A multimode-fibre laser-Doppler anemometer for highly spatially resolved velocity measurements using low-coherence light

摘要

The advantageous employment of multimode fibres for beam delivery in laser-Doppler anemometers (LDAs) is presented. Multimode fibres allow the transfer of significantly higher power into the LDA measurement volume and need less alignment effort than do the single-mode fibres which are usually employed. Powerful laser diodes can now be applied for LDA set-ups, allowing sensitive velocity measurements of fluid flows. It is demonstrated for the first time that the length of the measurement volume is less than that of the volume of intersection of the two laser beams because of the low spatial coherence of the multimode light. Highly spatially resolved measurements of variations in velocity in flows can be achieved. As a first result, a measurement volume with a diameter of 584 μm, length 40 μm and 280 fringes is presented. This allows precise frequency measurements and the determination of movements of accelerated particles. The speckle pattern of the multimode beam is sufficiently suppressed by choosing high-aperture fibres with high intermodal dispersion and by the use of laser diode arrays with low coherence lengths. A high accuracy in the determination of velocity gradients in laminar or turbulent boundary layers can be achieved with the multimode-fibre LDA.