Erbium-doped Circular Photonic Crystal Fiber Design for the Amplification of 20 OAM Modes

We propose a fiber design consisting of ring-shaped erbium-doped core surrounded by circularly arranged holes of varying radius in fused silica glass to amplify higher-order orbital angular momentum (OAM) modes. We numerically demonstrate simultaneous amplification of 20 OAM modes (l=5, 6, 7, 8, 9) with more than 20 dB gain at 1550 nm wavelength while having differential modal gain of 0.12 dB. We also demonstrate amplification of modes over the C-band with gain excursion of less than 2 dB, differential modal gain of less than 0.2 dB, and noise figure less than 4.2 dB. The proposed erbium-doped fiber amplifier would be useful in high-data-rate space-division multiplexing optical-communication systems.