Frequency Research Articles

Ultrahigh energy efficiency from a supersonic underwater ultrasound source

The present work is dedicated to an experimental and theoretical study of an innovative underwater ultrasound source. The source works using a technique in which a pulsed power generator using the impedance mismatch of a long high-voltage coaxial cable generates a train of voltage impulses with a very high pulse repetition frequency of the order of a few MHz. Applying this train of voltage impulses to a pair of underwater electrodes generates a streamer-initiated breakdown of water and, subsequently, a plasma column connecting the electrodes over a very large inter-electrode gap of 55 mm. The interaction of the long plasma column thus formed with the surrounding water produces a rapidly expanding vapor bubble, an “instrument” producing a strong pressure wave with an overall energy efficiency of 24%, an order of magnitude higher than most underwater pressure sources reported in the literature.

Phenological mismatch between trees and wildflowers: Reconciling divergent findings in two recent analyses

Abstract Recent evidence suggests that community science and herbarium datasets yield similar estimates of species' phenological sensitivities to temperature. Despite this, two recent studies by Alecrim et al. (2023) and Miller et al. (2022) found very different results when using different data sources (community science and herbarium specimens, respectively) to investigate whether warming threatens wildflowers with phenological mismatch in relation to shading by deciduous trees. Here, we investigated whether differences between the two studies' results could be reconciled by testing four hypotheses related to model design, species, spatiotemporal data extent and phenophase. Hybrid model structures brought results from the two datasets closer together but did not fully reconcile the differences between the studies. Neither the species nor the phenophase selected for analysis seemed to be responsible for differences in results. Cropping the datasets to match spatial and temporal extents appeared to reconcile most differences but only at the cost of much higher uncertainty associated with reduced sample size. Synthesis: Our analysis suggests that although species‐level estimates of phenological sensitivity may be similar between community science and herbarium datasets, inherent differences in the types and extent of data may lead to contradictory inference about complex biotic interactions. We conclude that, until community science data repositories expand to match the range of climate conditions present in herbarium collections or until herbarium collections match the spatial extent and temporal frequency of community science repositories, ecological studies should ideally be evaluated using both datasets to test the possibility of biased results from either.

Fast and flexible solid-state linear transformer driver for plasma discharge based on metal oxide semiconductor field effect transistor.

A pulsed power supply with a short rise time and high repetition frequency is favorable to driving diffusive plasma for strongly oxidizing radical (O3, OH) generation and increasing the system's energy efficiency. In this paper, a 10-stage solid-state linear transformer driver (LTD) with a nanosecond rise time is developed to drive plasma for wastewater treatment. To decrease the rise time, a control system with low jitter is developed to improve the synchronization of pulses using an optocoupler isolation chip. A 10-stage LTD with a rise time of 6.2ns is realized in the case that the rise time of the single-stage LTD is 5.4ns. The results show that the LTD can generate pulses on a 300Ω resistive load with a repetition frequency of 10kHz, an amplitude of 8.80kV, an overshoot less than 3.97%, and a reverse overshoot less than 4.82%. The rise time (6.2-33.0ns), the pulse width (35.9-200.0ns), and the fall time (10.5-27.6ns) can be adjusted flexibly and independently by controlling the drive signals of metal oxide semiconductor field effect transistors. The pulsed generator is utilized to drive plasma in the needle-water electrode system. The preliminary experimental results show that the plasma includes abundant oxygen atoms and hydroxyl radicals with high activity, and it is suitable for wastewater treatment.

Intercity Traffic Travel Mode Identification Method Based on Mobile Signalling Data

With the popularity of mobile devices, the signalling data generated by them provides significant opportunities for studying intercity travel behaviour in terms of data scale and information continuity. However, due to the low quality of the data in spatial accuracy, temporal frequency, and traffic semantics, the accuracy of identifying individual travel modes is low and it is difficult to extend to complex traffic scenarios. In this paper, we propose a new framework for identifying individual intercity travel modes based on mobile signalling data. The framework includes components for data pre-processing, geo-information mapping, feature and attribute extraction, and travel mode recognition. We utilize a comprehensive detection model to identify users’ multimodal intercity transport behaviour. Using two modules, Random Forest Embedding (RFE) and Bidirectional Long Short-Term Memory (Bi-LSTM), the model can capture the spatiotemporal characteristics and complex multi-stage associations in intercity travel chains. A large-scale mobile phone dataset from Jiangsu Province, China, was used for verification. The results showed that, on average, the method was able to detect travel mode with 92% accuracy. This study provides valuable support for further research on individual travel behaviour and the enhancement of transportation planning.

