Laser Power Stabilization for Improved Ablation Depth Uniformity

Due to low heat deposition into the surrounding material, ps-lasers are well suited for a precise ablation of micro structures. However, to obtain constant removal rates the laser parameters as well as the environmental conditions need to be kept stable. For our investigations we used a frequency tripled ps-laser operating at 355 nm. Without any additional power stabilization system, long term micro structuring processes of OFE-copper typically resulted in depth variations of about +/- 10 % which are strongly correlated with detected laser power fluctuations. The developed system for laser power stabilization takes advantage of the linear polarized radiation of our laser source. With a fast photoelectric detector the power fluctuations are measured. A specifically designed feedback control system turns a half wave plate mounted in a piezo driven rotation stage to decouple fluctuating power fractions via a polarizing beam splitter. We demonstrate a nearly constant laser power at the work piece. Substantially improved ablation depth uniformity is verified by a profilometric analysis of directly ablated diffractive structures.