High Repetition Frequency Research Articles

ICESat-2 single photon laser point cloud denoising algorithm based on improved DBSCAN clustering

AbstractThe Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) has great potential for development due to its advantages of the use of multiple beams, low energy consumption, high repetition frequency, and high measurement sensitivity. However, the weak photon signal emitted by the photon counting lidar is susceptible to the background noise caused by the sun and the atmosphere, which can seriously affect the processing and application of laser data. This paper proposes an improved DBSCAN clustering algorithm for denoising single photon laser point clouds in mountainous areas. Firstly, a grouping method based on elevation and distance statistics is proposed to reduce the influence of terrain undulations on denoising accuracy. Finally, an automatic radius search method is put forward to determine clustering radius of each group, automatically find the optimal radius, and improve the existing DBSCAN clustering method. The method proposed in this paper is compared with the classical DBSCAN algorithm. The results show that the proposed algorithm significantly improves denoising accuracy in mountainous areas and effectively filters out most background noise. Graphical Abstract

Ultrafast 550-W average-power thin-disk laser oscillator

SESAM modelocked oscillators are interesting for applications in strong-field physics such as high-harmonic generation and attosecond science at high repetition rates or frequency combs in the ultraviolet. Here we present a SESAM modelocked ultrafast thin-disk laser oscillator providing 550W of average output power with 852fs pulses at 5.5MHz repetition rate. To reach this significant power scaling, a replicating cavity design for modelocked oscillators is utilized. The oscillator delivers 103 MW of peak power with a pulse energy of 100 µJ at a beam quality of M2<1.2, with a high optical-to-optical efficiency of 35%. The advances in SESAM design and manufacturing that enabled this result are discussed, as well as practical challenges when scaling oscillators to the kW-class. When combined with established pulse compression technologies, this oscillator can enable simpler systems by avoiding the complexity of chirped pulse amplifier chains. Additionally, high power oscillators support a much lower noise floor due to the reduced influence of shot noise, which may provide a route to more sensitive pump-probe measurements.

Influence factors and improvement scheme on the breakdown behavior of pseudospark switch

Abstract High-power pulse generators are widely used in civil and military fields. The main switch directly determines the output characteristics of the high-power pulse generators, such as the voltage front time (tf). Pseudospark switches (PSS) show a promising future for middle voltage, high repetitive frequency pulse power applications. However, how to further improve the breakdown behavior without reducing its advantages is a challenging task. In this paper, the influence of operating parameters (anode voltage UA and gas pressure p) and structural parameter (number of cathode holes) on the breakdown behavior are investigated, the related mechanism are explained, and specific improvement schemes are proposed. It is found that the tf of the single channel PSS (SCPSS) decreased significantly with increasing p, but hardly varied with UA under moderate p. However, it is not a sound solution to increase the p excessively to reduce tf. Besides, increasing the number of cathode holes can obtain a shorter tf at low pressures (which implies superior repetition frequency performance). However, at 25 Pa, the jitter (which is defined as the standard deviation of tf in multiple tests) of the 2-channel PSS is larger than that of the SCPSS. And the jitter of the 4-channel and 8-channel PSS is also greater than 6 ns and 2 ns, respectively. Through experimental and simulation analyses, it can be explained as the stepwise penetration of the virtual anode and the non-simultaneous ignition of the channels. A scheme to increase the trigger energy (ε) has been adopted to improve the simultaneous ignition probability, while shortening tf and reducing jitter. After optimization, the good ignition probability of the 4-channel PSS has been improved to 82% and the jitter has been reduced to less than 1 ns at 25 Pa and 14.7 mJ.

Multiwavelength Photoacoustic Doppler Flowmetry of Living Microalgae Cells.

Photoacoustics can provide a direct measurement of light absorption by microalgae depending on the photosynthesis pigment within them. In this study, we have performed photoacoustic flowmetry on living microalgae cells to measure their flow characteristics, which include flow speed, flow angle, flow direction, and, more importantly, the photoacoustic absorption spectrum, all by observing the photoacoustic Doppler power spectra during their flowing state. A supercontinuum pulsed laser with a high repetition frequency is used as the light source: through intensity modulation at a specified frequency, it can provide wavelength-selectable excitation of a photoacoustic signal centered around this frequency. Our approach can be useful to simultaneously measure the flow characteristics of microalgae and easily discriminate their different species with high accuracy in both static and dynamic states, thus facilitating the study of their cultivation and their role in our ecosystem.

