Study of femtosecond-laser-written waveguides in glasses

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