A nonlinearity common in DWDM systems where multiple wavelengths MIX together to form new wavelengths, called interfering products. Interfering products that fall on the original signal Wavelength become mixed with the signal, mudding the signal, and causing attenuation. Interfering products on either side of the original wavelength can be filtered out. FWM is most prevalent near the zero-dispersion wavelength and at close wavelength spacings.