Four-wave mixing induced gain suppression and noise elevation in a distributed Raman amplifier

We observed experimentally four-wave mixing (FWM) between 1470 nm pump and 1520 nm signal in a forward-pumped distributed Raman amplifier (DRA) over a 25 km of non-zero dispersion shifted fiber with zero dispersion wavelength of 1495 nm. The 100 mW pump Fabry–Perot (FP) spectrum centered at 1470 nm is reproduced via FWM around the single-wavelength probe signal at 1520 nm, generating spectrally nonuniform FWM induced noise floors. The suppression of Raman gain by about 2.4 dB was also observed with maximum FWM at minimum phase mismatching between pump and signal. The new phase mismatching is derived and combined with the product counts of adjacent pump mode powers to estimate the FWM generated power ratio with maximum and less FWM efficiencies. We find these theoretical calculations are consistent with the experimental results.