Amplification of 14 orbital angular momentum modes in ring-core erbium-doped fiber with high modal gain.

We propose and demonstrate an orbital angular momentum (OAM) fiber amplifier supporting 14 OAM modes based on a fabricated ring-core erbium-doped fiber (RC-EDF) with a tailored erbium-doped profile. Theoretical analyses and numerical simulations suggest that the proposed RC-EDF provides relatively uniform gain larger than 20 dB for all 14 modes. With a core pump configuration, we experimentally characterize the performance of the RC-EDF-assisted OAM fiber amplifier, which can acquire a high modal gain up to 33.4 dB and low differential modal gain less than 1.8 dB at the wavelength of 1550 nm. The obtained results indicate successful implementation of the RC-EDF-assisted OAM fiber amplifier for 14 OAM modes with favorable performance.