Hybrid Raman and phase-sensitive amplifier for ultra-low-noise few-mode amplification and modal crosstalk mitigation in MDM systems

Ultra-low-noise amplification and modal crosstalk mitigation are critical for advancing mode division multiplexing (MDM) systems. We propose and experimentally demonstrate a few-mode hybrid optical amplifier that combines a few-mode distributed Raman amplifier (FM-DRA) with a single-mode phase-sensitive amplifier (PSA). The FM-DRA implemented in a 100-km few-mode fiber provides preliminary low-noise amplification, while the subsequent PSA leverages its unique phase-sensitive gain property to further enhance signal gain, improve optical signal-to-noise ratio, and suppress modal crosstalk. The system achieves average on-off gains of 20.3 dB and 14.8 dB across a 35-nm bandwidth (1530 to 1565 nm) for the <i>LP</i>₀₁ and <i>LP</i>₀₂ modes, respectively. Remarkably, the effective noise figures (NFs) for the <i>LP</i>₀₁ and <i>LP</i>₀₂ modes are below -2.8 dB and -3.7 dB, respectively, with the lowest effective NFs reaching -5.9 dB and -6.8 dB, outperforming the standalone FM-DRA. Additionally, the PSA demonstrates significant modal crosstalk reduction for each mode across the 35-nm bandwidth. These results demonstrate the potential of the proposed scheme to significantly enhance the performance of MDM systems.