We experimentally showed that the π/2-period oscillation of femtosecond laser ablation threshold with laser polarization orientation can be observed in calcium fluoride (CaF2). While the lowest single pulse ablation threshold, 5.3 J/cm2, was observed when laser polarization is along the 010-crystal axis, the highest one, 5.8±0.1 J/cm2, was found when angle between polarization and the 010-crystal axis were π/4 or 3π/4. The π/2-period oscillation of ablation threshold tends to be eliminated (yet still detectable) at high pulse fluence. The anisotropic ablation is possibly resulted from the different rate of avalanche ionization in different crystal axes.
We carried out a comparative study on ablation threshold behavior of femtosecond laser twin double-pulse processing of typical transparent material, semiconductors and metallic materials. Based on the change of ablation area with pulse temporal separation (100 fs-15ps) under the same spatially distributed Gaussian beam, influence of pulse-separation on normalized double-pulse ablation threshold (which is normalized to the single-pulse ablation threshold) was demonstrated qualitatively. Special attention paid on the variation characteristics of normalized double-pulse ablation threshold in the sub-picosecond pulse-separation range, as well as its value in pulse-separations comparable or larger than the electron-phonon (or ion) coupling time. We show that the ablation threshold behavior of femtosecond laser twin double-pulse is strongly material-specie dependent, however, can be summarized in to several ones. The difference in ionization and/or deionization mechanisms for the non-metal compared is possibly the physical origins for the contrasted behavior of double-pulse ablation threshold.
Carbon fiber reinforced polymer(CFRP) is a typical hard to machine material. High modulus CFRP is one of the most widely used types in spacecraft structures. In this paper, the ultrafast laser precision cutting technology for two kinds of high modulus materials with different thermal conductivity is studied. The single factor ablation experiments are carried out on two kinds of plates with the thickness of 1mm. The threshold value of the two materials and the influence law of different laser fluence and scanning speed parameters on the entrance cutting size and heat-affected zone are obtained, and the physical mechanism of the above laws is described. The results show that the huge difference in the thermal conductivity of the two materials has a certain influence on the difficulty of processing the material and the selection of process parameters.
We carried out a comparative study on laser cutting of fiber-reinforced plastic and its honeycomb sandwich structure widely used in aeronautic industry. The influence of pulse duration (CW, ns, ps and fs), wavelength (MIR, NIR, and NUV) on cutting quality (characterized by the HAZ) and productivity was experimentally studied. It was demonstrated that for either the aramid fiber reinforced plastics/polymers (AFRPs) or high-modulus carbon fiber reinforced plastics/polymers (CFRPs) and its honeycomb sandwich structure, tradition laser cutting was unable to satisfy the requirements of the appearance qualities in the astronautic industry due to the thermal damage induced discoloration, in spite of the less extension of damage than the contact machining used currently. The pulse duration effects on HAZ, the wavelength effects on cutting quality and productivity, the differences and similarities for low-HAZ cutting of AFRPs and high-modulus CFRPs, was studied, and the possible physical origins was discussed.
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