Toward Taper-Free Structures in Ultrashort Pulse Laser Ablation Using Liquid Crystal Spatial Light Modulators

Conventional ultrashort pulse laser microstructuring or cutting is inherently prone to the generation of tapered walls.This effect can only be compensated for by means of a defined manipulation of the angle of incidence (AOI) of the focused beam.This study explores the feasibility of using liquid crystal on silicon spatial light modulators (SLM) to achieve vertical wall angles in laser ablation.In the proposed method, the raw beam is passed sequentially over two cascaded SLMs to create a parallel offset that results in an altered AOI on the workpiece.This offers capabilities beyond established solutions such as simultaneous beam splitting and shaping.In a first step, investigations on laser cutting of stainless steel sheets are carried out to analyze the wall angle as a function of the most influencing parameters.Results indicate a minimum AOI of 3 is essential for taper-free structures, which is respected in the optics design considerations for the double-SLM module.Furthermore, the tradeoff between optical efficiency and beam path length is quantitatively presented.Finally, recommendations for selecting the focal length of the f-theta lens and the scanning system are provided based on analytical calculations and optical simulations.