Spatial coherence measurement and control in multimode fibers

Light sources based on multimode fibers allow for controlling the degree of spatial coherence, which can be crucial for applications in sensing and imaging. Understanding the underlying mechanisms of coherence reduction and control in multimode fibers is then essential for optimized performance in these applications. In this study, we perform spatial coherence measurements across the full beam of multimode fiber based light sources using a wavefront-folding interferometer. By generating interferograms at different optical path difference positions from the fiber far-field output, the degree of spatial coherence can be quantified across the entire beam from the fringes' visibility. An approach for quantitative control of coherence is also developed providing insights for applications requiring tailored coherence properties. Our experimental results are further confirmed by numerical simulations using a linearly polarized eigenmode expansion and random modal coupling model.