Recast-Free Helical Drilling Of Fused Silica Using Shg Picosecond Laser Pulses

Fused silica has an excellent optical and thermal properties and therefore is widely used in a variety of industrial applications. With the development of ultrafast laser technology, helical drilling with ultrashort laser pulses has demonstrated great advantages in terms of precision and reproducibility. In this paper, we used a rotating-Dove-prism-based helical optics together with a picosecond laser working at second harmonic generation (SHG). Three types of helical processes: classic helical drilling, helical contour cutting and helical trepanning based on identical helical optics are developed and applied to drill a 200 lam hole in 0.5 mm thick fused silica. The classic helical drilling process has a constant revolution diameter and requires higher pulse energy and longer duration to achieve multi-photon absorption. Therefore, a bulk of recast layer with height of 10 lam is generated surrounding the inlet of the micro hole. The height and radial width of recast layer predominantly depend on the focus position and applied pressure of process gas. The helical contour cutting of circular hole can be understood as a combination of helical drilling and circular contour cutting with Computer Numerical Control (CNC) program and precision motion stages. As a result of dynamic cutting front, the molten material are constrained in the cutting kerf and driven out from the hole outlet under pressure of process gas, which contributes to a recast-free drilling. The helical trepanning process is characterized by a through start-hole at beginning and the continuously increased helical diameter of laser pulses during the process. The molten volume within the hole diameter are completely removed and driven out from the pilot-hole. By means of that the drilling process can be accelerated with an enhanced surface quality. Moreover, the entrance of boreholes are free of recast. The influence of the energy deposition in the three drilling strategies on the drilling results regarding the recast layer was investigated by numerical simulation of superposed laser intensity distribution on the helical path.