Laser Pulse Frequency Research Articles

Design and develop an optical setup related to the pump-probe technique for spectroscopic study

In this research, an optical system using Pump-Probe technology was designed to study the effect of changing the Frequency values, integration time, delay time, and laser pulse width on the pumping pulse energy on an arbitrary laser dye material with an absorption coefficient of about 650 nm. we found that decreasing the pumping time to 10% of the pulse time is not suitable for the pump-probe technique because there is no resonance between the pumping time and the detector response to the source used. Also, we indicate that increasing the pumping time to 20% of the pulse time results in the highest possible detector response at an integration time 150 μs in the pump-probe technique. therefore, we noted that the detector response becomes more suitable for working in the pump-probe technique as the pumping time increases, as well as the efficiency of the detector, which increases significantly as the pumping time increases, as well as the integration time for the same delay time, it is more stable at 20% pumping time and collection time 150 μs at a delay time of 12.4 ns.

Building ultra-thin Inconel 718 sheet joints using low-frequency PLBW scheme: Weld bead quality, keyhole dynamics, and solidification characteristics

Low-frequency pulsed laser beam welding (PLBW) was employed in building 0.32 mm-thick Inconel 718 superalloy butt joints which are frequently applied in manufacturing high-temperature functional units in the aviation industry. Several process parameter combinations concerning peak laser power, welding speed, and pulse frequency were implemented to explore the macro morphology, microstructure, and mechanical properties of the welded joints. Examinations exhibit that the resulting weld beads possess high continuity at the surface with an overlap factor ranging from 0.58 to 0.73. The cross-sectional profile changes from a V-shape to an H-shape with increasing heat input. Finer grain structures composed of γ matrix and γ/Laves eutectic penetration are observed within the FZ center, as compared to that near the fusion line. Excellent performance is achieved in microhardness and tensile tests. Numerical simulation based on an integrated mathematic model reveals periodic keyhole and weld pool dynamics depending on the pulsing laser power. During a typical pulse interval, keyhole geometry collapses within a fraction of a millisecond, leading to a sharp cooling stage at ∼4 × 106 K/s, and then, the melt cools due to the thermal convection with a normal cooling rate of 105 K/s. The full growth of the adjacent weld spot results in the secondary melting zone (SMZ), wherein the columnar dendrites can be refined mainly due to the heterogeneous nucleation as compared to the primary melting zone (PMZ).

The role of asymmetric metal flow on weld formation and solidification characteristics during pulsed laser butt welding with assembly tolerance

In comparison to other laser-assisted welding techniques, the pulsed laser welding process (PLBW) provides cost-effectiveness and ease of operation while effectively connecting butt-assembled thin plates with manufacturing tolerances (both air gap and edge misalignment). A combination of numerical simulations and experimental studies was conducted to analyze the forming mechanism and solidification behavior of the weld. In this paper, a transient three-dimensional heat-flow-metallurgical model has been established for a PLBW process of mild steel plates with a thickness of 1.8 mm. The predicted weld pool profiles were consistent with the experimental findings. The finding suggested the molten metal, driven by surface tension and recoil pressure, successively generated liquid bridges on the rear and front sides of the pool. The two sheets were joined as the molten metal gradually solidified. The energy absorptivity was highly correlated with the weld pool and keyhole dynamics, and the laser energy absorbed by the metal was mainly transferred in the form of thermal convection. The kinematic features of the melt were observed by the liquid phase mass transfer rate and surface fluctuations. According to the Fourier analysis results, the molten pool oscillated at the characteristic frequency of the laser pulse frequency. The spatial and temporal distributions of solidification parameters aided in analyzing the solidification features within the pool. The asymmetric flow had a greater influence on the temperature gradient. These findings addressed the roles of PLBW on fluid flow, heat behaviors, and solidification parameters of welds with assembly tolerances, as well as the reasons for improved weld formation.

Experimental Study on Carbon Fiber-Reinforced Polymer Groove Machining by High-Power Water-Jet-Guided Laser.

