Comparative study of bulk modifications in borosilicate glass induced by femtosecond laser in single pulse, MHz-, and GHz-burst regimes

In this work, investigations of the interaction between borosilicate glass and three laser regimes are presented: repetitive single pulse, MHz-burst, and GHz-burst. Experiments were carried out using an ultrafast industrial laser system and galvanometric scanning to inscribe bulk modifications inside borosilicate glass (N-BK7) samples. By varying pulse or burst energy and scanning velocity, we produced a wide range of in-volume modification morphologies. This approach enabled to classify modifications and establish processing maps for each regime. Results show that the single-pulse regime is more suited for generating large, defect-free bulk modifications with a teardrop shape, while the MHz-burst regime increases the probability of defect formation, including voids and local cracking. These defects become even more pronounced under GHz-burst irradiation, where the lower energy per pulse results in absorption mainly near the focal point, leading to crack formation. Overall, this work provides a comprehensive understanding of glass behavior under femtosecond laser irradiation and provides guidelines for selecting laser conditions suitable for transparent glass welding.