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- Laskin, Alexander, et al.
(författare)
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Beam shaping of focused radiation of multimode lasers
- 2018
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Ingår i: High-Power Laser Materials Processing. - : SPIE - International Society for Optical Engineering. - 9781510615366
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Konferensbidrag (refereegranskat)abstract
- Performance of various laser technologies like welding, metal sheet cutting based on the use of powerful multimode fiber lasers, fiber-coupled solid-state and diode lasers can be improved by manipulation of energy distribution perpendicular and along optical axis. Welding, cladding with 0.5-2 mm size working spots benefit from "inverse-Gauss" intensity profiles, and doubled or tripled spots perpendicular to axis are good solutions to provide more uniform temperature profiles on a workpiece. Thick metal sheet cutting, some types of welding get benefits from distributing the laser energy along the optical axis resulting in more efficient usage of laser energy, higher and more stable cutting edge quality, faster processing. Since the radiation of multimode lasers is of low spatial coherence, characterized by big Beam Parameter Products (BPP) or M2 values, it is difficult to control the intensity distribution by methods other than imaging of a fiber end using a collimator and focusing objective. A promising suggested solution is combining of imaging the fiber end and geometrical separation of focused spots either perpendicular to or along the optical axis using dedicated optical components. Thus, energy of high power lasers is distributed among multiple foci. To provide reliable operation with multi-kW lasers and avoid damages the multi-focus optical devices are designed as refractive elements with smooth optical surfaces. The paper presents descriptions of multi-focus optics as well as examples of intensity profile measurements of beam caustics and application results
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| 2. |
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| 3. |
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| 4. |
- Laskin, Alexander, et al.
(författare)
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Multispot optics for beam shaping of high-power single-mode and multimode lasers
- 2021
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Ingår i: Journal of laser applications. - : AIP Publishing LLC. - 1042-346X .- 1938-1387. ; 33:4
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Tidskriftsartikel (refereegranskat)abstract
- The performance of various laser technologies, such as welding, laser powder bed fusion (LPBF), brazing, cladding, and sheet metal cutting, based on the use of high-power multimode fiber lasers, fiber-coupled solid-state, and diode lasers, can be improved using the patent pending beam-shaping optics providing optimal energy distributions by splitting the laser beam into several separate spots in the working plane and variable energy sharing between these spots. Various patterns, such as square, line, and rhombus, consisting of four or nine separate spots, are expected to eliminate or reduce spatter and to realize optimum temperature distributions in the melt pool and stabilizing the processes in the welding of tailored blanks, copper and aluminum parts in the production of batteries, zinc-coated steel, cladding, and LPBF. Because multimode lasers have a comparably low spatial coherence characterized by large beam parameter products or beam quality (M²) values, it is difficult to control the intensity distribution by methods other than imaging the fiber end with a collimator and a focusing objective. The proposed solution is a combination of fiber end imaging and geometrical separation of focused spots perpendicular to the optical axis using special optical components and creating a working spot as a combination of several spots. Varying the energy portions in separate spots and the distances between them make it possible to optimize common spot intensity distributions for particular applications. To ensure reliable operation with multi-kW lasers, the refractive optical components of the multispot devices are implemented from athermal optical materials characterized by insignificant thermal lensing and, hence, negligible thermal focus shift and spherical aberration. The article presents descriptions of multispot optics and examples of intensity profile measurements and application results, while the reduction in spattering was observed using multispot laser welding. It is concluded that the melt pool flows homogenize when applying several laser spots compared to a single spot. The possibility of tailoring melt pool dimensions in LPBF was shown.
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| 5. |
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| 6. |
- Laskin, Alexander, et al.
(författare)
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Comparison of the thermal focus shift and aberration between the single-mode and multimode lasers
- 2021
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Ingår i: Journal of laser applications. - : AIP Publishing LLC. - 1042-346X .- 1938-1387. ; 33:4
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Tidskriftsartikel (refereegranskat)abstract
- Thermal lensing is a well-known but typically undesired effect during the use of laser optics. Nonuniform (gradient) heating due to absorption of high-power laser radiation in optical elements causes thermal lensing, paraxial focus shift, and aberration leading to changes in size and intensity profile of the focused spot in optics. Therefore, an analysis of primary physical effects of geometrical deformation of optical surfaces in the form of aspheric bulges and transformation of the material into a gradient refractive medium was conducted to quantitatively estimate the focus shift and aberrations. Since focus shift effects are different in the case of single-mode and multimode lasers, for both laser modes, the optimal relationships between the physical properties of optical materials for reduction in thermo-optical effects through compensating the material thermal expansion by the change in the refractive index—condition of self-compensation or athermalization were formulated. A comparison of the characteristics, namely, temperature coefficient of the optical pathlength and thermo-optical ratio allowed determination of the optimal materials for the optics for both single-mode and multimode high-power lasers: athermal crystalline quartz and specialty glasses, sapphire with extremely high thermal conductivity ensure minimal temperature gradients. Optics made of these materials exhibit a minimized thermal focus shift and aberration even during the absorption of laser energy in the bulk material and coatings by contamination, scratches, and other surface defects. Weak birefringence of crystalline quartz and sapphire does not prevent their successive use in laser optics.
