A systematic investigation concerning the onset of microchannel formation in fused silica was performed using three Lithosil samples (100,150 & 200mm in length; 10, 20 and 40mJ/cm2 at 1kHz repetition rate). The onset was detected in-situ by naked eye as well as by 543.5nm light scattering and ArF laser transmission measurements (193nm). Light scattering measurements were found to be more sensitive than transmission measurements. ArF laser beam intensity distributions registered at the exit surface of the sample revealed a significant beam distortion in fused silica after irradiation with half of the laser pulses necessary for microchannel formation. Self focusing due to nonlinear effects within single laser pulses like the Kerr effect or stimulated Brillouin scattering could be excluded as reasons for microchannel formation.
