Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271LASER CUTTING WITH FIBER GUIDED LASER RADIATIONWolfgang SchulzNonlinear Dynamics of Laser Processing, RWTH Aachen University, GermanyEmail: wolfgang.schulz@nld.rwth-aachen.deAbstract. Laser cut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g is an industrially established production technology. Nevertheless, there aregaps in understanding the influence of dynamical processes on production quality regarding hig<strong>hc</strong>ut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g speeds assuring efficient facility utilization as well as for low cut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g speeds, which are used tocut contours accurately. Experimental diagnostics is found to be crucial for finding suitable controldirectives in laser cut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g. Physical modeling is therefore always based on experimental evidence.Quality features such as surface roughness or adherent dross are caused by the dynamics of theprocess. The various process states and characteristic <strong>ph</strong>enomena (e.g. evaporation, drop formation,gas flow) are visualized in a process simulation and have experimentally been validated. The processsimulation reproduces experimental details, observed by the diagnostic system and makes it possibleto retrieve possible strategies to avoid defects like adherent dross. In this talk, state-of-the-art andrecent advances in modeling, simulation and diagnostics are presented. The effects of radiationwavelength on the cut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g quality are thoroughly discussed. Therefor, a numerical tool for simulationof the process has been developed which is based on a model describing the dynamical behavior ofmoving interfaces. This tool can be used to predict the cut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g quality induced by ripple formationsalong the cut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g front. Additionally, a rigorous mathematical analysis of the process can be applied onbasis of so-called reduced models obtained by using various mathematical analysis tools such asintegral, spectral and asymptotic expansion methods. Ripple formation along the cut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g front isinvestigated by a mathematical stability analysis showing process parameter dependencies on thecut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g quality. Finally, the process will be investigated in the context of metamodeling. Thistechnique bridges the gaps between technology tables and numerical simulation as well as betweenreduced modeling and empiricism allowing for analysis and optimization of the process. A recentlydeveloped tool based on symbolic regression will be used to demonstrate the applicability in lasercut<s<strong>trong</strong>>tin</s<strong>trong</strong>>g.28
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271GENERATION AND APPLICATION OF PHASE MATCHED SMALLBANDWIDTH COHERENT EXTREME ULTRAVIOLET RADIATIONDao Van Lap, Dinh Ba Khuong and Peter HannafordAustralian Research Council Centre of Excellence for Coherent X-Ray Science, Centre for Atom Opticsand Ultrafast Spectroscopy, Swinburne University of Technology, Melbourne, Australia 3122.Abstract. We are able to generate by high-harmonic generation (HHG) just a few harmonic orders inthe spectral range 10 - 35 nm with a <strong>ph</strong>oton flux of ~ 2.10 12 <strong>ph</strong>otons/(harmonic cm 2 s) for argon and ~10 10 <strong>ph</strong>otons/(harmonic cm 2 s) for helium. The few-harmonic-order radiation is used for coherentdiffractive imaging directly or with a small band-width mirror. A spatial resolution down to 50 nm canbe achieved.I. INTRODUCTIONCoherent extreme ultraviolet (XUV) radiation can now be produced by high-order harmonicgeneration (HHG) of highly energetic femtosecond laser pulses in noble gases [1-3]. Such tabletop sources will complement large installations such as X-ray free-electron lasers (XFELS)currently under development. The unique properties of HHG emission present opportunities fornew applications in atomic and molecular spectroscopy, condensed matter <strong>ph</strong>ysics, imaging onthe nano- and subnano-scale, and plasma <strong>ph</strong>ysics [4-6].The brightness (which is related to the conversion efficiency from the near-infrared laser lightto XUV light), the spatial and temporal coherence properties and the <strong>ph</strong>oton energy areimportant properties of the source from the point of view of potential applications. The mainchallenge for the field at this time is to improve and control the properties of the source and toenhance the <strong>ph</strong>oton flux and degree of coherence. Significant enhancement of the efficiency ofHHG has been achieved by several groups through quasi <strong>ph</strong>ase-matching and the “absorptionlimit” for which re-absorption limits the effective length of the medium has now been reached.However, the conversion efficiency depends on the HHG configuration even within theabsorption limit and the ultimate optimization conditions are still far from being fullycharacterized.For many applications, such as coherent diffractive imaging (CDI) [7,8], a monochromaticbeam or a beam with a narrow and well characterized bandwidth is an important requirement ofthe source. In conventional CDI it is normally assumed in the image reconstruction that thesource is monochromatic with λ/∆λ >1000 [7]. Since the HHG source consists of manyharmonic orders, the above assumption represents a fundamental limitation of the applicability ofCDI to imaging. A spectrometer is typically used to select the monochromatic radiation;however the transmission of spectrometers in the XUV region is low. The generation of just afew harmonics, and especially a single harmonic, that can be used directly without any spectralselection would be very advantageous for such applications.In this paper we show that a semi-infinitive gas cell can be used to generate a high flux ofnarrow-bandwidth coherent XUV HHG radiation. We also demonstrate that the small bandwidthmulti-wavelength HHG sources can be applied to produce very high quality coherent diffractiveimages.29
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Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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