What is Optical Heterodyne Detection


Heterodyne detection (also called coherent detection) is a detection method which was originally developed in the field of radio waves and microwaves. There, a weak input signal is mixed with some strong “local oscillator” wave in some nonlinear device such as a rectifier, and the resulting mixing product is then detected, often after filtering out the original signal and the local oscillator frequency. The frequency of the mixing product is the sum or the difference of the frequencies of the signal and the local oscillator.

Optical heterodyne detection involves optical signal and local oscillator waves, whereas the mixing product is an electrical signal. The mixing product is not obtained by mixing the signal and local oscillator wave in a nonlinear crystal, but rather simply by detecting the linearly superimposed waves with a square-law photodetector, typically a photodiode. For example, one uses a beam combiner (or beam splitter) and aligns the two beams such that they are mode-matched. This means not only that their intensity profiles overlap, but also that their wavefronts have the same curvature on the detector, so that the interference conditions are uniform over the full detector area. Of course, this is possible only if the two beams are spatially coherent.In a fiber-optic setup, a fiber coupler would be used instead of the beam splitter, and all fibers would be single mode fibers, possibly of polarization-maintaining type; the mode matching is then guaranteed without a special alignment.

Applications of Heterodyne Detection

Some examples for the application of optical heterodyne detection are:

  • Heterodyne detection is used for Coherent Doppler LiDAR measurements (and related laser radar techniques), where very weak light scattered in the atmosphere needs to be detected. It is possible, for example, to accurately measure wind speeds based on the Doppler shift. By deriving the signal and local oscillator beams from the same single frequency fiber laser source, it is possible to resolve very small Doppler shifts.
  • There are other applications in laser spectroscopy. Examples are solar radiometry and cavity ring-down spectroscopy.
  • The linewidth of a fiber laser, for example, can be measured with the self-heterodyne technique, where a frequency-shifted copy of the fiber laser beam is superimposed with a temporally delayed copy on a photodetector.
  • In optical fiber communications, the phase sensitivity of heterodyne and homodyne detection allows one to demodulate phase-encoded signals, based on techniques like frequency shift keying (FSK) (→ coherent communications). Also, it the basis for some forms of electronic dispersion compensation.
  • Radio and Microwave over fiber sometimes uses heterodyne detection.