Orbital Angular Momentum Mode-Multiplexed Amplification and Transmission Based on a Ring-Core Erbium-Doped Fiber

Recently, orbital angular momentum (OAM) multiplexing has attracted much attention for providing an exponential growth in optical fiber capacity. As an irreplaceable component to promote long-haul OAM mode fiber transmission system, the on-line OAM mode-multiplexed fiber amplifier need to be developed. In this paper, we propose a Gaussian-like erbium doping distribution design method for ring-core erbium doped fiber (RC-EDF) to optimize the differential modal gain (DMG), which can reduce the fabrication complexity of the fiber preform. With a good match with original design, the fabricated RC-EDF was characterized by all the supporting four OAM modes ( $\vert l \vert$ = 1, 2, 3, 4) multiplexed amplification, which achieves an ultra-low DMG less than 0.3 dB at 1550 nm. The gain is above 15 dB and DMG is less than 1.2 dB for four OAM modes across the C-band under a 500 mW pump power. As a proof for transmission, 32 channels (8 wavelengths × 4 OAM modes) carrying 32-Gbaud quadrature phase-shift keying (QPSK) OAM modes multiplexed-WDM transmission over the OAM-EDFA is demonstrated. The proposed OAM-EDFA with the average OSNR penalties of 0.34 dB may promote the practical implementation for the long-haul OAM modes fiber transmission system.