Ring-core fiber-based coupling and amplification of OAM modes: realizing efficient second-to-first and second-to-second order mode conversion

Two fully fiber-coupled orbital angular momentum (OAM) mode amplifiers are designed using side-polishing and bonding techniques with dual ring-core erbium-doped fibers (RC-EDFs). Mode matching conditions at 1550 nm are selected to facilitate power coupling between the two RC-EDFs, enabling the coupling, transmission, and amplification of both second-to-first order OAM modes and second-to-second order OAM modes. By designing the structural parameters and ring spacing of the two RC-EDFs, two types of all-fiber in-line OAM amplifiers are developed, each achieving a gain exceeding 30 dB. This design approach establishes a foundation for separating and amplifying higher-order OAM modes and advances the application of all-fiber coupled amplifiers in high-capacity mode-division multiplexed optical communication systems.