Experiment Laser Doppler Anemometry (LDA)

The Laser Doppler Anemometry (LDA) is a contactless technique for velocity measurement. Its principle is based on the phenomenon of Doppler frequency shift of light (laser) scattered by moving particles. The frequency shift is directly related to the speed of the particles, making it an absolute measurement technique that requires no calibration. This kit requires careful optical adjustments and alignments to obtain the highest possible contrast at the beats. However, in case of degraded signals, the beat frequency can still be estimated either by measuring the temporal difference of the oscillations, or by the fast Fourier transform function of the oscilloscope. Topics covered: - Doppler shift, velocity measurement - Two-wave interference, light scattering - Particle size, frosted or striped disk - Influence of focusing on the beats - Fourier Transform Principles and objectives: The objective of this kit is to demonstrate and study the operating principle of a Laser Doppler Anemometer (LDA). In general, this technique is used to determine the velocity of a flow (air or liquid). In the proposed experiment, a laser beam is divided into two coherent rays that will intersect and interfere at the measurement point to create a network of periodically spaced bright and dark fringes. A rotating acrylic disk is placed in this measurement zone. Dust particles stuck on the surface of the disk allow the scattering of light. When the particle passes through a clear zone of the interference field, it reflects the light ... then when it passes through a dark zone, it no longer reflects it. In the end, a detector collects a periodic signal whose frequency allows the deduction of the particle velocity and therefore the angular rotation speed of the disk. The modulation amplitude depends on the particle size and semi-quantitative conclusions can be drawn. Another application concerns the measurement of the velocity of a liquid flow passing through a cell connected to a small pump.

Composition:
▪ 1 optical rail 0.8 m, with graduated ruler
▪ 1 laser diode in mount with adjustment and support on rider
▪ 1 laser diode controller with integrated oscilloscope
▪ 1 variable attenuator filter with support on rider
▪ 1 beam splitter and optical redirection with support on rider
▪ 1 focusing optic with support on rider
▪ 1 motorized rotation system with acrylic dispersion discs on support
▪ 1 rotation system controller
▪ 1 collimation lens with support on rider
▪ 1 photodiode with support on rider
▪ 1 experiments manual (in English)