The Impact of Spectral and Temporal Degradation on Vocoded Speech Recognition in Early-Blind Individuals.

This study compared the impact of spectral and temporal degradation on vocoded speech recognition between early-blind and sighted subjects. The participants included 25 early-blind subjects (30.32 ± 4.88 years; male:female, 14:11) and 25 age- and sex-matched sighted subjects. Tests included monosyllable recognition in noise at various signal-to-noise ratios (-18 to -4 dB), matrix sentence-in-noise recognition, and vocoded speech recognition with different numbers of channels (4, 8, 16, and 32) and temporal envelope cutoff frequencies (50 vs 500 Hz). Cortical-evoked potentials (N2 and P3b) were measured in response to spectrally and temporally degraded stimuli. The early-blind subjects displayed superior monosyllable and sentence recognition than sighted subjects (all p < 0.01). In the vocoded speech recognition test, a three-way repeated-measure analysis of variance (two groups × four channels × two cutoff frequencies) revealed significant main effects of group, channel, and cutoff frequency (all p < 0.001). Early-blind subjects showed increased sensitivity to spectral degradation for speech recognition, evident in the significant interaction between group and channel (p = 0.007). N2 responses in early-blind subjects exhibited shorter latency and greater amplitude in the 8-channel (p = 0.022 and 0.034, respectively) and shorter latency in the 16-channel (p = 0.049) compared with sighted subjects. In conclusion, early-blind subjects demonstrated speech recognition advantages over sighted subjects, even in the presence of spectral and temporal degradation. Spectral degradation had a greater impact on speech recognition in early-blind subjects, while the effect of temporal degradation was similar in both groups.

Pilot Experiment on Non-Invasive Non-Thermal Disintegration of Human Mucinous Breast Carcinoma Ex Vivo Using Boiling Histotripsy.

We present the results of a pilot study demonstrating the feasibility of non-invasive non-thermal disintegration of human mucinous carcinoma of the breast ex vivo using sequences of high-intensity focused ultrasound pulses in boiling histotripsy regimen. The target volume was sonicated by focusing ultrasound pulses (n=20) of 1.5 MHz frequency, 10-msec duration and 1-sec pulse repetition period, 517 W acoustic power within the pulse, and 103 MPa shock front amplitude at the focus into each node of a volumetric grid 4×4×1 mm. Sonication was visualized and controlled using B-mode ultrasound imaging, total time of the treatment was 21 min. Histological hematoxylin and eosin and Masson's trichrome staining revealed the absence of tumor elements in the treated region confirming destruction of cancer cells and their nuclei after boiling histotripsy procedure.

Studies on selective laser quenching by high repetition frequency lasers with scanning galvanometer

This study introduces a novel material processing methodology termed High Repetition Frequency Selective Laser Quenching (HRF-SLQ). This technique utilizes high-power lasers with a maximum power of 5 kW in conjunction with a scanning galvanometer, operating at elevated repetition rates up to 200 Hz, to specifically treat surfaces of high and medium carbon steels. The application of this method leads to the creation of allotropic metal matrix composite materials (A-MMCs). In general, the scanning speed of a galvanometer can range from hundreds to thousands of times per second. Based on the high acceleration and rapid jumping characteristics of the scanning galvanometer, this technology transforms the high-power laser heating of metal plates from continuous heating mode to parallel pulse heating mode, which repeats heating 30 to 200 times per second. This transformation ensures the quenched layer avoids melting even under prolonged irradiation times by high-power lasers. Consequently, it can significantly enhance processing efficiency by selective laser quenching (SLQ), ensuring that the depth of the hardened layer reaches 0.88–0.94 mm and keeping the surface smooth without melting. This paper systematically investigates the influence of various processing parameters, such as laser power (P), single heating time (t1), and the number of processing units (N), on the depth of the SLQ and the quenching efficiency. It also examines the impact of HRF-SLQ technology on the properties of A-MMCs and analyzes temperature field variations from room temperature to melting point through simulation methods combined with laser quenching experiments.

