High-speed speckle averaging for phase-only beam shaping in laser materials processing

A phase-only spatial light modulator (SLM) is a powerful tool to shape arbitrary intensity distributions and finds application in various research fields and industry. However, since phase-only beam shaping cannot control the full complex light field, a speckle pattern overlays the shaped target intensity and strongly impairs the result.
Several methods have been developed to diminish speckle noise, all of which have their advantages and disadvantages. Usually they offer a compromise between quality and efficiency. We present a method for phase-only beam shaping which achieves near-perfect and highly efficient speckle averaging with a liquid crystal on silicon SLM based on a single phase mask. A galvanometer scanner scans the laser beam over the active area of the SLM to achieve high-speed averaging. The applied angle resulting from the scanning movement is compensated by a proper optical setup to maintain a static target image with speckle averaging out. As an additional benefit of the scanning movement, artifacts from light not diffracted by the SLM disappear in the target plane. The setup is designed for high-energy laser beams as intermediate foci are avoided at all optical components. Besides optical averaging on camera images, we validate our method with ablation experiments where we make a profound analysis of the involved system parameters and the achieved quality. Thereby, it becomes apparent that the proposed method not only stands out by the high-quality results but also by its simplicity in implementation.