Femtosecond laser Bessel beam welding of transparent to non-transparent materials with large focal-position tolerant zone

It is known that ultrashort laser welding of materials requires an accurate laser beam focusing and positioning onto the samples interface. This puts forward severe challenges for controlling the focus position particularly considering that the tightly focused Gaussian beam has a short, micron-sized Rayleigh range. Here we propose a large-focal-depth welding method to bond materials by using non-diffractive femtosecond laser Bessel beams. A zero-order Bessel beam is produced by an axicon and directly imaged on the interface between silicon and borosilicate glass to write welding lines, ensuring a non-diffractive length in the 500 μm range and micron-sized FWHM diameter. The focal-position tolerant zone for effective welding increases thus many-fold compared to traditional Gaussian beam welding. The shear joining strength of the sample welded by this method could be as high as 16.5 MPa. The Raman spectrum and element distribution analyses within the cross section of welding line reveal that substance mixing has occurred during laser irradiation, which is considered as the main reason for femtosecond laser induced bonding.