USP-glass welding in the filamentation regimeTuesday (23.06.2020) 14:20 - 14:40 Room 2
Welding of optically contacted glasses has received serious scientific attention in the past years. As can be seen in literature, ultrashort laser pulses enable heat accumulation in small volumes (e.g. 20 x 20 x 100 µm³) of glass samples up to the point where welding of previously contacted surfaces occurs. Therefore, the USP-laser beam is focused into the volume of a transparent sample by a microscope objective. Usually, the pulse energies are chosen low, so that no self-focusing occurs. Near the focal region, nonlinear absorption occurs due to the high intensity. The electrons absorb the energy of several pulses and heat up the glass lattice in a short time. This results in a rapid melting and solidification that can be used for welding. This process promises great potential for the joining of glasses and other materials without macroscopic thermal influences, since the heat affected zone is significantly smaller than one millimeter. One serious limitation of this process is the short working distance that results from the short focal length, which is necessary to reach optical breakdown while avoiding self-focusing. Also, when using a microscope objective for beam focusing, the user has to move the sample whereas a movement of the laser beam via galvoscanner would allow for higher beam velocities, higher accelerations and larger workpieces.
Here, a galvoscanner with an F-Theta-lens of 65 mm focal length was used for USP-welding. Due to the relatively soft focusing and higher pulse energies of several µJ at 515nm wavelength and 1ps pulse duration, the laser intensity is in the filamentation regime. Contrary to the well-known breakdown regime, the molten zone then grows in the direction of the laser beam with the pulse energy rather than towards the laser beam. Also, much longer molten zones of up to 2 mm are possible. Measurements of the mechanical strength of bonded samples will be presented.
For application relevance, due to the long focal length i.e. working distance, the welding seam can be much deeper in the workpiece relative to the optically accessible surface. Since the molten filament is rather long, it is also easier to hit the interface of the workpieces to be welded. Application examples will be shown.
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