Towards an understanding of the mechanisms of therapeutic ultrasound on biomimetic models of cancer

Therapeutic ultrasound is transforming the treatment of a range of malignancies in a non-invasive and non-systemic manner. Low-intensity ultrasound (LIUS) has been proposed to selectively eradicate cancer cells but the underlying biological mechanisms remains unknown. To gain an understanding of this phenomenon, 2D breast cancer monocultures were sonicated at varying acoustic intensities (0.1–0.5 W·cm−2) and excitation times (1–10 minutes). Additionally, 2D monocultures consisting of healthy cell lines were sonicated at varying acoustic intensities (0.1–0.5 W·cm−2) to determine any distinguishing biological responses. To begin recapitulating in vivo conditions, breast cancer cells were also seeded into 3D collagen hydrogels. At a 1 MHz frequency, 20% duty cycle, 100 Hz pulse repetition frequency, a significant drop in cancer cell viability is observed at a sonication intensity of 0.5 W·cm−2 and over 10 minute excitation time. Healthy counterparts subjected to the same parameters revealed no distinguishing effects. Sonication of breast cancer cells seeded in 3D collagen hydrogels revealed no effect in cell viability compared to non-sonicated controls. The acoustic wave propagation software OptimUS was used to determine the influence culturing plates have on ultrasound propagation, revealing these materials can significantly vary the acoustic field at frequencies relevant to LIUS.

Pulse-stretching out of the CANREB EBIS

The CANadian Rare isotope facility with Electron-Beam ion source (CANREB) at TRIUMF is set to deliver rare isotope beams in high charge states. In the Electron Beam Ion Source (EBIS) ions are charge-bred by collisions with an electron beam of up to 500 mA. A strong magnetic field (up to 6T) maximizes the overlap between ions and electron beam and increases the breeding efficiency. Ion confinement is maintained by a combination of an electrostatic field and the electron beam space-charge potential. Ions are released by lowering the trapping potential with a step function. The system is operated at a pulse repetition frequency up to 100 Hz. Due to the short trap length, this fast extraction scheme produces pulses shorter than 10 µs with high instantaneous rates that can saturate detectors in experiments. Stretching the pulse can be done using a slowly varying voltage function to modify trap electrode potentials instead of a step function. The ideal function produces a pulse with a flat top distribution and can be calculated by knowing the ion energy distribution inside the trap. The latest pulse-stretching results will be discussed including the latest pulse duration up to 1.4 ms that have been produced. The slow extraction scheme has also been used for a measurement of the effective energy distribution of the ions inside the trap.

Miniaturization design of laser detection device for naval gun composite fuse

Naval gun weapons are the only weapon system on board that can undertake various combat tasks such as shore fire support, sea attack, air defense and missile defense, and rapid response to emergencies. A new type of naval gun laser-radio composite fuse(naval gun composite fuse) is proposed to address the issue of naval gun fuses being susceptible to interference and failure in a single radio proximity system. This improves the multiple operational effectiveness of naval guns and the adaptability of naval gun ammunition to electromagnetic and other interference environments. In response to the limitation of small size of naval gun fuses, the overall structure and miniaturized laser detection device were designed separately. Firstly, a miniaturized 4-channel narrow pulse high repetition frequency laser detection method with uniform forward circumferential distribution is adopted to achieve compatibility with the original naval gun fuse structure, the simulation results indicate that the structural deformation and strength meet the requirements. Secondly, by designing a laser emission collimating lens that is glued together with a small spherical lens and a small concave cylindrical mirror, the outer envelope size is only Φ5x3mm. The simulation results show that at a distance of 20mm from the laser emission window, the spot diameter is 0.66mm and the divergence angle is 1.9 °, achieving good laser collimation and miniaturization of the emission optical system. Finally, the receiving optical system reduces its volume by adding a receiving objective and a field mirror. The effective aperture of the receiving objective is 10mm, the diameter of the field mirror is 5mm, the focal length is 11.1mm, and the receiving field angle is 5 °. The simulation results show that the laser spot radius on the photosensitive surface of the photodetector is 0.06mm, which meets the requirements of photodetectors.