Due to the excellent properties of carbon fiber-reinforced polymers (CFRPs), such as high strength and strong corrosion resistance, the traditional water-jet-guided laser (WJGL) technology has problems with fiber pull-out and has a small cutting depth when processing CFRPs. Therefore, in this study, we used high-power water-jet-guided laser (HPWJGL) technology to perform groove processing experiments on CFRPs. The effects of four key process parameters, high laser power, pulse frequency, feed rate, and water-jet pressure, on the cutting depth were investigated by a single-factor experiment. The formation mechanism of groove cross-section morphology and the processing advantages of high-power water-jet-guided lasers were analyzed. On this basis, the mathematical prediction model of cutting depth was established by using the response surface method (RSM), and the optimal combination of process parameters was obtained. The mathematical prediction model was verified by experiments, and the error was only 1.84%, indicating that the model had a high reference value. This study provides a reference for the precision machining of HPWJGL technology.

Relationship between laser-generated micro- and nanostructures and the long-term stability of bonded epoxy-aluminum joints

ABSTRACT To improve the mechanical performance and to address current shortcomings of adhesive bonds such as bond degradation due to aging, a pulsed laser surface pretreatment of the metal surfaces of aluminum AW 6082-T6 joints with epoxy adhesive E320 is investigated. The surface treatment of the specimens resulted in increased single-lap shear (SLS) strengths before and after hydrothermal aging in 80°C hot water compared to non-pretreated reference specimens. In order to reveal the correlations of laser parameters, resulting surface morphologies and the SLS strength, differently laser pretreated surfaces were characterized at the micro- and nanoscale using optical and scanning electron microscopies. The surface enlargement was quantified with a digital image analysis of cross-sections prepared from the joint interfaces. An analysis of variances (ANOVA) of the SLS results indicated that the laser parameters power and pulse frequency were most critical for obtaining high SLS strengths. Pretreated joint surfaces with a high micro- and nano-surface enlargement and deep solidification structures provide high SLS strengths of up to 50 MPa and almost negligible aging losses of merely 4%. Undercut structures on the pretreated surfaces were found to be beneficial for the mechanical and aging properties when only limited micro- and nanostructuring was applied.

The first meta-analysis research on the effects of endovenous laser ablation combined with sapheno-femoral junction high ligation of the great saphenous vein.

Endovenous laser ablation (EVLA) is a common minimally invasive technique used to treat varicose veins. The most commonly used laser wavelengths for EVLA/EVLT of varicose veins are 810 nm and 1470 nm. The laser pulse frequency is typically set to continuous wave (CW) mode, with a pulse duration of a few milliseconds (ms) delivered in a radial mode. The energy delivered per pulse is typically set between 40 and 120 Joules, with a power setting of 10 to 30 watts and an intensity setting of 40 to 120 J/cm2. The controversy exists regarding the benefits of performing saphenofemoral junction (SFJ) ligation prior to EVLA to decrease the recurrence rate of varicose veins. This meta-analysis aims to investigate the effectiveness of combining EVLA with high ligation versus using EVLA alone in treating lower extremity varicose veins. We conducted a systematic search of four databases from their inception until July 1, 2022, for randomized controlled trials and prospective controlled trials evaluating the advantages and disadvantages of EVLA with or without high ligation for the treatment of lower extremity varicose veins. In analyzing binary data, rate difference (RD) is used, while odds ratio (OR) is used for evaluating the confidence interval (CI) of binary data. A P value of less than 0.05 is deemed statistically significant. Heterogeneity is assessed using the chi-square test. If the I2 statistic, which reflects statistical heterogeneity, is greater than 50%, a random-effects model should be used. In the absence of significant statistical heterogeneity, a fixed-effects model should be used if I2 is less than 50%. We used the Cochrane risk-of-bias tool to assess the quality of the studies and Review Manager 5.4 for the primary and secondary outcome analysis. The meta-analysis was conducted in accordance with the Cochrane Handbook. There were no significant differences in the rate of major complications (RR = 1.63; 95% CI, 0.40-6.69; P = 0.50) or in the frequency of minor complications (RR = 1.07, 95% CI, 0.87-1.31; P = 0.52) between the EVLA with high ligation (EVLA/HL) group and the EVLA group. However, the rate of vein occlusion was significantly lower in the EVLA group than in the EVLA/HL group (RR = 1.06; 95% CI, 1.03-1.09; P = 0.0004). Our meta-analysis indicates that combining EVLA with high ligation provides stable long-term clinical efficacy in treating varicose veins of the lower extremities, although it increases the invasiveness of the surgery. The use of EVLA alone may be less effective in preventing vein occlusion.

