Bulk heating of transparent materials using a high-repetition-rate femtosecond laser

Femtosecond laser pulses can locally induce structural and chemical changes in the bulk of transparent materials, opening the door to the three-dimensional fabrication of optical devices. We review the laser and focusing parameters that have been applied to induce these changes and discuss the different physical mechanisms that play a role in forming them. We then describe a new technique for inducing refractive-index changes in bulk material using a high-repetition-rate femtosecond oscillator. The changes are caused by a localized melting of the material, which results from an accumulation of thermal energy due to nonlinear absorption of the high-repetition-rate train of laser pulses.