Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses

The refractive index changes in doped silica are investigated. We observed that the permanent isotropic index change threshold (T1) is not significantly dependent on the doping. We show that strong birefringence (permanent linear birefringence) exists in doped silica but its threshold (T2) exhibits significant dependence on the used dopants. In our conditions, comparing with silica (0.31 mu J/pulse here), for 1.5 at% Ge-doped silica the T2 threshold is smaller (0.14 +/- 0.05 mu J/pulse). For a silica doped with 0.3 at% of fluorine, T2 is close to 1.20 +/- 0.05 mu J/pulse. An interpretation is given not only about threshold variation but also about RIC for energies beyond. It is based on the overcoming of relaxation time in the volume interaction. (C) 2011 Optical Society of America