Optimal Laser Energy Research Articles

Spectral characteristics of underwater laser-induced breakdown spectroscopy under high-pressure conditions

Laser-induced breakdown spectroscopy (LIBS) has been proven to be an attractive technique for in situ oceanic applications. However, when applying LIBS into deep-sea, the pressure effect caused by different ocean depths is inescapable and could have great influence on the LIBS signals. In this work, spectral characteristics of underwater LIBS were investigated as a function of pressure in the range of 0.1–45 MPa. A high-pressure chamber built in the laboratory was used to simulate the high-pressure deep-sea environment. Optimal laser energy and detection delay were first determined under different pressure conditions and were shown to be independent of the external pressure. The increase in pressure has a significant impact both on the peak intensity and line broadening of the observed spectra. The peak intensity of Na, Li and K lines increases with the increasing pressure until a maximum intensity is reached at 12.5 MPa. Above this value, the peak intensity decreases gradually up to 45 MPa. For Ca line, the maximum intensity was observed at 30 MPa. The line broadening keeps constant at low pressures from 0.1–10 MPa, while it increases linearly at higher pressures, indicating a higher electron density caused by the compression effect of the high external pressure. We also compared the spectral data obtained from the high-pressure chamber and from the field sea trials, and the good consistency between the laboratory data and sea-trial data suggested the key role of pressure effect on underwater LIBS signals for practical deep-sea applications.

Importance of Internal Tensile Stress in Forming Low-Loss Fiber Draw-Tower Gratings

Internal tensile stress inside fiber core can impact significantly on formation of Draw-tower gratings (DTGs). We carry out online dynamic preparation experiments of DTG to analyze the effect of internal stress on grating reflectivity, as well as offline multi-pulse accumulation experiments to further investigate the underlying mechanisms for grating formation. The optimum laser energy density used to grating writing and the variation of reflectivity with exposure extent were obtained, which is instrumental for their optimization and rapid prototyping. A stress modulation model was proposed as the dominant mechanism to describe the formation of the grating structure by a single laser pulse in the low-loss fiber, which is of great significance for the research of the grating photosensitivity mechanism.

Mechanical properties of Invar 36 alloy additively manufactured by selective laser melting

Invar 36 alloy is widely used in aerospace engineering, owing to its extremely low coefficient of thermal expansion. This work aims to explore the underlying influence of process parameters on the microstructures and mechanical properties of Invar 36 additively manufactured by selective laser melting (SLM) with island scanning strategy. A full factorial design for a wide range of SLM process parameter sequence was established. Density and the mechanical properties including hardness, tensile strength as well as microstructures, fractography were characterized. The results confirm that the increment of exposure time and the reduction of point distance remarkably induce stable melting and achieve superior density and hardness. The laser energy density shows a pronounced effect on the microstructures, as extremely low laser energy density induces considerable lack-of fusion pores, unmelted powder particles and blurry boundaries of islands. While, very high laser energy density induces a few keyhole pores, and makes the laser scanning tracks and island boundaries transfer to be irregular. The optimal laser energy density is recommended to be 99.2 J/mm3, and the as-fabricated Invar 36 shows the relative density of 99.5%, Vickers hardness of 1.8 GPa and ultimate tensile strength of 480 MPa those are very comparable to conventionally fabricated one. The testing temperatures of 200 °C and 600 °C induce the coarsening of γ phase, and result in a significant degeneration of tensile properties. The correlations between the SLM process and mechanical properties provide experimental basis of the additive manufacturing Invar 36 alloy.

