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materials. ‘I was very surprised to find out that with such a large intensity window we can actually weld a lot of different types of glass with different nonlinear absorption properties,’ he said. ‘This means that, from an industrial perspective, this process is very reproducable.’


High application potential The process has a plethora of applications in numerous sectors. The ability to bond glass to metal can be used in the manufacture of lasers and sensors in instrumentation for sectors such as aerospace, defence, satellite communications, and surveillance, or in the packaging of laser diodes, photonic integrated circuits and micro- electromechanical systems. The technique can also


be used to achieve hermetic sealing, which opens up numerous applications in, for example: creating vacuum insulated glazing windows (Karnakis noted that skyscraper builders have expressed interest in this); moisture ingress prevention in avionic systems, photovoltaic panels, watches and inorganic LEDs (for harsh environments such as vehicle headlights and streetlights); medical implant assembly (devices encapsulated with special quality glass that go inside the body); sealing pharmaceutical vials; achieving pressure capability for underwater systems; and manufacturing OLED devices. For this last application, the


interest comes from the fact that manufactures of OLED devices have to go to extreme lengths to encapsulate them –


which, according to Karnakis, is a source of great expense that contributes considerably to the overall cost of producing OLED devices. ‘So if we had a technique to encapsulate these using glass, which is a natural encapsulant, then that would go a long way,’ he remarked. The technique has also


generated a lot of interest in the space sector in the assembly of instrumentation, satellite communication systems and camera sensors. The Ultraweld partners have even had discussions with NASA regarding the adhesives it currently uses. According to Karnakis, due to the extreme conditions of space, NASA is only able to use a handful of specific adhesives based on the amount of outgassing they exhibit. As a result, NASA is investigating laser microwelding as an alternative to using adhesives.


Putting it to the test As part of the Ultraweld project, Oxford Lasers has developed a Class-1 laser safe prototype microwelding machine at Technology Readiness Level 6 (demonstrated in a relevant environment). The system is based on Oxford Lasers’ C-Series Tool and uses an industrial-grade ultrafast infrared laser with adjustable pulse duration between 300fs and 10ps. The system uses a flexible beam delivery system with variable focus and high- resolution optical scanning, a custom software interface and various beam diagnostics to tailor the process as needed. The prototype is now available to industry for proof-of-concept


Quartz wave plates welded to a stainless steel base


feasibility and pre-production trials.


Using the machine, the


researchers were able to bond together a range of different material combinations including, among others: crystal quartz to stainless steel, NBK7 glass to aluminium or stainless steel, fused silica to stainless steel, sapphire to stainless steel, and calcium fluoride to stainless steel. A range of demonstrator


components made from different materials were


“There’s a lot of interest in this process and definitely a great opportunity for industrial uptake”


produced to demonstrate the bonding, with the researchers using a range of tests to characterise the bonds. Samples faced tests such as repeated thermal cycling between -65°C and +95°C, prolonged vibration tests, shear tests at varying forces, hermeticity tests and stress birefringence tests. Karnakis showed attendees


Left: an N-BK7 lens welded to an aluminium ring mount. Right: A BK7 wedge prism welded to an aluminium base.


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some of these components: A BK7 wedge prism (maximum thickness 18mm) welded to an aluminium base, which demonstrated an average shear bond strength exceeding 75N; a 9mm thick, 10 x 10mm sapphire cube welded to stainless steel, which withstood 200N of shear force; a 75mm diameter, 9mm thick fused silica window welded to stainless steel that also withstood 200N of shear force; a 9mm thick, 10 x 10mm calcium


fluoride cube welded to stainless steel that withstood 50N of shear force; two 2 x 1mm-thick quartz waveplates welded to a stainless steel metal base; and a 40mm diameter, 4mm thick N-BK7 lens welded to an aluminium metal ring mount. ‘We’ve had great


characterisation results so far that show promise in terms of bond strength, hermeticity and other performance metrics,’ said Karnakis. ‘There’s a lot of interest in this process and definitely a great opportunity for industrial uptake.’


The future of ultrafast laser microwelding While ultrafast laser microwelding has been proven and is now ready for industry uptake, the physics of this technique are still not yet fully understood, according to Karnakis. ‘A lot of work has happened so far to understand this process, but we are still in early days at the moment and don’t understand it very well,’ he said. ‘This is a little bit problematic because we cannot expand on the design rules until we understand the physics further. Process design criteria are mostly based on empirical data so far.’ Oxford lasers plans to


expand its inhouse bond characterisation capabilities to fine-tune ultrafast laser microwelding across different applications. It also plans to expand the matrix of material combinations it can weld, including other glass-metal combinations, glass-ceramics, glass-semiconductors and semiconductors-metals. The firm will also be working to optimise toolpaths for performing large area welding, and for welding intricate shapes. l


SUMMER 2021 LASER SYSTEMS EUROPE 13


Oxford Lasers & Gooch and Housego


Oxford Lasers and Gooch & Housego


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