Pulsed laser drilling in steel was explored in a wide range of repetition rates (up to 200 kHz) at variable pressure of the
ambient air. A critical repetition rate was established, exceeding of which drastically increased productivity of ablative drilling. The phenomenon was attributed to formation of a long-living domain (> 100 μs) with hot rarefied atmosphere in the vicinity of the ablated spot or inside the drilled channel. This rarefication was shown to reduce the plasma screening effect, which allows creation of quasi-vacuum conditions for laser ablative technology avoiding vacuum-pumped facilities.
Ablation of Fe, Al, Ni, and Cu by laser pulses at durations of 0.1, 1, and 5 ps is investigated experimentally. The laser fluence used vaires from below the ablation threshold up to 100 J/cm<sup>2</sup>. The ablation rate depends on the laser pulse duration at laser fluences above several J/cm<sup>2</sup> as the shorter pulse produces higher ablation rate. A change of the ablation regime with the laser fluence increase is also observed. The presence of molten material is clearly expressed at fluences above 10 J/cm<sup>2</sup> for all pulse durations used. These effects can be referred to the contribution of the electron heat diffusion in the distribution of the absorbed energy. The traces of solidified molten material suggest for realizations of melt ejection mechanism of ablation.
Influence of pulse duration on microprocessing of Al is studied. Results show noticeable differences in terms of quality, burr height and remolten or recast matter into micromachined grooves at high fluence regime for 120fs and 4,5 ps pulse duration. At 120 fs experimental results of penetration depth are found to be 2 or 3 times higher than the theoretical optical penetration depth and is lowered to this value with increasing pulse duration. At high fluence regime up to 2 J/cm<sup>2</sup>, ablation thresholds are found to be in the range 10 times higher than for the case of 1 J/cm<sup>2</sup>. Penetration depths are higher by a factor 10 to 20 than the theoretical optical penetration depth. The ablation rate is nearly constant until 1 ps and then falls down to 2 times lower values and decreases regularly until 4,5 ps. This time is supposed to correspond to a critical pulse width between ultrashort and short regime.
On the fast growing market of precision micro-machining of metals lasers do not only compete with other methods of structuring. There is also strong competition among different laser-processing strategies and, especially, among laser sources with different pulse duration. A comprehensive study of laser micro-machining with nanosecond, picosecond, and femtosecond laser pulses will be presented with a focus on fundamental aspects of the processes and on their practical consequences. An analysis will be given of the potential or the limitations of these laser processes with respect to their industrial application.
We have studied ablation plumes generated by femtosecond and picosecond-laser pulses using various optical methods for both single-pulse ablation as well as for drilling with up to 1 kHz repetition rate. Time-resolved shadow and resonance-absorption photographs visualize the plume- and vapor-expansion behavior in the nanosecond- and microsecond-time domains whereas the detection of Mie-scattered 308 nm-radiation allows to qualify the vapor movement and accumulation up to several milliseconds, i.e. well beyond the corresponding pulse-to-pulse separation at 1 kHz repetition rate.
This contribution examines the basic concepts and results of two laser ablation models based on commercially available hydrodynamical codes. In both cases the different material phases are described continuously by a single numerical algorithm. The first approach uses a finite-element model for the simultaneous description of solid and melt. It is thereby particularly suited for the description of melt formation and ejection. The results indicate a slow acceleration of the melt during the laser pulse up to velocities of some 10m/s followed by a rather steady-going ejection which is finally cut off by the resolidification. Although it was possible to examine this expulsion process, the model showed considerable numerical stability problems for higher intensities and the ultrasonic vapor expansion cannot be included. To overcome these shortages another model is proposed which is based on an equation of state for the target material in combination with a special pressure-based solver. Besides the continuous description of the material states, it also includes a continuous treatment of the beam propagation and energy coupling by solving Maxwell's equations. Although the work on this model is still going on, some of its basic prospects and limitations can already be discussed.
This work investigates the role of ambient atmosphere in material ablation by ultra-short intense laser pulses. It is shown, that ablative action of femtosecond pulses reveals limitations imposed by nonlinear optical response of gases resulting in significant modification or the incident laser beam. This phenomenon called conical emission (CE) manifests itself as strong scattering or emission of radiation in the forward direction developed at focusing of intense pulses of Ti:Sa laser (π=110÷1500 fs) in air. Transformation of the nearly Gaussian spatial profile into a wide angle cone is followed by spectral conversion of the fundamental laser frequency into a broad spectrum with relatively shorter wavelengths extending up to the visible range. Thresholds, converted energy, spectra and profiles of scattered radiation were measured at variable laser pulse duration and the ambient pressure. It was found, that more than 70% of the incident pulse energy can be scattered at conventional focusing of the beam by a long focal length lens. Effect of CE on material ablation in air was investigated, and the data obtained allowed to explain paradox morphology of steel channels drilled by high power femtosecond pulses.
The accuracy of laser drilled holes in metals is limited by a relatively large amount of molten material which is produced when lasers with pulse durations in the range of nanoseconds or longer are used. In general, shortening the pulse duration down to the picosecond or femtosecond regime promises to overcome these problems. In this contribution different influences on hole quality such as energy density, beam quality, and polarization as well as processing strategies for high precision drilling of steel with ultra-short pulses are presented and discussed. A new method of polarization control is demonstrated by which the hole geometry can significantly be improved and ripples in the surface of the hole walls can be avoided during helical drilling. Furthermore, results of investigations on the influence of the ambient pressure will be presented.
