The influence of self-focusing and filamentation on refractive index modifications in fused silica using intense femtosecond pulses

The interaction of focused femtosecond infrared laser pulses at 1 kHz repetition rate with bulk fused silica is thoroughly investigated. The interplay between self-focusing and filamentation of the laser pulses is analyzed for a broad range of focusing conditions. It is shown that even in the case of very tight focusing, filamentation is observed as evidenced by the scanning electron microscope (SEM) pictures. Preliminary results show that using such a tight focusing geometry and at input powers above the critical power for self-focusing in silica, waveguide structures with elliptical cores are inscribed within the glass by moving the sample perpendicular to the laser beam propagation direction.