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Micro-hole drilling on glass plates by femtosecond laser pulses

Holes of few tens of micrometers have been drilled through a glass plate by femtosecond laser pulses. A femtosecond laser (Clark-MXR, CPA 2001) which delivered 775 nm, 150 fs pulses at 1 kHz was used to machine holes through glass plates of about 150 micrometer thick. Laser pulses were focused by a plano-convex lens on to the glass plate. An electric shutter controlled number of irradiating laser pulse. Entrances of the drilled hole were crack-free but shell-like cracks appeared at exit end of them. Shapes of the drilled holes were examined in detail by taking replicas of them using silicon-rubber based material. Small debris scattered around the drilled hole and its deposition range depended on the laser peak power. Development of the hole with increasing pulse numbers showed several distinct stages. The first few tens of pulses drilled a shallow, flat bottom hole with a deeper channel at the center of a flat bottom. Then, the flat bottom and the channel emerged into a deep, funnel shaped hole with tapered wall. The hole became deeper and finally went through to the rear side at certain pulse numbers, which depended on pulse energy and focusing conditions. Position of the focus relative to the glass surface affected the shape of drilled holes. This means that the shape of the drilled hole is very sensitive to laser intensity distribution with a well defined threshold and would suggests the possibility of drilling designed-shape holes with proper diffractive optics for modifying the intensity distribution of the laser light.