Plasma propagation velocity dependence on driving and restricting forces

According to the time-averaged expression for an alternating electric field, the normalized electromagnetic pressure is proportional to the square of the voltage intensity and inversely proportional to the square of the voltage repetition frequency. Moreover, the plasma propagation velocity is either directly proportional, inversely proportional, or nonproportional to the normalized electromagnetic pressure at all neutral gas flow rates. Because the plasma current is only directly proportional to the normalized electromagnetic pressure at all neutral gas flow rates, the dependence of the plasma density on the electromagnetic pressure changes to obtain a balance of dependence. In the momentum transfer equation, plasma density does not originally depend on electromagnetic pressure, but the dynamic pressure associated with the neutral gas flow also exerts a force on the plasma through collisions. Therefore, when the ionization generation of plasma by collisions between the plasma and neutral particles is dominant over recombination by collisions, the plasma density is square proportional or directly proportional to the electromagnetic pressure.

Modulation of the neuronal response in human primary visual cortex by re-entrant projections during retinal input processing as manifest in the visual evoked potential

Initial deflections in the visual evoked potential (VEP) reflect the neuronal process of extracting features from the retinal input; a process not modulated by re-entrant projections. Later deflections in the VEP reflect the neuronal process of combining features into an object, a process referred to as ‘object closure’ and modulated by re-entrant projections. Our earlier work indicated that the VEP reflects independent neuronal responses processing temporal – and spatial luminance contrast and that these responses arise from an interaction between forward and re-entrant input. In this earlier work, changing the temporal luminance contrast property of a stimulus altered its spatial luminance contrast property. We recorded the VEP in 12 volunteers viewing image pairs of a windmill, regular dartboard or an RMS dartboard rotated by either Π/4, Π/2, 3Π/4 or Π radians with respect to each other. The windmill and regular dartboard had identical white to black ratio, while the two dartboards identical contrast edges per unit area. Rotation varied temporal luminance contrast of a stimulus without affecting its spatial luminance contrast. N75, P100, N135 and P240 amplitude and latency were compared and a source localisation and temporal frequency analysis performed. P100 amplitude signals a neuronal response processing temporal luminance contrast that is modulated by re-entrant projections with fast axonal conduction velocities. N135 and P240 signal the neuronal response processing spatial luminance contrast and is modulated by re-entrant projections with slow axonal conduction velocities. The dorsal stream is interconnected by fast axonal conduction velocities, the ventral stream by slow axonal conduction velocities.

Algorithm for the Weak Target Joint Detection and Ambiguity Resolution Based on Ambiguity Matrix

The looking-down mode of space airship bistatic radars faces complex sea–land clutter, and the mode of wide-range surveillance and the over-sight detection of the satellite platform generates a low SNR and range–Doppler ambiguity. The method traditionally used involves the transmission of multiple Pulse Repetition Frequencies (PRFs) and correlating them to solve the ambiguity. However, with a low SNR, the traditional disambiguation fails due to the large number of false alarms and target omissions. In order to solve this problem, a new algorithm for multi-target joint detection and range–Doppler disambiguation based on an ambiguity matrix is presented. Firstly, all possible state values corresponding to the ambiguous sequence are filled into the ambiguity matrix one by one. Secondly, the state values in the matrix cell are divided into several groups of subsequences according to the PRF. By disambiguating multiple sets of subsequences, performing subsequence fusion, and then undertaking point aggregation, the targets can be effectively detected in scenarios with a strong clutter rate, the false alarms can be suppressed, and the disambiguation of the range and Doppler is completed. The simulation shows that the proposed algorithm has the strong ability to detect targets and perform ambiguity resolution in the scenario of a multi-target and multi-false alarm.

