Multimode optical fibers have seen increasing applications in communication, imaging, high-power lasers and amplifiers. However, inherent imperfections and environmental perturbations cause random polarization and mode mixing, making the output polarization states very different from the input one. This poses a serious issue for employing polarization sensitive techniques to control light-matter interactions or nonlinear optical processes at the distal end of a fiber probe. Here we demonstrate a complete control of polarization states for all output channels by only manipulating the spatial wavefront of a laser beam into the fiber. Arbitrary polarization states for individual output channels are generated by wavefront shaping without constraint on input polarizations. The strong coupling between spatial and polarization degrees of freedom in a multimode fiber enables full polarization control with spatial degrees of freedom alone, transforming a multimode fiber to a highly-efficient reconfigurable matrix of waveplates.
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