Four-Wave Mixing is one of the four Non-linear attributes in optical fiber communication. As the name indicates, the fourth wave gets generated when three (or even two) distinct signals travel together. Four-wave mixing (FWM), which is also called four-photon mixing, occurs when the interaction of two or three optical waves at different wavelengths generates new optical waves, called mixing products or sidebands, at other wavelengths.
This interaction of waves occurs between signals in multiple-channel systems, between OA ASE noise and a single channel, as well as between the main mode and side modes of a single channel. In the case of two signals, the intensity modulation at their beat frequency modulates the fiber refractive index and produces a phase modulation at a difference frequency.
The phase modulation creates two sidebands at frequencies given by this difference. In the case of three signals, more and stronger mixing products are produced, which will fall directly on adjacent signal channels when the channel spacings are equal in frequency. Two optical waves propagating along a fiber produce FWM with high efficiency if the phase matching condition is achieved between sidebands and initial signals.