Interferential devices

Question 3 : Speed measurement

Refraction index of the flow medium is of 1,33. The laser used is a continuous ionized argon laser which dominant beams are, in term of power, of , , , and . The wavelength beam is selected. The laser beam of diameter is divided into two parallel and same intensity beams with no, or two very slight, course difference at the output of the wavefront-splitter. Both beams are distant from the other of . A focusing lens with a focus distant make the beams converge in its image focus where the measurement volume of diameter which value depends on the light wavelength in its medium, on optical emission focus and on diameter , is located:

The detector detects a frequency. Figure 53 gives the experimental configuration.


   
    Figure 53 : Velocimeter experimental configuration
Figure 53 : Velocimeter experimental configuration [zoom...]Info

Question

Deduce of these experimental conditions:

a) Angle

b) Interfringe

c) Measurement volume diameter

d) Fluid speed

Remarques :

  1. The circuitry describes above does not make it possible to determine the speed direction. In the exercise, the direction was to be known. If it is not the case, in order to find this out, we make the fringes scroll slightly altering one of the optical frequencies of one of the two beams using a Bragg cell or an acousto-optic modulator.

  2. Using the different line of laser emission in the same measurement volume, three interferences fringes networks of different colors a directions can be obtained and so make it possible to detect the three components of an unknown direction speed.

  3. The adjustment of the beams convergence must be done carefully so that the beams intersection is located at the level of the latter's “waist” to obtain flat, equidistant and well contrasted fringes (figure 54). In that case only, the measured frequency will not depend on the particle position in the measurement volume.


   
    Figure 54 : Field of interferences (http://www.onera.fr/dafe/velocimetrie-laser)
Figure 54 : Field of interferences (http://www.onera.fr/dafe/velocimetrie-laser) [zoom...]Info

For further information the reader should consult the bibliographical references [7.8].

Solution
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