H. Hügel

Measurements of wavelength-dependent transmission in excimer laser-induced plasma plumes and their interpretation

Short-pulse laser ablation in air at 0.1 MPa leads to intense evaporation of the target material. The ablated material compresses the surrounding gas and leads to the formation of a shock wave. The incident laser radiation interacts with the partially ionized material vapour and the condensed material clusters embedded therein and affects the efficiency and quality of the ablation. Methods to increase the efficiency and quality of the ablation process require knowledge of these mechanisms. Therefore, the transmissivity of a laser-induced plasma plume was investigated in the wavelength range between 440 nm and 690 nm with a spatial resolution of about . The results show a weak dependence of the extinction coefficients over the investigated wavelength range. The spatial resolution allowed us to identify the regions behind the shock wave with the highest extinction for the visible wavelength range probed. These regions correspond to areas with high free-electron densities. To understand the mechanisms that are responsible for the heating and ionization of the vapour at the start of the excimer laser pulse, a simplified stationary model was applied. The experimental results were interpreted using Mie scattering theory on condensed material clusters, inverse bremsstrahlung absorption and absorption due to photoionization of excited material vapour atoms. The modelling shows that extinction of the laser light in a plasma with the assumed thermodynamic parameters is dominated by Mie absorption on condensed material clusters for wavelengths less than about 430 nm and is dominated by photoionization absorption and inverse bremsstrahlung above 430 nm.

Analytical investigations on geometrical influences on laser drilling

An analytical model for laser drilling is proposed which includes three-dimensional heat conduction in a simplified manner. For that purpose, the heating of the curved surface is locally described by that of a spherical cavity with comparable curvature within an infinite medium. Additionally, the absorption of laser radiation on the inclined side wall is taken into account. Using these components, it is possible to calculate the evolution of the hole shape from pulse-to-pulse in an iterative way. Therefore, this model is suitable to study the main aspects of deep drilling such as ablation rates and hole shapes without the disadvantage of long computational times. As a drilled hole deepens and the walls become steeper, its surface area grows and, thereby, in principle the absorbed intensity drops. This can lead to a considerable reduction of ablation rate. At the same time, extremely curved surface areas will heat much faster or slower than plane ones which, again, results in local changes of drilling velocity. It is shown that the former is particularly of interest for the description of the resulting hole shape while the latter has a considerable influence on the ablation velocity at the tip of the hole. To verify the analytical model, its results are compared with those of three-dimensional numerical simulations. It is shown that the simplified assumptions introduced here are, up to some extent, suitable to explain the final surface shapes for blind holes as well as the experimentally observed dependence of ablation rate on hole depth.

Two-wavelength interferometry on excimer laser induced vapour/plasma plumes during the laser pulse

Abstract Ablation-using short-pulse lasers, e.g., excimer lasers and solid state lasers, is becoming an important technology for micro-machining, thin film formation and fine particle generation. Hence, there is a great interest to understand the interaction mechanisms between the radiation field and the evaporated material. Especially the laser-induced material vapour influences the efficiency and the quality of the ablation, as shown in earlier contributions [G. Callies, P. Berger, J. Kastle, H. Hugel, Proc. SPIE, Vol. 2502, p. 706; G. Callies, H. Schittenhelm, P. Berger, F. Dausinger, H. Hugel, Proc. SPIE, Vol. 2246, p. 126]. Two-wavelength interferometry, shadowgraphy and resonance absorption photography allowed us to investigate the whole laser induced region with each probe-laser pulse. The experiments were performed in ambient air, helium and argon at a pressure of 10 5 Pa. Earlier, shadowgraphy experiments indicated several discontinuities within the plume arising during the laser pulse. To get more information about the nature of these discontinuities and their expansion behaviour and to obtain the free electron density distributions within the shock wave, interferometry with two wavelengths was applied. The results show spatially-separated regions of high free electron densities and therefore, high temperatures within the plasma plume. The observed regions correspond to those found by shadowgraphy and resonance absorption photography: the region of material vapour directly behind the contact front, the plasma core near the target surface with high electron densities, and two more regions separated by discontinuities. A variation of the ambient gas causes a drastic change in the electron density. In an argon atmosphere, a formation of a laser supported detonation wave, instead of a shock wave, arises for energy densities higher than 20–25 J/cm 2 . The interferometry yields, for this case, a very high electron density within the material vapour near the contact front. A comparison of the electron density distribution with ablation rates in helium and nitrogen indicate the independence of the interaction of the excimer laser radiation with the electrons.

Time-resolved interferometric investigations of the KrF-laser-induced interaction zone

Abstract In earlier ablation experiments the excimer laser-induced interaction zone was observed using Schlieren-photography. A spherical blast wave and, additionally, several regions separated by discontinuities behind the shock front were discovered during the pulse. This article will present experimental investigations using a Michelson interferometer in order to measure the phase front distortion of a probe laser radiation field crossing the mentioned interaction zone. The experiments were realized during the pulse (FWHM=30 ns, basis width=60 ns) under different ambient gas pressures (104–105 Pa), different energy densities (5–45 J/cm2) and with a time resolution of 500 ps. The index of refraction n was calculated using the optical path lengths obtained from the interference pattern, considering a radial symmetry. The results show a high refractive index at the shock front and a steep decrease towards the target closely behind the shock front. In this region the index of refraction tends to a lower value than the index of refraction of vacuum due to a high concentration of free electrons. A comparison with the results of Schlieren-photography gives reasonable explanations for the different regions within the interaction zone.

