Reinforcing a Direct Bond between Optical Materials by Filamentation Based Femtosecond Laser Welding

We propose a joining process using femtosecond laser micro-welding to reinforce a direct bond. The process, which is executed at ambient temperature and can be applied to both similar and dissimilar material combinations, consists in inscribing weld lines in a closed loop forming a sealing pattern at the outskirts of the direct bond, thus preventing it from lifting off. We first characterized the size of material modification following exposure to focused femtosecond laser pulses at the direct bonded interface and its dependence on processing parameters. The derived optimal joining conditions are then applied to join fused silica-fused silica and fused silica-BK7 assemblies. Statistical measurements show an approximate threefold reinforcement factor after writing 11 weld lines in a rectangular pattern with rounded corners. This data was fitted with the two-parameter-Weibullmodel which gives insight on the performance and reliability of the process.