MICRO-GROOVING OF ALUMINUM-BASED COMPOSITES USING Nd:YAG LASER MACHINING

This paper describes the pulsed Nd:YAG laser machining characteristics on 6351 aluminum reinforced with silicon carbide (SiC) and boron carbide (B4C) particles. The composites are prepared using stir casting route by varying the weight percentage of B4C (0, 5, and 10 wt.%). During experimentation, the traverse speed (5, 15, and 30 mm/s) and the laser pulse frequency (5, 9, and 15 kHz) are considered to evaluate the groove width. The results reveal that the lower pulse frequency produced poor groove surface quality. Higher thermal energy dissipation at lower traverse speed may also result in the formation of recast layer and heat-affected zone. This is evident from the microscopic image and the EDS analysis. Thus, the optimum condition (composite with 10 wt.% B4C machined at 30 mm/s and 15 kHz) to achieve minimum grove width with improved surface morphology is identified by desirability analysis. Additionally, the regression model is developed to predict future values ([Formula: see text] at 91.86% and [Formula: see text] (adj) at 87.55%). Finally, the probability plot confirms the effectiveness of the proposed model at 95% confidence level.

Simultaneous distributed acoustic and temperature sensing system based on ultra-weak chirped fiber Bragg grating array.

The quasi-static temperature and dynamic acoustic signals based on ultra-weak chirped fiber Bragg grating (CFBG) array is proposed and experimentally demonstrated, which can be employed to detect distributed acoustic and temperature signals simultaneously. Distributed temperature sensing (DTS) was realized by cross-correlation operation to measure the spectral drift of each CFBG, and distributed acoustic sensing (DAS) was achieved by gauging the phase difference between adjacent CFBG. Using CFBG as the sensor unit can protect acoustic signals from the fluctuations and drifts induced by temperature change without deterioration of signal-to-noise ratio (SNR). Applying least square mean adaptive filter (AF) can enhance harmonic frequency suppression ratio and increase the SNR of system. In the proof-of-concept experiment the SNR of acoustic signal is higher than 100 dB after the digital filter and frequency response is from 2 Hz to 1.25 kHz with repetition frequency of laser pulses of 10 kHz. Temperature measurement from 30°C to 100°C is achieved with demodulation accuracy of ±0.8°C. The spatial resolution (SR) of two-parameter sensing is 5 m.

Scoring molecular wires subject to an ultrafast laser pulse for molecular electronic devices.

A nonionizing ultrafast laser pulse of 20-fs duration with a peak amplitude electric-field ±E = 200 × 10-4 a.u. was simulated. It was applied to the ethene molecule to consider its effect on the electron dynamics, both during the application of the laser pulse and for up to 100 fs after the pulse was switched off. Four laser pulse frequencies ω = 0.2692, 0.2808, 0.2830, and 0.2900 a.u. were chosen to correspond to excitation energies mid-way between the (S1 ,S2 ), (S2 ,S3 ), (S3 ,S4 ) and (S4 ,S5 ) electronic states, respectively. Scalar quantum theory of atoms in molecules (QTAIM) was used to quantify the shifts of the C1C2 bond critical points (BCPs). Depending on the frequencies ω selected, the C1C2 BCP shifts were up to 5.8 times higher after the pulse was switched off compared with a static E-field with the same magnitude. Next generation QTAIM (NG-QTAIM) was used to visualize and quantify the directional chemical character. In particular, polarization effects and bond strengths, in the form of bond-rigidity vs. bond-flexibility, were found, for some laser pulse frequencies, to increase after the laser pulse was switched off. Our analysis demonstrates that NG-QTAIM, in partnership with ultrafast laser irradiation, is useful as a tool in the emerging field of ultrafast electron dynamics, which will be essential for the design, and control of molecular electronic devices.