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| 7. |
- Laskin, Alexander, et al.
(författare)
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On the selection of materials for high-power laser optics with reduced thermal lensing
- 2022
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Ingår i: High-Power Laser Materials Processing: Applications, Diagnostics, and Systems XI. - : SPIE - International Society for Optical Engineering.
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Konferensbidrag (refereegranskat)abstract
- Thermal lensing is a well-known but undesired effect in high power laser optics for welding, 3D-printing and other technologies. Stability and performance of laser processing depend on the possibility to control and minimize the thermo-optical effects induced by non-uniform (gradient) heating due to absorption of laser energy in optical elements: paraxial focus shift and thermally induced aberration, which lead to a change in size and intensity profile of the focal spot. Analysis of primary physical effects: geometrical deformation of optical surfaces and the material transformation into a gradient refractive medium, allows the quantitative estimation of the wavefront beam distortion leading to focus shift and aberration. It also allows formulating an optimal relationship between the physical properties of optical materials to reduce the change in the wavefront through mutual compensation of thermo-optical effects induced by the thermal expansion and the refractive index change - athermalization condition. Athermal optics exhibit minimized thermal focus shift and aberration even when absorbing laser energy in the bulk material and coatings, by contamination or scratches. Considering physical characteristics the Temperature Coefficient of the Optical Pathlength and ThermoOptical Ratio allows determining the optimal materials for optics: athermal crystalline Quartz and specialty glasses, Sapphire with high thermal conductivity. Weak birefringence of Quartz and Sapphire doesn’t prevent their successive use in laser optics. The comparison of the theoretical analysis and experimental validation results of optics made of Fused Silica, N-BK7, crystalline Quartz and Sapphire confirm the theoretical method for reducing the thermal focus shift and effectiveness of the suggested approach.
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| 8. |
- Shrivastava, Manish, et al.
(författare)
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Recent advances in understanding secondary organic aerosol : Implications for global climate forcing
- 2017
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Ingår i: Reviews of Geophysics. - 8755-1209. ; 55:2, s. 509-559
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Tidskriftsartikel (refereegranskat)abstract
- Anthropogenic emissions and land use changes have modified atmospheric aerosol concentrations and size distributions over time. Understanding preindustrial conditions and changes in organic aerosol due to anthropogenic activities is important because these features (1) influence estimates of aerosol radiative forcing and (2) can confound estimates of the historical response of climate to increases in greenhouse gases. Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, represents a major fraction of global submicron-sized atmospheric organic aerosol. Over the past decade, significant advances in understanding SOA properties and formation mechanisms have occurred through measurements, yet current climate models typically do not comprehensively include all important processes. This review summarizes some of the important developments during the past decade in understanding SOA formation. We highlight the importance of some processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing, including formation of extremely low volatility organics in the gas phase, acid-catalyzed multiphase chemistry of isoprene epoxydiols, particle-phase oligomerization, and physical properties such as volatility and viscosity. Several SOA processes highlighted in this review are complex and interdependent and have nonlinear effects on the properties, formation, and evolution of SOA. Current global models neglect this complexity and nonlinearity and thus are less likely to accurately predict the climate forcing of SOA and project future climate sensitivity to greenhouse gases. Efforts are also needed to rank the most influential processes and nonlinear process-related interactions, so that these processes can be accurately represented in atmospheric chemistry-climate models.
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| 9. |
- Volpp, Joerg, et al.
(författare)
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Beam shaping solutions for stable laser welding : Multifocal and multispot beams to bridge gaps and reduce spattering
- 2021
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Ingår i: PhotonicsViews. - : John Wiley & Sons. - 2626-1308 .- 2626-1294. ; 18:5, s. 38-41
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The laser beam is a highly flexible tool that is used for many material processing applications. New spatial beam shaping technology enables even further functionality to create up to four focal spots in an axial or lateral direction, which enables the distribution of the energy to be tailored. It was shown that certain beam shapes help to suppress spattering and increase the bridgeable gap size during deep penetration laser welding.
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