Effect of heat treatment on microstructure and mechanical behaviours of 18Ni-300 maraging steel manufactured by selective laser melting

Selective laser melting (SLM) of 18Ni-300 maraging steel is an important research area in view of its numerous applications in the automotive domain. Heat treatment plays a significant role in the microstructure and mechanical behaviour of maraging steels and is a major area of interest. This paper investigated the effect of heat treatment on microstructure and mechanical behaviour of SLM-built 18Ni-300 maraging steel. The experimental results showed that the densest parts with the smallest number of defects were fabricated at optimum laser energy density of 70 J/mm3 and laser power of 275 W. When the laser power was fixed at 275 W, lower laser energy density resulted in the formation of balling and irregular pores, while higher laser energy density induced spherical pores and microcracks. The as-built samples consisted of cellular and columnar microstructures due to the fast cooling and solidification rates during SLM. However, solution treatment led to changes in the typical microstructure and massive lath martensite phase. The tensile strength and microhardness decreased slightly due to grain growth and residual stress relief upon solution treatment; an opposite effect was observed when the samples were subjected to solution treatment followed by aging at 490 °C for 2 h. With regard to the tensile anisotropy, yield strength and ultimate tensile strength of the horizontally-built samples slightly exceeded those vertically-built. These findings are significant as they allow an informed prediction about the effect of various heat treatments on the microstructure and mechanical behaviour of components manufactured from 18Ni-300 maraging steel using the SLM process.

Influence of laser spot scanning speed on micro-polishing of metallic surface using UV nanosecond pulse laser

During laser micro-polishing of the metallic surface, it is very important to choose the optimal laser energy density. The optimal laser energy density is merely related to the laser spot scanning speed while the spot size and the irradiated laser energy are rated. In this paper, during micro-polishing on the metallic surface by using UV nanosecond pulse laser, the influence of laser spot scanning speed on the polishing effect was investigated in terms of the relationship with the laser energy density. The experimental and analytical considerations were shown that there is the optimal scanning speed of laser spot for the best laser micro-polishing effect when the laser energy density on the workpiece surface was rated. In addition, the optimal process parameters for the laser micro-polishing of Ti and Ni metallic surfaces were obtained and the laser micro-polishing experiments on those metallic surfaces were conducted. For Ti and Ni metallic surfaces, the surface roughness improvements of up to 56% and 52% were, respectively, obtained.

Study of pressure effects on ocean in-situ detection using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has attracted extensive attention as a new technique for in-situ marine application. In this work, the influence of deep-sea high pressure environment on LIBS signals was investigated by using a compact LIBS-sea system developed by Ocean University of China for the in-situ chemical analysis of seawater. The results from the field measurements show that the liquid pressure has a significant effect on the LIBS signals. Higher peak intensity and larger line broadening were obtained as the pressure increases. By comparing the variations of the temperature and salinity with the LIBS signals, a weak correlation between them can be observed. Under high pressure conditions, the optimal laser energy was higher than that in air environment. When the laser energy exceeded 17 mJ, the effect of laser energy on the signal intensity weakened. The signal intensity decreases gradually at larger delays. The obtained results verified the feasibility of the LIBS technique for the deep-sea in-situ detection, and we hope this technology can contribute to surveying more deep-sea environments such as the hydrothermal vent regions.

An automated design process for short pulse laser driven opacity experiments

Stellar-relevant conditions can be reached by heating a buried layer target with a short pulse laser. Previous design studies of iron buried layer targets found that plasma conditions are dominantly controlled by the laser energy while the accuracy of the inferred opacity is limited by tamper emission and optical depth effects. We developed a process to simultaneously optimize laser and target parameters to meet a variety of design goals. We explored two sets of design cases: a set focused on conditions relevant to the upper radiative zone of the sun (electron temperatures of 200 to 400 eV and densities greater than 1/10 of solid density) and a set focused on reaching temperatures consistent with deep within the radiative zone of the sun (500 to 1000 eV) at a fixed density. We found optimized designs for iron targets and determined that the appropriate dopant, for inferring plasma conditions, depends on the goal temperature: magnesium for up to 300 eV, aluminum for 300 to 500 eV, and sulfur for 500 to 1000 eV. The optimal laser energy and buried layer thickness increase with goal temperature. The accuracy of the inferred opacity is limited to between 11% and 31%, depending on the design. Overall, short pulse laser heated iron experiments reaching stellar-relevant conditions have been designed with consideration of minimizing tamper emission and optical depth effects while meeting plasma condition and x-ray emission goals.

