. Such a correlation is valuable for predicting and designing chalcogenide glasses for integrated optical applications across THz and IR regions.

Jitter-calibrated second-order correlation measurement for pulsed excitation with low repetition rate

Second-order correlation [g2(τ)] measurement has been utilized to characterize a nanolaser with high spontaneous emission coupling factor. Pulse excitation with low repetition rate is usually preferred for nanolasers due to its heat accumulation or photobleaching induced problem; however, introducing pulse-to-pulse timing jitter issue hinders the g2(τ) measurement from functioning. Here, a jitter-calibrated method is proposed by adding an extra reference time channel so that the g2(τ) can be calculated accurately. To verify the validity of this calibration method, the lasing threshold of a CdSe nanowire excited by a pulsed laser with a repetition rate of 1 kHz is studied. The descent of g2(0) to 1 coincides with the kink in light-in vs light-out curve, which confirms the effectiveness of this method.

Compact acousto-optical Q-switched Yb:ScBO3 laser with giant pulse energy.

In this Letter, a continuous-wave Yb:ScBO3 laser was enhanced with a maximum output power of 1.63 W and a slope efficiency of 48.97%, pumped by a continuous-wave 965 nm diode laser. Subsequently, the first, to the best of our knowledge, acousto-optically Q-switched Yb:ScBO3 laser was realized with an output wavelength of 1022 nm and repetition rates ranging from 0.04 to 1 kHz. The characteristics of pulsed lasers modulated by a commercial acousto-optic Q switcher were comprehensively demonstrated. With a low repetition rate of 0.05 kHz, the pulsed laser was generated under an absorbed pump power of 2.62 W, having an average output power of 0.44 W and giant pulse energy of 8.80 mJ. The pulse width and peak power were 80.71 ns and 109 kW, respectively. The findings manifest that the Yb:ScBO3 crystal is a gain medium with great potential for Q-switched laser generation with high pulse energy.

Generation and decay of N2(A3Σu +) molecules in reacting CO2 and CH4 plasmas

Time-resolved number densities of N2(A3Σu +, v = 0, 1) molecules in diffuse ns pulse discharge plasmas in N2, N2–H2, N2–CH4, N2–CO2, and N2–CO2–CH4 are measured by tunable diode laser absorption spectroscopy (TDLAS). The first series of measurements is made in the discharge pulse bursts at a relatively low pulse repetition rate (3 kHz), when the N2(A3Σu +) generation and decay after individual discharge pulses is fully resolved. The second set of data is taken during a sequence of two pulse bursts generated at a higher pulse repetition rate (100 kHz), for different delay times between the first and second bursts. This approach is used to determine the effect of accumulation and decay of reacting species generated in the plasma, including N, H, and O atoms, CO molecules, and C2 hydrocarbon product species, on the rate of N2(A3Σu +) production and quenching. The effect of these species can be isolated since the rates of N2(A3Σu +) quenching by the initial reactant species (H2, CH4, and CO2) are slow. Comparison of the measurement results with the kinetic modeling predictions is used to obtain insight into the plasma chemical reaction kinetics. The results complement the measurements of N, H, O, and CO in high-pressure reacting plasmas, and help quantify the plasma chemical processes driven by the electron impact dissociation, electronic excitation, and reactive quenching of the excited electronic states. The present results may be used for the development and validation of higher fidelity kinetic models of reacting plasmas, incorporating state-specific electronic and vibrational energy transfer and chemical reactions.

Flexible control of pulse intensity and repetition rate for multiphoton photostimulation

In deep tissue imaging, pulsed near-infrared lasers commonly provide high peak powers needed for nonlinear absorption, but average power and linear absorption can be limiting factors for tissue damage through heat. We implemented intra-cavity dumping within a mode-locked Ti:Sapphire laser used for two-photon computer generated holography stimulation. This system enables photostimulation in which pulse energy, average power, and repetition rate can each be varied and harnessed as degrees of freedom. We demonstrate how this system provides a new dimension of temporal control in photostimulation experiments to alter the ratio of nonlinear to linear light-tissue interactions, namely by tuning the laser repetition rate between single-shot and ≈ 3 MHz. Repetition rates below 1 MHz, yielding pulse energies over 60 nJ, facilitated holographic projections with more regions of interest than the highest repetition rate of 3 MHz. Even lower repetition rates ( ≈ 10 kHz) diminished thermal load on the sample, as characterized by quantification of heat shock protein expression in zebrafish tissue.

