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). Degenerate four-wave mixing 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. 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, 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. Applications of FWM 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. See also Optical phase conjugation, phase conjugate mirror Supercontinuum generation Kerr frequency comb Optical Kerr effect Lugiato–Lefever equation References External links Encyclopedia of Laser Physics and Technology Nonlinear optics Photonics Optical fiber Frequency mixers