Spot arrays for uniform material ablation with ultrashort pulsed lasers

Phase-only beam shaping with a spatial light modulator (SLM) improves precision and increases efficiency during ultrashort pulsed laser materials processing. Speckle noise appears as not the full complex light field can be controlled by phase-only beam shaping. Adding up several patterns of spot arrays is one method to overcome speckle. Therefore, the targeted pattern is divided into several arrays of spots where adjacent spots within the same pattern are separated far enough to not interfere with each other. We show that a pulse-by-pulse change of the individual spot arrays during material ablation is crucial to achieve a uniform surface of high quality. Furthermore, the amount of ablated material and thus the ablation efficiency increases by a factor of 2 when each laser pulse is shaped with another spot pattern. To realize those rapid changes, we divide the SLM into independent sub-sections and sequentially illuminate the corresponding areas instead of changing phase masks on the SLM. To maintain the resolution while illuminating less area, we investigate the influence of the pixel count on the SLM.