Misconceptions in IONM Part III: Stimulation Repetition Rate Effects on Intraoperative Somatosensory Evoked Potential Amplitude and Latency

ABSTRACT The rate at which stimulation is applied to peripheral nerves is critical to generating high-quality intraoperative somatosensory evoked potentials (SSEPs) in a timely manner. Guidelines based on a limited study and anecdotal evidence present differing, incorrect, or incomplete stimulation rate recommendations. We examined the effect stimulating the ulnar and tibial nerves at 1.05, 2.79, 5.69, and 8.44 Hz had on cortical, subcortical, and peripheral response amplitude and latency in 10 subjects with neuromuscular blockade (NMB) and 10 without NMB in the operating room under general anesthesia. As the stimulation repetition rate increased, the amplitude of upper and lower extremity cortical responses decreased equally in both groups. The ulnar nerve N20 cortical response amplitude decreased 27.9% at 2.79 Hz, 48.8% at 5.69 Hz, and 53.8% at 8.44 Hz. The tibial nerve P37 cortical response amplitude decreased 30.3% at 2.79 Hz, 53.8% at 5.69 Hz, and 56.8% at 8.44 Hz. Neither upper or lower extremity peripheral or subcortical amplitudes nor upper and lower extremity subcortical or peripheral latencies were affected by increasing repetition rate in either group. Low SSEP stimulation repetition rates ensure the highest quality cortical responses.

Generation of high-frequency pulse train by designing a buried SOI waveguide

Silicon photonics has attracted tremendous research interest in recent times because of its compact size, easy-to-build nature, and high values of nonlinearity. Though the study of pulse interaction inside a planer SOI waveguide is done earlier, the confinement of light should be better in the case of a buried core waveguide. Such a buried rectangular SOI waveguide is designed here, which possesses a small value of group velocity dispersion (β2) ∼2.432 (ps2/m) and a quite high value of nonlinear coefficient (γ) of ∼ 441.19 (W.m)−1. When the propagation of a pair of pulses of lower repetition rate (∼GHz) is investigated through it, we have found that the pulse pair interact with each other as they overlap and generate a high-frequency (∼THz) pulse train. The effect of timing jitter on high-frequency generation is also studied. The study of interactions of pulse pairs inside an SOI waveguide helps us to explore the potential applications of such devices as tunable high frequency (THz) pulse generators and many more.

Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL.

The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.

Window filtering algorithm for a low repetition rate pulsed laser coherent combination system.

The multi-dithering method has been well verified in the phase-locking of polarization coherent combination experiments. However, it is difficult to apply to low repetition rate pulsed laser coherent combination, since there exists an overlap in the frequency domain between the pulse laser and the large amplitude-phase noise resulting in traditional filters being unable to effectively separate the phase noise. Aiming to solve the problem, we propose, to the best of our knowledge, a novel method of pulse noise detection, identification, and filtering based on the autocorrelation characteristics between noise signals. The self-designed adaptive window filtering algorithm can effectively filter the pulse signal doped in the phase noise around 0.1 ms. After the pulses are filtered out, the remaining phase noise signal is used as the input signal of the multi-dithering method for phase locking; the phase difference of two pulsed beams (10 kHz) is successfully compensated to zero; and the coherent combination of the closed-loop phase lock is realized. Simultaneously, the phase correction periods are short, the phase lock effect is stable, and the intensity of the final combined pulses rises to the ideal value (0.9Imax). In addition, the adaptive window filtering algorithm we proposed can be applied to the coherent combined system of large array fiber lasers and further lay the foundation for fiber phased array lidar.

Pulse picking of a fiber laser enables velocimetry of biomass-laden jets at low and ultra-high repetition rates

The introduction of fibrous aspherical biomasses for the substitution of coal as a sustainable solid fuel causes complex particle-turbulence interactions, which need to be captured using minimally invasive laser-based diagnostics. High-resolution two-phase particle image velocimetry (PIV) of high-velocity biomass-laden turbulent flows require laser inter-pulse times of only a few microseconds, which is usually achieved with separate laser systems for low-speed statistical and time-resolved measurements. In this work, we introduce a novel approach to achieve low-speed dual-pulse and ultra-high-speed time-resolved two-phase PIV using a single laser system. The flexible repetition rate laser output is realized by coupling a continuously pulsing fiber laser to an acousto-optic deflector (AOD) for pulse picking. The selective deflection of two adjacent laser pulses into the measurement volume is presented at a camera repetition rate of 25 Hz and laser repetition rate of 312.5 kHz resulting in an inter-pulse time of 3.2 µs. Temporal limitations of the AOD pulse picking system are evaluated beyond the access time limit for partially overlapping pulse pairs. To demonstrate the feasibility of the diagnostic system, low-speed measurements and additional ultra-high-speed measurements at 200 kHz are performed at high spatial resolutions to resolve individual biomass particles and their surrounding flow field in a non-reacting turbulent round jet. Simultaneous diffuse back-illumination measurements using a pulsed LED for particle size measurements of a high-aspect-ratio biomass (miscanthus) are conducted. The diagnostic system offers the capability of determining the slip velocity field and projected particle size, enabling a direct measurement of the local particle Reynolds number under high-velocity turbulent conditions at low and ultra-high repetition rates.

