Figure 3.5: Heterodyne Configuration of LDV.

For visible light MHz and the photo-optics will not respond to such high frequencies. Hence the electrical output will oscillate as the at a frequency . These frequencies can be measured using one of the following methods:

1. Spectroscopy (in which case an electrical signal is not generated).

2. Counting the number of zero crossings.

3. Digitizing the electrical signal and applying FFT algorithm to obtain the frequency spectrum.

In (3) the dominant frequency in the spectrum corresponds to the Doppler shift. Convenient instruments such as a apectrum analyzer are available for making such measurements.