The output of an ultrashort pulse (< 15 picoseconds) laser is focused to a small spot within the substrate. The very high laser intensity achieved produces self-focusing of the beam (due to the Kerr optical effect) within the glass. This self-focusing further increases power density, until, at a certain threshold, a low-density plasma is created in the material. This plasma lowers the material refractive index in the center of the beam path and causes the beam to defocus. If the beam focusing optics are properly configured, this focusing/defocusing effect can be balanced to repeat periodically and self-sustain. This forms a stable filament, that is, a line of tiny voids, which extends over several millimeters in depth into the glass. The typical filament diameter is in the range of 0.5 µm to 1 μm.
Example system: 50 W of average output power at a wavelength of 1064 nm, 100 mm/s – 2 m/s.
Selective laser-induced etching (SLE)
An ultra-short pulsed laser (Satsuma HP2, Amplitude Systèmes, Pessac, France) with a central wavelength of 1030 nm and the maximum output power of 20 W was used. The maximum laser pulse energy was 40 μJ at the pulse repetition rate of 500 kHz and the pulse width variation was from 370 fs to 10 ps. The pulse repetition rate was variable up to 2000 kHz. For 3D fabrication machine, the laser amplifier was integrated with a 2-axis (XY) galvano scanner (DynAXIS, ScanLab, Puchheim, Germany) and an air bearing 3-axis(XYZ) servo motion stage with a controller (A3200, Aerotech, pittsburgh, USA). A focusing objective lens (NA = 0.4, model No. 378-867-5, Mitutoyo, Kawasaki, JAPAN) was assembled with the galvano scanner for high scan speed. The focused laser beam size is estimated about 2 μm. The combined scan speed of the scanner and 3D stage is up to 200 mm/s and the field size is 100 mm × 100 mm. After laser modification process, the exposed glass substrate was etched using 8 mol potassium hydroxide (KOH) at 85 °C in an ultrasonic bath for uniform concentration control. (Kim et al, 2019)
Possible laser: Osram SPL PL90 3 pulsed laser diode. It is constructed from three epitaxially stacked emitters with a laser aperture of 200 μm by 10 μm and has a peak output power of 75 W, a wavelength of 905 nm, and a maximum pulse width of 100 ns. The rated duty cycle is 0.1%, but this has been exceeded without damage to the diode. $20. datasheet, (Parziale_et_al, 2015)
Diodes with ps pulses are low energy mW or less, so coupling and amplification through a fiber laser is needed.