Microstructural characteristics and mechanical properties of carbon nanotube reinforced AlSi10Mg composites fabricated by selective laser melting

To obtain CNTs/AlSi10Mg alloys with homogeneous microstructure, a new process combines ball milling and selective laser melting (SLM) has been developed. The crystallographic texture, microstructure evolution and mechanical properties of the CNTs/AlSi10Mg alloys produced by SLM at different laser energy densities were investigated. Because of the high temperature and rapid solidification, the special structures of CNTs are destroyed by evaporation and decomposition during the SLM process. Some carbon atoms tend to diffused in the molten pool, which inhabit the grain growth during the SLM process. Under the optimal laser energy density of 131J/mm3, a fine microstructure with equiaxed crystal was obtained, exhibiting elevated microhardness of 143.7 HV0.1. Compared with that of the unreinforced AlSi10Mg part, the incorporation of CNTs enhanced the Al-based composites strength, but simultaneously deteriorated the ductility of the CNTs/AlSi10Mg parts.

Low power continuous wave-laser seed irradiation effect on Moringa oleifera germination, seedling growth and biochemical attributes

Recently, laser application in agriculture has gained much attention since plant characteristics were improved significantly in response of pre-sowing seed treatment. Pre-sowing laser seed treatment effects on germination, seedling growth and mineral profile were studied in Moringa olifera. M. olifera healthy seeds were exposed to 25, 50, 75mJ low power continuous wave laser light and grown under greenhouse conditions. The seedling growth and biochemical attributes were evaluated from 10-day-old seedlings. The germination parameters (percentage, mean germination time), vigor index, seedling growth (root length, seedling length, shoot fresh weight, root fresh weight, shoot dry weight, root dry weight) enhanced considerably. The laser energy levels used for seed irradiation showed variable effects on germination, seedling growth and mineral profile. The mineral contents were recorded to be higher in seedling raised from laser treated seeds, which were higher in roots versus shoots and leaves. The effect of laser treatment on seedling fat, nitrogen and protein content was insignificant and at higher energy level both nitrogen and protein contents decreased versus control. Results revealed that M. olifera germination, seedling growth and mineral contents were enhanced and optimum laser energy level has more acceleratory effect since at three laser energy levels the responses were significantly different. Overall the laser energy levels effect on germination and seedling growth was found in following order; 75mJ>50mJ>25mJ, where as in case of fat, protein and nitrogen contents the trend was as; 25mJ>50mJ and 75mJ. However, this technique could possibly be used to improve the M. olifera germination, seedling growth, and minerals contents where germination is low due to unfavorable conditions.

Selective laser melting of weak-textured commercially pure titanium with high strength and ductility: A study from laser power perspective

In this study, fully dense commercially pure titanium (CP Ti) parts were successfully fabricated by selective laser melting (SLM) using the same optimal laser energy density Ep¯ but with two different laser powers: high laser power EH ~250W and low laser power EL ~50W. It was found that at the same Ep¯ different laser powers led to different phase formation, microstructure, texture and mechanical properties of the selective laser melting fabricated (SLMed) CP Ti. A weak-textured CP Ti with isotropic mechanical properties was achieved using EH while a strong-textured CP Ti with anisotropic mechanical properties was obtained using EL. The underlying mechanism was attributed to the formation of α' phase in the CP Ti as a result of the higher cooling rates at EH. The formation of α' phase also contributed to the observed high ultimate compressive strength ~1.1GPa and high compressive strain ≥50% in the SLMed weak-textured CP Ti at EH. This study provides important insights into the role of laser energy in the SLM fabrication of CP Ti with tailorable crystallographic texture and thus mechanical properties.