Optimization and Application of English Word Memory Algorithm Based on Reinforcement Learning

The optimization and application of an English word memory algorithm based on reinforcement learning involve refining a system that utilizes reinforcement learning techniques to enhance vocabulary retention. By leveraging reinforcement learning, the algorithm can adapt its strategies for presenting and reviewing English words based on users' responses and performance. This adaptive approach enables personalized and effective learning experiences, wherein the algorithm adjusts the frequency and timing of word repetitions to maximize memorization efficiency. Applications of this optimized algorithm span language learning platforms, educational apps, and personalized tutoring systems, offering learners tailored support to strengthen their English vocabulary acquisition and retention skills. This paper introduces the Spider Swarm Optimized English Word Memory Algorithm (SSOEWMA) algorithm, a novel approach aimed at optimizing language processing and learning tasks. Leveraging principles of reinforcement learning and swarm intelligence, SSOEWMA demonstrates its effectiveness in enhancing various aspects of language processing, including word difficulty assessment, sentence estimation, memory retention, and student performance evaluation. Through a series of experiments and evaluations, we showcase SSOEWMA ability to optimize dataset attributes, accurately estimate sentence characteristics, and improve memory retention performance. Simulation results achieved an average word recall accuracy of 85% across multiple evaluations. Through its utilization of reinforcement learning principles and swarm intelligence, SSOEWMA significantly improved memory retention performance, reducing recall latency by an average of 20% compared to baseline measures. Additionally, the algorithm demonstrated strong learning efficiency, converging to optimal solutions with a speed increase of 30% compared to traditional methods.

Influence of working parameters on multi-shot femtosecond laser surface ablation of lithium niobate

This paper presents an experimental investigation on the ultrashort pulsed laser ablation of 128° Y-cut Lithium Niobate (LiNbO3) using multi-shot fs-laser pulses at different pulse repetition frequencies (1, 10 and 100kHz). The ablation threshold fluence was observed to rapidly decrease as the number N of incident laser pulses increased, regardless of the repetition frequency. This behavior, compatible with the incubation effect, was accurately modeled by a power law. The calculated single pulse ablation threshold Fth,1=1.98±0.15J/cm2 is consistent with values reported in existing literature. The incubation coefficient S∗ appears to be independent of the repetition frequencies. In contrast, the asymptotic ablation threshold Fth,∞ decreased as the repetition frequency was increased. The study delves deeper into the impact of laser operational variables, including pulse energy, repetition frequency, total pulse count, and scanning speed, on the surface roughness and milled depth of fs-laser micromilled zones on LiNbO3 substrates. A discernible trade-off between achieving control over the obtained depth and the surface finish of the process was identified, providing valuable insights for achieving precise control over fs-laser processing of LiNbO3 surfaces.

Research on laser repair technology for surface defects of flywheel disc steel plate

Flywheel disc steel plate, used in the rear end of the engine disc-shaped parts, is one of the important components that make up the engine. However, during the manufacturing process, its surface is prone to defects such as scratches and dents. The defects position of the steel plate will produce the phenomenon of stress concentration, which directly leads to its scrapping. To solve this problem, a laser repair method is proposed, which uses laser with high energy density to repair surface defects. Firstly, the repairable defect depth under different laser parameters is predicted by finite element simulation. The temperature of the steel plates increases with the increase of the laser power density and decreases with the increase of the scanning speed. When the laser power density is 5.57×1011 W m−2 and the scanning speed is 120 mm s−1, surface defects of various depths can be repaired, and the size of the melt pool is moderate. Then, process experiments were conducted, and the results showed that good repair effects can be achieved when the laser power is 14 W, the repetition frequency is 130 kHz, the scanning speed is 120 mm s−1, and the scanning spacing is 0.02 mm. Finally, the surface with original surface roughness of 2.370 μm can be repaired by laser to obtain a flawless smooth surface with surface roughness of 0.731 μm, which provides an effective method for enterprises to repair flywheel disc steel plates.

Integer-locking condition for stable dual-comb interferometry in situations with fluctuating frequency-comb repetition rates

To make dual-comb interferometry usable in a wide range of applications, it is important to achieve reproducible measurement results even in non-ideal environments that affect the repetition-rate stability. Here, we consider dual-comb interferometry based on a pair of fully referenced optical frequency combs (OFCs) and investigate the impact of fluctuations in the OFC repetition frequencies on the peak position of the center burst in the interferogram. We identify a phase-locking scheme that minimizes the impact of these fluctuations through choosing a special combination of phase-locked frequencies, and the resulting type of operating condition is termed integer-locking condition. Under the integer-locking condition, the number of sampling points in each interferogram remains constant regardless of repetition-rate variations, and this enables more stable phase-resolved measurements in non-ideal environments. We demonstrate the application of this approach using absolute path-length measurements and discuss the accuracy limit imposed by the integer-locking condition. Our findings offer a strategy for robust dual-comb interferometry outside metrology laboratories.

