Femtosecond pulses focused in glass substrate induces various modifications in the material depending on the fluence level and laser exposure conditions. Upon increasing pulse energies above the energy threshold for non-linear absorption, femtosecond laser exposure leads respectively, to an increase of the refractive index , the formation of nano-gratings  or to micro-explosions . The intensity of the modifications also depends on the amount of energy deposited in the material . Depending on the pulse repetition rate), cumulative effects are eventually observed for repetition rates typically above 1MHz [5, 6]. © 2011 IEEE.