Ultra-short pulsed laser welding of crystals, glasses, metals, and more

Optical components are now used in an enormous range of products and devices. A well-known problem in the assembly of these devices is how to join the optical components either to other optical materials, or to structural materials like metals. Considerable resource and effort have been spent in developing reliable bonding methods, often using adhesives. Despite attempts to standardise these process solutions are typically bespoke to the components involved and still exhibit serious issues in terms of outgassing, creep, accuracy and aging. An alternative solution, preferably avoiding the requirement for an interlayer material is therefore desirable. Ultra-short pulse laser welding is just such an alternative technology. First demonstrated in 2005 for glass-glass welding [1], and of glass-metal welding in 2008 [2] there is now considerable industrial interest in this emergent technology.
The combination of ultra-short pulses and high NA focussing optics allows for extreme peak energy densities, typically in the order to TW/cm2. This energy density allows for strong multi-photon absorption in the otherwise transparent optical material in combination with linear absorption on the surface of opaque structural materials. This generates a highly confined plasma surrounded by a thin layer of melt, typically in the order of a few 10’s of microns. By continuously translating the focus across the material interface the resulting highly confined melt-plasma zone allows for welding of materials with highly dissimilar thermal properties, like glass and metal.
The process does, however, have several challenges associated with it. Firstly, close contact is required between the parts at the interface, without this the high-pressure plasma is liable to escape and the result will be ablation rather than welding. Secondly, although the confined thermal zone allows for welding highly dissimilar materials thermal stress during the component’s lifetime (e.g. thermal cycling or ambient temperature changes) can produce significant stress at the interface leading to poor performance or failure. Thirdly, the welding process induces some stress within the optical component and for high precision polarisation-sensitive applications the resulting stress induced birefringence may be detrimental to performance.

Figure 1 (a) Exemplar test sample of ultra-fast laser welding. (b) Exemplar micrograph of a spiral “spot weld”, CF Figure 1(a). (c) Exemplar stress induced phase retardance of a glass-metal weld measured with a polariscope in side-view, adapted from [3].

At Heriot-Watt University we have carried out research aimed at investigating and addressing these issues; investigating surface preparation and mounting conditions required to allow for close contact and developed a unique optical polariscope quantify and to analyse the stress induced birefringence from welding and subsequent thermal cycling. This presentation will focus on recent developments including new material combinations and the measurement and analysis of weld-induced stress within the optical components, as well as thermal-induced stress under extreme environmental temperatures.

[1] T Tamaki, W Watanabe, J Nishii and K Itoh, Welding of transparent materials using femtosecond laser pulses, Japan. J. Appl. Phys. 44, pp 687-689, (2005)
[2] Y Ozeki, T Inoue, T Tamaki, H Yamaguchi, S Onda, W Watanabe, T Sano, S Nishiuchi, A Hirose and K Itoh, Direct welding between copper and glass substrates with femtosecond laser pulses, Appl. Phys. Express, 1, 082601, (2008)
[3] N Macleod, S N Hann, A Dzipalski, I F Elder, I J Thomson, N Weston, R M Carter, R A Lamb, D P Hand, M Troughton and M J Daniel Esser, Birefringence analysis of aluminium-to-BK7 bonding methods under thermal stress, Opt. Cont., 1(12), pp 2621-2636 (2022)
The authors would like to acknowledge funding from; ESPRC under grants EP/K030884/1, EP/V01269X/1, and Innovate UK under grant TS/R000417/1