Speckle-free laser marking of metals with liquid-crystal-based spatial light modulator

A liquid-crystal-based spatial light modulator (LC-SLM) is used for laser beam patterning and manipulation in order to produce very small, speckle-free 2D marks on metal surfaces. To avoid speckles within the laser marked area, which is a typical drawback of current SLM-based laser marking processes, an array of "non-interfering" beamlets is produced by the LC-SLM and used for generating micro-patterns. The micro-patterns (e.g. 20*20 pixel datamatrices with overall dimensions of less than 320?m by 320?m) are generated in a series of 16 steps, using a Fresnel zone lens (FZL) which is written into a computer-generated hologram (CGH) that produces an array of beamlets. By shifting off-axis the whole kinoform (FZL+CGH) for each step, the array of beamlets is spatially moved along the imaging plane, producing the required micro-pattern. In comparison to other SLM-based laser marking approaches already reported in the literature, our method not only eliminates (or at least significantly reduces) unwanted speckle interference but also reduces the laser power required for marking.