Femtosecond laser irradiation enables modification of transparent materials and is widely used to fabricate optical waveguides and optofluidic devices. In this study, the propagation of 50 fs laser pulses at 800 nm in fused silica was modeled using the nonlinear Schrödinger equation coupled with a rate equation for free-electron density. Because fused silica exhibits negligible linear absorption at 800 nm, energy deposition was attributed to nonlinear ionization and subsequent absorption by generated free electrons. The deposited-energy distribution was then used as the heat source in a non-Fourier heat-conduction model to investigate temperature evolution during repeated irradiation. The repetition rate, pulse energy, and scanning speed effects on heat accumulation were examined.

The results show that repeated irradiation can produce temperature increases even when the deposited energy is insufficient to reach the softening or melting range. During scanning, temperature approaches a quasi-steady state after several pulses and then changes along the scan path. Increasing pulse energy raises the temperature, broadens the temperature distribution, and prolongs cooling time.

Increasing the repetition rate from 100 kHz to 1 MHz produces a more symmetric temperature distribution and extends the affected region beyond the focal volume. At 100 kHz, increasing pulse energy from 0.3 to 1 μJ increases temperature, with limited expansion of the affected region. Higher repetition rates enhance heat accumulation and raise the quasi-steady temperature, whereas higher scanning speeds reduce interaction time and accumulated heat. When pulses per point remain constant, increasing repetition rate and scanning speed shortens stabilization time while increasing accumulated temperature.