MULTISOURCE AND MULTITEMPORAL LAND COVER MAPPING OF GREATER LUZON ISLAND USING GOOGLE EARTH ENGINE

Abstract. A variety of research endeavors and practical applications necessitate the use of land cover maps. These maps are valuable for tasks such as change detection, forest monitoring, urban expansion monitoring, natural resource mapping, catering to diverse user requirements. While satellite sensors offer essential data for comprehending spatial and temporal variations in land cover, relying on a single satellite system can be limiting, especially considering the potential hindrance of cloud cover in the case of optical sensors. To enhance temporal frequency, it becomes essential to utilize multiple satellite systems, albeit requiring harmonization to ensure consistent outcomes. This study presents a large-scale annual land cover mapping which utilizes harmonized Landsat-8 and Sentinel-2 satellite imagery, in conjunction with supplementary data, and a machine learning algorithm. In addition, the use of powerful computational processing platforms such as Google Earth Engine and Google Colaboratory is now a requirement to manage big geospatial data as well as to run different algorithms for processing and analysis.

Longitudinal Evaluation of Congenital Cardiovascular Surgical Performance and Skills Retention Using Silicone-Molded Heart Models.

Objective: Hands-on surgical training (HOST) for congenital heart surgery (CHS), utilizing silicone-molded models created from 3D-printing of patients' imaging data, was shown to improve surgical skills. However, the impact of repetition and frequency of repetition in retaining skills has not been previously investigated. We aimed to longitudinally evaluate the outcome for HOST on two example procedures of different technical difficulties with repeated attempts over a 15-week period. Methods: Five CHS trainees were prospectively recruited. Repair of coarctation of the aorta (CoA) and arterial switch operation (ASO) were selected as example procedures of relatively low and high technical difficulty. Procedural time and technical performance (using procedure-specific assessment tools by the participant, a peer-reviewer, and the proctor) were measured. Results: Coarctation repair performance scores improved after the first repetition but remained unchanged at the follow-up session. Likewise, CoA procedural time showed an early reduction but then remained stable (mean [standard deviation]: 29[14] vs 25[15] vs 23[9] min at 0, 1, and 4 weeks). Conversely, ASO performance scores improved during the first repetitions, but decreased after a longer time delay (>9 weeks). Arterial switch operation procedural time showed modest improvements across simulations but significantly reduced from the first to the last attempt: 119[20] versus 106[28] min at 0 and 15 weeks, P = .049. Conclusions: Complex procedures require multiple HOST repetitions, without excessive time delay to maintain long-term skills improvement. Conversely, a single session may be planned for simple procedures to achieve satisfactory medium-term results. Importantly, a consistent reduction in procedural times was recorded, supporting increased surgical efficiency.

171st-order harmonic soliton erbium-doped fiber laser based on 2H-MoTe2 saturable absorber

We have demonstrated a high order harmonic mode-locked erbium-doped fiber laser by utilizing 2H-MoTe2 film as a saturable absorber (SA). The SA is synthesized by depositing 2H-MoTe2 film on a D-shaped fiber. The nonlinear optical properties of the 2H-MoTe2 SA are characterized, showing a modulation depth of 14.7% and a saturation intensity of 3 GW cm−2. Through optimizing the pump power and intra-cavity polarization state, a 171st-order harmonic soliton at 0.9 GHz repetition frequency with a pulse width of 1.06 ps was obtained. The fundamentally mode-locked pulse train had a pulse width of 1.03 ps and a repetition rate of 5.33 MHz. To the best of our knowledge, the 171st-order harmonic soliton is the highest operating repetition frequency that has ever been demonstrated in a 2H-MoTe2 SA mode locked fiber laser. The results indicate the potential of 2H-MoTe2 SA at high repetition rates regimes, which could benefit for future possible application.