Fibre optic beam delivery system for high peak power laser PIV illumination

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Abstract
Diffractive optical elements (DOEs) are used to couple Q-switched and frequency-doubled Nd:YAG laser beams into optical fibres to achieve significantly increased damage thresholds, enabling fibre optic beam delivery for high-speed particle image velocimetry (PIV) measurements. For single fibre delivery systems, the maximum pulse energy density that can be transmitted is increased by a factor of 5-10 compared with the best that can be achieved using conventional optics, of up to 10 mJ in a m core diameter fibre at 532 nm. We also applied the DOE arrangement to our previously developed bundle delivery system, comprising nineteen m core diameter fibres. On testing, over 1000 pulses with energies of 30 mJ were successfully transmitted, with no indication of damage. This allows fibre delivery to become a practical option for many air-flow PIV applications, as demonstrated here with measurements of flow in a square duct.