Төрт толқынды араласу: Сызықты емес оптика және толқындық бөлісу жүйелеріндегі әсері

Four wave mixing (FWM) is an intermodulation phenomenon in nonlinear optics, whereby interactions between two or three wavelengths produce two or one new wavelengths. It is similar to the third order intercept point in electrical systems. Four wave mixing can be compared to the intermodulation distortion in standard electrical systems. It is a parametric nonlinear process, in that the energy of the incoming photons is conserved. FWM is a phase sensitive process, in that the efficiency of the process is strongly affected by phase matching conditions.

Two common forms of four wave mixing are dubbed sum frequency generation and difference frequency generation. In sum frequency generation three fields are input and the output is a new high frequency field at the sum of the three input frequencies. In difference frequency generation, the typical output is the sum of two minus the third. 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. In many configurations the sum of the first two photons will be tuned close to a resonant state. It is often very hard to satisfy this in the sum frequency configuration but it is more easily satisfied in the difference frequency configuration (where the pi phase shifts cancel out). As a result, difference frequency is usually more broadly tunable and easier to set up than sum frequency generation, making it preferable as a light source even though it's less quantum efficient than sum frequency generation. The special case of sum frequency generation where all the input photons have the same frequency (and wavelength) is Third Harmonic Generation (THG).

Four wave mixing is also present if only two components interact. In this case the term

couples three components, thus generating so called degenerate four wave mixing, showing identical properties to the case of three interacting waves.

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, 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.

FWM finds applications in optical phase conjugation, parametric amplification, supercontinuum generation, Vacuum Ultraviolet light generation and in microresonator based frequency comb generation. Parametric amplifiers and oscillators based on four wave mixing use the third order nonlinearity, as opposed to most typical parametric oscillators which use the second order nonlinearity. Apart from these classical applications, four wave mixing has shown promise in the quantum optical regime for generating single photons, correlated photon pairs, squeezed light and entangled photons.