In light scattering, heterodyning refers to mixing of the scattering light wave with a fraction of the incident light wave to elucidate an improvement in signal quality and/or measurability.
As an example of how one would use heterodyning, let us consider an APD, the typical detector used to monitor the scattering intensity of a sample in a light scattering instrument. The intensity is proportional to the square of the wave field. For light generated from a 633 nm laser, the frequency is on the order of 1014 Hz, resulting in an oscillation frequency at the APD on the order of 2 x 1014 Hz. The APD response rate however is on the order of 0.1 ms or 107 Hz. So if the goal is monitor subtle changes or Doppler shifts in the frequency of the scatter light, as in an electrophoretic light scattering measurement, one could not accomplish this by attempting to monitor the frequency directly with the APD. The response rate is simply too slow. However, if the Doppler shifted scattered light wave is heterodyned with a fraction of the incident light wave, a pulsed or beating wave can be generated, the frequency of which can easily be monitored with an APD.
The schematic shown below is given to assist with a visualization of the effects of the method of heterodyning. The base (or reference) wave from the incident light source is shown in blue and the Doppler shifted scattering wave is shown in green. The wave shown in red is that generated upon addition of the base and scattered waves. The oscillating bold red line, is representative of the average signal that would be monitored at the APD.
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