Plasma and vapor plumes generated by ultrashort laser pulses have been studied by various optical methods for both single pulse ablation as well as high-repetition rate drilling. Time-resolved shadow and resonance absorption photographs enable to determine the plume and vapor expansion behavior and, by means of an analytical shock wave model, allow to estimate an energy balance that can be refined by plasma transmission measurements. The results furthermore suggest that several types of laser-induced plasmas can be distinguished according to their origin: the material vapor plasma originating at the ablated surface even at moderate intensities, a breakdown plasma at increased power densities occurring in cold vapor or dust particles left from previous ablations during repetitively-pulsed processing and, finally, the optical breakdown in the pure atmosphere at high intensities. The latter also gives rise to nonlinear scattering phenomena resulting in a strong redistribution of the energy density in the beam profile.
The detailed study of the role of air pressure in deep hole drilling by femtosecond and picosecond intense laser pulses (Ti:Al<sub>2</sub>O<sub>3</sub> and Nd:YAP lasers) was performed in the range 1÷1000 mBar. Steel sample plates were mostly tested, experimental data obtained for ceramic materials is also presented. The following ablation parameters were measured and analyzed: ablation rates and their dependence on the channel depth, ablated crater morphology, optical transmission in channels after through hole formation. Both percussion and helical drilling regimes were used.
Special attention was paid to two strong gas assisted effects typical of sub-picosecond and sub-nanosecond material ablation, which are low threshold gas breakdown in deep channels and nonlinear interaction of ultra-short intense pulses with air resulting in conical emission. Unwanted aspects of both phenomena were shown to disappear in a moderate vacuum of ~100 mBar. A new approach to formation of such a vacuum in drilled channels was also proposed and experimentally modeled using ultra-high repetition rate nanosecond laser pulses.
The presence of melt during the laser drilling process always signifies a balance between an efficient material removal in molten form and a reduction of quality due to recast on the hole walls and near the crater entrance. Earlier investigations have demonstrated that by reducing the laser pulse duration the amount of produced melt can be decreased and hence, the precision increased. Nevertheless, they also demonstrate that melt can never be avoided completely. Therefore, to achieve an optimum balance between efficiency and quality by a preferably complete expulsion of melt the physical fundamentals of its generation and ejection have to be understood. By applying several different analytical and numerical models ranging from simple estimations to multi-dimensional simulations, the authors will outline the peculiarities of the melt formation and dynamics during the drilling with short and ultra-short laser pulses. Since these calculations demonstrate the importance of the consideration of melt acceleration and geometric aspects, special interest will be taken in these matters. While the evaporation stops soon after the laser pulse, the melt ejection may continue until the complete solidification of the material. For a better understanding and verification, the results of the models will be compared to experimental data.
The drilling process in different materials (diamond, steel, ceramics and PMMA) was studied for a large range of pulse lengths from about 100 fs to 10 ns using different approaches. In transparent materials the penetration process was visualized with high-speed video analysis and microscopy. The drilling rate as well as the relation between processing energy density and ablation threshold were determined in situ. The penetration of the laser beam inside the channel and the influence of laser-ignited plasma were investigated by transmission measurements. Mechanisms of energy coupling and heat losses were examined by applying simple analytical calculations. Proposals for the basic understanding of the drilling process are presented.
At the end of 1999 a German National Project called PRIMUS was established, the most important aim of which is to analyze the potential advantages of ultrashort pulses in combination with different drilling strategies and to obtain a better understanding of the ablation and drilling processes. This contribution will present the first results of this project. The advantages of short and ultrashort pulses in view of quality and efficiency will be discussed. It will be shown, that the use of suitable drilling technologies, such as e.g. helical drilling, and a specifically designed trepanning optic can significantly increase the quality of holes as well as expand the possible range of applications.
Experiments on deep drilling of steel by 300 ps, 1 ps and 125 fs laser pulses are reported. The ablation rate dependence on the channel depth was studied and energy losses in through channels for different radiation parameters were measured. The low-threshold cluster-assisted air breakdown was revealed to play an important role in ablation by 300 ps pulses. The ablated particles remaining inside the channel between laser shots provide substantial reduction of the air breakdown threshold. Laser-induced spark produces noticeable shielding effect and, presumably, is main reason of observed deep channel widening. Pronounced strengthening of light shielding by laser-induced spark was observed under steel target ablation comparing with pure air without target for ultrashort (125 fs, 1 ps) laser pulses. The dramatic reduction of the drilling rate in deep channels was observed for all examined pulsewidths. In the case of 300 ps pulses, the drilling rate falls down sharply by two order of magnitude at a certain critical channel depth increasing with the incident laser fluence. It was found that the integral plasma transmittance (breakdown plus ablation) remains unchanged when the drilling rate decreases.
Shadowgraphic and interferometric analysis of vapor/plasma plumes generated by pulses of a ns-Nd:YAG laser display a distinctive morphology for each of three different processing wavelengths (1064 nm, 532 nm, and 355 nm). Analytical models of shock wave expansion can be applied to determine the total energy stored within the plumes. Electron density distribution measurements reveal the regions of absorption and, using a simple model of inverse bremsstrahlung and photoionization, allow us to estimate absorption coefficients.