Strength of Joining Seams in Glass Welded by Ultra-fast Lasers Depending on Focus Height

Glass welding by ultra short laser pulses allows joining without the need of an absorber or a preand post-heating process. However, in order to achieve high joining strength the processing parameters such as feed speed, repetition rate, laser power and focal height must be chosen correctly. We show in this work that albeit the shear force of the joining seam depends on the displacement between the focal position and the joining interface the focal height can be chosen in a relatively wide range to provide welding seams with good quality. Furthermore, two different ways are examined in order how to measure the shear strength of a welding seam inside optically contacted area.