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Artigo IPEN-doc 24968 Backscattered light properties during femtosecond laser ablation and development of a dynamic interferometric focusing system2018 - RAELE, MARCUS P.; SAMAD, RICARDO E.; FREITAS, ANDERSON Z.; PRETTO, LUCAS de; AMARAL, MARCELLO M.; VIEIRA JUNIOR, NILSON D.; WETTER, NIKLAUS U.The backscattered light originated when machining with femtosecond laser pulses can be used to accurately measure the processed surface position through an interferometer, as recently demonstrated by our group, in a setup that uses the same laser beam for ablation and inspection. The present work explores the characteristics of the laser light reflected by the target and its interaction with the resulting plasma to better understand its propagation physics and to improve the dynamic focusing system. The origin of this returning radiation was studied and has been traced, mainly, from the peripheral area of the focal spot (doughnut-like). By means of a Mach-Zehnder setup, the interferometric pattern was measured and analyzed aiming to access the influences of the plasma on the laser beam properties, and therefore on the retrieved information. Finally, the wavefront of the laser that creates and propagates through the plasma was characterized using a Shack-Hartmann sensor.Artigo IPEN-doc 23822 Development of a dynamic interferometric focusing system for femtosecond laser machining2017 - RAELE, MARCUS P.; PRETTO, LUCAS R. de; SAMAD, RICARDO E.; FREITAS, ANDERSON Z. de; ROSSI, WAGNER de; VIEIRA JUNIOR, NILSON D.; WETTER, NIKLAUS U.Taking advantage of the inherent characteristics of femtosecond laser used for machining, we developed an interferometric system able to evaluate and correct the focal position with an accuracy of a few microns, implementing a technique based on low coherence interferometry. This approach measures at the exact spot that the laser is machining, in real time, and is sensitive to any sample that acts as a scatterer to the wavelength in use. The experimental evaluation was divided in two steps: in the first a system based on a superluminescent LED was mounted to check the viability and develop the controlling software; in the second part a setup was mounted employing a femtosecond laser, and several kinds of samples using the active focus control, among which the results obtained with glass sample and a bovine tooth are meticulously described in this paper. The system was able to improve the performance in both samples, keeping them in the confocal region for an extended positioning range, resulting in better engraving by the laser.