Editing Four-wave mixing (section)

== Adverse effects of FWM in fiber-optic communications ==
FWM is a fiber-optic characteristic that affects [[wavelength-division multiplexing]] (WDM) systems, where multiple optical wavelengths are spaced at equal intervals or channel spacing. The effects of FWM are pronounced with decreased channel spacing of wavelengths (such as in dense WDM systems) and at high signal power levels. High [[chromatic dispersion]] ''decreases'' FWM effects, as the signals lose [[coherence (physics)|coherence]], or in other words, the phase mismatch between the signals increases. The interference FWM caused in WDM systems is known as interchannel [[crosstalk]]. FWM can be mitigated by using uneven channel spacing or fiber that increases dispersion.  For the special case where the three frequencies are close to degenerate, then optical separation of the difference frequency can be technically challenging.

:<math> f_{ijk} = f_{i} + f_{j} - f_{k}, \mathrm{where}\, i, j \neq k </math>