Distortion of the intensity profile at the processing image plane of a focused femtosecond laser beam
Résumé
Femtosecond laser processing of surfaces is an efficient technique for rapid surface structuring on the micro or submicrometer scale with high potential for applications [1,2]. For surface treatment, the technique has been employed with success in various fields of application including biology, tribology, wettability, color marking or material ablation and cutting. The physical properties of the laser-irradiated surfaces can be controlled by adjusting the irradiation conditions. In applications where the generation of laser-induced nano/micro structures is involved, the dependence of their dimensional properties with the laser fluence has been reported on various materials, showing the need for precise control of the laser intensity distribution in the processing plane. In order to obtain a homogenous intensity distribution on the sample, we and several research groups, have developed beam shaping methods [3,4] based on amplitude and/or phase modulation using a spatial modulator or lens association. However, these techniques increase the complexity of the laser machining set up, with limitations on the maximum power and fluence applicable on spatial modulators. A straightforward and frequently employed method consists in positioning an arbitrarily shaped diaphragm on the laser path before the focusing lens and to position the surface to be machined at the conjugated image plane. Despite the loss of energy, the obtained intensity distribution in the image plane corresponds to a truncated Gaussian distribution with a quasi-constant fluence level. Here, we put the light on the limitations of this technique for high fluence levels commonly attained with amplified industrial femtosecond lasers. There, a strong deterioration of the laser intensity profile and the micromachining quality occurs. We provide a robust and easily implementable method based on geometrical optics to overcome this problem. Illustrations are shown with laser processing of cavities on stainless steel (316L) using white light microscopy [1] B. N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. Tuennermann, Femtosecond, picosecond and nanosecond laser ablation of solids, Applied Physics A, vol. 63, pp. 109-115, (1996). [2] D. Dufft, A. Rosenfeld, S. K. Das, R. Grunwald, J. Bonse, Femtosecond laser-induced periodic surface structures revisited: A comparative study on ZnO, Journal of Applied Physics A, vol. 105, pp. 034908-1-9, (2009). [3] N. Sanner, N. Huot, E. Audouard, C. Larat, J.-P. Huignard, B. Loiseaux, Programmable focal spot shaping of amplified femtosecond laser pulses, Optics Letters, vol. 30, pp. 1479-1481 (2005). [4] C. Mauclair, G. Cheng, N. Huot, E. Audouard, A. Rosenfeld, I. V. Hertel, R. Stoian, Dynamic ultrafast laser spatial tailoring for parallel micromachining of photonic devices in transparent materials, Optics Express, vol. 17, Issue 5, pp. 3531-3542, (2009).