Acoustical focal positioning for micromachining using a picosecond pulsed UV-laser

Picosecond pulsed UV-lasers enable micromachining down to a few microns in nearly all kinds of solid materials like metals, ceramics and polymers. However, precise machining results with high surface quality require a defined sublimation process [1]. Problems arise by accurate focal positioning on the work piece surface and by decreasing laser power for long machining times caused by laser and temperature influence on optics and thus misalignment. Therefore, control units for monitoring and controlling of significant process parameters like focal position and laser power are necessary. While laser ablation emitted acoustic airborne-sound emissions are used for the control unit. A precise adjustment of the focal depth which is +/-20 µm onto the work piece surface has been investigated for cemented carbide and an Al₂O₃+ ZrO₂-ceramic by recording of acoustic emissions. The analysis of the airborne-sound emissions by Fast Fourier Transformation gives information about the frequency spectrum and the correlation to the focal position. A subsequent fitted algorithm search allows an automated acoustical focal positioning. Furthermore, a power measuring instrument has been integrated into the working area for automatically measuring and adjusting of actual laser power at the focal spot due to the fact that the laser power has to be kept constant for acoustical focal positioning.