Amplification of Multi-Order OAM Modes With High Gain and Low Differential Modal Gain

In this letter, we proposed and fabricated a single ring uniform erbium-doped ring-core fiber with trench assisted to achieve high modal gain and low differential modal gain (DMG) in amplifying orbital angular momentum (OAM) modes. This is enabled by confining all modes effectively in a highly similar area to improve the overlap integral of erbium ions and mode fields. Theoretical analyses and experimental results suggest the proposed fiber can support four OAM mode groups (
$\vert \text{l}\vert $
= 1 - 4, 14 OAM modes in total). Moreover, the negligible distortion in the intensity and phase patterns of the OAM modes after amplification indicates a stable and effective amplification process. Furthermore, the experimental results exhibit that the RC-EDF amplifier acquires some favorable performances with a high modal gain more than 23 dB and low DMG less than 1 dB for all 14 OAM modes across the whole C-band.