Estimation of the refractive index change in glass induced by femtosecond laser pulses

It is known that local refractive index change occurs when femtosecond lase r pulses with extremely high peak power are launched into glass. me focused 130-femtosecond laser pulses of 800 nn into the bulk of glass and examined the shape of the induced refractive index change. me found that the length of the spot of the refractive index change along the optical axis reached about 30 mu m despite the diameter being about 2 mu m. To estimate the dist ribution of induced refractive index change, we fabricated Bragg grating by scanning the focused spot and calculated the amount of the change by apply ing Kogelnik's coupled mode theory to the measured diffraction efficiencies of the higher order diffracted beams.