Mechanism and elimination of bending effect in femtosecond laser deep-hole drilling

In this work, a comprehensive study of the bending effect, which remains one of the most critical challenges during deep-hole drilling, was conducted. The experimental statistics indicate that polarization is not the main factor in bending, but the deviation of the hole tends to be perpendicular to the polarization direction. Also, the dynamic ablated material/plasma was studied. Straight microholes were obtained by extending the interval between laser pulses to avoid dynamic ablated material existing in the millisecond time domain. Therefore, we speculated that the disturbance of the laser beam at the dynamic ablated aerosol, which have not sufficiently dispersed in the millisecond domain, is the main mechanism of bending. However, to more efficiently reduce the disturbance factor, a rough vacuum environment was applied; and the bending effect was also eliminated. The critical pressure for eliminating bending was about 2 × 10⁴ Pa that is about one order of magnitude lower than the atmosphere. The fabricated high-quality microhole arrays without bending show that the proposed drilling method is convenient and efficient with high repeatability and controllability.