Research on Polarization Modulation of Electro-Optical Crystals for 3D Imaging Reconstruction.

A method for enhancing the resolution of 3D imaging reconstruction by employing the polarization modulation of electro-optical crystals is proposed. This technique utilizes two polarizers oriented perpendicular to each other along with an electro-optical modulation crystal to achieve high repetition frequency and narrow pulse width gating. By varying the modulation time series of the electro-optical crystal, three-dimensional gray images of the laser at different distances are acquired, and the three-dimensional information of the target is reconstructed using the range energy recovery algorithm. This 3D imaging system can be implemented with large area detectors, independent of the an Intensified Charge-Coupled Device (ICCD) manufacturing process, resulting in improved lateral resolution. Experimental results demonstrate that when imaging a target at the distance of 20 m, the lateral resolution within the region of interest is 2560 × 2160, with a root mean square error of 3.2 cm.

Design and optimization of handheld alloy analysis instrument based on microjoule high pulse repetition frequency LIBS.

We briefly describe the design of a handheld metal detection instrument based on microjoule high repetition frequency laser-induced breakdown spectroscopy. The instrument uses a Raspberry Pi as the control core and a laser with a frequency of 10kHz and a single pulse energy of 100 µJ as the excitation source. In addition, a mini-putter is built into the instrument to move the laser, allowing the ablation of the sample surface line area without external auxiliary equipment. The excitation-generated plasma radiation is collected by a simple optical path and transmitted directly to the spectrometer. We also constructed and trained a Backpropagation Artificial Neural Network (BP-ANN) model based on 12 different grades of alloys and transplanted the feedback process of the BP-ANN to the Raspberry Pi, which realized the rapid classification of the 12 alloys with >95% classification accuracy on the handheld instrument.

Fast Space Charge Measurement of Power Module Packaging Insulation Under Square Wave Voltage Based on High Repetition Frequency Pulsed Electroacoustic Method

Fast Space Charge Measurement of Power Module Packaging Insulation Under Square Wave Voltage Based on High Repetition Frequency Pulsed Electroacoustic Method

Control of Sitophilus oryzae (L.) using argon and helium atmospheric non-thermal plasma

The rice weevil, Sitophilus oryzae Linnaeus, 1763 (Coleoptera: Curculionidae), is a major pest of stored cereals worldwide that causes significant grain loss by feeding internally on seeds. In this study, we propose the use of an atmospheric pressure cold plasma generated from dielectric barrier discharge (DBD) as an emerging technology in insect pest management. Series of experiments were conducted to test the effectiveness of such plasmas on the lethality of adult S. oryzae by measuring the mortality rate and examining the biochemical markers that impact the insects. Plasma treatments (24 kV input voltage at frequency of 2.5 or 7.5 kHz) were carried out in He and Ar, used as carrier gases, with exposure time ranging from 30 to 120 s. Results showed a high level of mortality (60–100%) for each treatment, with a better efficiency observed when Ar was used as carrier gas. Furthermore, at the highest repetition frequency (7.5 kHz), equivalent to 60 W of electrical power consumption, the insecticidal efficiency reached 100% insect mortality at only 1 h after a 30 s plasma treatment. The mortality rate appeared to be correlated with a notable decrease in protein content and an increase in catalase enzyme levels within the insects' tissue, indicating that insecticidal mechanisms involve oxidative stress. This study demonstrates that low-energy atmospheric cold plasma has detrimental effects on rice weevils and could be considered a promising tool for pest control in stored food, which will help reduce global dependence on chemical pesticides.

