Fundamental processes of refractive index modifications during femtosecond laser waveguide writing

By using focused ultrashort pulsed laser radiation refractive index modifications are induced in glass in order to generate optical components. The understanding of physically fundamental processes induced by laser radiation is the basis for the systematic control and maximization of the refractive index change for the realization of three-dimensional, optical components for integrated optics like in-volume waveguides. In this paper fundamental processes which are induced by focused laser radiation in the volume of borosilicate glass D263 and fused silica are investigated. The glass materials are structured by laser radiation in the infrared spectral range (lambda=1045nm). By using femtosecond laser pulses with high repetition rates (f = 500 kHz), thermal processes like heat accumulation effects are induced leading to heat affected zones and thus waveguide cross sections with dimensions larger than the focal spot. The absorptivity during modification in relation to the applied pulse energy is measured for different repetition rates in both glass materials. Furthermore, the laser induced structural change in the glass matrix by the increase of three- and four-membered ring structures is proved with Raman spectroscopy