Hi.
I am an trying to understand how a Heterodyne Laser Doppler Vibrometer works. I am aware of the basic design. I am just worried that I don't understand the details of signal detection and processing.
As I understand it, The Detector in the vibrometer will receive a mixture of the reference signal and the object signal (The object is vibrating with some frequency). The intensity fluctuation at the detector has a certain equation to represent it. Please See the attached file
My question is, what is the signal the detector receives ? and how it looks like after detection ?
(a) Does the detector receive a bright spot that changes its intensity based on the object vibration ?
(b) Does it detect a fringe pattern that changes due to vibration ?
If the answer is (a) then I assume the problem is simple, but if the answer is (b) then I am not sure how I would extract the vibration information from the signal. I read some literature about fringe counter, I am not sure how it can be implemented in a Laser Doppler Vibrometer.
The detector is, in most cases, a simple photodetector and detects only the intensity of the laser light. So it is case (a) from your question.
While the laser frequency is normally very high and the shift at the sample very small you employ the beat of the interference between reference and signal to measure the dynamics at your sample. The beat frequency is much lower and easier to detect. In the end you measure the fluctuation of your beat frequency.
Sources: https://en.wikipedia.org/wiki/Laser_Doppler_vibrometer and an old LDV in the lab next to my office.
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