Laser Doppler Velocimetry / Laser sheet tomography

 

LDV Barricau ENG

Laser Doppler technique optical principle

 

two wavelength interference pattern laser sheets

488nm and 515nm interference fringe patterns

 

LDV lasers setup

Two lasers 488nm and 515 nm

Credits, Dr Ph Barricau ISAE France

 

Laser Doppler Velocimetry / Anemometry (LDV) / LDA):

Measurement of the velocity fields 1D, 2D, 3D in liquid flow, gas flow, wind tunnel, combustion process, spray analysis…Historically, Argon lasers have been used in this application because of their single frequency linewidth and high visible powers. The laser beam must have a sufficiently large coherence length (to see the interferences) and a high pointing stability in order to ensure stability of the static interference pattern in the movement region. The laser must also have a high power stability because the measurement is based on small fluctuations in the reflected light.

Laser sheet tomography:

Flow visualization, quantitative measurement of combustion, Photo Laser-Induced Fluorescence (PLIF),…These measurements may be combined with Particle Image velocimetry (PIV). In this application, a high coherence length is not needed, but a high pointing and power stability are required, as well as an excellent spatial mode quality. A low noise version may also be interesting (single frequency version) for the most demanding setup with high sensitivity detectors.

a.      ALS technological advantages

  • Thanks to its cooler less laser head, its fiber technology and its single pass frequency doubling process, the ALS lasers offer an excellent pointing stability and spatial mode quality/stability which are crucial parameters for LDV and PLIF.
  • The long coherence length (over hundreds of meters) of the ALS lasers is well enough to produce stable interferences pattern (even on long distance like in wind tunnel) and the stability of the central frequency is excellent to keep these interferences over a long period.
  • Historically Argon lasers have been used for these application which means that a lot of systems are currently in place all over the world (hundred(s) in France, few thousands in the World) and the main suppliers are Dantec and TSI. ALS lasers have been qualified by Dantec to replace their Argon lasers. It is also possible to couple Dantec/TSI systems with ALS lasers to upgrade performances and reduce maintenance costs.
  • The lowest the noise level, the better the measurements.
  • Power stability. Best on the market (< ±0,2% short term and < ±0,3% long term)
  • Ease of integration (no cooling) and ease of transport (small size and standard electricity).

 

  Lasers versions

ALS-VISIBLE 500mW to 10W@488nm, 515nm, 532nm – single frequency or narrow bandwidth

ALS-OCEAN 500mW to 2W@488nm, 515nm, 532nmSingle frequency or narrow bandwidth

https://en.wikipedia.org/wiki/Laser_Doppler_velocimetry
http://velocimetry.net/ldv_principles.htm
http://www.onera.fr/en/actualites/zoom-in-the-lab/laser-measure-noise
http://www.vidix.se/Fluid_Mech.htm