Laser Spatial Coherence Suppression With Refractive Optical Elements Toward the Improvement of Speckle Reduction by Light Pipes

We study laser speckle reduction by light pipes, and we propose a new method to improve its efficiency. Proof-of-concept refractive optical elements (ROEs) with a staircase-like structure are introduced before a holographic diffuser to split a laser beam into laser sub-beams. Optical paths of the laser sub-beams after transmitting through the ROEs are different, and these partially correlated (or uncorrelated) laser sub-beams are added in intensity basis because of the folded mirror reflections by the light pipe. Thus, laser spatial coherence is suppressed, which helps to reduce speckle. We demonstrate this method in a simplified laser projection system, where subjective speckle contrast is reduced from 0.33 to 0.24 before and after introducing a two-dimensional ROE, respectively. Comparing with other improved speckle reduction methods by light pipes, the proposed method is motionless and simpler.