DOP Characterization of Mode Division Multiplexing Signals with Application to Differential Mode Gain Control for Few-Mode Erbium-Doped Fiber Amplifiers

Degree of polarization (DOP) is often used to measure the polarization properties of single-mode signals and the classical entanglement of orbital angular momentum (OAM) modes or vector modes. The paper investigates the ensemble DOP (eDOP) of mode division multiplexing (MDM) signals consisting of linearly polarized (LP) modes in an optical fiber, which is dependent on their respective DOPs, powers and polarization angles. The expression for eDOP is deduced, and the theoretical results are verified by the experimental data. Furthermore, we experimentally realize the automatic differential mode gain (DMG) control for a two-mode erbium-doped fiber amplifier (EDFA) by monitoring the eDOP of the amplified MDM signals before adjusting the pump power in a feedback way. The cases for three-mode and six-mode amplification are also analyzed by simulation. It is shown that the eDOP-based automatic DMG control technique is promising for the commercial FM-EDFAs in the future.