When children are playing while striking the vertical bars of a picket fence with a stick it gives a characteristic burst like noise. The faster they are running the shorter the burst is however the frequency of the strikes is higher. The same principle Yeh and Cummins exploited 1964 when they invented their Laser Doppler Anemometer. Anemos is a Greek word and means “wind”. Consequently a Laser Doppler Anemometer (LDA) can be considered as a “wind meter” using a laser
Consequently a Laser Doppler Anemometer (LDA) can be considered as a “wind meter” using a laser. However the LDA cannot detect pure wind as a clean air stream, it needs to have particles moving with the wind, These particles are guided to move through two crossing laser beams created from one source. Due to the coherence of the laser and thus also of the two beams a spatial interference pattern appears within the crossing zone looking like zebra stripes. When particles are moving through the stripes they scatter the light in preferred directions. Instead of playing children the set-up uses an ultra sonic particle generator. Via a nozzle the "dry" water particle are perpendicular crossing the interference zone. A photodetector combined with a fast and sensitive amplifier is used to detect the scattered light. A storage oscilloscope is required to display and store the individual burst for the subsequent analysis.
Special care must be taken in the selection of the laser source as well as the beam splitting section. Both components contribute significantly to the contrast of the interference fringes in the beam crossing zone. The laser source should be a single mode laser with a Gaussian beam profile in TEM00 mode. Furthermore the polarisation of beam A and B must be the same which requires a careful selection of the beam splitting and bending prism. Another design criteria is the aberration minimized imaging of the focusing lens. Finally the beam stop and the photodiode sensitivity with subsequent amplifier determine a good signal to noise ratio.
Another key is the provision of a suitable particle jet consisting of small and “dry” particles. The experiment comes with a two stage ultrasound particle generator with adjustable jet flow speed. The Fig. 2.30 shows a burst event with simultaneous FFT. It will take a while to catch such events since a lot more events exist. It is important to set the trigger of the oscilloscope to single events with a trigger level high enough not to react on small bursts. Also one trace will contain probably more than one burst, but a digital oscilloscope can be focused and expanded to a desired event. The storage possibility of the oscilloscope allows also storing of the data on a USB stick for further evaluation with Excel or other software.