The Spectral Gain Performance of C-Band EDFA under Dual-Wavelength Pumping

Abstract This paper presents a combined experimental and numerical investigation of the spectral gain performance of a C-band erbium-doped fiber amplifier (EDFA) employing a dual-wavelength bidirectional pumping scheme, in which a 980 nm laser diode is launched in the forward direction and a 1480 nm laser diode is injected in the backward direction. The amplifier was characterized under both single-tone and nine-channel WDM input conditions, covering small-signal (−30 and −10 dBm) and saturated (0 dBm) input regimes. The backward 1480 nm pump power was systematically varied to quantify its effect on C-band spectral gain uniformity, and gain flatness was evaluated using four metrics: gain variation range (ΔG), standard deviation, gain ripple, and gain slope. The results demonstrate a clear trade-off between overall gain enhancement and spectral uniformity as the backward pump power increases under saturated operation: higher backward pump power boosts the total gain but amplifies the gain peak near 1530 nm, leading to greater spectral nonuniformity. Under saturation, the best flatness was achieved at a backward pump power of 40 mW (ΔG = 0.87 dB, ripple = 5.89%). Under nine-channel WDM loading, gain nonuniformity was found to be more severe than in the single-tone case due to the unequal gain compression across channels. Signal-off ASE spectra recorded at the amplifier output decrease monotonically with increasing backward pump power, providing an independent experimental signature of the underlying mechanism. All observations are reproduced by a Giles-model numerical analysis with no fitted parameters, which traces the small-signal gain behavior to the quasi-two-level transparency of the 1480 nm transition and the multi-channel nonuniformity to cross-gain saturation. These findings provide experimentally grounded guidance for backward pump power optimization in bidirectionally pumped C-band WDM systems.