Speckle reduction techniques in holographic beam shaping for accurate and efficient picosecond laser structuring

Holographic beam shaping using a spatial light modulator (SLM) provides flexible adaptation of the intensity profile in laser material processing. This dynamic beam shaping is advantageous regarding the adaptation of accurate and efficient ultrashort laser based material ablation processes. However, speckles occur due to the pixelated display of the SLM and consequently discretized phase shifts. Speckles reduce the quality of a shaped intensity profile and the accuracy of generated microfeatures and therefore have to be suppressed. Against this background, selected speckle reduction techniques are applied, modified, and evaluated regarding the quality of a desired top-hat intensity profile. This beam shape is relevant for the generation of friction influencing microfeatures. Holograms are calculated by the iterative Fourier Transformation algorithm. The criteria for top-hat evaluation such as flatness, speckle contrast, and edge steepness are applied according to DIN EN ISO 13694. Furthermore, the effects of speckles on a defined microfeature geometry generated in a steel alloy are presented. The quality and the ablation efficiency including the diffraction efficiency of the SLM are evaluated and compared to conventional micromachining with the Gaussian intensity profile. The speckle reduction techniques of deterministic shift-averaging and time-averaging which is based on averaging of the reconstruction of different independently calculated holograms result in a high flatness factor and high quality of material removal. The number of holograms is determined, which is necessary to generate microfeatures of sufficient accuracy and low roughness. In contrast, stochastic shift-averaging leads to intensity profiles with higher speckle contrast and microfeatures with higher roughness. These averaging techniques limit the processing speed of microstructuring due to numerous hologram variations at low switching frequencies of the SLM. Therefore, an additional method is applied. Sufficient speckle reduction is achieved for a single hologram. As a result, defined microfeatures can be generated by an averaging of the reconstruction of different holograms, which enables higher ablation efficiency for microstructuring.