The influence of laser ablation plume at different laser incidence angle on the impulse coupling coefficient with metal target

A calibrated pendulum measuring device and a dimensionless analysis method were used to measure the impulse coupling coefficient at different laser intensities with aluminum, steel, and iron targets. The experiment was performed with a pulsed laser with the wavelength of 1.06 μm and the pulse duration of 7 ns. The experimental measurements of the variation of the impulse coupling coefficient versus the laser energy density agree with the theoretical prediction, and the optimum laser energy density correlated with the maximum impulse coupling coefficient corresponding to the theoretical predictions. The impulse coupling coefficients with laser incidence angles of 0° and 45° are compared for understanding of the effects of the ablation plume on the impulse coupling effect, and the experimental result shows that the impulse coupling effect grows as the incidence angle changes from 0° to 45°. Furthermore, the transmittance of the incident laser through the ablation plume in front of the target is deduced from the impulse measurements, and the effect of the ablation plume on the impulse coupling at high laser intensity is discussed. In order to investigate the weak impulse coupling effect, which is difficult to obtain from the experiments, the impulse coupling coefficient at low laser energy density was calculated by the finite element simulation.

Sensitivity, stability, and precision of quantitative Ns-LIBS-based fuel-air-ratio measurements for methane-air flames at 1-11 bar.

Nanosecond laser-induced breakdown spectroscopy (ns-LIBS) is employed for quantitative local fuel-air (F/A) ratio (i.e., ratio of actual fuel-to-oxidizer mass over ratio of fuel-to-oxidizer mass at stoichiometry, measurements in well-characterized methane-air flames at pressures of 1-11 bar). We selected nitrogen and hydrogen atomic-emission lines at 568 nm and 656 nm, respectively, to establish a correlation between the line intensities and the F/A ratio. We have investigated the effects of laser-pulse energy, camera gate delay, and pressure on the sensitivity, stability, and precision of the quantitative ns-LIBS F/A ratio measurements. We determined the optimal laser energy and camera gate delay for each pressure condition and found that measurement stability and precision are degraded with an increase in pressure. We have identified primary limitations of the F/A ratio measurement employing ns-LIBS at elevated pressures as instabilities caused by the higher density laser-induced plasma and the presence of the higher level of soot. Potential improvements are suggested.

Influence of femtosecond laser pulse energy on the surface reflection of black silicon in alkaline solution

A highly efficient approach for reducing the reflection of the black silicon surface is demonstrated, in which the black silicon is fabricated in alkaline solution via a femtosecond laser irradiation. The junglelike microstructures are formed on the surface of the black silicon. Compared to the polished silicon, the black silicon can significantly suppress the surface reflection. Throughout the region of visible light, the average minimum reflectance of the blackened surface is about 5.6%. Meanwhile, in order to get an optimal laser energy, we investigated the evolution on silicon surface as a function of incident pulse energy. Taking into account the height of junglelike microstructures and the reflectance of black silicon samples, the optimal laser energy is 1400 μJ. By choosing the right laser energy, it is possible to fabricate the highly absorptive black silicon. These results are of extreme importance in the control of surface morphology and the modification of material surface.

Effect of Powder Size and Shape on the SLS Processability and Mechanical Properties of a TPU Elastomer

This work investigates the influence of powder size/shape on selective laser sintering (SLS) of a thermoplastic polyurethane (TPU) elastomer. It examines a TPU powder which had been cryogenically milled in two different sizes; coarse powder (D50∼200μm) with rough surfaces in comparison with a fine powder (D50∼63μm) with extremely fine flow additives. It is found that the coarse powder coalesces at lower temperatures and excessively smokes during the SLS processing. In comparison, the fine powder with flow additives is better processable at significantly higher powder bed temperatures, allowing a lower optimum laser energy input which minimizes smoking and degradation of the polymer. In terms of mechanical properties, good coalescence of both powders lead to parts with acceptable shear-punch strengths compared to injection molded parts. However, porosity and degradation from the optimum SLS parameters of the coarse powder drastically reduce the tensile properties to about one-third of the parts made from the fine powders as well as those made by injection molding (IM).

Laser lithotripsy of salivary stones: Correlation with physical and radiological parameters.

