Fiberooptic Laser Doppler Velocimeter

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The discussion revolves around understanding the functioning of a Laser Doppler Velocimeter (LDV) system, specifically its light source and the role of a 3 dB coupler. The light from the laser is scattered by particles in a flow, allowing the system to measure particle velocity through the Doppler effect. Questions arise regarding how the light is divided after passing through the coupler and how returning light is handled. There is a mention of a broken image link that may hinder understanding. Clarification on these technical aspects is sought from those knowledgeable about LDVs.
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Hi,

I have problem understanding some things about this particular LDV system(a Laser Dopler velocimeter).

google.jpg

Source is a light source - laser. On the right there is a flow seeded with particles that scatters the light.

So here we have a simple directional 3DB coupler. Light goes thought it, part of light goes thought the all fiber's length and on the distant end it comes out, scatters on a particle inside the flow and then it goes back to the fiber, comes thought the coupler and then reaches the detector. Detector determines particle velocity thought difference of frequencies of the light because of the Doppler-effect in the process of scattering.

Providing this is a 3 db counter, how is the light divided after passing thought it?
When the light comes back from the fiber after the scattering(in different direction than the laser light) how is it divided?

I hope somebody learned in school about such LDV's.
 
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Your picture link appears to be broken. At least to me.
 
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