We are setting up a three component LDV by TSI, two fiberoptics probes, one with 4 beams (green and blue, shifted and unshifted), the other one with 2 beams (purple shifted and unshifted). The two probes are on a transverse system in air, the measurements are in a wave flume with water inside through a glass window. While the green-blue channels, with the axis of the probe orthogonal to the glass window, work perfectly, the purple channel, with the probe inclined of 28 degrees, does not work. We have carefully aligned the fibers in air. The purple channel works satisfactorily if the probe is orthogonal to the glass window. I suppose that the problem is the refraction, which modifies the angles of the two beams and also the Field of View of the backscatter. Should we align the fiberoptics in water? Did anybody encounter the same problem we had? Thanks
Dear Sandro,
My colleague Muriel, in my Laboratory, had the same problem. She tried to set LDV with probes non orthogonal to the glass window. But she failed. I attach here a small picture that she gave me, with an example of alignement at 28° angle of emission head, focal length 310 mm, initial beam space 35 mm. One major problem is that the beamwaist of beam 1 (where reception is focused) , is not exactly the beamwaist of beam 2, it is also different from the location where the two beam intersect in water. (see figure in the pdf file).
Hope you can find a solution.
Best regards
hubert
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Dear Sandro and Hubert, If I properly understand the situation, this is ray trace and spot diagram for beams with spacing of 35mm. They are shifted with each other in the water vertically and horizontally at 28 degree incidence. You will also observe coma and astigmatism aberrations. Shigeo
Dear Shigeo, Dear Hubert, you are right. However, we do not need a high quality image reproduction device. It is enough the overlap of the two beams more or less in the focus plane of the backscatter optics. The electronics should do the job of cleaning the signal. We shall try a fiberoptics alignment in water in the same identical configuration we would use in the wave flume. We will use a waterproof microscope to detect the focus and the overlap. In principle it should work.
A second possible solution is to add an internal and an external prism on the two sides of the glass, in order to compensate for refraction.
Thanks for your comments
ok, with a waterproof probe you may find a way to add some corrections.
Another point : as the situation is asymmetrical, the optical paths of the two beams (of the same wavelength) must not be exactly the same ... So, the interference fringes could be modified (?) I am not an optics expert .
Dear Huber,indeed I meant a waterproof microscope to make the alignment in water, but with a probe in air. The microscope could work not perfectly as in air, but it should do the job.
If you consider a LDV with two beams orthogonal to the glass windows, you have only to consider that the focal of the lenn is increased. In fact, the wavelength is reduced in water, with respect to the air, but also the half-angle between the two beams is reduced in the same way, hence the fringe spacing d=\lambda/(2 sin(theta/2)) does not change. The light scattered from the particles moving amongst the fringes has the same frequency in water and in air, and since the signal processing is always in the time or frequency domain, no correction is requested. I have not made the computation for the non symmetric case, but I feel that the results should be angle-invariant. After all, the beam rays in water do not have memory of their source (inside or outside the water), hence only local geometry should control the process of fringes formation.
Sandro, Hubert's comment on influence on inteference fringe is true, when you consider wavefront aberration such as coma and astigmatism caused by asymmetry in the water. In presence of these, at least fringe visibility would be much reduced. Shigeo
Dear Shigeo, I agree that fringes quality should be strongly reduced. The other relevant point is that, despite of my hopes and thoughts of my past message, also fringes spacing is modified if the two beams encounter the glass wall with a different angle. It makes everything even trickier.