Sialolithiasis is a common disease of the major salivary glands. Owing to the variety of conservative and minimally invasive techniques, it is now possible to treat most cases of sialolithiasis without removal of the affected salivary gland. One treatment option is the endoscopic removal of the calculi. In cases of larger concretions, intraductal disintegration using laser-induced shock waves can be appropriate to allow endoscopic removal. In the present study, we investigated whether physical and radiological parameters of salivary stones can effectively predict the applicability of laser lithotripsy. Furthermore, we determined to what extent the applied laser energy resulted in tissue damage. In addition to basic parameters like size and density, we analysed 47 salivary stones using fluorescence spectroscopy, infrared spectroscopy, Raman spectroscopy, and dual-energy computed tomography. Subsequent fragmentation of all stones was performed with a Ho:YAG laser in a near-contact manner. Fragmentation rates were calculated and correlated with the previously measured physical and radiological parameters. Finally, to test for tissue damage, we performed HE-histology of salivary duct mucosa treated with the same laser energy used for stone fragmentation. Blue light excitation induced either green or red fluorescence emission. Dual-energy CT resulted in evidence of calcium-containing material. Infrared spectroscopy and Raman spectroscopy, both identified carbonate apatite as the main component of salivary stones. Disintegration into pieces smaller than 2 mm was possible in all cases. Fragmentation rates depended on the energy per pulse applied but not on any of the analysed physical and radiological parameters. In contrast to lithotripsy with 500 mJ per pulse, which was associated with no tissue damage, lithotripsy with 1,000 mJ per pulse resulted in damage of salivary duct mucosa. This suggests that the optimal laser energy for stone fragmentation is between 500 and 1,000 mJ per pulse. Laser lithotripsy using Ho:YAG laser is a highly efficient treatment, at least in vitro. All salivary stones could be disintegrated irrespective of their physical and radiological composition.

A rabbit vocal fold laser scarring model for testing lamina propria tissue‐engineering therapies

To develop a vocal fold scarring model using an ablative laser in the rabbit as a platform for testing bioengineered therapies for missing or damaged lamina propria. Prospective controlled animal study. An optimal laser energy level was first determined by assessing the depths of vocal fold injury created by a Holmium:YAG laser at various energy levels on fresh cadaveric rabbit larynges. The selected energy level was then used to create controlled unilateral injuries in vocal folds of New Zealand white rabbits, with the contralateral folds serving as uninjured controls. After 4 weeks, the larynges were harvested and subjected to excised-larynx phonation with high-speed imaging and immunohistochemical staining for collagen types I and III, elastin, and hyaluronic acid (HA) with quantitative histological analysis. A total of 1.8 joules produced full-thickness injury of the lamina propria without extensive muscle injury. After 4 weeks, the injured vocal folds vibrated with reduced amplitude (P = 0.036) in excised-larynx phonation compared to normal vocal folds. The injured vocal folds contained a higher relative density of collagen type I (P = 0.004), higher elastin (P = 0.022), and lower HA (P = 0.030) compared to normal controls. Collagen type III was unchanged. With its potential for higher precision of injury, this laser vocal fold scarring model may serve as an alternative to scarring produced by cold instruments for studying the effects of vocal fold lamina propria bioengineered therapies.

Effect of laser irradiation on gas sensing properties of sol–gel derived nanocrystalline Al-doped ZnO thin films

The effect of laser irradiation on the performance of gas sensor made with sol–gel derived Al-doped ZnO thin films was investigated. The films with desired thicknesses were deposited on the alkali-free glass substrates by a sol–gel process. A pulsed laser system with a wavelength of 532nm, a pulsed duration of 8ns, pulsed frequency of 5kHz and the laser fluence in the range of 1.06–3.58J/cm2 was used as the irradiation source. The microstructure, optical transmittance, surface morphology, electrical conductivity and gas sensor performance of the as-deposited and laser-irradiated Al-doped ZnO films were studied as a function of laser energy level. The X-ray diffraction results indicated that low laser energy significantly enhanced the crystallinity and promoted grain growth, whereas high laser energy irradiation resulted in deterioration of crystalline quality. It was also found that the laser irradiation affected the surface morphology and electrical conductivity of ZnO films. The gas sensor performance of Al-doped ZnO sensors was examined in terms of ZnO film thicknesses and H2 concentrations in the air at an operating temperature of 130°C. It was found that the sensing response of Al-doped ZnO sensors varied depending on the film thickness as well as the laser energy level. An optimum laser energy level resulted in high and rapid response characteristics of gas sensors for the detection of H2. The results also suggested that the crystallinity was critical to achieve the optimum sensor performance.

