High power ultrafast lasers are nowadays able to provide high ablation rates and meet the technical requirements and performances needed for many industrial applications. However depositing more energy into the target may induce heat accumulation, and consequently enhances the ablation efficiency as well as introduces some detrimental effect on the processing quality. So, the key issue is to improve throughput while maintaining the processing quality and preventing from any deformation of the work piece. In this paper we present some comparative results on surface ablation of Aluminum, Copper, Molybdenum and stainless steel in both picosecond and femtosecond regimes. Trials have been performed using two high power Yb-doped fibre ultrafast lasers which are tunable in pulse duration (350 fs to 10 ps, up to 20 W). Samples have been characterized with confocal microscopy and scanning electron microscopy. We discuss the effect of pulse duration, repetition rate, fluence and energy dose per mm on ablation efficiency, processing quality and ripples formation.

We report on the first direct measurements of the laser induced temperature distribution after the absorption of multiple ultrashort laser pulses at high repetition rates in borosilicate glass. To this end, we developed an in-situ micro Raman setup to determine the temperature dependent ratio between Stokes and Anti-Stokes scattering. The results indicate a critical influence of the pulse energy on the induced temperature. In borosilicate glass, the maximal temperature directly after the excitation (pulse energy of 1100 nJ, repetition rate of 1 MHz, wavelength of 1044 nm, pulse duration of 600 fs, 2000 pulses per laser spot) is more than 5000 K and rapidly cools down within several hundreds of ns

When a transparent material is irradiated by focused femtosecond laser pulses, various types of structural modification can occur inside the material. Recently, it has been reported that asymmetric structures, which are composed of two regions with positive and negative refractive index changes (Δn), can be inscribed by irradiation of multiple femtosecond laser pulses at high (>MHz) repetition rates. Interestingly, the amount of positive Δn in this type of modification is larger than those in conventional structural modifications by one order of magnitude. However, the mechanisms underlying such modifications are still unclear. In this paper, we describe that similar asymmetric structures can be inscribed by using femtosecond laser pulses at a low repetition rate. Then, we examine the formation and spectroscopic characteristics of the asymmetric structures. The observation of the processed areas reveals that the asymmetric structures are formed after the movement of voids. Furthermore, micro-Raman spectroscopy suggests a large increase of three-membered ring structures in the region with positive Δn, which may account for the large Δn.

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