Spatial dimension expansion of incoherent optical field in a multimode fiber scheme for speckle suppression

Speckle noise is a major limitation of laser projection and imaging systems, and be suppressed using multimode fiber (MMF); however, speckle suppression is not sufficient when only using MMF. Here, we propose a scheme for decoherence of an optical field in 2D using an MMF, retardation plate, and a pair of microlens arrays. We built a theoretical model to design the retardation plate and evaluate the performance of the scheme. Theoretical results demonstrate that decoherence of the distal optical field of the MMF in 2D can significantly increase the number of uncorrelated sub-beams. The effectiveness of the proposed scheme is demonstrated by experiments; speckle suppression efficiency improves by >20%. The incoherent areas distribution of the optical field at the distal end of the MMF shifts from 1D to 2D as the length of the MMF increases. This is significant for understanding the evolution of a partially coherent light field within an MMF.