Soğuk Haddelenmiş Strenx 700 CR Çeliğinin Nd:YAG Lazer Kaynak Faktörlerinin Taguchi Yöntemi ile Optimizasyonu

In this study, cold rolled Strenx 700 CR steel with a minimum yield strength of 700 MPa, which is typically used in load bearing structures to produce stronger and lighter structures, is butt welded with Nd:YAG laser welding. The effect of welding factors on tensile strength is investigated by using laser power rate (20%, 40% and 60%), pulse duration (2, 4 and 6 ms) and pulse frequency (3, 5 and 7 hz). By using Taguchi L9 orthogonal experimental design, the number of experiments, which should be 27 when full factorial, is reduced to 9 experiments. The tensile strength test results are optimized by the Taguchi method's larger is better control characteristic. The optimum test combination for maximum tensile strength is determined as A3B2C2 (60%-4 ms-5 hz). The effect of the welding factors on the tensile strength results is analyzed using the analysis of variance (ANOVA) method and it is found that the most effective factor is the laser power rate with 84.26%.

Experimental Study on Coaxial Waterjet-Assisted Laser Scanning Machining of Nickel-Based Special Alloy

The problems of the recast layer, oxide layer, and heat-affected zone (HAZ) in conventional laser machining seriously impact material properties. Coaxial waterjet-assisted laser scanning machining (CWALSM) can reduce the conduction and accumulation of heat in laser machining by the high specific heat capacity of water and can realize the machining of nickel-based special alloy with almost no thermal damage. With the developed experimental setup, the laser ablation threshold and drilling experiments of the K4002 nickel-based special alloy were carried out. The effects of various factors on the thermal damage thickness were studied with an orthogonal experiment. Experimental results have indicated that the ablation threshold of K4002 nickel-based special alloy by a single pulse is 4.15 J/cm2. The orthogonal experiment results have shown that the effects of each factor on the thermal damage thickness are in the order of laser pulse frequency, waterjet speed, pulse overlap rate, laser pulse energy, and focal plane position. When the laser pulse energy is 0.21 mJ, the laser pulse frequency is 1 kHz, the pulse overlap is 55%, the focal plane position is 1 mm, and the waterjet speed is 6.98 m/s, no thermal damage machining can be achieved. In addition, a comparative experiment with laser drilling in the air was carried out under the same conditions. The results have shown that compared with laser machining in the air, the thermal damage thickness of CWALSM is smaller than 1 μm, and the hole taper is reduced by 106%. There is no accumulation and burr around the hole entrance, and the thermal damage thickness range is 0-0.996 μm. Furthermore, the thermal damage thickness range of laser machining in the air is 0.499-2.394 μm. It has also been found that the thermal damage thickness is greatest at the entrance to the hole, decreasing as the distance from the entrance increases.

Laser Pulse’s Frequency Effect on Plasma Parameters for Titanium Dioxide Produced by FHG of a Q-Switched ND: YAG Pulse Laser

This work studies the relationship between laser irradiance and pulse frequency effect on plasma features of the TiO2. This target was irradiated by a Q-switched nanosecond Nd: YAG laser with the first harmonic generation (FHG) wavelength (1064[Formula: see text]nm), laser energy 500[Formula: see text]mJ, and pulse frequency ranging from 6[Formula: see text]Hz to 10[Formula: see text]Hz at atmospheric pressure. The Boltzmann plot and the Stark broadening methods calculated the plasma parameters ([Formula: see text] and [Formula: see text]. The findings were examined in light of the previously published experiments and theories, and it was discovered that they agreed with the hypothesis of the local thermodynamic equilibrium (LTE); on the other hand, research was conducted on the other basic plasma properties such as the Debye length ([Formula: see text]), the Debye sphere ([Formula: see text]), and the plasma frequency ([Formula: see text]). We observed that all plasma parameters are influenced by pulse frequency. The results clarify the linear change in electron temperature at increasing pulse frequency for TiO2 plasma. In contrast, the broadening of the line profiles related to electron density was evident with pulse frequency, causing an increase in electron density.

