Partially coherent beam smoothing using a microlens array

The main issue in applying a microlens array as a beam homogenizer is the high intensity contrast resulting from the periodic structure. We present a practical solution to eliminate speckle patterns by using a diffuser that can convert a coherent source into a Gaussian Schell source with an adjustable coherence length. Based on numerical simulation and experimental results, the microlens should have the Fresnel number above 15, a large pitch when the target plane is fixed, and an appropriate D/sigma(f) ratio to obtain a spot with high uniformity. Moreover, a slight defocus of the observation surface within the range of +/- 1mm has a negligible effect on the uniformity of shaped spots the homogenized spot. Finally, the applications of the findings are presented. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.