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