Bragg cell in laser doppler vibrometer

In summary: I tried to confirm that idea, but I found a warning that AOMs are not good for velocity sensing.In summary, the Bragg cell used in a laser doppler vibrometer (LDV) is able to provide information about the direction of an object's velocity by shifting the frequency of the laser beam. This is due to the light scattering from moving planes and the Doppler effect. However, there may be some limitations to using the Bragg cell for velocity sensing.
  • #1
zaimir83
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0
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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  • #2
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"

Welcome to PF zaimir83
 
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  • #3
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?
 
  • #4
In some AOMs, two acoustic waves travel in opposite directions in the material, creating a standing wave.
If you have this situation, then you would be able to see in which direction you accelerate the device. You would get frequencies shifted up in one direction, and frequencies shifited down in the other direction. I think.
 

1. What is a Bragg cell in a laser doppler vibrometer?

A Bragg cell is an essential component of a laser doppler vibrometer, which is a non-contact measurement device used to assess the vibration and displacement of objects. The Bragg cell is responsible for generating two laser beams with slightly different frequencies, which are used to measure the Doppler shift of the reflected light from the vibrating object.

2. How does a Bragg cell work in a laser doppler vibrometer?

The Bragg cell utilizes the principle of acousto-optics, where a high-frequency sound wave is generated and sent through a crystal. This sound wave creates a diffraction grating within the crystal, causing the incoming laser beam to split into two beams with slightly different frequencies. The reflected light from the vibrating object is then compared, and the Doppler shift is calculated to determine the object's vibration and displacement.

3. What are the advantages of using a Bragg cell in a laser doppler vibrometer?

The use of a Bragg cell allows for non-contact and non-invasive measurements, making it suitable for delicate or hazardous environments. It also provides highly accurate and precise measurements, making it a valuable tool for scientific and industrial applications. Additionally, the Bragg cell can measure a wide frequency range and is not affected by environmental factors such as temperature or electromagnetic interference.

4. Are there any limitations to using a Bragg cell in a laser doppler vibrometer?

One limitation of using a Bragg cell in a laser doppler vibrometer is that it can only measure the velocity of the vibrating object and not its displacement directly. This limitation can be overcome by using additional components, such as a position sensor, in the vibrometer setup. Additionally, the accuracy of the measurements may be affected by the angle of incidence of the laser beam on the vibrating object.

5. What are some common applications of laser doppler vibrometers using Bragg cells?

Laser doppler vibrometers using Bragg cells have various applications in industries such as aerospace, automotive, and structural monitoring. They are used to measure the vibration and displacement of machinery, vehicles, and structures to detect any defects, faults, or deviations. They are also useful in scientific research, such as studying the dynamics of structures, analyzing the effects of vibrations on materials, and evaluating the performance of musical instruments.

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