Four-wave mixing

Four-wave mixing can be compared to the intermodulation distortion in standard electrical systems.

A condition for efficient generation of FWM is phase matching: the associated k-vectors of the four components must add to zero when they are plane waves.

This becomes significant since sum- and difference-frequency generation are often enhanced when resonance in the mixing media is exploited.

[1] However, close to resonances the index of refraction changes rapidly and makes addition four co-linear k-vectors fail to add exactly to zero—thus long mixing path lengths are not always possible as the four component lose phase lock.

In gaseous media an often overlooked complication is that light beams are rarely plane waves but are often focused for extra intensity, this can add an addition pi-phase shift to each k-vector in the phase matching condition.

The effects of FWM are pronounced with decreased channel spacing of wavelengths (such as in dense WDM systems) and at high signal power levels.