Refractive index modification using fs-laser double pulses

Buried waveguides in glass are manufactured by irradiation with femtosecond laser double pulses. The refractive index change Deltan is determined by measuring the numerical aperture NA of the waveguides and by through light microscopy. The value of Deltan shows a significant dependency on the time delay Deltat of the fs-laser double pulses. A Deltan of up to 2x10(-3) in fused silica is reached at a Deltat between 400 and 800 ps. Based on the results of the double pulse experiments the initial effects of the refractive index change are discussed, taking into account thermal effects and the formation of self trapped excitons (STE) and transient color centers and their interaction with the next laser pulse.