Available Photonics Experiments:

P5884 Laser Doppler anemometer
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  • Dual Beam interference
  • Interference pattern
  • Doppler Effect
  • Light Scattering
  • Recording Particle Speed
  • Fourier Transformation
  • Examples of investigations and measurements
Principle of operation
The emission of a DPSSL of 532 nm is divided into two beams. Both beams are recombined be means of a focussing lens producing a spatial interference pattern. Particles passing this zone creating timely modulated scattered light whereby the frequency depends on their speed as well as on the constant periodic pattern structure. A set of lenses focuses the scattered light onto a high sensitive and fast photodetector. The initial beams are blocked and do not reach the detector.
Anemos is a word coming from Greek and means “wind”. Consequently, a Laser Doppler Anemometer (LDA) may be termed as a “wind meter” using a laser based on the physical effect that Christian Doppler discovered in 1842. However, the LDA cannot just detect pure wind as a clean air stream; it needs to have particles moving with the wind. These particles are scattering the laser light and in fact their speed is measured. Due to the coherence of the laser light a spatial interference pattern can be generated within the crossing zone which look like interference pattern. When particles are moving through this pattern, they scatter the light resulting in specific bursts. Yeh and Cummins exploited the same principle in 1964 when they invented their Laser Doppler Anemometer.
OX The set-up
uses an ultra sonic particle seeder (10) and a nozzle (6) to create a stream of particles to be measured. The nozzle is covered by a transparent plastic cover to keep the moisture enclosed. The speed of the water vapour jet can be adjusted in such a way that a clear series of bursts are obtained. The scattered light is passed via the collimating (7) and focussing lens (8) to the extreme sensitive photodiode. The amplifier is built into its housing (9) and the signal as well as the operation voltage is connected via a multi pin connector. The laser beam is provided by a DPSSL (1) with an output power of 10 mW and a coherence length of 4 mm. By means of an arrangement of prisms (4) the beam is divided into two rays. The plano convex lens (5) deviates and focuses the beam in one spot where the interference pattern is created. A digital oscilloscope is required to display and store the individual burst for subsequent analysis.
  • P5884 Laser Doppler Anemometer (LDA) consisting of:
18X-XP151Manual Laser Doppler Anemometer
2ED-00101Adaptive power supply APS-05
3ED-01501Ultrasonic particle seeder controller USC-01
4ED-03101Photodetector, ultrafast with amplifier
5MC-02601Optical rail MG 65, 500 mm
6XM-00341Module - collimating optics, f=60 mm
7XM-00401Module C - Focussing optics, f=60 mm
8XM-02801Dimo diode laser module, 532 nm (green), HC
9XM-08701LDA Beam splitting assembly
10XM-08761LDA beam divison and focussing lens module
11XM-08801Ultrasonic particle seeder
 Required Options:  
 TP-01001Oscilloscope 100 MHz digital, two channel