Study of femtosecond-laser-written waveguides in glasses

Mechanisms of the femtosecond-laser-induced refractive-index change (Δn) were investigated for fused silica and a borosilicate glass. Experiments were based on two exposure situations: (a) high repetition rate and low pulse energy and (b) low repetition rate and high pulse energy. The calculated temperature rise based on model (b) was above 1000 °C, whereas for situation (a) it was negligible. The results do not support a thermal origin of the induced Δn; rather, heat may limit the magnitude of the change. Correlation between color-center formation and Δn in both glasses suggests that defects contribute substantially to the index increase. However, annealing studies have shown that the induced Δn persisted beyond the disappearance of the color centers. Analysis of the induced stress showed that densification plays a small role in this change.