Robust Synthesis of Higher-Order Polarization States Through Partial-Coherence Engineering

Higher-order polarization states, arising from the nonseparable coupling between the spatial and polarization degrees of freedom, have important applications in structured-light science. However, when directly carried by fully coherent beams, their spatial polarization distributions are readily distorted by atmospheric turbulence, obstructions, and other propagation disturbances. This review discusses partial-coherence engineering as an alternative strategy for robust polarization-state synthesis. In this approach, a prescribed higher-order polarization state is encoded into the two-point coherence structure of a partially coherent vector source and subsequently reconstructed through propagation. We review the description of higher-order Poincaré-sphere states, their behavior in atmospheric turbulence, and the theoretical foundations of vectorial spatial coherence engineering. We then explain how coherence-to-polarization conversion enables the target state to emerge in the far field, a focal plane, or a prescribed propagation plane and compare representative experimental implementations. Finally, we assess current limitations and outline future directions and potential applications of robust higher-order polarization states. Overall, partial coherence emerges as an active degree of freedom for the robust synthesis and transmission of spatially structured polarization.