Femtosecond Welding of Glasses and Crystalline Materials to Metals

The use of epoxies in space-based instruments is often unavoidable in situations where the bonding of dissimilar materials such as glass and metal is required. While there are epoxies that exhibit low total mass loss (TML) and collected volatile condensable materials (CVCM) in vacuum, in some applications they can still be a source of problematic contamination. Epoxies can also be incompatible with exposure to chemical environments some space instrumentation may be exposed to. In high power laser instruments such as LIDAR systems where optical components must be securely bonded to metal mounts, the impact of epoxy outgassing can be especially acute. Even with very low outgassing levels, the intense laser can break down the outgassed material and preferentially deposit it on optics that handle high optical power. This laser induced contamination in turn leads to laser induced damage, leading to degradation of optical components and reducing the reliability and operational lifetime of laser instruments [1-6]. Alternative bonding methods that avoid introducing additional contaminants could greatly improve reliability and operational lifetime of space instruments. We present the results of our investigations of the use of femtosecond laser welding as an alternative to epoxies in the construction of spaceflight optical instruments. Unlike conventional laser welding, femtosecond laser welding uses extremely short (~100fs) low energy (10’s of micro-Joules) pulses- as a result there is negligible heating of the materials being welded. The bond is not created by melting of materials, instead it is created by the extremely rapid formation of a plasma in which the two materials being bonded are mixed and rapidly recombine. The process we used to create these bonds will be describes as well as analysis of the bond strength. Electron microscope images take of the resulting bonds show significant mixing between the metal and crystalline materials being welded. We have demonstrated that the bonds created via femtosecond welding can have strengths comparable to epoxy and can be hermetic. Furthermore, we demonstrate that the femtosecond laser welding process allows bonds that are not possible using traditional methods. We have bonded Calcium Fluoride to stainless steel, diamond to Invar, sapphire to titanium, sapphire to Invar, Zerodur to Invar and fused silica to copper to name just a few combinations. We plan to pursue this technique and demonstrate its utility in the fabrication of space-flight instrumentation.