LII in an aero-engine exhaust using a low peak power fibre laser

A commercial fibre laser (SPI G3 Laser) delivering 192 ns long pulses at a rep rate of 30 KHz has been used to observe LII in the exhaust of a modified helicopter engine. Images of LII, collected at 90o, from a focused Gaussian beam were recorded at average laser powers 2.2, 11 and 21 W.The waist diameter of the focused beam was 0.15 mm, corresponding to fluencies of 0.13, 0.64 and 1.21 J cm-2 for each power condition. As power increases, the distance along the beam over which LII can be detected increases and at 21 W a dip in LII intensity, expected due to particle vaporization at high fluence, was observed. By examining the signal level at different distances from the focus, fluence curves were constructed and compared with predictions1. During engine experiments gating of the camera was not possible and the minimum time integrated images were obtained over 0.01 s (300 pulses). The temporal behavior of LII using the SPI laser with a particle generator will be discussed.