Detection of coherent optical radiation

Sensitive coherent detection techniques, identical in principle to the methods applicable in the RF or microwave regions, are being investigated at optical wavelengths. Since we already can reach the natural limit of being able to count individual photons in both the short-wavelength visible and near-ultraviolet regions, there is no possibility for sensitivity improvement in those spectra. However, at increased wavelengths, where the detectors are several orders of magnitude away from being able to count photons, much higher sensitivity can be achieved through the use of coherent detection methods. In optical superheterodyne detection the mixing in the superheterodyne receiver arises because optical detectors measure the intensity rather than the amplitude of the light waves, and thus square-law detection results. Another detection method is based on a truly nonlinear effect, in which the electric field associated with the incident radiation is so strong that it drives the polarization out of the linear region. Parametric amplification of the optical signal then becomes possible.