Spatial coherence manipulation via the transient state in cascaded multimode-single-mode fibers

We present a simple fiber-based scheme that enables the manipulation of spatial coherence by exploiting the leaky-mode attenuation within the spatial transient state of a cascaded multimode-single-mode fiber structure. Shortening the single-mode fiber length reduces both the coherence length and the global coherence parameter, with the beam evolving from a leaky-mode-dominated regime to a fundamental-mode-dominated steady state, in good agreement with leaky-mode theory. The scheme exhibits strong polarization robustness against variations in incident polarization and environmental perturbations, requiring no active polarization control. This straightforward approach offers a versatile and practical solution for applications such as ghost imaging, turbulent-atmosphere communication, and speckle-free imaging, where stable and controllable spatial coherence is essential.