Micro processing of metals using a high repetition rate femto second laser: from laser process parameter study to machining examples Paper #M102 J. Schille 1,2 , L. Schneider 1 , U. Loeschner 1 , R. Ebert 1 , P. Scully 2 , N. Goddard 2 , B. Steiger 1 , H. Exner 1 1 University of Applied Sciences Mittweida, Laser Institute, Technikumplatz 17, 09648 Mittweida, Germany 2 The University of Manchester, The Photon Sciences Institute & CEAS, Oxford Road, M13 9PL Manchester, GB Abstract The paper presents a study of laser micro processing of metals by using a high repetition rate femto second laser. On stainless steel (AISI 304), copper and aluminium the impact of the significant laser processing parameters onto the machining process was investigated, such as laser fluence, repetition rate, lateral pulse distance and polarisation. The machining results were evaluated by the ablation rate, surface roughness, process efficiency, material removal rate and the wall-angle. For complementary discussions the experimental data were compared with results achieved in theoretical analysis. Outgoing from the results appropriate laser processing parameters were derived in order to optimise the machining process. With the application of ultra short laser pulses high- quality machining results with a minimal thermal load and a roughness R a of the laser processed surface of only some hundreds nano meter were obtained. On other hand high machining throughputs were achieved due to application of high repetition rates. Finally, the possibilities and the limits of the high repetition rate femto second laser technology in laser micro processing are demonstrated by means of three- dimensional micro structured machining examples. Introduction The application of high repetition rate femto second lasers in micro machining joins together the advantages of ultra short laser pulses and the industrial need of short processing times. The advantages of ultra short laser pulses in laser processing are a precisely defined energy input, confined ablation thresholds, and a small debris deposition 1-6 . Furthermore the thermal load of the work piece due to the laser process is minimized in comparison to laser machining with longer laser pulses. Thus high-quality processing results with negligible heat affected areas can be achieved. On other hand, the recent development of high repetition rate ultra short pulse laser systems with average laser powers of hundreds Watts 7 enables short processing times and high machining throughputs. However, the application of high repetition rate ultra short pulse lasers in material processing of metals shows novel laser matter interaction mechanisms, such as heat accumulation or particle shielding effects 8-11 . Heat accumulative effects enhance the ablation behaviour and both lowered ablation thresholds and higher ablation rates were reported. As a disadvantage irradiation of high laser powers induces the formation of irregular conical micro structures at the bottom of the laser processed surface 12 . These phenomena affect negatively both the ablation process and the processing qualities. Hence the basic understanding of the laser ablation process and the use of appropriate laser parameters ensure optimised laser machining processes. Among others process optimisation in ultra short pulse laser processing was demonstrated in the pico second pulse duration regime by Neuenschwander et.al. 13 . In this work the impact of the laser processing parameters onto the machining outcomes is studied in order to find out optimised processing conditions. Particularly the influence of the laser fluence, repetition rate, lateral pulse distance and polarisation was investigated in both theoretical and experimental considerations. From the results appropriate processing parameters are derived with respect to the machining qualities, the processing time and the ablation efficiency. The possibilities and the limits of the high repetition rate femto second laser technology are demonstrated in stainless steel by means of 3D micro machining examples.