Influence of anisotropic Raman gain on four-wave mixing signals

Gain of Raman-amplified four-wave mixing (FWM) signals in optical fibers was found to be different from less than the third power of the signal gain at and near spectral positions of local minima when the spacing of two input signals is a few nm. The gain there was enhanced which was unaccountable with theoretical analyses. It is likely that wavelength regions corresponding to dips are sensitive to the distribution of local Raman gain and that increased Raman gain for FWM signals was caused by anisotropic Raman gain over a fiber.