Ablation by ultrafast lasers results from a series of complex nonlinear phenomena of absorption and transfer of energy that take place in the surfaces of materials upon irradiation. Provided that a good window of processing parameters is chosen, the resulting thermal effects are in general negligible, making ultrafast lasers excellent micromachining tools applicable to most types of materials. It is thus beneficial to understand how ablation is affected by the laser processing parameters and the material properties, in order to optimize the micromachining processes.
We propose an engineering model to estimate the dimensions of ablation, taking into account on the one hand the material properties such as the ablation threshold, penetration depth and the refractive index and, on the other hand, the processing parameters namely the pulse energy and beam diameter, scanning speed, repetition rate and angle of incidence. The model considers as well the effects of incubation, changes of topography during multi-pulse irradiation, surface reflectivity and Gaussian beam diameter variation with the distance to the focal plane.
The model is able to simulate the profiles of ablation surfaces produced by normal or tilted laser beam, either for spot, line and area processing. The results obtained are validated by comparison to the ones obtained experimentally. Both the model and the experiments focus on stainless steel. The predictions of the model also allow for the optimization of the micromachining process, both energy and time wise.
The today available ultra-short pulsed laser systems offer average power in the range of 100 W or even more resulting in high pulse energies. In contrast to treat metals only moderate peak fluences are required to work at the well-known optimum point, where the ablation process is most energy efficient. In a standard setup the laser beam is deflected by a galvo scanner. The achievable scan speed is limited and therefore also the usable repetition frequency and average power. The use of pulse bursts instead of single pulses is a possibility to further increase the ablation rate, i.e. using higher average power. This further increase is crucial for the usage of the ultra-short pulses in industrial applications. It was shown in previous publications that the number of pulses in the burst have a significant influence on the specific removal rate in case of ps pulses. It was observed, that the second pulse of a 2-pulse burst is shielded by the particle plume of the first pulse of the burst. It is believed that the shielding effect depends on the particle density of the plume, thus the effect should be stronger if more material is ablated. As already known, a decrease of the pulse duration to a few hundreds of fs leads to an increase of the specific removal rate for single pulses. In this work we investigate the influence of pulse bursts on the specific removal rate as well as the pulse duration on the ablation process using pulse bursts.
We study the surface topographical, structural, and compositional modifications induced in bovine cortical bone by femtosecond laser ablation. The tests are performed in air, with a Yb:KYW chirped-pulse-regenerative amplification laser system (500 fs, 1030 nm) at fluences ranging from 0.55 to 2.24 J/cm2. The ablation process is monitored by acoustic emission measurements. The topography of the laser-treated surfaces is studied by scanning electron microscopy, and their constitution is characterized by glancing incidence x-ray diffraction, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and micro-Raman spectroscopy. The results show that femtosecond laser ablation allows removing bone without melting, carbonization, or cracking. The structure and composition of the remaining tissue are essentially preserved, the only constitutional changes observed being a reduction of the organic material content and a partial recrystallization of hydroxyapatite in the most superficial region of samples. The results suggest that, within this fluence range, ablation occurs by a combination of thermal and electrostatic mechanisms, with the first type of mechanism predominating at lower fluences. The associated thermal effects explain the constitutional changes observed. We show that femtosecond lasers are a promising tool for delicate orthopaedic surgeries, where small amounts of bone must be cut with negligible damage, thus minimizing surgical trauma.