The basic principle behind FWM involves the interaction of three optical waves (frequencies) within a medium, leading to the generation of a fourth wave. Mathematically, if the original waves have frequencies \( \omega_1 \), \( \omega_2 \), and \( \omega_3 \), the new wave will have a frequency \( \omega_4 \) given by: \[ \omega_4 = \omega_1 + \omega_2 - \omega_3 \] This process relies on the nonlinear properties of the medium, which means its refractive index changes with the intensity of the light passing through it.
