Beam-profile evolution of trapped femtosecond pulses in Kerr media

High energy femtosecond pulses can be trapped in Kerr media for a distance larger than the confocal parameter without optical breakdown. Single filament trapping was maintained by self-focusing for several millimeters. To resolve this trapping, the spatial profile evolution of the self-focused beam was measured. An 18 degree sapphire wedge was used as Kerr medium. Multifilament beams were produced at higher intensity, whereas at lower intensity the beam cannot be trapped. As the beam propagates through the medium, the beam profile gradually separates into a high intensity central core and an outer region of low intensity. Because of the size limit of sapphire wedge, the length of trapping that extends for all the beams was not measured.