Motionless speckle reduction using a three-dimensional printed micro-optical structure

Speckle suppression is crucial in laser displays because it degrades image quality. This study establishes a theoretical framework for speckle suppression based on a three-dimensional printed micro-optical structure (3DP-µOS) that utilizes the spectral characteristics of a multi-longitudinal-mode laser with an arbitrary intensity spectral distribution. The stepped microstructured elements of the 3DP-µOS introduce optical path differences, leading to speckle reduction through the superposition of different speckle fields. The corresponding 3DP-µOSes with repeated arrays are designed, printed, and used for speckle suppression experiments, revealing that the experimental results match theoretical predictions. We further reveal that the number of cells in the repeated array is fundamentally constrained by the ratio between the total optical path difference span and the minimum spacing required for effective decorrelation of laser sub-beams. Beyond this limit, further increasing the number of cells does not lead to additional speckle reduction, and the achievable speckle contrast asymptotically approaches a saturation limit.