https://publik.tuwien.ac.at/files/PubDat_198014.pdf

Metal Ablation with Short and Ultrashort Laser Pulses

Laser materials processing with ultra-short pulses allows very precise and high quality results with a minimum extent of the thermally affected zone. However, with increasing average laser power and repetition rates the so-called heat accumulation effect becomes a considerable issue. The following discussion presents a comprehensive analytical treatment of multi-pulse processing and reveals the basic mechanisms of heat accumulation and its consequence for the resulting processing quality. The theoretical findings can explain the experimental results achieved when drilling microholes in CrNi-steel and for cutting of CFRP. As a consequence of the presented considerations, an estimate for the maximum applicable average power for ultra-shorts pulsed laser materials processing for a given pulse repetition rate is derived.

Heat accumulation during pulsed laser materials processing.

Electro-optic and Acousto-optic Laser Beam Scanners

Surface Structuring with Ultra-short Laser Pulses: Basics, Limitations and Needs for High Throughput

We study the incubation effect during laser ablation of stainless steel with ultrashort pulses to boost the material removal efficiency at high repetition rates. The multi-shot ablation threshold fluence has been estimated for two pulse durations, 650-fs and 10-ps, in a range of repetition rates from 50 kHz to 1 MHz. Our results show that the threshold fluence decreases with the number of laser pulses N due to damage accumulation mechanisms, as expected. Moreover, approaching the MHz regime, the onset of heat accumulation enhances the incubation effect, which is in turn lower for shorter pulses at repetition rates below 600 kHz. A saturation of the threshold fluence value is shown to occur for a significantly high number of pulses, and well fitted by a modified incubation model.

Role of heat accumulation on the incubation effect in multi-shot laser ablation of stainless steel at high repetition rates

Influence of the Pulse Duration in the ps-Regime on the Ablation Efficiency of Metals

Ultra short (ps, fs) laser pulses are used, when high requirements concerning accuracy, surface roughness, heat affected
zone etc are demanded for surface structuring Ps-laser systems that are suited to be operated in industrial environments
are of great interest for many practical applications Here results in the field of 3-d structuring (metals and transparent
materials), induced processes and structuring of flexible solar cells will be presented
Beside the pulse duration, which is given by the laser system, the user has a wide variety of optimization parameters such
as fluence, repetition rate and wavelength Based on a simple model it will be shown, that there exist optimum laser
parameters to achieve maximum volume ablation rates at a given average power To take benefit of these optimum
parameters and to prevent harmful effects like plasma shielding and surface melting, adapted structuring strategies,
depending on the requirements, have to be used Today's ultra short pulsed systems have average powers from a few W
up to a few 10W at high repetition rates The actual available beam guiding systems are limited and can often not fulfill
the requirements needed for high throughput structuring with optimized parameters Based on the achieved results, the
needs for future beam guiding systems will be discussed

Processing of metals and dielectric materials with ps-laserpulses: results, strategies, limitations and needs

Ever since industrially applicable ps laser systems have been available, cold ablation with ultra short laser pulses is of huge interest when high requirements concerning accuracy, defined surface roughness and small heat affected zone are demanded. Interesting applications are in the fields of surface and 3-d structuring with direct and induced processes. For a profitable industrial use of this technology high process efficiency is required, which is enabled by the development of systems with high average power of more than 100 W. The process efficiency directly scales with the average power when the repetition rate of the system is properly chosen. But aside from process efficiency often a high surface quality (low surface roughness, minimized surface melting and no oxidation processes) is desired. These measures are not only strongly affected by the laser parameters but also by the strategy of structuring. Especially for surface structuring the corresponding requirements for the equipment of the beam guiding system are often not accomplishable and therefore there is a strong demand for new technologies which have to be developed.

Processing of dielectric materials and metals with PS laserpulses

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.

Urs Hunziker

Papers

Influence of the Pulse Duration in the ps-Regime on the Ablation Efficiency of Metals

Processing of metals and dielectric materials with ps-laserpulses: results, strategies, limitations and needs

Processing of dielectric materials and metals with PS laserpulses

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

Influence of Pulse Bursts on the Specific Removal Rate for Ultra-fast Pulsed Laser Micromachining of Copper