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?