Four wave mixing

Four-wave mixing is an intermodulation distortion in optical systems, similar to the third order intercept in electrical systems. Four-wave mixing can be compared to the intermodulation distortion in standard electrical systems. When three wavelengths (λ1, λ2, and λ3) interact in a nonlinear medium, they give rise to a fourth wavelength (λ4) which is formed by the scattering of the incident photons, producing the fourth photon. It is easier to illustrate this if we use frequency instead of wavelength.
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