Transition from repetitive to cumulative thermal processing in femtosecond laser induced machining of embedded waveguides

Presentation Date: 

Saturday, January 25, 2003


Photonics West (San Jose, California)

Presentation Slides: 

In previous work, we have demonstrated laser writing of embedded waveguides in silicate glasses with only nanojoules of energy. The laser system is an unamplified 25 MHz laser oscillator generating 24-nJ, 55-fs pulses. Laser machining at this high repetition rate results in a cumulative thermal mechanism of material modification which leads to structural index of refraction changes beyond the focal volume. We present a parametric study of the role of the laser repetition rate in the size of the machined structures. The material used for the study is a chalcogenide glass, As_2S_3. We identify two distinct regimes of processing in the kHz to MHz range. As the time interval between pulses is reduced, we observe a transition from a repetitive modification process (identical to what is frequently called multiple shot damage) to a cumulative thermal mechanism. In the repetitive regime, each pulse acts independently and the energy deposited diffuses out of the focal volume before the next pulse arrives. In the cumulative thermal regime, the heat deposited is additive and leads to structures of significantly different morphology whose size increases dramatically with the decrease of the time interval between pulses. We compare our experimental results with a thermal diffusion model.