An optical fibre delivery system for pulsed laser particle image velocimetry illumination

We demonstrate the design and implementation of a fibre-optic beam delivery system for particle image velocimetry applications. The system is designed for the transmission of pulses of high peak power from a Q-switched and frequency-doubled Nd:YAG laser with energies typically in the range of 9 mJ; it is hence suitable for illuminating air flows. Practical laser-induced damage thresholds for large-core step-index multimode fibres are determined, and through exploitation of inter-modal dispersion in the fibre, intensity variations in the output from the fibre due to laser speckle are suppressed to a visibility of <3.5%. Effective use of the fibre-optic beam delivery system for particle image velocimetry was demonstrated for a turbulent air flow seeded with corn oil droplets of the order of 1-2 mu m in diameter; the mean air velocity was 5.5 m s^-1.