PassTCN-PPLL: A Password Guessing Model Based on Probability Label Learning and Temporal Convolutional Neural Network.

The frequent incidents of password leakage have increased people’s attention and research on password security. Password guessing is an essential part of password cracking and password security research. The progression of deep learning technology provides a promising way to improve the efficiency of password guessing. However, the mainstream models proposed for password guessing, such as RNN (or other variants, such as LSTM, GRU), GAN and VAE still face some problems, such as the low efficiency and high repetition rate of the generated passwords. In this paper, we propose a password-guessing model based on the temporal convolutional neural network (PassTCN). To further improve the performance of the generated passwords, we propose a novel password probability label-learning method, which reconstructs labels based on the password probability distribution of the training set and deduplicates the training set when training. Experiments on the RockYou dataset showed that, when generating passwords, the coverage rate of PassTCN with password probability label learning (PassTCN-PPLL) reached 12.6%, which is 87.2%, 72.6% and 42.9% higher than PassGAN (a password-guessing model based on GAN), VAEPass (a password-guessing model based on VAE) and FLA (a password-guessing model based on LSTM), respectively. The repetition rate of our model is 25.9%, which is 45.1%, 31.7% and 17.4% lower than that of PassGAN, VAEPass and FLA, respectively. The results confirm that our approach not only improves the coverage rate but also reduces the repetition rate.

Efficient Gene Transfection by Electroporation—In Vitro and In Silico Study of Pulse Parameters

Gene electrotransfer (GET) is a widely used method for nucleic acids’ delivery into cells. We explored, evaluated, and demonstrated the potential use of different pulse durations for introducing plasmid DNA (pDNA) into cells in vitro and compared the efficiency and dynamics of transgene expression after GET. We performed experiments on cell suspensions of 1306 fibroblasts and C2C12 myoblasts with four ranges of pulse durations (nanosecond, high frequency bipolar (HF-BP), and micro- and millisecond). Six different concentrations of pDNA encoding green fluorescent protein were used. We show that GET can be achieved with nanosecond pulses with a low pulse repetition rate (10 Hz). The GET’s efficiency depends on the pDNA concentration and cell line. Time dynamics of transgene expression are comparable between millisecond, microsecond, HF-BP, and nanosecond pulses but depend greatly on cell line. Lastly, based on the data obtained in the experiments of pDNA concentration effect on GET the model of the probability of pDNA and cell membrane contact during GET was developed. The model shows that pDNA migration is dominated by diffusion for nanosecond and HF-BP pulses and by electrophoresis for micro- and millisecond pulses. Modeling results can provide valuable guidance for further experiments and interpretations of the results obtained by various pulse protocols.

Deep-tissue two-photon microscopy with a frequency-doubled all-fiber mode-locked laser at 937 nm

In two-photon microscopy, low illumination powers on samples and a high signal-to-noise ratio (SNR) of the excitation laser are highly desired for alleviating the problems of photobleaching and phototoxicity, as well as providing clean backgrounds for images. However, the high-repetition-rate Ti:sapphire laser and the low-SNR Raman-shift lasers fall short of meeting these demands, especially when used for deep penetrations. Here, we demonstrate a 937-nm laser frequency-doubled from an all-fiber mode-locked laser at 1.8 μm with a low repetition rate of ∼9 MHz and a high SNR of 74 dB. We showcase two-photon excitations with low illumination powers on multiple types of biological tissues, including fluorescence imaging of mouse brain neurons labeled with green and yellow fluorescence proteins (GFP and YFP), DiI-stained and GFP-labeled blood vessels, Alexa Fluor 488/568-stained mouse kidney, and second-harmonic-generation imaging of the mouse skull, leg, and tail. We achieve a penetration depth in mouse brain tissues up to 620 μm with an illumination power as low as ∼10 mW, and, even for the DiI dye with an extremely low excitation efficiency of 3.3%, the penetration depth is still up to 530 μm, indicating that the low-repetition-rate source works efficiently for a wide range of dyes with a fixed excitation wavelength. The low-repetition-rate and high-SNR excitation source holds great potential for biological investigations, such as in vivo deep-tissue imaging.

Hydrogen targetry in laser-plasma physics

The development of various types of cryogenic target systems opens new opportunities for laser-matter interaction experiments. Existing systems of solid hydrogen targets, which are used with high-power laser systems for various experiments, including laser acceleration of protons, are considered. The details of target formation techniques are discussed. We also discussed some most challenging issues in target fabrication at low temperature and laser operation high repetition rate.