Conversion of laser energy to nuclear energy driven by Coulomb explosion of nanostructures

Tabletop nuclear fusion reactions in the chemical physics laboratory can be induced by ultrafast, high-energy dynamics of multicharged nanostructures. Compelling experimental and theoretical evidence emerged for nuclear fusion driven by Coulomb explosion (NFDCE) of multicharged deuterium, containing nanostructures generated by ultraintense, femtosecond, near-infrared laser pulses. NFDCE constitutes the conversion of laser energy to nuclear energy mediated by the dynamics of molecular nanostructures. Theoretical-computational studies of tabletop laser-driven nuclear fusion of high-energy (upto 15 MeV) deuterons with 7Li, 6Li and D nuclei demonstrate the attainment of high fusion yields within a source–target reaction design. This constitutes a source of Coulomb exploding deuterium nanodroplets and a solid, hollow cylindrical target containing the second element. The fusion yields and efficiencies were maximised for the nanodroplet size and the laser parameters which accomplish optimal laser energy deposition into single nanodroplets and into an assembly of nanodroplets. The reaction design attains the highest tabletop fusion efficiencies (upto 4 × 109 J−1 per laser pulse) obtained to date. The highest conversion efficiency of laser energy to nuclear energy (10−2 to 10−3) for tabletop fusion in the source–target design, with a source of exploding large deuterium nanodroplets (initial size of 300 nm) driven by superintense lasers (peak intensity: 5 × 1019 W cm−2), is comparable to that for DT fusion currently accomplished for ‘big science’ inertial fusion setups.

The effects of Pulsed Green Laser Annealing for Carbon NanoWalls (CNWs)

ABSTRACTThe effects of the pulsed green laser annealing at ambient nitrogen for two different heights-CNWs grown on silicon substrate were investigated on the crystallinity and morphology using Raman spectroscopy, SEM, TEM and XPS. For the 1μm height-CNWs, the peak intensity of D-band spectra decreased as the laser energy density increased up to 1.3Jcm-2, ID/IG ratio decreased from 2.5 to 0.7. The crystallinity of CNWs was improved by the laser irradiation. For the 1μm height-CNWs irradiated above 1.5Jcm-2, the height of CNWs decreased gradually as the laser energy density increased, it was clarified that the surfaces of CNWs were vaporized by the laser irradiation. For the 20μm height-CNWs, the peak intensity of D band spectra also decreased until the laser energy density increased up to 0.8Jcm-2, ID/IG ratio decreased from 1.6 to 0.5. From the TEM observation of CNWs irradiated at 0.8 Jcm-2, it was confirmed that the laser irradiation changed CNWs to be highly oriented crystal structure. However above 0.8Jcm-2, the crystallinity was deteriorated due to the vaporization of CNWs as the same as the 1μm height-CNWs. The pulsed green laser annealing is effective to improve the crystallinity of CNWs on optimal laser energy density for both height-CNWs, the higher laser energy densities vaporized the CNWs and changed the morphology and crystallinity of CNWs.

The effect of laser energy on the preparation of iron oxide by a pulsed laser ablation in ethanol

Recently the preparation magnetic nanoparticles by a pulsed laser ablation in liquid has gained much attention because it is easy to control experimental parameters. Iron oxide magnetic nanoparticles have been prepared by a pulsed laser ablation of <TEX>${\alpha}-Fe_2O_3$</TEX> target in ethanol at different magnitude of laser energy of 1, 20, 40 and 80 mJ/pulse. It revealed that particle size increases with increasing laser energy. It could be concluded that 40 mJ/pulse is an optimum laser energy for the preparation of iron oxide nanoparticles with uniform size distribution. The nanoparticles are homogeneously dispersed in ethanol and their stability maintained for several months.