Differential mode-gain equalization for FM-EDFA with real-time on-demand mode pumping via spatial light modulator

We demonstrate a real-time on-demand mode pumped few-mode erbium-doped fiber amplifier (FM-EDFA) with reconfigurable differential mode gain control. A 980 nm high-order mode system excitation platform based on spatial light modulator (SLM) has been developed, which can efficiently convert the output of a single-mode pump laser into LP_11a, LP_11b, LP_21a, LP_21b, and LP₀₂ modes online. The effects of each pumping mode on the gain characteristics under single-mode individual amplification and mode-multiplexed amplification are analyzed in detail. The experimental results show that an average gain of 21.21 dB and a low differential modal gain (DMG) of 0.335 dB at 1550 nm can be achieved by online modulation of the LP₀₂ pump mode. In the range of signal input variation from -20 to -10 dBm, the DMG is consistently below 0.77 dB, with a minimum DMG of 0.1 dB. When 8 wavelength channels are amplified simultaneously, an average gain of up to 20.86 dB with a DMG of less than 2.47 dB can be achieved over the C-band. In addition, when the azimuthal-dependent pump is selected, the DMG under mode multiplexing shows a significant difference. The constructed amplifier configuration provides guidance on implementing differential mode gain equalization for the expansion of mode division multiplexing (MDM) transmission to a larger number of mode channels and longer transmission distances.