Rami Arieli: "The Laser Adventure" Chapter 9.3.1 page 2 - comment

Heterodyne Detection
When two signals with close frequencies are mixed, the result is a signal with intensity proportional to the product of the intensities of the two signals, and its frequency is proportional to the frequency difference between the two signals.

The mathematical description of the two signals:

E1 = Amplitude of first signal.
E2 = Amplitude of second signal.

w1 = Frequency of first signal.

w2 = Frequency of second signal.
The intensity of the detected signal (I) is proportional to the product of the sum of the signals and their complex conjugate:
After the mathematical manipulation:
w = w1-w2 is the frequency difference between the signals, and is called Beat Frequency.
In practice, a strong oscillator (laser) is used as a Local Oscillator.

The signal from the local oscillator is mixed with the weak signal coming from the target.

The result is a strong signal at the beat frequency.

When optical signals are involved, their frequencies are of the order of 1014-1015 Hz.

Electronics can not response at such frequencies, but if the two signals are close enough in frequency, electronics can measure the beat frequency.

The DC signal can be filtered easily, and the signal at the sum frequency is not detected by the electronics.

Pevious Page