Birefringence analysis of aluminum-to-BK7 bonding methods under thermal stress

Ultrafast laser welding, hydroxide catalysis bonding (HCB) and adhesive bonding techniques were used to bond N-BK7 cubes to aluminum bases. Using a thermal chamber and polariscope, the stress induced birefringence was measured at temperatures from -50°C to +80°C. Adhesive samples made with different industrial assembly methods were compared for thermal birefringent behavior. An ultrafast laser welded sample was thermally cycled in increasing temperature ranges, surviving the test with reduced retardation, indicating fatigue. An HCB sample was thermally cycled like the laser welded sample, and had a linear birefringent relation to temperature initially, and failed by fatigue after a fracture event.