Enhanced surface performance of insulating ceramic by plasma polymerization with nanosecond-pulse dielectric barrier discharge: Insight into the effect of the repetition frequency

Insulating ceramics are widely used in power systems, but their high polarity makes them prone to wet and pollution flashover. In this paper, an Ar/polydimethylsiloxane dielectric barrier discharge excited by a parametric nanosecond-pulse power source is utilized for plasma polymerization to enhance ceramic surface insulating performance. Diagnosis of the discharge characteristics and surface physiochemical properties is conducted at different repetition frequencies to investigate the mechanism associated with the relationship between repetition frequency and the plasma polymerization process. The results indicate that a superhydrophobic surface can be achieved at repetition frequencies above 2 kHz. This transformation brings about multiple effects, including a decrease in surface polarity, an increase in charge dissipation, and an improvement in surface dry and wet flashover voltages. It is discovered that the discharge mode shifts from homogeneous to filamentary due to the memory effect of high repetition frequencies. Additionally, several filaments are generated simultaneously during a single pulse, facilitating the polymerization reactions. For high repetition frequencies, a low-polarity silicon-containing film with micro-nanometer structures is deposited on the ceramic surface, while scattered polymer fragments are generated on the surface without a cross-linked film for repetition frequencies below 2 kHz, despite longer treatment durations.

All-PM Yb-doped mode-locked fiber laser with high single pulse energy and high repetition frequency

We demonstrate an all-polarization-maintaining (PM) ytterbium (Yb)-doped fiber laser with a figure-of-9 structure to generate mode-locked pulses with high single pulse energy and high repetition frequency. By exploiting the nonlinear amplifying loop mirror, a stably self-started mode-locking is achieved with a spectrum bandwidth of 13 nm and a pulse duration of 4.53 ps. The fundamental frequency is 97.966 MHz at the maximum output power of 143 mW in single pulse mode-locked operation, corresponding to the single pulse energy is 1.46 nJ. The output pulses maintain both high repetition frequency and high single-pulse energy. This laser oscillator can be an ideal seed source for applications such as high-energy amplifiers.

A solid-state pulse power sub-nanosecond SiC DSRD-based generator with high-voltage and high repetition frequency for pulse discharge water treatment

Water treatment is one of the most important issues for all walks of life around the world. The unique advantages of the solid-state power electronic pulses in water treatment make it attractive and promising in practical applications. The output voltage, rising time, repetition rate, and peak power of output pulses have a significant impact on the effectiveness of water treatment. Especially in pulse electric field treatment and pulse discharge treatment, the pulse with fast rising time achieves the advantage of generating plasma without corona, which can avoid water heating effect and greatly improve the efficiency of the pulse generator. High repetition rate can significantly reduce the peak power requirement of the pulse in water treatment application, making the equipment smaller and improving the power density. Therefore, the study developed a high-voltage high frequency sub-nanosecond pulse power generator (PPG) system for wastewater treatment. It adopts SiC DSRD (Drift Step Recovery Diode) solid-state switches and realize modular design, which can achieve high performance and can be flexible expanded according to the requirements of water treatment capacity. Finally, an expandable high-voltage PPG for water treatment is built. The output parameters of the PPG include output pulse voltage range from 1 to 5.28 kV, rise time <600 ps (20%–90%), repetition up to 1 MHz. The experiment results of PPG application for pulse discharge water treatment is presented. The results indicate that the proposed generator achieves high-efficiency degradation of 4-Chlorophenol (4-CP), which is one of the most common chlorophenol compounds in wastewater. From experiment, the homemade system can degrade 450 mL waste water containing 500 mg/L 4-CP in 35 min, with a degradation rate of 98%. Thereby, the requirement for electric field intensity decreased. Through the further quantitative analysis, the impact of frequency, voltage, and electrode spacing on the degradation effect of 4-CP is confirmed.

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.

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.

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.

4.4 kW Nd:YAG slab amplifier with a high repetition frequency and high beam quality

Using the self-developed fused indium wetting technology and planar waveguide, the uniform heat dissipation of the slab crystal and uniform pumping of the pump light were achieved, respectively. Based on the master oscillator power amplification (MOPA) scheme, the power was then amplified when the seed light source passed through the Nd:YAG slab crystal three times. Additionally, the image transfer system that we added to the amplified optical path achieved high beam quality. Finally, we obtained a rectangular pulsed laser with an output average power of 4461 W, a repetition frequency of 20 kHz, a pulse width of 62 ns, an optical-to-optical conversion efficiency of 26.8%, and a beam quality of β x =7.0 and β y =7.7.

