Why is Direct Detection of Doppler Frequency Shift Impractical in Laser Doppler Anemometry?

[leila]

I am doing an experiment in the lab at the moment on laser doppler anemometry and a few questions have popped into mind, was wondering if anyone had any ideas/insights on them:

Direct detection of the Doppler frequency shift using conventional prism or grating spectrometers is not usually practical, why? How could I show this?

Also, why use just one beam? And not two for laser doppler anemometry?

 

[nicp]

In order for me to try to answer at your questions plese explain me how it works laser anemometry.
I suppose that you use reflection of light by the air and you make reflected and original beam interfere with each other. Please explain me into more details.
And how do you measure the frequency shift?

 

[astrokreng]

Laser Doppler Anemometry (LDA) is a powerful technique for measuring fluid velocity in a non-intrusive manner. However, the direct detection of the Doppler frequency shift using conventional prism or grating spectrometers is not practical for a few reasons. Firstly, the Doppler frequency shift is very small and can be easily masked by other noise signals. Secondly, the Doppler shift is a function of the angle of incidence, making it difficult to accurately measure with a single spectrometer system. Finally, the required resolution for detecting the Doppler shift is typically in the MHz range, which is beyond the capabilities of most conventional spectrometers.

To demonstrate this, one could set up a simple experiment using a spectrometer and a laser source. By varying the angle of incidence and measuring the Doppler shift, one can see that it is dependent on the angle and thus, not easily detected with a single spectrometer.

As for the use of one beam in LDA, this is because the technique relies on the interference between the two beams to create a signal proportional to the fluid velocity. Adding a second beam would complicate the interference pattern and make it difficult to accurately measure the velocity. Additionally, using two beams would require more complex optics and increase the cost and complexity of the system. Overall, using a single beam is the most efficient and accurate method for LDA measurements.