B. Jaeggi

Ultra-high-precision surface structuring by synchronizing a galvo scanner with an ultra-short-pulsed laser system in MOPA arrangement

For surface and 3D structuring the ultra short pulsed laser systems are mostly used in combination with galvo scanners. This work reports on the synchronization of the scanner mirror motion with the clock of the laser pulses, which is usually in the range of 100 kHz and higher, by a modification of the electronic scanner control. This synchronization facilitates the placement of the small ablation craters from single pulses with the precision of about 1 μm relative to each other. The precise control of the crater positions offers the possibility to test and optimize new structuring strategies. Results of this optimization process with respect to minimum surface roughness, steepness of wall, accuracy to shape and efficiency will be presented.

High Throughput and High Precision Laser Micromachining with ps-Pulses in Synchronized Mode with a fast Polygon Line Scanner

To be competitive in laser micro machining, high throughput is an important aspect. One possibility to increase productivity is scaling up the ablation process i.e. linearly increasing the laser repetition rate together with the average power and the scan speed. In the MHz-regime high scan speeds are required which cannot be provided by commercially available galvo scanners. In this work we will report on the results by using a polygon line scanner having a maximum scan speed of 100 m/s and a 50 W ps-laser system, synchronized via the SuperSync™ technology. We will show the results concerning the removal rate and the surface quality for working at the optimum point i.e. most efficient point at repetition rates up to 8.2 MHz.

Burst mode with ps- and fs-pulses: Influence on the removal rate, surface quality, and heat accumulation

The burst mode for ps and fs pulses for steel and copper is investigated. It is found that the reduction of the energy in a single pulse (in the burst) represents the main factor for the often reported gain in the removal rate using the burst mode e.g. for steel no investigated burst sequence lead to a higher removal rate compared to single pulses at higher repetition rate. But for copper a situation was found where the burst mode leads to a real increase of the removal rate in the range of 20%. Further the burst mode offers the possibility to generate slightly melted flat surfaces with good optical properties in the case of steel. Temperature simulations indicate that the surface state during the burst mode could be responsible for the melting effect or the formation of cavities in clusters which reduces the surface quality.

Influence of the pulse duration and the experimental approach onto the specific removal rate for ultra-short pulses

To be competitive in industrial applications the throughput is a key factor in laser micro machining using ultra-short pulsed laser systems. Both, ps and fs laser systems are suitable for industrial applications. Therefore one has to choose the right pulse duration for highest ablation efficiency. As shown in earlier publications the efficiency of the ablation process can be described by the specific removal rate, which has a maximum value at an optimum fluence. But its value often bases on a calculation using the threshold fluence and energy penetration depth deduced by measuring the depth of ablated cavities machined with different fluences and number of pulses. But this calculated specific removal rate often differs from the one deduced from ablated squares as recently shown in literature. Further an unexpected drop of the specific removal rate was reported for stainless steel when the pulse duration was reduced from 900 fs to 400 fs. Thus the influence of the pulse duration in the fs and low ps regime onto the specific removal rate is investigated with different methods for industrial relevant materials

Picosecond-laser bulk modification, luminescence and Raman lasing in single-crystal diamond

Bulk modification and micro-structuring of diamonds using ultra-short laser pulses is of great interest due to its potential in photonic applications, radiation detectors and diamond gem marking. We report on bulk micro-structuring and stimulated Raman scattering (SRS) in type IIa single crystal diamond with multi pulse irradiation by picosecond-laser pulses at the wavelength 532nm (10ps & 44ps). The experiment was expanded by additional setups for on-line video imaging and spectroscopic measurements during laser irradiation and structure growth in the bulk diamond from the backside of the crystal. We discuss the influence of the crystal orientation ({100} and {110}) relative to the laser beam onto (i) the optical breakdown threshold, (ii) the character of the structural modifications and (iii) generation of SRS during irradiation. We show that the formation of bulk microstructures dramatically influences the behavior of the SRS and that the structure growth and the laser-induced breakdown in the bulk are governed by the dielectric breakdown mechanism. We will further present the conditions for efficient SRS lasing depending on the different pulse durations. Based on the Stokes-to-anti-Stokes intensity ratio in the recorded SRS spectra we will finally propose a method of local temperature measurements in the bulk of diamond to determine the “pre-breakdown” temperature.

Improvements in ultra-high precision surface structuring using synchronized galvo or polygon scanner with a laser system in MOPA arrangement

In earlier work the capabilities of synchronizing a galvo scanner or a polygon line scanner with a picosecond laser system in MOPA arrangement were presented. However these systems only enabled precise positioning of laser pulses on the target relatively to each other. Since then a novel approach to increase the absolute precision in positioning has been developed. This improvement enables new and more efficient process strategies such as bidirectional processing or high precision structuring of large areas in combination with additional mechanical axes. These improvements represent a major step towards large scale industrial applications in laser based micromachining.