wavelengthconversion

Wavelength conversion, in optics, is the process of changing the wavelength of light while preserving its essential properties. It relies on nonlinear interactions in media with nonzero second- or third-order susceptibility to transfer energy between photons at different frequencies.

In second-order processes, second-harmonic generation doubles the frequency (λ becomes half); sum-frequency generation combines two inputs at ω1 and ω2 to produce a photon at ω3 = ω1 + ω2; difference-frequency generation yields ω3 = |ω1 − ω2|. In third-order processes, four-wave mixing can translate frequencies by interacting two pump photons with a signal to generate an idler at ωi = 2ωp − ωs. Efficiency requires phase matching, a condition that ensures momentum conservation among interacting waves. Techniques to achieve phase matching include angle tuning, birefringent phase matching, and quasi-phase matching via periodic poling.

Common platforms for wavelength conversion include bulk nonlinear crystals, periodically poled crystals such as PPLN and PPKTP, integrated waveguides on materials like lithium niobate or silicon nitride, and fiber-based nonlinear media. Engineers tailor dispersion and mode confinement to enhance interaction length and nonlinearity, enabling practical devices.

Applications span telecommunications, where wavelength translation helps interconnect systems operating at different bands, and quantum information, where converting single photons allows interfacing quantum memories with long-distance telecom channels. Wavelength conversion also supports spectroscopy, sensing, and certain forms of optical signal processing. Noise and added photons from spontaneous processes can limit performance, particularly in quantum-level conversions, requiring careful design and filtering.