Fibre Optic Delivery of High Peak Power Q-Switched Laser Pulses for Flow Measurement

High peak power frequency-doubled Nd:YAG laser pulses have been delivered using a bundle of optical fibres for the purpose of particle image velocimetry (PIV) of high speed flows inside automotive internal combustion engines. Data loss due to fibre speckle has been minimised by ensuring that the individual fibres in the bundle differ in length by more than the coherence length of the laser being used. This means that although the speckle patterns from each fibre will overlap, they will not interfere. Furthermore, we have shown by modelling that it is the modulation depth in the speckle (contrast between light and dark regions) that is the limiting factor rather than the size of the speckle regions involved. The issue of damage to the fibres has been addressed by the design and manufacture of a diffractive optical element to condition the laser beam and prevent the fundamental problem of self-focussing. Using this method, delivery of pulse energies in excess of 25 mJ is possible. Fibre optic bundles and associated light-sheet forming optical arrangements were designed and constructed for a series of in-cylinder PIV measurements in an internal combustion engine, including flows inside the cylinder head, where conventional free-space beam delivery could not be used. Results from these experimental trials are presented, and comparisons made with measurements generated using conventionally-delivered laser beams.