Abstract
The advantageous employment of multimode fibres for beam delivery inlaser-Doppler anemometers (LDAs) is presented. Multimode fibres allow thetransfer of significantly higher power into the LDA measurement volume andneed less alignment effort than do the single-mode fibres which are usuallyemployed. Powerful laser diodes can now be applied for LDA set-ups, allowingsensitive velocity measurements of fluid flows. It is demonstrated for thefirst time that the length of the measurement volume is less than that of thevolume of intersection of the two laser beams because of the low spatialcoherence of the multimode light. Highly spatially resolved measurements ofvariations in velocity in flows can be achieved. As a first result, ameasurement volume with a diameter of 584 µm, length 40 µm and280 fringes is presented. This allows precise frequency measurements and thedetermination of movements of accelerated particles. The speckle pattern ofthe multimode beam is sufficiently suppressed by choosing high-aperture fibreswith high intermodal dispersion and by the use of laser diode arrays withlow coherence lengths. A high accuracy in the determination of velocitygradients in laminar or turbulent boundary layers can be achieved with themultimode-fibre LDA.
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