This work presents designed and fabricated silica few-mode optical fiber (FMF) with induced twisting 10 and 66 revolutions per meter, core diameter 11 µm, typical "telecommunication" cladding ...diameter 125 µm, improved height of quasi-step refractive index profile and numerical aperture 0.22. Proposed FMF supports 4 guided modes over "C"-band. We discussed selection of specified optical fiber parameters to provide desired limited mode number over mentioned wavelength range. Some results of tests, performed with pilot samples of manufactured FMF, are represented, including experimentally measured spectral responses of laser-excited optical signals, that comprise researches and analysis of few-mode effects, occurring after fiber Bragg grating writing.
The whole-phase heat transport model of laser-based directed energy deposition (L-DED) with multichannel coaxial powder feeding is the basis of investigating the underlying physical mechanisms, such ...as molten pool evolution, thermal history, grain growth, stress and deformation evolution. The existing heat transport models usually ignore or simplify some important physical processes, which limits the simulation accuracy of the models to some extent. In this paper, a modeling framework for whole-phase heat transport for coaxial L-DED is developed. Considering the interaction among the laser beam, coaxial multipowder stream and substrate, a comprehensive whole-phase heat transport model including a parametric model of powder-scale laser-powder coupling and deposition heat transport model is developed. A parametric model for laser-powder coupling can enable one to avoid the influence of the parameters of the coaxial multichannel powder-fed head on the number and structure of the model. The spatial relations between the laser beam, coaxial multipowder stream and substrate are modeled based on homogeneous transformation theory, which effectively addresses coordinate transformation on the complex spatial position and pose. The influence of two typical spatial laser beam profiles (SLBPs): the Gaussian profile (GP) and super-Gaussian profile (SGP), on the whole-phase heat transport is studied. The results show that under the conditions of the SGP and GP, the peak temperature of the coaxial multipowder stream on the deposition surface is below the melting point. The peak temperature distribution and powder heat flux for the coaxial multipowder stream shows a distribution with a lower center, higher sides and cruciform tower-like shape. The peak temperature and heat flux of the coaxial multipowder stream under the SGP are less than that under the GP. Dimensionless number analysis indicates that under both the SGP and GP, the Marangoni convection and thermal conduction jointly dominate the heat transport behavior within the molten pool. Compared to the GP, the Marangoni effect and the heat accumulation capacity of the molten pool under the SGP are weaker, but the heat dissipation capacity of the molten pool is stronger. The use of the SGP is favorable for reducing the thermal deformation of various parts during L-DED.
The effects of the two types of typical spatial laser beam profiles (SLBP), including the super-Gaussian profile (SGP) and Gaussian profile (GP), on the thermal-fluid transport during laser-based ...directed energy deposition (L-DED) were numerically investigated. Considering the laser-powder-pool coupling effect, a comprehensive heat transport model including an input source model, laser-powder coupling model and molten pool thermal-fluid transport model is established. It is found that the SLBP has an important influence on the temperature gradient at the solidification interface and the flow field. Compared with the GP, the temperature gradient at the solidification interface under the SGP is high. Because the Marangoni stress and fluid momentum are in opposite directions, the fluid velocity distribution on the molten pool surface presents an 'M' shape. The fluid velocity on the molten pool surface under the SGP is obviously different from that under the GP.
A three-dimensional transient heat and mass transfer model for laser-powder coupling is created based on the lumped parameter method. A super-Gaussian beam model that is closer to the fiber laser ...beam profile for welding is employed instead of a frequently-used Gaussian beam. A beam characteristic parameter identification method is proposed to identify the beam characteristic parameters such as the focal spot radius and Rayleigh length. The spatial distribution of the beam power density is reconstructed. The super-Gaussian beam model has a better goodness-of-fit index than the Gaussian beam model. Finite difference method is used to solve the heat and mass transfer model developed. Some powder particles, under the action of three types of laser beams, have evaporated before falling into the laser-induced pool. The average heating rate for the super-Gaussian beam and non-ideal Gaussian beam is lower than that of the ideal Gaussian beam. Although the heated powder distribution zone and the melted powder distribution zone on the workpiece surface for the super-Gaussian beam are larger, the maximum powder mass loss by evaporation is the smallest. The effect of the identified super-Gaussian beam and non-ideal Gaussian beam on the heat and mass transfer zone and evaporation-induced mass loss is not significantly different, except for the ideal GB.
Laser parameters in high resolution MALDI Imaging were investigated and evaluated to understand ion formation in the micrometer spatial resolution range.
