Abstract

In previous studies, we have shown that the grating photo-induced by femtosecond laser pulses in a SF59 lead silica glass is not formed instantaneous and continues to evolve at the millisecond time scales after excitation. The grating reflectivity was shown to depend on the pump power density and on the number of exciting pulses [1]. The grating was directly written onto the SF glasses by imaging a master grating, which is illuminated by femtosecond laser pulses (1mJ, ~50fs, 1KHz, ~800nm) [2]. The formation of the grating was shown to result from defects photo-induced by the two photon-absorption of the femtosecond laser pulses [1,2]. Theses defects induce a change of the real (Δn_r) and imaginary (Δn_i) part of refractive index of the glass. However our previous experiments were not able to reveal the time scale for the formation of these defects or to evidence the impact of the two photons absorption on Δn_r or Δn_i. Indeed for a given laser power density, one expects to observe an increase of Δn_r and therefore of the grating reflectivity as two-photon absorption coefficient α² is increased.

© 2013 IEEE