Polarization-independent deterministic mode localization in a photonic lantern
Coherent interference in multimode photonic systems underpins scalable, high-fidelity control for beam shaping, power delivery, and signal processing, yet most existing approaches rely on bulky adaptive optics or polarization-sensitive waveguides. Here, we demonstrate an all-fiber, polarization-independent coherent mode-recombination scheme that deterministically localizes Gaussian-like spots with a Gaussian similarity index up to 0.95 at three distinct positions on the multimode facet of a commercial three-mode graded-index photonic lantern (PL). The device coherently combines the lantern's individual outputs using piezoelectric phase shifters and a reciprocal Faraday-mirror feedback loop, which enforces polarization reciprocity and eliminates alignment sensitivity. This configuration achieves near-unity (100%) relative mode-conversion efficiency, three-spot switching, and long-term stability with sub-micron centroid drift ($0.55\mu m$) without active feedback. The phase-locked profiles maintain high Gaussian correspondence, strong spatial confinement, and high single-mode coupling efficiency, demonstrating robustness under laboratory-scale perturbations. Numerical simulations quantitatively reproduce the experimental recombination dynamics and further establish scalability through six-mode commercial-lantern modeling. The polarization-insensitive, compact, and low-loss architecture establishes PLs as practical engines for coherent beam forming and deterministic spatial localization, enabling turbulence-resilient beam delivery, reconfigurable mode-division multiplexing, biomedical imaging and sensing, and quantum photonics, while reducing system complexity and preserving efficiency.