Heterodyne detection





In heterodyne detection, one modulates, usually by a frequency shift, one of two beams prior to detection. A special case of heterodyne detection is optical heterodyne detection, which detects the interference at the beat frequency. The AC signal now oscillates between the minimum and maximum levels every cycle of the beat frequency. Since the modulation is known, the relative phase of the measured beat frequency can be measured very precisely even if the intensity levels of the beams are (slowly) drifting. This phase is identical in value to the phase one measures in the homodyne case. There are many additional benefits of optical heterodyne detection including improved signal to noise ratio when one of the beams is weak.



In standard interferometry, the interference occurs between two beams at the same wavelength (or carrier frequency). The phase difference between the two beams results in a change in the intensity of the light on the detector. Measuring the resulting intensity of the light after the mixing of these two light beams is known as homodyne detection.
Homodyne Detection


In homodyne detection for a given relative phase shift, the output is a constant (DC) signal level. This level is indirectly related to the phase shift. If the minimum and maximum possible values of the signal level are known (through calibration) then one can compute the relative phase shift. In practice, precise calibration is difficult since the optical beams
1) may not be perfectly aligned
2) are not true plane waves, or
3) usually undergo unknown time varying attenuation on one arm of the interferometer.