Effect of surface texture on quality of stainless steel-PET transmission welding

Metal-plastic joints are widely used in industrial production. Laser lap welding using polyethylene terephthalate (PET) side can effectively connect 304 stainless steel to PET. However, there are still several issues in this transmission welding. In this study, the scanning speed and pulse frequency of laser were optimized for producing the best texture grooves. The influences of surface texture grooves on laser absorption and strength of 304 stainless steel/PET joints were then investigated. The obtained results showed that the scanning speed and pulse frequency of laser have a great influence on the width and depth of texture grooves. On the premise of ensuring the formation of quality of a groove, the depth of the groove can be allowed to reach 69 μm. In the laser absorption rate experiment, the deeper and narrower the grooves, the higher was the laser absorption rate. This was consistent with the simulation results of the temperature field. The average temperature in the melting zone of the textured surface was 15.27 % higher than that of the non-textured surface. After welding, the tensile strength of the textured material in the joint was 35.26 MPa, which was 4.67 MPa higher than that of the non-textured material. This also showed that the texture treatment of stainless steel surface can not only improve the laser absorption rate of the surface, but also provide a mechanical anchoring effect and improve the tensile strength. Compared with that under non-textured material, the tensile strength of textured material under low energy parameters was found higher, which was conducive to the high-strength connection between metal and plastic.

Dual beam transient thermal lens spectroscopy with high repetition pulsed IR-Laser Excitation: Photothermal and fluorescence quantum yields determination

Based on experimental results, we demonstrated that the most widely used model for the description of the dual beam transient thermal lens spectroscopy technique also works well when a high repetition frequency pulsed pump laser is used instead of a commonly employed continuous wave source. For this, for water samples colored with a tartrazine dye, we compared the optical absorbance obtained using a continuous and a pulsed laser source emitting at the same excitation wavelength. A close to 90 % photothermal quantum yield was obtained as a function of the dye concentration. As an application, we measured the near-infrared fluorescence quantum yield spectrum of a CF®800 dye (Biotium) dissolved in water. The thermal lens signal was recorded as a function of the photon’s wavelength using a tunable MAI TAI BB laser oscillator (Spectra Physics) for pumping that generates light pulses of about 80 fs at a repetition frequency of 80 MHz between 710 and 980 nm. The quantum yield spectrum and a comparison of the optical absorbance spectra measured by the thermal lens technique and a conventional transmittance experiment show a significant fluorescence quenching due to water optical absorption.

Predicting Characteristics of Dissimilar Laser Welded Polymeric Joints Using a Multi-Layer Perceptrons Model Coupled with Archimedes Optimizer.

This study investigates the application of a coupled multi-layer perceptrons (MLP) model with Archimedes optimizer (AO) to predict characteristics of dissimilar lap joints made of polymethyl methacrylate (PMMA) and polycarbonate (PC). The joints were welded using the laser transmission welding (LTW) technique equipped with a beam wobbling feature. The inputs of the models were laser power, welding speed, pulse frequency, wobble frequency, and wobble width; whereas, the outputs were seam width and shear strength of the joint. The Archimedes optimizer was employed to obtain the optimal internal parameters of the multi-layer perceptrons. In addition to the Archimedes optimizer, the conventional gradient descent technique, as well as the particle swarm optimizer (PSO), was employed as internal optimizers of the multi-layer perceptrons model. The prediction accuracy of the three models was compared using different error measures. The AO-MLP outperformed the other two models. The computed root mean square errors of the MLP, PSO-MLP, and AO-MLP models are (39.798, 19.909, and 2.283) and (0.153, 0.084, and 0.0321) for shear strength and seam width, respectively.

Understanding and controlling the depth sensitivity of scanning probe based infrared imaging and nanospectroscopy for buried polymeric structures.

Atomic force microscopy paired with infrared spectroscopy (AFM-IR) is a robust technique for investigating complex polymer blends and composites' nanoscale surface topography and chemical composition. In this work, we measured bilayer polymer films to study the effect of laser power, laser pulse frequency, and laser pulse width on the depth sensitivity of the technique. Unique bilayer polystyrene (PS) and polylactic acid (PLA) samples with various film thicknesses and blend ratios were prepared. The depth sensitivity characterized by the amplitude ratio of the resonance bands of PLA and PS was monitored as the thickness of the top barrier layer was incrementally increased from tens of nanometers to hundreds of nanometers. In addition, incrementally increasing the incident laser power resulted in greater depth sensitivity due to the enhanced thermal oscillations generated in the buried layer. In contrast, incrementally increasing the laser frequency increased the surface sensitivity, as indicated by a reduced PLA/PS AFM-IR signal ratio. Finally, the dependence of the depth sensitivity on the laser pulse width was observed. Consequently, by precisely controlling the laser energy, pulse frequency, and pulse width, one can finely control the depth sensitivity of the AFM-IR tool from 10 nm to 100 nm. Our work provides the unique capability to study buried polymeric structures without the need for tomography or destructive etching.

