Optical Heterodyne Detection

Optical heterodyne detection is the implementation of heterodyne detection principle using a nonlinear optical process. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" (LO) that is set at a close-by frequency. The desired outcome is the difference frequency, which carries the information (amplitude, phase, and frequency modulation) of the original higher frequency signal, but is oscillating at a lower more easily processed carrier frequency.

Optical heterodyne detection has special temporal and spatial characteristics that pragmatically distinguish it from conventional Radio Frequency(RF) heterodyne detection. Electrical field oscillations in the optical frequency range cannot be directly measured since the relatively high optical frequencies have oscillating fields that are much faster than electronics can respond. Instead, optical photons are detected by energy or equivalently by photon counting, which are proportional to the square of the electric field and thus form a non-linear event. Thus when the LO and the signal beams impinge together on the surface of a photodiode they "mix", producing heterodyne beat frequencies directly via the physics of energy absorption. While an old technique, key limiting issues were solved only as recently as 1994 with the invention of synthetic array heterodyne detection.