# Bragg cell in laser doppler vibrometer

## Main Question or Discussion Point

I know that Bragg cell used in laser doppler vibrometer (LDV) is used to give information about the direction of the velocity of the object that we measure. I also know that Bragg cell will shift the frequency of the laser beam(modulation frequency is reduced) when the object is moving towards the interferometer. But the problem is I want to know how Bragg cell can reduced the modulated frequency and next increased the frequency so that we know the velocity direction of the object. Can anybody explain?

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dlgoff
Gold Member
I think the vibration frequency your are trying to measure is what is used to do the modulation?

One difference from Bragg diffraction is that the light is scattering from moving planes. A consequence of this is the frequency of the diffracted beam f in order m will be Doppler-shifted by an amount equal to the frequency of the sound wave F.
http://en.wikipedia.org/wiki/Acousto-optic_modulator" [Broken]

Welcome to PF zaimir83

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Thank you dlgoff.

I still could not comprehend the statement well. For example, if I have a laser beam entering the bragg cell at 40MHz and the beam coming out of the cell have a frequency of 50MHz. Is the frequency coming out of the cell always has a fixed value? Or it will shift up and down over time?

dlgoff