On-line Interferometric Observation of Thermomechanically Induced Refractive Index Changes during Glass Welding by Ultra-short Laser Pulses

• Published in: Journal of laser micro nanoengineering 2018-12, Vol.13 (3), p.301-308
• Main Authors: Cvecek, Kristian, Heberle, Johannes, Miyamoto, Isamu, Bergler, Michael, Ligny, Dominique de, Schmidt, Michael
• Format: Article
• Language: English
• Subjects: Cameras; Cold; Cold welding; Dimensional changes; Fusion welding; Glass; Heat; Heat affected zone; Interferometry; Laser beam welding; Lasers; Optics; Phase distribution; Plasma; Pressure distribution; Refractivity; Simulation; Temperature; Thermography
• ISSN: 1880-0688; 1880-0688
• DOI: 10.2961/jlmn.2018.03.0027

Glass welding by ultra-short puled lasers is known to exhibit a strongly localized heat affected zone, thereby allowing the fusion welding of glass parts with thermally sensitive components nearby. Moreover, the shape of the refractive index change induced by the thermomechanical gradients has an impact on the laser beam propagation into the molten zone and could cause e.g. an eventual focus shift or deteriorate the beam quality. While the temperature of the heat affected zone can be estimated by simulations, measurements of the Raman shift or thermography cameras, the shape of the temperature and stress gradients cannot be completely recovered by the aforementioned techniques due to specific limitations. In this work we present an interferometric approach to measure a 2-dimensional change of the refractive index around the molten zone before, during and after welding. Moreover comparisons between phase distribution inside the cold and unmodified glass with the hot and modified as well as cold and modified glass make it possible to estimate the thermal and the stress-induced refractive index modifications around the molten zone separately. Keywords: pump-probe, glass, ultra-short pulsed lasers, refractive index, phase, welding, laser processing, thermal evolution