A standardised surface preparation technique for ultrashort pulse laser welding and its application to bonding CTE-matched alloys and optics

Ultrashort pulsed laser welding presents an attractive alternative to the adhesive bonding currently used for metal to optical bonding in the manufacture of precision optical systems. However, the ability to translate the process into high yield industrial manufacturing, and particularly to readily apply it to new material combinations, has been hindered by the lack of a standardized surface preparation method for the metal component. To overcome this, we developed a multistage surface preparation methodology, and we demonstrate the versatility of this approach with three metal alloys that have very different properties: Invar, Kovar, and Inconel. Following surface preparation, these alloys were each welded to an optical material chosen to have a well-matched coefficient of thermal expansion: Invar to fused silica glass; Kovar to BK7 borosilicate glass; and Inconel to crystalline quartz. Using a 5 ps pulsed 1030 nm laser and following fine tuning of the laser parameters for each material combination, we demonstrate 100 % yield for a total sample set of sixty welds (twenty of each material combination). All of these welds also survive subsequent environmental cycling tests conforming to ISO 9022–2:2015 severity 6 i.e. temperature cycling from −40 ± 3 °C to 70 ± 2 °C with a temperature ramp of 2 °C/min for both heating and cooling, for a total of 5 full cycles. The weld stress-induced optical birefringence was analysed and as expected was lowest for the Kovar-BK7 combination which has the best thermal expansion match. Importantly, the birefringence of all three welded material combinations meets the ISO 10110–18:2018 guidelines for permissible birefringence retardation values of precision optics and astronomical instruments.