Laser beam modulation with a fast focus tunable lens for speckle reduction in laser projection displays

Abstract We propose a laser speckle reduction method using a fast focus tunable lens (FTL). Different laser beams are generated after modulating the FTL. Thus, when the laser beams are used to illuminate a diffuser, various speckle images are obtained, and the summed speckle images yield a reduced speckle contrast ratio. To maximize the tuning range of the focal length of the FTL for efficient speckle reduction, the FTL operates in a resonant mode at the resonant frequency at 97 Hz. The effect of the change of the distance between the FTL and the diffuser on the efficiency of speckle reduction is investigated. Experimental results show that the lowest objective speckle contrast ratio is 0.05. After the application of this speckle reduction component in a laser projector, the subjective speckle contrast ratio is reduced from 0.16 to 0.09. In comparison with other speckle reduction methods, the method proposed in this study is simple and effective, and can be used in practice in laser projection displays.