Andreas Michalowski

Microdrilling in steel using ultrashort pulsed laser beams with radial and azimuthal polarization

A linear to radial and/or azimuthal polarization converter (LRAC) has been inserted into the beam delivery of a micromachining station equipped with a picosecond laser system. Percussion drilling and helical drilling in steel have been performed using radially as well as azimuthally polarized infrared radiation at 1030 nm. The presented machining results are discussed on the basis of numerical simulations of the polarization-dependent beam propagation inside the fabricated capillaries.

Heat accumulation in ultra-short pulsed scanning laser ablation of metals.

High average laser powers can have a serious adverse impact on the ablation quality in ultra-short pulsed laser material processing of metals. With respect to the scanning speed, a sharp transition between a smooth, reflective and an uneven, dark ablated surface is observed. Investigating the influence of the sample temperature, it is experimentally shown that this effect stems from heat accumulation. In a numerical heat flow simulation, the critical scanning speed indicating the change in ablation quality is determined in good agreement with the experimental data.

Comparison between ray-tracing and physical optics for the computation of light absorption in capillaries – the influence of diffraction and interference

Ray-tracing is the commonly used technique to calculate the absorption of light in laser deep-penetration welding or drilling. Since new lasers with high brilliance enable small capillaries with high aspect ratios, diffraction might become important. To examine the applicability of the ray-tracing method, we studied the total absorptance and the absorbed intensity of polarized beams in several capillary geometries. The ray-tracing results are compared with more sophisticated simulations based on physical optics. The comparison shows that the simple ray-tracing is applicable to calculate the total absorptance in triangular grooves and in conical capillaries but not in rectangular grooves. To calculate the distribution of the absorbed intensity ray-tracing fails due to the neglected interference, diffraction, and the effects of beam propagation in the capillaries with sub-wavelength diameter. If diffraction is avoided e.g. with beams smaller than the entrance pupil of the capillary or with very shallow capillaries, the distribution of the absorbed intensity calculated by ray-tracing corresponds to the local average of the interference pattern found by physical optics.

Monitoring of the micro-drilling process by detection of laser-induced shock waves in air

Ablation and drilling utilising short pulsed solid state lasers are becoming increasingly important technologies for micro-machining, micro-structuring and micro-drilling. Ablative drilling using pulses shorter than 10 ps results in a high-precision processes and outstanding quality due to negligible heating of the workpiece and consequently a reduced formation of recast. The advantages of ultra-short pulsed lasers have thus proved to be attractive for industrial applications. Corresponding to this development, process monitoring and control turn out to be an essential requirement. Different aspects of the drilling process such as borehole depth, moment of break through and end of the process have to be monitored.In this paper we present a method that permits the determination of borehole depth as well as the detection of completion of desired channel morphology. The method utilizes the detection of shock waves generated during the interaction between laser beam and irradiated material. The results prove that the proposed method offers a potential for on-site diagnostics of the laser drilling process.Ablation and drilling utilising short pulsed solid state lasers are becoming increasingly important technologies for micro-machining, micro-structuring and micro-drilling. Ablative drilling using pulses shorter than 10 ps results in a high-precision processes and outstanding quality due to negligible heating of the workpiece and consequently a reduced formation of recast. The advantages of ultra-short pulsed lasers have thus proved to be attractive for industrial applications. Corresponding to this development, process monitoring and control turn out to be an essential requirement. Different aspects of the drilling process such as borehole depth, moment of break through and end of the process have to be monitored.In this paper we present a method that permits the determination of borehole depth as well as the detection of completion of desired channel morphology. The method utilizes the detection of shock waves generated during the interaction between laser beam and irradiated material. The results prove ...

Laser surface structuring with long depth of focus

Theoretical and experimental results concerning the use of axicons for laser-ablation are reported. Analytical formulas allow to predict the generated fluence distributions and the expected ablation widths. The influence of an imperfect axicon tip is discussed. The long depth of focus of the generated beam enables easy small-sized laser marking in dimension of the laser wavelength.

Improvement of nanosecond laser ablation quality by pulse shaping technique

A novel approach to restrain the formation of the burr during nanosecond laser ablation is reported in this paper. An assistant laser pulse, separated from the primary processing laser pulse with the pulse duration of 21 ns by temporal pulse shaping method, is used to control the formation of the melt deposit. The effect of the assistant pulse on the morphologies of the melt pools is investigated with the aid of microscope. The results of machining grooves on steel samples with the shaped pulses show a reduction of the burr at the boundary of the ablation zone. The contribution indicates a potential method for obtaining an efficient ablation as well as good processing quality in short pulse laser microfabrication.

Model of the final borehole geometry for helical laser drilling

Abstract A model for predicting the borehole geometry for laser drilling is presented based on the calculation of a surface of constant absorbed fluence. It is applicable to helical drilling of through-holes with ultrashort laser pulses. The threshold fluence describing the borehole surface is fitted for best agreement with experimental data in the form of cross-sections of through-holes of different shapes and sizes in stainless steel samples. The fitted value is similar to ablation threshold fluence values reported for laser ablation models.

Processing Techniques and System Technology for Precise and Productive Microdrilling in Metals

The fabrication of microstructures in metals employing ultrashort laser pulses has been under study since the middle of the 1990s. Over many years, extraordinary precision and machining quality have been demonstrated in surface patterning, cutting and drilling applications. However, apart from some exceptions, micromachining applications with ultrashort laser pulses could not be used in industrial production because of insufficient throughput until the late 2000s. This was the case particularly for microdrilling applications, where processing times easily reached several minutes per hole. This situation has changed fundamentally in recent years. In 2008, industry standard picosecond laser sources with average powers of 50 W combined with repetition rates up to 1000 kHz have been launched onto the market. In the present chapter, it is shown that these newly available laser specifications can be utilized for efficient and high-quality drilling techniques, the basic principles of which will be explained in combination with the necessary engineering expertise. A beam steering system for helical drilling with rotating beam profile will be presented. Finally, various diagnostic methods will be discussed with regard to visualization and fundamental understanding of ablation processes, but also as a basis for process monitoring and control.

Theoretical and experimental studies of ultra-short pulsed laser drilling of steel

Methods for the machining of metals based on the use of ultra-short pulsed laser radiation continue to gain importance in industrial production technology. Theoretical considerations and experimental studies on laser drilling of steel are discussed. The applicability of geometrical optics to calculate the absorbed energy distribution inside small blind holes is investigated theoretically. A model for melt transport during ultra-short pulsed drilling is proposed and verified experimentally. It confirms that helical drilling is advantageous for machining burr-free holes.

Efficient Laser Drilling with Double-Pulse Laser Processing

A significant improvement of drilling efficiency is observed in laser drilling thin steel plate with double-pulse laser technique. The laser used in the experiment is a 1047 nm Q-switched laser with pulse duration of 21 ns and interpulse separation of 52 ns. The maximum ablation rate of double-pulse is about 30 times higher than that in the conventional single-pulse drilling at room temperature. The enhancement ratio of ablation efficiency of double-pulse to single-pulse with the laser energy density, repetition rate, thickness of the samples and the ambient gas pressure is studied. These results can be important for practical applications in efficient laser drilling.