A spectroscopic comparison of femtosecond-laser-modified fused silica using kilohertz and megahertz laser systems

Abstract

Waveguides were written in fused silica using both a femtosecond fiber laser with a 1MHz pulse repetition rate and a femtosecond amplified Ti:sapphire laser with a 1kHz repetition rate. Confocal Raman and fluorescence microscopies were used to study structural changes in the waveguides written with both systems. A broad fluorescence band, centered at 650nm, associated with nonbridging oxygen hole center (NBOHC) defects was observed after waveguide fabrication with the megahertz laser. With the kilohertz laser system these defects were only observed for pulse energies above 1μJ. Far fewer NBOHC defects were formed with the megahertz laser than with kilohertz writing, possibly due to thermal annealing driven by heat accumulation effects at 1MHz. When the kilohertz laser was used with pulse energies below 1μJ, the predominant fluorescence was centered at 550nm, a band assigned to the presence of silicon clusters (Eδ′). We also observed an increase in the intensity of the 605cm−1 Raman peak relative to the total Raman intensity, corresponding to an increase in the concentration of three-membered rings in the lines fabricated with both laser systems.