downconversion

Downconversion is the process of converting a signal or photon from a higher frequency to a lower frequency. In radio frequency and microwave systems, it is typically achieved by mixing the incoming signal with a local oscillator in a nonlinear device such as a mixer. The mixer output contains both sum and difference frequencies; a filter selects the lower, intermediate frequency (IF) for amplification and detection. This spectral shift enables easier, more selective processing of high-frequency signals and improves compatibility with fixed-frequency amplifiers and converters.

In optics and quantum optics, downconversion refers to nonlinear processes that transform higher-energy photons into lower-energy photons. A common example is spontaneous parametric down-conversion (SPDC), where a pump photon at frequency ωp is converted into two photons at ωs and ωi such that ωp = ωs + ωi. Difference-frequency generation is another pathway. Downconversion is a principal method for producing correlated or entangled photon pairs for quantum communication, imaging, and metrology. Realization relies on nonlinear materials and phase-matching techniques to achieve efficient conversion.

Applications and considerations: In RF systems, downconversion allows fixed-frequency amplification and easier filtering, but can introduce conversion loss, image-frequency interference, and LO leakage. In optical systems, efficiency depends on the nonlinear coefficient, pump intensity, crystal length, and phase matching; generated photons often exhibit spectral correlations that must be managed for a given use.

Downconversion thus encompasses both electronic signal processing and nonlinear-optical frequency conversion, serving to bring high-frequency signals or photons into more accessible, lower-frequency domains for detection, processing, or photon-pair generation.