Real-time holographic 3D beam manipulation for ultrashort pulse laser processing

Abstract

Modern ultrashort pulsed high power (USP) lasers enable advanced and highly parallel micromachining processes. Due to constraints of the system technology, parallelization and beam shaping is mostly static during the process, which limits many interesting applications. The increasing power limits of liquid crystal on silicon (LCoS) spatial light modulators (SLMs) now open the door to dynamic beam shaping and splitting in the machining process. Although the frame rates of high-performance LCoS displays are typically in the range of 60 Hz and above, these devices are still mainly used as programmable diffractive optical elements (DOEs) and usually remain static during the fabrication process due to the high computational cost of conventional computer-generated hologram (CGH) algorithms. Herein, we report on the application of real-time CGH algorithms for dynamically generating and modifying 3D spot-distributions for advanced micromachining processes with high power USP lasers. We use a recently demonstrated algorithm based on compressive sensing combined with a highly parallelized computation on graphics processing units (GPUs). This allows for the calculation of phase holograms on the timescale of typical SLM response time and enables an online adjustment of the beam shape and spot distribution. In a detailed investigation of different CGH generation strategies, the impact of hardware response times, calculation speed and heat accumulations on the parallelized multi-spot laser process is experimentally evaluated on silicon. The presented results reveal the challenges and limits of on-the-fly CGH computation in USP laser material processing with SLMs and assesses its gain in processing flexibility for versatile USP micromachining applications.