High-purity amplification of circularly polarized orbital angular momentum modes in an active spun ring-core tapered fiber

Abstract: Structured light, optical fields engineered in their spatial, polarization, or phase degrees of freedom, has become a key resource across advanced communication, sensing, imaging, and quantum technologies. Optical fibers nowadays play an essential role in this landscape, providing stable and scalable platforms for guiding, and amplifying complex modes such as vector and orbital angular momentum (OAM) beams. In this work, we demonstrate an active spun ring-shaped tapered fiber as a gain medium for efficient amplification of OAM modes preserving their modal purity and polarization topology. OAM beams with topological charges l = 1 and l = 2 carrying 60 ps pulses at 15 MHz repetition rate at 1030 nm wavelength are amplified over 1.2 W average power with modal purity over 95%. The spatially resolved measurement of the OAM beam polarization topology revealed small distortion due to the coupling in to neighbour modes. These results demonstrate the high potential of active spun ring-shaped tapered fibers for power scaling of complex beams, preserving their phase and polarization structure simultaneously.