Mix with four waves

The mixture with four waves is an effect non-linear Optique, which consists of an intermodulation between three electromagnetic waves which generate or amplify a fourth of it. When three waves of frequencies $\ nu_1 \,$ , $\ nu_2 \,$ and $\ nu_3 \,$ interact in a non-linear medium (an agreement of phase is necessary), they can produce the frequencies $\ nu_1 \ pm \ nu_2 \ pm \ nu_3$

Characteristics of the process

As third order process (i.e proportional to $\ chi_ {3} E^3 \,$ ), it is generally of rather low intensity, interesting only in the absence of process of second order (for example in a material Centrosymétrique, where the non-linear susceptibility of order two $\ chi_2 \,$ is null).

the agreement of phase is in general carried out not colinéairement in the solids and colinéairement of use the abnormal Dispersion in gases.

the process can be largely more effective in the presence of a resonance with one or two photons (i.e excited level of the crystal considered near to the energy brought by one or two of the incidental photons)

Uses

The mixture with four waves plays a big role in the generation of Supercontinuum in fiberoptics with photonic crystal, particularly in the case of long impulses (duration higher than the picosecond)

It is also a cause of major embarrassment in the field of telecommunications per fiberoptic, particularly in the case of Multiplexage in wavelength

Mix with four wave degenerated in frequency

When two of the incidental frequencies are identical, one speaks about mixture with four waves degenerated: it is an exploitable effect for the realization of mirrors to conjugation of phase.

When the three incidental frequencies are identical, it is in fact a Génération of third harmonic.

References

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