Dispersive coherent Brillouin scattering spectroscopy

Frequency- and time-domain Brillouin scattering spectroscopy are powerful tools to read out the mechanical properties of complex systems in material and life sciences. Indeed, coherent acoustic phonons in the time-domain method offer superior depth resolution and a stronger signal than incoherent acoustic phonons in the frequency-domain method. However, it requires scanning of delay time between laser pulses for pumping and probing coherent acoustic phonons. Here, we present Brillouin scattering spectroscopy that spans the time and frequency domains to allow the multichannel detection of Brillouin scattering light from coherent acoustic phonons. Our technique traces the time-evolve Brillouin oscillations at the instantaneous frequency of a chromatic-dispersed laser pulse. The spectroscopic heterodyning of Brillouin scattering light in the frequency domain allows a single-frame readout of gigahertz-frequency oscillations with a spectrometer. As a proof of concept, we imaged heterogeneous thin films and biological cells over a wide bandwidth with nanometer depth resolution.

Ultrafast serrodyne optical frequency translator

The serrodyne principle enables an electromagnetic signal to be frequency shifted by applying a linear phase ramp in the time domain. This phenomenon has been exploited to frequency shift signals in the radiofrequency, microwave and optical regions of the electromagnetic spectrum over ranges of up to a few gigahertz, for example, to analyse the Doppler shift of radiofrequency signals for noise suppression and frequency stabilization. Here we employ this principle to shift the centre frequency of high-power femtosecond laser pulses over a range of several terahertz with the help of a nonlinear multi-pass cell. We demonstrate our method experimentally by shifting the central wavelength of a state-of-the-art 75 W frequency comb laser from 1,030 nm to 1,060 nm and to 1,000 nm. Furthermore, we experimentally show that this wavelength-shifting technique supports coherence characteristics at the few hertz-level while improving the temporal pulse quality. The technique is generally applicable to wide parameter ranges and different laser systems, enabling efficient wavelength conversion of high-power lasers to spectral regions beyond the gain bandwidth of available laser platforms.

Controlling Floquet states on ultrashort time scales

The advent of ultrafast laser science offers the unique opportunity to combine Floquet engineering with extreme time resolution, further pushing the optical control of matter into the petahertz domain. However, what is the shortest driving pulse for which Floquet states can be realised remains an unsolved matter, thus limiting the application of Floquet theory to pulses composed by many optical cycles. Here we ionized Ne atoms with few-femtosecond pulses of selected time duration and show that a Floquet state can be observed already with a driving field that lasts for only 10 cycles. For shorter pulses, down to 2 cycles, the finite lifetime of the driven state can still be explained using an analytical model based on Floquet theory. By demonstrating that the amplitude and number of Floquet-like sidebands in the photoelectron spectrum can be controlled not only with the driving laser pulse intensity and frequency, but also by its duration, our results add a new lever to the toolbox of Floquet engineering.

Reducing Surface Roughness of 3D Printed Short-Carbon Fiber Reinforced Composites.

A 100 W fibre laser source was used to minimize the surface roughness of 3D-printed Onyx parts. Furthermore, this study aimed to determine the mechanism of surface finishing, the influence of the laser process parameters (laser power, pulse frequency, and laser scanning path) on the surface morphology, and the influence of the scanning path on the dimensional accuracy of the investigated Onyx 3D-printed specimens. A significant reduction in surface roughness of 91.15% was achieved on the S3 Onyx 3D-printed specimen following laser surface polishing treatment using a 50 W laser power and a frequency of 50 kHz. The laser scanning path had little influence on the surface roughness, but had a major impact on the geometrical deviation of the treated sample.