Spatial Coherence Characterization in Multimode Fibers

Spatial coherence is a key property of light for imaging and sensing. Conventional approaches to control the degree of spatial coherence typically rely on diffusers, which is not practical from an application viewpoint. Multimode fibers where multiple modes interact along propagation provide on the other hand a convenient means to alter spatial coherence in a guided and compact geometry. Previous studies of spatial coherence in multimode fibers however have been limited to coordinate separation within the beam profile, providing only partial information on the coherence of the beam [1]. Here, using scanning wavefront folding interferometry, we characterize experimentally for the first time the spatial coherence of a step-index multimode fiber (MMF) coupled broadband light across the full beam profile. This allows to reveal how the spatial coherence and coherence area of the beam are affected by the number of excited modes with increased residual coherence at the beam edges. Our experimental observations are in very good agreement with numerical simulations using a random mode coupling theoretical model [2].