US Patent 9873628 Filamentary cutting of brittle materials using a picosecond pulsed laser

In pulsed-laser apparatus for filamentary cutting a glass-sheet, a laser-beam is focused by a plano-convex focusing lens into the glass-sheet through a first surface of the sheet to a point close to a second surface of the sheet. Pulses from the laser are delivered in repeated bursts. The laser-beam fills the clear-aperture of the lens such that uncorrected spherical aberration in the focusing lens causes radial modulation of the beam between the lens and the focus point. This provides that each burst of pulses generates a filament extending between the first and second surfaces of the sheet.