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Glass is a widely used material in different industrial domains thanks to some of its main properties such as high thermal and chemical resistance, biocompatibility and transparency. Glass bonding methods have a lots of applications in various domains such as architecture and design, optics, microfluidics or pharmaceutical. Compared to conventional glass bonding methods e.g. adhesive, fusing or anodic junction, ultrashort laser glass welding presents many advantages in terms of mechanical, chemical and thermal resistance, absence of additive material, process speed and miniaturization. Ultrashort laser pulses glass welding consists in irradiating the interface of the samples to weld through the glass with a focused beam. Glass is initially transparent to the laser beam, until a nonlinear absorption of the laser energy due to the high intensity reached in the focusing volume, which increases locally the temperature inside the material. A thermal accumulation effect is induced at high repetition rate (> 100 kHz), increasing the temperature up to the melting point of the glass. The junction is obtained by the blending of the material at the interface, followed by a fast cooling of the melting pool.
Our study focuses on the relevance of using a long focal length focusing device, as opposed to a more classical microscope objective, for femtosecond laser glass welding at high repetition rate. Femtosecond laser welding is performed on borosilicate glass plates using a scanner head with a 100 mm F-theta lens. The low numerical aperture focusing device generates a wide elongated focusing spot at the interface of the glass plates. Compared to a microscope objective as a focusing device, such an F-theta lens presents several advantages. The long Rayleigh length, up to hundreds of micrometers, reduces the level of accuracy generally needed when positioning the focusing spot at the interface. The integration of the lens in a scanner head gives a large freedom of geometries and patterns, at high scanning speed, with a high positioning accuracy. Our thermomechanical modeling shows that the wide elongated focusing spot, combined with the thermal accumulation effect occurring at high repetition rate (500 kHz), generates a low temperature increase at each pulse arrival, up to the melting temperature. On the opposite, in the literature, the use of a microscope objective induces single pulse material modifications by reaching temperature largely over the melting point. Our experimental characterizations by photoelasticimetry confirm that the low temperature increase reduces the residual thermal stress inside the material. On this basis, a design of experiments has been carried out considering the influence of pulse duration, pulse energy, wavelength and repetition rate with tensile tests, photoelasticimetry and transmission measurement characterizations.
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