Laser focus diameters (optical resolution) ...and dependence of ion signal intensities on laser fluence were investigated for MALDI imaging mass spectrometry at high spatial resolution (optical focus diameters in the range of 1.1–8.4
μm). Results are of interest in the light of MALDI mechanisms and of methodological optimization, since no data was available yet for such small laser spots. The dependence of ion signal intensities on laser fluence was found to be in line with data published for larger laser spot diameters. Results argue for a common general mechanism in MALDI, with a steady trend of parameter values when going from larger (>100
μm) to smaller (<10
μm) spot sizes.
The scattering effect of laser-transparent part has significant influence on the intensity profile of heat source during laser transmission welding (LTW). The purpose of the present study is to ...propose a method for modeling the laser heat source considering light scattering. The knife-edge experiment is served to obtain the normalized power flux distribution (NPFD) without considering light scattering, and the non-contact line scanning experiment is used to describe the NPFD considering light scattering. Subsequently, the energy transformation algorithm is presented to transform the normalized line energy intensity into the normalized point energy intensity. Then the heat source considering light scattering can be modeled based on the distribution of point energy intensity. Compared with the heat source without considering light scattering, the intensity profile of heat source considering light scattering shows a wider width and a lower peak height. Especially when the laser-transparent part contains reinforcements, the difference of intensity profile is more evident than the unreinforced laser-transparent part. This indicates that light scattering has a significant influence on the laser intensity at the weld interface. The proposed method for modeling the heat source, considering light scattering, can optimize the laser source in the numerical simulation of LTW.
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•The knife-edge and non-contact line scanning methods are served to obtain the line energy intensity distribution.•An energy transformation algorithm is proposed to transform 1-D line energy intensity into 2-D point energy intensity.•The heat source considering light scattering is modeled at the weld interface.•The effect of thickness and reinforcement on light scattering and laser intensity is investigated.
We have investigated the use of a Gaussian beam laser for MALDI Imaging Mass Spectrometry to provide a precisely defined laser spot of 5 μm diameter on target using a commercial MALDI TOF instrument ...originally designed to produce a 20 μm diameter laser beam spot at its smallest setting. A Gaussian beam laser was installed in the instrument in combination with an aspheric focusing lens. This ion source produced sharp ion images at 5 μm spatial resolution with signals of high intensity as shown for images from thin tissue sections of mouse brain.
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This work presents fabricated silica microstructured optical fiber with special equiangular spiral six-ray geometry, an outer diameter of 125 µm (that corresponds to conventional commercially ...available telecommunication optical fibers of ratified ITU-T recommendations), and induced chirality with twisting of 200 revolutions per minute (or e.g., under a drawing speed of 3 m per minute, 66 revolutions per 1 m). We discuss the fabrication of twisted microstructured optical fibers. Some results of tests, performed with pilot samples of designed and manufactured stellar chiral silica microstructured optical fiber, including basic transmission parameters, as well as measurements of near-field laser beam profile and spectral and pulse responses, are represented.
This work presents the first instance of a silica few-mode microstructured optical fiber (MOF) being successfully fabricated with a hollow GeO2-doped ring core and by strongly inducing twisting up to ...790 revolutions per meter. Some technological issues that occurred during the manufacturing of the GeO2-doped supporting elements for the large hollow cores are also described, which complicated the spinning of the MOFs discussed above. We also provide the results of the tests performed for the pilot samples—designed and manufactured using the untwisted and twisted MOFs described above—which were characterized by an outer diameter of 65 µm, a hollow ring core with an inner diameter of 30.5 µm, under a wall thickness of 1.7 µm, and a refractive index difference of Δn = 0.030. Moreover, their geometrical parameters, basic transmission characteristics, and the measurements of the far-field laser beam profile patterns are also provided.
This work presents a fabricated silica few-mode microstructured optical fiber (MOF) with a special six GeO2-doped core geometry, an outer diameter of 125 µm (that corresponds to conventional ...commercially available telecommunication optical fibers), and improved induced twisting up to 500 revolutions per 1 m (under a rotation speed of 1000 revolutions per meter with a drawing speed of ~2 m per minute). The article discusses some technological aspects and issues of manufacturing the above-described twisted MOFs with complicated structures and geometry as GeO2-doped silica supporting elements for them. We present results of some measurements performed for fabricated samples of chiral silica six-GeO2-doped-core few-mode MOFs with various orders of twisting and both step and graded refractive indexes of “cores”. These tests contain research on MOF geometrical parameters, attenuation, and measurements of the far-field laser beam profile.
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