Predistortion Phenomenon of the Field Distortion Switch at High Repetition Frequency

Predistortion Phenomenon of the Field Distortion Switch at High Repetition Frequency

Optical–Mechanical Integration Analysis and Validation of LiDAR Integrated Systems with a Small Field of View and High Repetition Frequency

Integrated systems are facing complex and changing environments with the wide application of atmospheric LiDAR in civil, aerospace, and military fields. Traditional analysis methods employ optical software to evaluate the optical performance of integrated systems, and cannot comprehensively consider the influence of optical and mechanical coupling on the optical performance of the integrated system, resulting in the unsatisfactory accuracy of the analysis results. Optical–mechanical integration technology provides a promising solution to this problem. A small-field-of-view LiDAR system with high repetition frequency, low energy, and single-photon detection technology was taken as an example in this study, and the Zernike polynomial fitting algorithm was programmed to enable transmission between optical and mechanical data. Optical–mechanical integration technology was employed to obtain the optical parameters of the integrated system under a gravity load in the process of designing the optical–mechanical structure of the integrated system. The experimental validation results revealed that the optical–mechanical integration analysis of the divergence angle of the transmission unit resulted in an error of 2.586%. The focal length of the telescope increased by 89 μm, its field of view was 244 μrad, and the error of the detector target surface spot was 4.196%. The continuous day/night detection results showed that the system could accurately detect the temporal and spatial variations in clouds and aerosols. The inverted optical depths were experimentally compared with those obtained using a solar photometer. The average optical depth was 0.314, as detected using LiDAR, and 0.329, as detected by the sun photometer, with an average detection error of 4.559%. Therefore, optical–mechanical integration analysis can effectively improve the stability of the structure of highly integrated and complex optical systems.

High repetition frequency dual-wavelength pulse Pr3+:YLF laser

High repetition frequency dual-wavelength pulse Pr3+:YLF laser

Emerging PbSnS2 pulse modulators for ultrafast and high repetition frequency photonic applications

Two-dimensional ternary metal sulfides receive growing concern due to the diverse physicochemical properties and broad application potential. In this study, ultrafast nonlinear optical properties of two-dimensional lead tin sulfide (PbSnS2) nanosheets are demonstrated. PbSnS2 nanosheets with the thickness less than 3 nm were fabricated by liquid-phase exfoliation, and PbSnS2 nanosheets based saturable absorber (SA) was constructed by photodeposition. The nonlinear optical absorption properties such as modulation depth (21 %) and saturation intensity (14.2 MW/cm2) of PbSnS2-SA were investigated. Dispersion management was applied to Er3+-doped fiber laser (EDFL) coupled with PbSnS2-SA. The EDFL with net dispersion −0.36 ps2 outputs 928 fs mode-locked pulses. With a cavity dispersion of −0.58 ps2, tunable 1st to 9th order harmonic mode-locked (HML) is realized by tuning the pump power only. The EDFL (net dispersion −0.98 ps2) produces the highest repetition frequency at 362.59 MHz (corresponding to 80th order HML). The results indicate that two-dimensional PbSnS2 may be widely used as ultrafast photonics material for photonics and engineering applications research.

Extremely Low Threshold Quasi‐CW Amplified Spontaneous Emission Achieving and Its Mechanism Study

AbstractThe development of lasing materials with weak pile‐up effect from triplet excitons, low lasing threshold, and long operational lifetime is much significant for truly continuous wave operation and electrically pumping organic lasers. However, most of organic gain media has been verified to have extreme quenching effect from triplet excitons due to its wide‐band distribution of triplet absorption spectrum. Here, it is found that the phenanthroimidazole derivatives as gain media have congenital advantages on handling triplet's issues. A very low amplified spontaneous emission (ASE) threshold of 0.56 µJ cm−2 and a long operational lifetime of 5.77 × 105 pump pulses in air atmosphere are well obtained. When increasing the pump repetition frequency from 20 Hz to 20 kHz, the operational lifetimes of the thin films have small reduction. Under a high pump repetition frequency of 200 kHz, which is quasi‐continuous wave operational regime, the PI2PhDPO thin film exhibits very low ASE threshold of 0.78 µJ cm−2 due to the tiny overlap between triplet absorption and ASE spectra.