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Stress induced birefringence of glass-to-metal bonded components
We report on the stress induced birefringence of 10 mm BK7 cubes bonded to 15 mm x 15 mm x 5 mm aluminium coupons using ultrashort pulse laser welding, hydroxide catalysis bonding, and an optical adhesive using a standard approach used in industry. It was observed that ultrashort pulse laser welding results in a low level of stress induced birefringence within an 85% optical aperture of the 10 mm cube. These levels are suitable for use in photography and microscopy applications as defined by the relevant ISO standard for permissible stress induced birefringence limits in optics. Hydroxide catalysis bonding was shown to approach and possibly exceed this limit, however, it must be noted that the bonding area of the hydroxide catalysis samples greatly exceeds that of the welding and adhesive bonded samples by a factor of 20 and thus will likely experience grater mechanical stress from defects on the metal surface. The induced stress in the adhesively bonded samples was found to be well-confined, with the retardation falling below the maximum acceptable limits for microscopy type applications of 10 nm/cm retardation within 0.5 - 1 mm of the glass-metal interface for the majority of samples tested. The hydroxide catalysis and ultrashort pulse laser welded samples have a significant increase in retardation in the region directly above the bonded surface compared to the adhesively bonded samples. This retardation decays rapidly as a function of distance from the bonded interface. In the case of ultrashort pulse laser welded samples, optimisation of the welding parameters has been shown to control the level of retardation within the interface region, thus reducing the overall effect of the weld on the bulk optic.
Hann Samuel, Macleod Nathan, Morawska Paulina, Dzipalski Adrian, Carter Richard, Elder Ian, Lamb Robert, Esser Daniel, Hand Duncan
Paper for Seminar/Symposium/Conference
SPIE - Security + Defence Digital Forum (21-25 September 2020, Online Only)
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