Laser-induced plasma as a source for an intensive current to produce electromagnetic forces in the weld pool

Laser beam welding performed with a CO2 laser by applying a magnetic field perpendicular to the welding direction influences the weld pool dynamics, which changes the seam properties significantly. From these results it was concluded that an intensive current density must exist in the melt. In repeating those welding experiments with a Nd:YAG laser, however, no significant effects could be observed. To explain this discrepancy, detailed trials with both CO2 and Nd:YAG lasers were carried out and led to an explanation which is presented in this paper. Looking at the welding process with radiation of 10.6 &mgr;m and 1.06 &mgr;m, the only significant difference is the presence of a laser-induced plasma plume above the workpiece in the case of the longer wavelength. Therefore, the investigations were concentrated on its possible role in establishing a current flow through the weld pool. This current was directly measured during the welding process (bead on plate): Two aluminum plates separated by an insulated gap of 0.85 mm were moved under the focused beam (3 kW; 5 m/min) and the signal was recorded as function of the gaps position. From these measurements were deduced values of current that amounted to approximately 0.3 A with CO2 and more then one order of magnitude less with Nd:YAG lasers.

Optical parameters of silicon carbide and silicon nitride ceramics in 0.2–1.3 μm spectral range

The optical properties—reflectance, absorptance, and transmittance—of silicon carbide (SiC) and silicon nitride (Si3N4) ceramics in the 0.2–1.3 μm spectral range were estimated by means of Monte Carlo simulation using single-crystal macroscopic optical constants assumed to remain uniform over a small unit of ceramic volume. The data obtained are in good agreement with the measurements made by other authors.

Structuring with excimer lasers—experimental and theoretical investigations on quality and efficiency

Manufacturing with excimer lasers is becoming an established technology in microstructuring, drilling, and laser vapor deposition. In this article an overview of the work will be given that was recently performed at the IFSW on excimer laser ablation concerning the quality and the efficiency of the ablation process. In a first part the setup used for the ablation experiments is presented. A beam homogenizer developed at IFSW is explained in more detail. The development of this device was initiated by the need for high beam quality along the whole propagation path. Results with modelings including three-dimensional heat conduction and multiple reflections of the incident beam at the hole walls will be compared with experimental findings. It will be shown that multiple reflections are responsible for the inhomogeneity of the bottom structure and can explain the quality of the ablated structure. Additionally, a few two- and three-dimensional structures obtained by using the presented setup and different work...

Aerodynamic window for high precision laser drilling

High precision laser drilling is getting more and more interesting for industry. Main applications for such holes are vaporising and injection nozzles. To enhance quality, the energy deposition has to be accurately defined by reducing the pulse duration and thereby reducing the amount of disturbing melting layer. In addition, an appropriate processing technology, for example the helical drilling, yields holes in steel at 1 mm thickness and diameters about 100 &mgr;m with correct roundness and thin recast layers. However, the processing times are still not short enough for industrial use. Experiments have shown that the reduction of the atmospheric pressure down to 100 hPa enhances the achievable quality and efficiency, but the use of vacuum chambers in industrial processes is normally quite slow and thus expensive. The possibility of a very fast evacuation is given by the use of an aerodynamic window, which produces the pressure reduction by virtue of its fluid dynamic features. This element, based on a potential vortex, was developed and patented as out-coupling window for high power CO2 lasers by IFSW1, 2, 3. It has excellent tightness and transmission properties, and a beam deflection is not detectable. The working medium is compressed air, only. For the use as vacuum element for laser drilling, several geometrical modifications had to be realized. The prototype is small enough to be integrated in a micromachining station and has a low gas flow. During the laser pulse, which is focussed through the potential flow, a very high fluence is reached, but the measurements have not shown any beam deflection or focal shifting. The evacuation time is below 300 ms so that material treatment with changing ambient pressure is possible, too. Experimental results have proven the positive effect of the reduced ambient pressure on the drilling process for the regime of nano- and picosecond laser pulses. Plasma effects are reduced and, because of the less absorption, the drilling velocity is increased and widening effects are decreased. So the process is more efficient and precise. Furthermore, the necessary pulse energy for the drilling of a certain material thickness is reduced and so laser power can be saved.

Continuous multiphase modeling of laser ablation with short pulses

Summary form only given. In laser drilling with short pulses many material phases with very different properties are simultaneously present which complicates the modeling of their interaction with each other. The demonstrated hydrodynamical model is based on a commercial code and uses a continuous description of solid, melt, and vapor by means of an equation of state. It is shown that the resulting ablation dynamics is very sensitive on the choice of the optical properties of the metal vapor. In order to consider the beam propagation within the plasma plume and the drilled hole, a numerical approach for solving directly Maxwell equations on a microscopic scale is presented, additionally.