Beat Jäggi

Processing of industrially relevant non metals with laser pulses in the range between 10ps and 50ps

In materials processing pulses in the ps range are an excellent tool to achieve high precision. Although for metals even shorter pulses would be desirable, the lower ps range is attractive from the point of view of the availability of industrial grade systems. In that sense we have analyzed metals and found that the increase of the pulse duration from 10ps to 50ps directly goes along with a significant drop of the maximum achievable volume ablation rate which may reduce the attractiveness of systems with pulse durations in the range of a few tens of ps. The situation changes for non metals like ceramics, polyether ether ketone (PEEK) and polycrystalline diamond (PCD). For PEEK and PCD the dependence of the ablation efficiency on the pulse duration in the investigated regime is strongly reduced. Silicon again shows a metal like behavior, which seems to be independent from the doping level. The porous form of the chosen zirconium oxide ceramic thwarted a systematic study to determine the thresholdfluence and the energy penetration depth at the used laser wavelength of 1064nm. Nevertheless it was possible to machine small structures into the zirconium oxide. The obtained results show, that laser systems with pulse durations in the range of a few tens of ps, which are expected to be fiber based and cost effective, may be a very attractive alternative to machine industrially relevant non metals.In materials processing pulses in the ps range are an excellent tool to achieve high precision. Although for metals even shorter pulses would be desirable, the lower ps range is attractive from the point of view of the availability of industrial grade systems. In that sense we have analyzed metals and found that the increase of the pulse duration from 10ps to 50ps directly goes along with a significant drop of the maximum achievable volume ablation rate which may reduce the attractiveness of systems with pulse durations in the range of a few tens of ps. The situation changes for non metals like ceramics, polyether ether ketone (PEEK) and polycrystalline diamond (PCD). For PEEK and PCD the dependence of the ablation efficiency on the pulse duration in the investigated regime is strongly reduced. Silicon again shows a metal like behavior, which seems to be independent from the doping level. The porous form of the chosen zirconium oxide ceramic thwarted a systematic study to determine the thresholdfluence an...

Picosecond-laser bulk modification induced enhancement of nitrogen-vacancy luminescence in diamond

We report on the enhancement of nitrogen-vacancy (NV) luminescence induced by the bulk structure modification of a type IIa single-crystal diamond with a visible picosecond laser. Using online monitoring of picosecond-laser-induced photoluminescence (PL) in the bulk regions we found that the integrated intensity of the NV PL is significantly increased after the bulk microstructure formation. The confocal PL spectroscopy investigations of the bulk microstructures have evidenced the enhanced NV luminescence and the splitting of the NV− emission. The increased concentration of the NV defects during picosecond-laser bulk modification correlates with the formation of sp3 carbon allotropic structures and other defect centers revealed in PL/Raman spectra of high-stress regions near the fabricated microstructures.

High throughput surface structuring with ultrashort pulses in synchronized mode with fast polygon line scanner

High precision laser micromachining requires an exact synchronization of the laser pulse train with the mechanical axes of the motion system to ensure for each single pulse a precise control of the laser spot position - on the target. For ultra short pulsed laser systems this was already demonstrated with a conventional two-axis galvanometer scanner. But this solution is limited by the scanner architecture to a marking speed of about 10m/s with a maximum scan line length of about 100mm. It is therefore not suited for average powers far beyond 10W when working at the optimum point with highest removal rate and machining quality is desired. A way to overcome this limitation is offered by polygon line scanners which are able to realize much higher lateral speeds at large scan line lengths.In this work we will report on the results with a polygon line scanner having a maximum moving spot velocity of 100m/s, a scan line length of 170mm, spot diameters of 45µm (1064nm) and 22µm (532nm) together with a 50W, 10-ps laser system. The precise control of the laser spot position i.e. the synchronization is realized via the new SuperSyncTM technology. Decoating, perforation and 3D patterning will act as benchmark processes to evaluate this scanning technology.High precision laser micromachining requires an exact synchronization of the laser pulse train with the mechanical axes of the motion system to ensure for each single pulse a precise control of the laser spot position - on the target. For ultra short pulsed laser systems this was already demonstrated with a conventional two-axis galvanometer scanner. But this solution is limited by the scanner architecture to a marking speed of about 10m/s with a maximum scan line length of about 100mm. It is therefore not suited for average powers far beyond 10W when working at the optimum point with highest removal rate and machining quality is desired. A way to overcome this limitation is offered by polygon line scanners which are able to realize much higher lateral speeds at large scan line lengths.In this work we will report on the results with a polygon line scanner having a maximum moving spot velocity of 100m/s, a scan line length of 170mm, spot diameters of 45µm (1064nm) and 22µm (532nm) together with a 50W, 10-p...

Beam delivery methods for highly efficient USP-laser micro structuring of large cylindrical surfaces for printing and embossing tools

Despite of today’s availability of high power USP lasers up to several hundred Watts, it is still a challenge to structure large surface areas as required on printing and embossing rollers within an acceptable processing time for industrial production. This paper shows and compares the results of two different approaches, the parallel treatment of the workpiece with multiple beam arrangements (each beam individually modulated at repetition rates of up to 2 MHz) and a second approach based on high pulse repetition rates from 6 MHz to 16 MHz combined with fast beam scanning techniques with a polygon scanner up to 100 m/s.

Ablation dynamics – from absorption to heat accumulation/ultra-fast laser matter interaction

Abstract Ultra-short laser radiation is used in manifold industrial applications today. Although state-of-the-art laser sources are providing an average power of 10–100 W with repetition rates of up to several megahertz, most applications do not benefit from it. On the one hand, the processing speed is limited to some hundred millimeters per second by the dynamics of mechanical axes or galvanometric scanners. On the other hand, high repetition rates require consideration of new physical effects such as heat accumulation and shielding that might reduce the process efficiency. For ablation processes, process efficiency can be expressed by the specific removal rate, ablated volume per time, and average power. The analysis of the specific removal rate for different laser parameters, like average power, repetition rate or pulse duration, and process parameters, like scanning speed or material, can be used to find the best operation point for microprocessing applications. Analytical models and molecular dynamics simulations based on the so-called two-temperature model reveal the causes for the appearance of limiting physical effects. The findings of models and simulations can be used to take advantage and optimize processing strategies.

Residual heat during laser ablation of metals with bursts of ultra-short pulses

Abstract The usage of pulse bursts allows increasing the throughput, which still represents a key factor for machining with ultra-short pulsed lasers. The influence of the number of pulses within a burst on the specific removal rate is investigated for copper and stainless steel. Furthermore, calorimetric measurements were performed to estimate the residual energy coefficient as well as the absorptance of machined surfaces for copper to explain the reduced specific removal rate for a 2-pulse burst and the similar or even higher rate for a 3-pulse burst compared to single pulse ablation. Based on the measurements, a description of the process using single pulses and pulse bursts with up to three pulses is presented.

Random surface texturing of mc-Silicon for solar cells with picosecond lasers; a comparison between 1064 nm, 532 nm and 355 nm laser emission wavelengths

Multicrystalline Silicon was textured with picosecond laser. Different laser wavelengths (λ = 1064, 532, 355 nm) where compared regarding laser-induced damage. We found that λ = 355 nm picosecond radiation resulted in shallower defect-reach region.