J. Lightwave Technol. 29, 3091–3098 (2011)

Although directly sampling an electric field at optical frequencies is impossible owing to the frequency limitation of electronic sampling devices, indirect sampling can be achieved through heterodyne processes that generate beat notes in the radiofrequency domain. Peter Delfyett and co-workers in the USA have now used frequency combs as local oscillators to downconvert and compress optical signals through multiheterodyne detection, a process by which two combs that share an optical reference are mixed to deduce the effect of a medium on the signal comb. The researchers used a commercially available 250 MHz erbium-doped fibre frequency comb as a local oscillator in three distinct experiments: mixing two mutually incoherent mode-locked laser combs; mixing a mode-locked comb and phase-modulated continuous-wave light; and performing spectral interferometry on downconverted white light. The researchers demonstrated spectral compression by factors of 1,600 for phase-modulated light and 17,000 for mode-locked pulses, during which carrier frequencies were converted from ∼200 THz to ∼100 MHz. They also showed that interference patterns can be obtained in the microwave regime by summing photocurrents, and that these patterns can be used for high-resolution white-light spectral interferometry.